UNIT 399

Medellin Colombia

                            After receiving numerous requests for 858SSB information, I decided to republish this page.  - 399. Click here to add your own text and edit me. It's easy.



The Uniden 858 ssb chassis was produced between1976 and 1978, and was sold under many brand names
including Cobra, President, Realistic, Teaberry, and Robyn, among others. This chassis was (and still is) the
BEST mass-produced type-accepted CB radio ever made!! It was designed for maximum performance (not lowest cost) and was built from high quality components. Because it could be easily modified to exceed FCC limits for output power, modulation, and frequency range, its manufacture was discontinued due to pressure from the FCC in late 1978.
I run 858 chassis on my base (Realistic TRC-457) and in my mobile (Cobra 138XLR). There are many mods for these rigs that will not be presented here because they are easily found on other web sites. What follows is a compilation of mods I personally have developed, with the main focus being increased reliability and on-the-air performance. Also, some of the circuitry exclusive to the 858 ssb chassis (like the speech clipper/modulation limiter) is explained in detail.

If anyone has any suggestions, questions, or whatever ...let me know. Email me at: unit.three99@gmail.com
- 399




This site is written for the experienced technician. If you don't know what you're doing and you screw things up ...




 Back in the late 70s, the CB craze was running full tilt. Everyone and his uncle had a Cb in their mobile, and many base stations were out there too.  There was a battle between the big time CB manufacturers (Stoner, CPI, et al, to build the ultimate cb radio. Uniden was the new kid on the block, and wanted to strut their stuff.  Uniden had an 858pll chassis in production, so, they upgraded this chassis, creating the PC196BD board model,  which proceeded to kick ass on-the-air and did so at a price far below what the elite manufacturers were asking.  This rig was a total success, and well over 2 million base/mobiles were sold. This was called the Generation 2 Upd858 SSB.   The gen 2 model 858 was sold under various brand names; Cobra, President, Realistic, Teaberry, Courier and  Robyn, and others.  All of the Gen 2 858 rigs are the same, with the exception of the Realistics. For some reason or another Radio Shack  decided to use a different (less powerful)  transformer in their base stations' power supplies than the ones available from other marketers. There are also some  component differences. These will be explained later. If you can get a factory service manual, do so as it contains a wealth of information and is very well written. Copies of the Realistic manuals are available from Radio Shack. You can download a copy of the TRC449 factory manual here:


 I am going to assume that your unit is operational.  Do not attempt to work on it without appropriate test
equipment, dummy load, and factory service manual or SAMS. Mark all changes on your schematics for future reference. If the rig that you have has been previously "peaked and tweaked", check to see if the mike amp limiter (TR18, 19, or D20) or the AM AMC (D23) has been clipped or removed. If so, restore the limiter circuit to stock, and then replace relay RL1, and C179. Chances are they've been damaged, and will fail sooner or later, so you might as well change them out now. The relay is available here:

858ssb replacement relay



The following changes are very important (especially the installation of Bergquist insulators), and  should be done to any 858 chassis to eliminate shortcomings and improve overall performance . The important thing to remember is that 858SSB radios in good condition are getting harder to find. Sets in mint condition are fetching a pretty penny, and will only continue to go up in price.  Many of the parts used on these rigs (driver, final, audio output chip, VCO, etc.,)  are NLA from the manufacturer. So protect your investment and keep your set in top condition. Some of the following mods might seem trivial, but they will help your 858 perform at 100%+  for a long time.




The first thing to do is to replace all of the thick white insulators from between the power transistors and the heat sinks. The white ceramic insulators were used as a replacement for mica insulators because they exhibit better thermal conductivity, and their thickness (.5 - 1.5mm) reduces capacitive coupling between the device and the heatsink. The drawbacks are that they are rigid, crack easily, and require the use of heatsink compound which is toxic and messy. Both the insulators and the compound contain berillium which is quite toxic. Replace the ceramics with Bergquist SIL-PADS or Bergquist type 800 insulating mat'l. These are thermally-conductive insulators which are grease-free, and flexible. They handle  high voltages, have superior thermal conductivity, and are virtually puncture resistant. I prefer to use the Bergquist Type 800 insulating material. It has double-sided adhesive and eliminates the need for any mounting hardware. It
comes in 12"x12" sheets, so it can be cut to size and used with any transistor or tabbed IC.  It's available from

DIGI-KEY 1.800.344.4539 FREE    Order Digi-Key part #Ber348-ND. About $20 per sheet.



On base station models, unbolt TR401 and D406 diode bridge from the chassis. Discard TR401's mica
insulator. Clean off all of the heat sink compound from the devices and the chassis. Remount both devices using Bergquist insulating material. Install a small clip-on heat sink on TR402.



Make sure that L6 and FT2 are installed in your unit. In some sets (TRC449, Cobra 138/139XLR) they were
omitted and replaced by a jumper (JP-37). These parts are absolutely necessary for best adjacent channel
rejection. Unfortunately, they are no longer available from Uniden America or Dynascan. 


NOTE:  Uniden America and Dynascan are no longer stocking these parts. I am searching for another
source and will post if/when I find one. 

Just as an additional note to persons looking for these parts. Coil L2 on the Cobra 142gtl, 148gtl and 2000gtl models is the same LA-179 coil used as L6 in the 858ssb sets. FT2 on these same models is the same FL-046 as on the 858. So maybe they can be scrounged from an old junker you have laying around.
- 399



Bridge a piece of buss wire across capacitors C202, 203, 220, 221, 222, 223, and 239. These caps are
connected between the ground plane on the main board and the chassis mounting points. Their only function is to isolate the chassis from the circuit board in the event that the radio is installed in a vehicle that has a positive ground electrical system. On base stations,  install a piece of buss wire from the ground plane to each of the mounting screw pads on the power supply board. Also install a piece of buss wire across C606 on the noise blanker pc board, and across C406 on the mic jack.


Grounding the chassis will improve its shielding ability, and help reduce receive noise.



Change C304 to a 10,000uf/50volt computer grade electrolytic (Base stations). Change  C107 to 3300uf/35v.

Change C102 to 1000uf/35volts.

 *************************************************Realistic sets only **************************************************


Locate D23 (next to VR7). In the Realistic sets (TRC449, 457, 458) a 220 ohm resistor is installed in the D23 location. Replace this resistor with a 1N914 diode (observe polarity). Replace C87 (2uf/10v  tantalum) with a 10uf/16volt electrolytic. This will make the Realistic modulation limiter circuit the same as on all of the
other 858SSB sets. This simple change really opens up the audio on the Realistic 858s. Your regular contacts
(especially locals) will be amazed at the improvement in your audio. However, do not attempt to defeat the AM modulation limiter, as this will degrade  performance. In this case, more is definitely not better.

Replace VR9, VR15, and VR16 with Cermet (or equiv.) multi-turn pc-mount variable resistors of the same values.This will allow more accurate setting of the transmit  frequency (VR9), and of the driver and final bias (VR15/16).  All of these parts are available (cheap!) through AMAZON.  

Search: 3296W Cermet potentiometer trimpot

Note: if a 300 ohm variable resistor is not available, use a 500 ohm  instead.





Replace D10,11,15,and 16 with 1N6263 schottky diodes. This will improve receive sensitivity measurably. DO NOT REPLACE TR5 WITH A 2SC2999 TRANSISTOR. The original equipment transistor (2SC1730L) has higher ac
current gain, and replacing it will degrade performance.




If your rig does not have a front panel mike gain control, install one. See SECRET CB Vol. 8 page 39 (on CBTRICKS website)  to see how to do it. 

Replace ALL of the electrolytic capacitors. These radios are over 40 years old, and the capacitors are well past
their rated life. When replacing the 2.2uf/10volt tantalum capacitors (C55, 68, 87, 179, 211), use tantalum caps of the same value, only 16v rating. Use high quality caps and not Chinese junk! Some techs also recommend upgrading all of the 2SC1419 transistors (2SD325E in the Realistics). I believe that this is a waste of time, as their high initial failure rate was due to insufficient heat sink compound. I've only had two of these transistors fail, and, in both cases,  the cause was a defective insulator (my fault). However, if you want to do this, go ahead. It won't hurt anything but your pocketbook.

Do a complete alignment per the factory manual/Sams.(CAUTION!! If you have a Pres Grant, Pres Washington, Cobra 138XLR or 139XLR read the
"Word to the Wise" below.) 

On base stations, adjust RT301 for a power supply output of 15.0 vdc. Adjust VR7 for maximum AM modulation. Set the AM and SSB outputs to your preference, but do not exceed 4 watts deadkey on AM or 12 watts pep on ssb. You will now have a rig that will kick ass on-the-air,  and be as reliable as the day is long.

Note: These sets will easily exceed 4 watts AM/ 12 watts ssb. However, running these rigs "balls to the wall" is
a mistake. All it accomplishes is to stress components to their breaking point. Especially the driver and final
transistors, and the T/R relay. The relay has only recently become available again, but for years many otherwise good 858 sets were sent to the junkpile for lack of a replacement. Maxing the output won't make you sound  much louder on the receive end. If you want more power ... run a linear. 






There has been a lot of confusion concerning the bias settings on the 858ssb chassis. The Sams for the Cobras and the Presidents Washington and Grant, as well as the Cobra service manuals, call for a driver bias of 40 ma, and a final bias of 70 ma. The Sams for the Realistics, Robyns, Teaberrys, and the Presidents Adams and Madison
(plus the Realistic service manuals) specify 40 ma and 15 ma respectively. The reason for this difference is that Dynascan (Cobra) and President placed their 858 orders with Uniden well ahead of the other marketers. Robyn and Teaberry waited until 3 months later to place their order. Tandy waited until June of ‘77 to place their order to give dealers time to sell existing stock, and to have the new rigs available for the ‘77 Christmas sales season. 


In the intervening time, Cobra and President were overwhelmed with warranty claims for blown drivers and finals. Uniden suspended production, set out to determine the problem and devise a solution.  It turned out that the original driver/final circuit design had too much power potential. When these rigs were set to the FCC limits of 4 watts AM and 12 watts pep SSB, they would cruise along all day no problem. But when the screwdriver jockeys
in the field cranked up the supply voltage, peaked out the transmitter, and clipped all of the limiters, these sets blew drivers like crazy. The final transistor, which ran warm anyway with the specified 70 ma bias current, started smoking from excessive keydown times. 


Uniden responded by redesigning the driver/final circuit for less output potential, and specified a new final bias
setting of 15ma. They issued field service bulletins 1208 and 
1208A which specified the necessary circuit changes, and sent the bulletins to their authorized service centers.  The Sams for the Cobras and the President Grant and Washington were published before the service bulletin,  and show the original circuitry and the 70 ma final bias
spec. On the other hand, the Sams for the Realistics, Robyns, Teaberrys, Pres. Adams and Madison (as well as the
Realistic service manuals) were published after the service bulletin and show the revised circuit and the new 15 ma bias spec.


BTW, the service bulletins are shown on page 5 of Sams #135.


Many techs are unaware of the service bulletins and are setting the bias wrong. If you have any doubt .....







Just recently, we had a HUGE lightning storm roll through our area of Colombia. We had a direct lightning hit on our power lines, and lost power for several hours. We have over-voltage surge protection on everything at our farm, and fortunately we didn't lose anything to the power surge.

Our neighbors weren't as fortunate, and lost their TVs, refrigerators, microwaves, and other electronics that weren't protected.

I always install a 130vac varistor on the line input of any new electronics we buy because it's CHEAP INSURANCE !! I purchased 100  GE V130LA20A 130V 6.5KA Metal Oxide Varistors from a seller on Ebay, and it was one of the best buys I ever made. Simply install one of the varistors across the AC line input to any device, and it will absorb any HV surges from lightning or whatever. 


I installed a varistor across the ac line input on all of my radio equipment, as well as across all of the wall plugs in our house and in my shop. Haven't lost a single piece of equipment since I did this. Persons that utilize cb or amatuer transceivers need to protect their equipment in the same manner. Having a tall antenna invites lightning, and protecting against it is a must !!!
Install a varistor across the ac line in ALL of your equipment. Just solder the varistor across the AC line
where it enters the equipment, and then breathe easy.




 As is the case with any transceiver, microphone selection is of paramount importance for best performance. A rig used primarily for dxing should have a mike with a response curve that favors voice frequencies, while a radio used mostly for local nighttime conversations needs a mic with a flat response curve to provide a more natural sound to the operator’s voice. 

When working in heavy skip, the band is saturated with QRM. When you keyup and throw your signal into this mess, you need your audio to be loud and clear. This way you will punch through and be heard. A microphone whose output only contains voice frequencies is imperative. Anything else just adds more noise and reduces your clarity. On the other hand, When operating locally at night when the band is quiet, a mike with flat response that makes your voice sound  “natural,” will give the listener the impression that you’re sitting next to him.

Because the 858SSB chassis has a powerful built-in mike amp, an amplified microphone is not necessary. In fact, it can actually negatively affect the clarity of your rig’s audio. The mike input on this chassis is the standard unbalanced 600-ohm low impedance. There are many great aftermarket “broadcast” mikes out there, and this is the best way to go if you don’t mind spending the $$$. A broadcast mike is the type used by announcers at live sports broadcasts. These mikes favor the announcer's voice and attenuate the crowd noise.The internet is full of reviews on mikes of this type, and I will leave it to you to make your choice. Remember, if you buy a “balanced” mike, you will have to use a balanced-to-unbalanced adapter.


I personally prefer a hand mike, even for base use. My rig is set up in my workshop, and I am usually copying the mail whenever I’m in there working. It’s convenient for me to be able to pick up the mike, keyup, and wave my hand without having to get up from my bench.

Pre-1985 Uniden hand mikes are a good match, because this

is what the 858SSB rig was designed to work with. The best

match for my voice that I have found is the CMP22 handmike

that came with the (Uniden-built) SUPERSCOPE Aircommand

640. Excellent voice clarity and amazing output for an

unamplified dynamic. Uniden mike element 6K44. This

element must have a magnet with extremely high flux density.



Surprisingly, the Realistic 21-1172 hand mike (pre-’85) is my

2nd favorite. Very sharp voice response and output just a little

shy of the CMP22. But, unless you’re running a realistic 858

rig, you’ll have to change out the 5-pin DIN plug. Zillions of

these mikes available CHEAP. Always use a pre 1985 model as 

the mike element in newer models is junk.

Experiment with different mikes and find what works best for you.






A line out circuit is a good addition to any CB radio, as it allows the user to record his contacts. In addition, it
allows the connection of audio denoising and/or amplifying gear to produce a better sounding receive output.


The circuit shown below is used in the Cobra 1000 and 2000GTL models




The factory issue power switch is a low reliability device, and cannot handle the additional power required by some of the following mods. After having several switches fail due to internal arcing, I developed the following circuit which uses a relay to switch the power. The relay is controlled by the
original power switch.




The following is the frequency chart and pinouts for the UPD858 pll chip
(used by permission of Custom Conversions)




With more operators using export or amatuer transceivers for 11 meter dxing, the use of frequencies 5kc below the standard CB channels  (i.e. 27.400 instead of 27.405) is becoming popular on single sideband. Typically, operators with regular cb rigs "unlock " the clarifier and swing the transmit frequency up or down 5 kc to get these "zero" channels. However, unlocking the clarifier opens a
Pandora's box of problems because using the clarifier to tune in an off-frequency station will then put your transmitter off-frequency too. This paper explains how to modify the popular Uniden 858SSB chassis to select frequencies in 5 kc steps rather than the standard 10 kc switching, allowing the
transmitter to pick up the "zero" frequencies without unlocking the clarifier.


Many of the older PLL chips, like the PLL02a and UPD858c were used in applications other than cb radios. Because of this, these chips had an input pin that could be programmed for 5 or 10 kc channel steps. On the 858 chip this is done by manipulating the logic state on pin 7: Logic 0 = 5 kc steps, Logic 1 = 10 kc steps. To perform the mod, first isolate pins 7, 19, 20, 21, and 22, and install 4.7k or 5.1k ohm
resistors as shown below. Connect the pins to 5 SPDT CENTER-OFF switches also as shown


Then set the logic state on the pins as follows:
Pin 7 - logic 0
Pin 19 - logic 1
Pin 20 - OFF
Pin 21 - logic 0
Pin 22 - Logic 1
The unit will now tune from 27.335 to 27.430 in 5 kc steps
on channel selector positions 24 to 40


For additional 5 kc freqs set the logic as follows:
Pin 7 - 0
Pin 19 - 0
pin 20 - 1
Pin 21 - 0
Pin 22 - 1
The unit will now tune 27.440 to 27.510 in 5 kc steps on
channel selector positions 8 - 22, 24, 25.


For even more freqs, set the logic at:
Pin 7 - 0
Pin 19 - 0
pin 20 - OFF
Pin 21 - OFF
Pin 22 - 1
And ... the unit will tune 27.510 to 27.730 in 5 kc steps on
selector positions 1 thru 40





The switch shown above works perfectly with the 5 kc steps mod shown above.  It is a 6-pole 4-position switch, and can be purchased on Ebay. Two of the switches cost about $10.00 w/ shipping.


Connect the switch as shown, and you're ready to go. 

The switch can be purchased HERE






The Uniden 858SSB chassis utilizes a form of audio processing called speech clipping to increase
average modulation. This is UNIDEN's most sophisticated clipping circuit, and should be left as is. 

The diagram below is a simplified drawing of the circuit.


The audio signal from the mic is fed to the uPC592H mic amp. The output from the amp is sampled, and then limited by TR18 and 19. This is a very "tight" limiter, such that the output from the mic amp is practically the same whether the operator shouts or whispers into the mic. The audio is run through a gain control, and is then clipped by the back-to-back diodes D21, and D22. With the mike gain control set to maximum, the audio output from the diodes is a constant 1.3 volt peak-to-peak almost-squarewave. This signal is then amplified by TR20 to a level sufficient to drive the output modulator to 100%+ modulation, and the low-pass filter (L10,11, C71 thru 75) rolls off the harmonics generated by the clipping. 


The result of the clipping is audio that is extremely loud and clear. The real beauty of this particular design is that it is functional on both AM and SSB.
The drawback to clipping is that the operator's voice loses it "natural" sound, and the overall audio bandwidth is restricted. The bandwidth is further restricted by the low-pass filter used to remove clipping harmonics.  
To open up the bandwidth, and have more natural sounding audio, it is possible to bypass the clipping circuit. The drawback to bypassing is that the AMC is disabled, forcing the operator to use the mike gain control to limit modulation. The following mod allows the clipper to be switched in/out.


The diagram below is a schematic of the clipping portion of the 858ssb audio circuit. The component designations have been removed to unclutter the diagram.


Cut the circuit traces between points A & B, C & D, E & F. Then connect a 3 pole double-throw relay or an
in/out switch as shown in the hookup diagram. After the mod is complete, it's a good idea to look at the output of the rig on a 'scope to determine the optimum mike gain setting for 100% modulation.


Without the AMC circuit, these radios will overmodulate somewhat regardless of the mic gain setting.
The limiter on the uPC592H mic amp will keep the modulation pretty much under control, but expect to hear reports that your rig is "bleeding over"  a little when the clipper is bypassed. Don't worry, the bleedover won't be excessive. I would suggest that you not do the MIC AMP LIMITER MOD #13 if you decide to bypass the clipper.


In any case, the clipper can now be switched in for dxing and noisy daytime conditions, and switched out for local nighttime conversations.




As mentioned above, L6 and FT2 are omitted in some of the 858SSB sets, and are replaced by a jumper (JP-37). Without these parts, adjacent channel rejection totally sucks in these rigs. Obtain the parts, and install them as shown below.

For even better rejection connect two FT2s in series and install them in the FT2 location. Adjust L6 for maximum receive output.

FT2 is a monolytic crystal filter. It is 2 crystal wafers on a single substrate mounted in a HC18/U package. A signal containing the filter’s resonant frequency (in this case, 7.8mhz) is fed into the filter and through the crystal to the output. Since the crystal will only vibrate at its resonant frequency, other freqs are attenuated and will not pass through. Connecting 2 crystals in series adds an additional stage of attenuation providing even better adjacent channel rejection






The clarifier on the 858SSB rigs is very sensitive, and it's difficult to easily lock on to a station's signal.  A 10-turn clarifier will work, but going from one side of a channel to the other is a slow PITA. This mod makes fine tuning easy. I found a 1k ohm pot in my parts box.  I used it to build the circuit below , and added it as shown. It allows a swing of a few hundred cycles above or below the
frequency selected by the main clarifier control, so the user can easily lock on to a station. . If you don't want to add another pot to your rig, use the dual clarifier control from the Cobra 2000GTL (available from Cobra/Dynascan)






All of the 858 base transceivers will operate on AC or DC power. As such they have a hardwired AC power cord, and a 13.8 VDC jack on the rear panel. When the radio is being operated on 110 vac, there is 13.8 vdc present at the dc power jack. This can be used as a power supply for another radio or 12v accessories.


As usual, the exception to this is the Realistic rigs. Their sets use a plug-in AC power cord, and have an AC and a DC jack on the rear panel. The AC power jack has a built-in switch that disconnects the DC jack whenever the AC power cord is connected. 


The first portion of this modification will show how to bypass the switch and make the Realistics' DC jack live whenever the AC supply is being used.  The second part of this mod shows how to add a relay to disconnect the DC jack whenever the rig is in transmit. I use an LED audio spectrum display and a 7-band car stereo equalizer to fine-tune my receive audio. These are powered from the DC
jack on the rear of the radio. When I keyup, the 2nd relay fires and disconnects power from the DC jack, so that the on-board supply delivers maximum power to the transmitter only.   


Remove the orange and the yellow wires from the rear of the AC power jack, and disconnect everything soldered to the positive (+) pin on the DC power jack. If you do not want the DC power at the jack to be switchable, simply solder the yellow and orange wires to the positive
(+) pin on the DC jack, and you're finished. 

If you want the DC power at the jack to be switchable:

1. Solder  the orange and yellow wires to the common contact pin on the relay as shown in Fig. 1.


2. Mount the relay to the chassis using double-sided adhesive tape. 


3. Run a red wire from the positive pin on the DC power jack to the NC contact pin on the relay. Connect another red wire to one of the coil terminals on the relay, and solder it to the point on the main board shown in Fig. 2. Connect a black wire from the other coil terminal on the relay to one side of a SPST switch. Connect a second black wire to the other side of the SPST switch and solder the other end of
the wire to the point shown on Fig. 2.

Mod complete.

To add a relay and switch to rigs other than Realistics:

1. Disconnect the wire going to the positive (+) terminal on the DC power jack, and solder it to the common contact pin on the relay.


2. Do steps 2 and 3 above, and the mod is complete. 






Many times techs will add a new feature to a rig (like a 10kc jump switch), and will disconnect an existing switch to use with the new feature. Other techs will drill holes in faceplates or cabinets and install toggle switches, which just reduces the value of the radio, and looks tacky too. The easy way to install new features is to replace a control pot, like the squelch, rf gain, clarifier, etc., with an identical value pot w/push-pull switch.


The Alpha potentiometers shown above are available in 5k, 10k, 20k, 50k, 100k,and 200k values and each incorporates a DPDT switch operated by pulling the switch shaft in/out.  


On my TRC-457, I replaced the rf gain, squelch, mike gain, clarifier, and SWR cal controls with new controls with switches. I was able to add a tape monitor, 10kc jump, DC output jack control, cooling fan switch, and a key-up tone switch to the rig without disabling the ANL, NB or PA/CB features. I had previously removed the headphone jack and installed a mike gain control with a factory 457
knob. So, the rig looks stock and unmodified.  I found the pots/switches on ALIEXPRESS. There are several companies that sell them, but one offers mix and match if you buy 25 or more. At $1.44 ea. with free shipping, it was a no-brainer. Only took 8 biz days from China to Colombia, door-to-door. Probably faster to the US.




On a stereo receiver, the Tape Monitor switch allows the user to take the audio signal from the receiver preamp, and process it using equalizers, audio de-noisers, DSP units, etc., in order to tailor the sound to the user's individual liking before sending it on to the final amplifier. Installing a "Tape Monitor" on a cb or amateur receiver is useful when Dxing, because it allows the operator to use the
same type of audio processing equipment to remove noise and boost voice frequencies. This helps pull those weak signals "out of the mud" and allows contacts that otherwise wouldn't happen. 


I first installed a tape monitor on my TRC458 back in 1985. The results using an equalizer were so good that I sent a writeup to SECRET CB. The writeup was published in Volume 23 on page 12. Noise reduction has improved measurably since '85, and now a monitor switch is an even more important addition to any radio.


Here's how to install a tape monitor on an 858SSB base or mobile. Other rigs are similar. 


Open up the radio, and install a stereo RCA jack (Radio Shack # 274-332) on the rear panel.  Find the
WHITE wire connected to the wiper of the volume control. This wire is the output of the AM and SSB detector. Remove this wire from the volume control. Solder another White wire to the wiper of the volume control, and connect the two white wires as shown in Figure 1 above. Use either a dpdt relay or
a dpdt switch. Your choice. If you use a relay, mount it as close to the stereo RCA jack as possible. Double-sided tape works good for this. Use one of the switches shown in Mod #9 to operate the relay.


Then connect an Eq, DSP, or any other de-noising equipment in-line, and hear the improvement it
makes in your receive. Switching the monitor In/Out lets you hear the difference.  Enjoy, enjoy, enjoy.





This conversion was developed and written by WWDx Forum member ExitThirteen, and is used with permission.

After doing some research, and a lot of testing, I have successfully done conversions on the upD858 AM/SSBchassis. For these conversions, I used IRF520Ns from International Rectifier. I'm sure you could substitute ERF2030s if you wanted to do so. For the upD858 chassis (President Washington, Madison, Grant, Cobra 138XLR, 139XLR, etc):

Parts you will need:


IRF520 (or ERF2030)

EKL companion part EN-369FN (You can build your own too)

56K ohm 1/2 watt resistor

39pf disc or mica capacitor (Optional, see installation text)

Remove old transistor at TR43
Remove R179
Remove L44
nstall IRF520 at TR43
Install EN-369FN companion part across final, positive side to gate, unmarked side to source
Install 56k ohm resistor from IRF520 gate to L31 (goes to far side of L31, furthest away from final, +13V)
Install 39pf capacitor across C167 (This is optional, will bump up output in some units, test radio without first)
Check voltage at gate of final, should read approx. 3.65V+/- .1V
Retune TX

That's it! Double check your work, and you should have the same output if not more than the old stock transistor.My President Washington did about 22W on SSB before, with the MOSFET in place, it now does 27W on SSB.


Nice Job, ExitThirteen. We appreciate your letting us publish your write-up here. 73s.
- 399




858 SSB Mike Amp Limiter Modification

Removing the mike amp limiter in these rigs is a no-no, but the limiter can be modified for a little more output. Locate D20 and remove it from the board. Replace it with two 1N914 diodes wired in series. Install the pair in the D20 location (observe polarity). 

This will reduce the drive to the limiter circuit, and will increase the mic amplifier output.









If there is any weak link in the 858SSB chassis, it is the 2SC1306 driver transistor. In the initial production run, rigs tuned to the max blew drivers like crazy. The solution is to upgrade the 2SC1306 to a 2SC2166. The collector current on the 1306 is rated at 3 amps max, and the max collector dissipation is 12 watts. The 2166 collector current is 4 amps max, and the dissipation is 12.5 watts. Although the power increase is small, the difference in collector current is a 33% improvement. This is a significant amount which provides the driver with additional headroom and works well with the High Power Modification shown below. 





(Circuit Board PC-196BD)

The Uniden 858SSB chassis went on sale in November of 1976, when the Cobra 138XLR and 139XLR, and the President Grant and Washington models were introduced. Almost overnight, Dynascan (Cobra) and President were deluged with warranty claims for blown drivers and finals. Production and sales were suspended, while Uniden analyzed the problem and devised a fix.

It turned out that the power output potential of the original circuit design was more than the driver and final could handle. At the FCC limits of 4 watts AM and 12 watts pep SSB, these rigs would cruise along all day no problem. But when the screwdriver jockeys in the field peaked the transmitter, cranked up the supply voltage, and clipped all of the limiters, drivers began blowing like crazy. The final transistor, which ran pretty warm anyway with the specified 70ma bias current, started burning out from the heat
generated by excessive keydown times. 
Uniden’s response was to redesign the driver/final circuit in order to reduce the transmitter’s power output potential. They also changed the final transistor bias to 15ma to let the final run cooler. 


On April 3, 1977, Uniden issued production service bulletin 1208 which was sent to Cobra and President service centers. This bulletin was revised to 1208A on June 21,1977. The changes specified in the bulletins were incorporated and production resumed. Approximately 5000 rigs were in the field and another 7000 or so were in the pipeline when production was suspended. Most of these were changed under warranty, so the chances of getting one with the original circuitry is slim to none. The thing is, radios with the original circuitry were kick-ass audio monsters on-the-air and were extremely bullet-proof when set up properly. 


So ... if you want one of these monsters, reverse the changes called out in the service bulletins.  




Use common sense when setting the AM and SSB outputs of your rig after this modification is complete. Do not exceed 22-23 watts peak output in either mode. If you crank it up to the max, I flat guarantee you will smoke the driver and final in a heartbeat. If you do not heed this warning, and you blow it up ...


NOTE: If the original 2SC1307 final transistor has been replaced with, say, a 2SC1969 or 2SC2312, you will have to set the peak output considerably lower. The 1307 has a max collector current of 8 amps. The 1969/2312 max out at 6 amps. So set the peak output accordingly.


If you decide to do this mod, definitely replace the 2SC1306 driver with a 2SC2166. When installing some of these parts, the holes in the circuit board may need to be enlarged. If so, do it by hand, NOT with an electric drill.  Realign the rig using the alignment values for the 138/139XLR models (Sams #126 or #127 or from the 139XLR Service manual on CB Tricks).  Remember to set the final bias to 70ma.  When the mod is complete, you will have a rig that will exceed 12 watts dead key on AM, and 35
watts peak on SSB. The 2SC1307 transistor used as a final in these radios is rated at 25 watts output max. Set the AM and SSB peak output to no more that 23 watts in either mode using an ACCURATE RF POWER METER. Exceeding 23 watts will guarantee that you will fry the driver or final or both!!!  It is a good idea to provide additional cooling for the driver and final when doing this mod. Using the
Bergquist insulators mentioned earlier is a must. On mobile units, adding a heat sink like the one used on the RCI 2950 should suffice. On base models, adding a fan that blows directly across the driver/final heatsink is imperative. A fan is a pretty good idea on mobile units too. 


A good way to prevent frying the 2SC1307 final when doing this mod, is to do the Mosfet conversion shown earlier. Use a 13N10 mosfet as it has a higher power capability than the one that Exit13 used in his mod. Doing this will enable the unit to operate at its full potential. The power supply may not be able to keep up, and I will be posting a power supply upgrade in a week or so. Good Luck !!


      QTY                      Description             
        1         10 ohm 1/2 watt carbon resistor (R204)
        1         47uf/25volt electrolytic capacitor (C181) BE SURE TO USE 25 VOLT CAP!
        1         180pf disc capacitor (C232)
        1         56 ohm 2 watt resistor (R180)
        1         220 ohm 1/2 watt resistor (R181)
        1         1 ohm 1/4 watt resistor (R205)
        2         KB162W Temperature compensating diode(D51,52), subs to NTE601
 note: It is ok to keep the MV-1Y diodes in the circuit. The MV diodes are an improvement over                      the KBs. Both the KBs and the MVs sub to the NTE601.

REMOVE the 47 ohm resistor (R219) connected between the base of TR43 final transistor and ground. This resistor along with two diodes is connected to the final on the solder side of the board.  REMOVE all three parts. See diagram below.


Also REMOVE the 470 ohm resistor (R218) connected between the base of TR44 driver transistor and ground.  


INSTALL: R204, R205, C181, C232. They were removed per the service bulletin. Install C232 in the location where R218 was installed. See installation diagram below.

Note: After production restarted, Uniden made an additional change, installing a 220 ohm resistor in the R204 location. This was done in order to make setting the final bias easier. if your rig has a 220 ohm resistor in the R204 position, remove it and install the 10 ohm resistor in its place.


 BTW - The 858 Main board layouts (bottom vs top views) in the TRC 449 Service Manual (pages 23 & 24) have some different components shown in the driver and final area. This mistake is only in the 449 manual, not the 457 0r 458.  See if you can spot the difference. - 399






This circuit is easy to build and produces a nice clean beep tone. 

                               PARTS REQUIRED:
                    QTY.         DESC.
                      1      small piece of  pc vector board
                      1      555 timer IC
                      1      2.2K ohm resistor
                      1      33 ohm resistor
                      1      10k ohm resistor
                      1      10k ohm pc pot
                      1      50k ohm pc pot
                      1      0.1 uf disc cap.
                      1      1 uf/ 16V electrolytic cap
                      1      470 uf/ 16V electrolytic cap
                      1      1N914 diode
                      1      SPST switch (optional)




Build the circuit according to the schematic shown above. Installation shown is for the Uniden 858 SSB chassis. It is important to connect the output of the RB circuit to the audio chain AFTER the speech clipper (as shown in the diagram), as this will result in a cleaner sounding beep tone.
Use an existing front panel switch, or install the SPST switch in a location of your choice to turn the RB On/Off.



The 555 timer is configured as an astable oscillator. When the mike is keyed, Vxmit turns on and powers up the RB circuit. Simultaneously, RESET (pin 4) on the 555 is pulled low (disabling oscillator output), and the unkey delay circuit is discharged. When the key is released, RESET goes high enabling oscillator output, and the beep is broadcast as the relay stays locked in transmit for a few milliseconds until the unkey delay circuit is sufficiently recharged.
Adjust the 10k ohm pot for your desired RB frequency, and adjust the 50k ohm pot for a loud beep tone w/o overdriving the modulator. 







This modification is quite easy, and will improve the 858SSB's receive sensitivity significantly. In stock form, Gate 2 of FET 1 (1st receive mixer) is connected to the low impedance tap on coil L4. Since the input impedance of a FET is high, there is a much better match when Gate 2 is connected to L4's high impedance output. 

Refer to the diagram. Wear a grounding bracelet when doing this mod. These old FETs are very susceptible to damage from static discharge.


Install a jumper wire from G2 to the unused tap on the coil using a grounded tip soldering iron. Do this BEFORE cutting the original circuit
trace to G2 of FET 1. 
Set your rig to channel 20 (27.205). Inject a 10uv signal at 27.205 modulated by a 1000 hz tone. Adjust coil L4 for maximum receive signal.

After modifying coil L4, remove R14 (1kohm), no replacement. Also remove R12 (1kohm) and replace it with a 100 ohm-1/4 watt resistor. 


Note: After completing this mod, the receiver may "motorboat" in receive without an antenna connected to the radio. This "motorboating" can be eliminated by ever so slightly detuning coil L4.



Modification # 18
The following modification was developed by CB Tricks member " NOMAD " (CHRIS) at NomadRadio, and he has been kind enough to permit us to post it on this site.
AM operators have complained that adjusting the TX Level control (VR8) on the 858 SSb chassis for a 1.5 to 2 watt deadkey also reduces the total pep output (swing). This is because Uniden used a simple regulator to control the voltage applied to the secondary of the AM modulation transformer. As the applied voltage is lowered, the deadkey is reduced. The problem is that as the voltage is lowered, the available current remains the same so the peak wattage output is also reduced (because P=EI ). 

The newer Uniden AM/SSB sets eliminate the AM modulation transformer, so transmit audio is direct-coupled into the modulator allowing full audio swing in AM mode regardless of deadkey. 

The following modification allows any setting of the AM deadkey on the 858 SSb chassis while still permitting maximum audio output. ( this modification applies to the Uniden 858SSB Chassis with the PC-196BD main board.)

Parts Required:


1    PNP Darlington Transistor (TIP120 or equiv.)
 1    1000uf/35 volt electrolytic capacitor
 2    680 ohm 1/4 watt carbon resistors
 1    10 kohm 1/4watt carbon resistor
 1    1 kohm pot.


Also, a short piece of heat shrink tubing and a few inches of 18 ga               
stranded hookup wire.



First, remove the following parts from the radio and discard them.




Then, refer to Figure 1 above, and install the new parts as shown. Install the TIP 120 in the TR25 location using a good ceramic insulator and heat sink compound. Drill a small hole as shown in the figure, and install the new (1000uf/35V) C102 on the COMPONENT side of the board (observe polarity). All of the other components are installed on the solder side of the board. Use the heat shrink tubing to cover all but the very ends of the 680 ohm resistor connected between the base of TIP120 and the output of the modulation transformer to prevent its shorting to other components.   Install the carrier control pot in a convenient location, and connect the wires to the wiper (center lug) and the clockwise lug connection

Note: On my TRC457, I removed the mike gain control (20k ohms) and replaced it with the dual control (20K ohm/1K ohm) used as the clarifier control on the Cobra 2000. 

Check your work, and fire it up. The deadkey can now be varied from 1 to 6 watts or so, and the rig will swing to full pep output regardless of the dead key setting
















If you have any corrections, suggestions, or just want to tell me what you think of the site, Email me at:  unit.three99@gmail.com


- 399

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"The PB-010 Chassis"



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