UNIT 399

MEDELLIN COLOMBIA

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UNDERSTANDING AND MODIFYING THE UNIDEN 858 SSB CHASSIS.

The Uniden 858 ssb chassis was produced between 1976 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 rigs 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 websites. What follows is a compilation of mods I have developed over the years, 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.

- 399

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WARNING

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

DON'T BLAME ME !!

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Getting started

All of the 858 chassis 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 station's power supplies than the ones available from other marketers. There are also some component differences. These will be explained later. If you don't have a factory service manual, get one as it contains a wealth of information and is very well written. Copies of the Realistic manuals are available from Radio Shack.

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.

IMPORTANT!!

The following changes are very important (especially the installation of mica insulators), and should be done to any 858 chassis rig to eliminate shortcomings and improve overall performance .

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The first thing to do is to replace all of the thick white insulators from between the power transistors and the heat sinks. Use mica insulators and a liberal amount of heat sink compound. Remove TR401 and remount it using a liberal amount of heat sink comp

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. They are still available from Uniden America: L6 - Uniden part # 2200-029, FT2 - #2100-002. Order an additional FT2 filter to use in the adjacent channel rejection mod shown later.

Change C304 to a 10,000uf/50volt computer grade electrolytic. Change C107 to a 3300uf/35volt. 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 with a 15uf/16volt electrolytic. This will make the Realistic modulation limiter circuit the same as on all of the other 858SSB sets. Do not attempt to modify or defeat the AM modulation limiter, as this will degrade performance.

IMPORTANT !! The driver/ final bias settings on the Realsitic 858 sets is different than all of the other 858 rigs. Set the driver bias to 15ma, and the final bias to 40ma on the 449,457, and 458 models.

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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)

Replace D10 and D11 with 1N6263 schottky diodes. DO NOT REPLACE TR5 WITH A 2SC2999 TRANSISTOR. The original equipment transistor (2SC1730L) has a lower noise figure than the 2SC2999, and replacing it will degrade receive 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.

Many techs recommend replacing all of the electrolytic capacitors in these sets due to their age. I think this is a total waste of time and money. I have had (and used) about thirty of these sets over the years, and have never had a capacitor failure. However, I do recommend replacing all of the 2.2uf tantalum capacitors (C55, 68, 87, 179, 211) because when they were made tantalum capacitor technology was in its infancy and they are more prone to failure. These same techs also recommend upgrading all of the 2SC1419 transistors (2SD325E in the Realistics). I believe that this is also a waste of time, as their high initial failure rate was due to the insufficient heat transfer capability of the thick white ceramic insulators and insufficient heat sink compound. I've only had two of these transistors fail, and, in both cases, the cause was a defective mica insulator (my fault).

Do a complete alignment per the factory manual/Sams. Adjust VR7 for maximum AM modulation. On base units set the power supply voltage at 14.8 vdc. 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.

*******************************HOWEVER**************************

If you MUST run the rig to the max, then make the following additional changes:

Replace R-112, R-113, R-161 with the same value but with 2W rating. Mount further off the board for better cooling. If you have to enlarge the holes in the circuit board, do not use an electric drill, do it by hand.

In the AM power regulator, change the tolerance, not the value: R-110 to 1/2 watt, D-26 to 5 watt, D-24 to 5 watt.

On base units change D-301 and D302 to 5 watt. Mount off the board for cooling also.

See if the unit will dead key 6 watts in AM, and do 16 watts in ssb. If not, decrease the values of C-167 and C-170 50pf at a time. Do not decrease either one over 100pf. When replacing use N750 type only.

GOOD LUCK:

- 399

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Links to the following pages will be added as I get the time:

If you have any suggestions or mods that you want posted, send me an email at [email protected].

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