Trc 457 Repair Help Needed

[giboni]

Neglected to check LSB before starting a Re-Cap & Mods. I used it in my car for a few months AM. I received it with lower channel mods.

10,000uf Elna - PSU board. I changed out all the large caps to new Japanese. Played with the Audio. Decided to remove the resistor & reconnect the clipper diodes. Shotky's R-43. New heat paste on all 202 transistors. Cleaned the PC board. Was going to install a Redco UFO. Then my first complete cb alignment with all my new used testing toys.

Before removing the lower channel mod checked USB/ AM working. LSB relay keys- no output.

Figure a lot of experience still floating around here on the 858 chassis.

Today on my day off. Going to work on my working Cobra 139XLR with Redco UFO. Put the Trc 457 on the shelf till I have time.

Any clues as to what I ought to check first ?

Thanks in advance

Best John

 

[Decoder]

First make sure its receiving on LSB, If it doesn’t then I would look at the crystal and varator circuit for LSB, If some opened up the clarifier (voice lock) they may have used an inductor to give it more slide and it could be lose or bad or the crystal could be bad. Remember 858's has a state for each AM, USB, LSB. This is just a start.

 

[giboni]

First make sure its receiving on LSB, If it doesn’t then I would look at the crystal and varator circuit for LSB, If some opened up the clarifier (voice lock) they may have used an inductor to give it more slide and it could be lose or bad or the crystal could be bad. Remember 858's has a state for each AM, USB, LSB. This is just a start.

Thanks a million. Varactor. Many times working on vintage radios. Cold solder Joints.

 

[Robb]

Just go over all of the test points first.
Usually, if voltages and/or frequencies don't look right, then that is the circuit area I would check thoroughly . . . fixed many radios using that method . . .

 

[unit_399]

I would check the mode switch first. Maybe an open contact, or connection. If not the switch, then I would check out the LSB oscillator, as it's the only part of the circuit exclusive to LSB.

- 399

 

[giboni]

Thanks 399. Makes sense. Well next project. After recapping a Teaberry Stalker 202.
will go back to this Trc457. Just finished a Cobra 139 XLR re-cap. top tier caps mostly
Japanese.

Very interesting. I have acquired slowly 139XLR, Madison 858, Washington 858, Trc 457,458 Robyn 520D Sams Photo facts. & yes I have them all + both 858 Madison Versions radios.

Their are differences in alignment instructions in the quoted Sams for the radios.

139XLR the Mic gain controls the clipper.

C-78 or is it C-75? is missing on the 139xlr. A jumper across where the trimmer for the clipper was.

So far both my 139XLR put out more audio on AM than 2x 457 & 2x 458. All 4 Trc I fallowed your Web Page instructions on making the audio circuit similar to other PC196 units.

I'm going to carefully go over the fold up schematics of both. & copy the 139xlr audio circuit to the Trc's. One first of course. Could be more than C-7? ?.?

Cobra139XLR ages better than other PC196 Radio's. The case does not have vent holes on the top. All my other radios require major cleaning of the PCB because of their vent holes being on top, Guess another thread one of these days. I owe I owe off to work I go !

Thanks again 399 ! Getting faster on the Re-Cap process !