When you power it on, does one of the relays pull in? The smaller relay is the receive preamp. The transistor that drives that relay is probably failed.
You could find the Normally Open poles of the main relay and, tack a wire across them and see if that helps you locate the problem.
Having operated a broadband Class AB2/C solid state 2M amp with receive preamp at a club special event station I can say the best thing is simply disable the receive preamp and forget it. All it did was drag in more noise and didn't improve the receive one bit. I'd say from that experience the only thing a receive preamp is useful for is when the rig has a crap receiver to begin with. Those cheap Radio Shack shortwave receivers sure need one, the 20dB gain 3 leg chip I used did the trick and I gave the souped up RX to a local kid who became a ham after hearing the world on a wire out the window. BTW the special event was a big disappointment being able to reach only one repeater being we were down in a deep valley, and HF was dead. The only Amateur Radio demonstration those poor kids got at the camp was me talking with a club member with an HT at the other end of the lake. <sniff>
Now we get to transmit that has problems of it's own. Throttling a 100W transmitter to drive a 200W CB amp is a total waste of time and effort to put another half an S unit on the receiving end. Remember signal increase with power is logarithmic, each doubling of power puts out another 3dB and one S unit equals 6dB, so all you get on the receive end is half an S unit with that amp. There's a good reason why CW/SSB rigs put out 100W, that's all that's needed on any band provided your antenna is efficient.
As an aside, I had fun testing a 500W CB amp after repairing it. It was the same sort of thing, I lowered the drive and audio gain on my scratchy Apache so I didn't push the grid driven 4CX250 into Class C. I doubt Dave could get away from his pill addiction long enough to even conceive of using those ubiquitous VHF/UHF transmitting tubes. The one S unit and a spit increase didn't do nearly as much QRM cutting as the audio when it's set up correctly, that clipper with an M derived pi section LP filter with a sharp cutoff at 3KHz cuts like a hot knife through butter. That 4CX250 put out a clean signal as opposed to the 12 sweep tubes in the D&A Phantom 500 aka The Green Dragon. I knew a guy who had one and couldn't use it, not only did it sound like shit, but it put the Tennessee Valley Indians on the warpath for blocks around, the neighbors nearly killed him.
D&A Phantom 500 amp inside.jpg
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That relay looks like an Omron DPDT of the type used in the FT-726R's various modules, the FT-980's RF board and other rigs of the period. They go bad due to their open-frame construction, which allows the contacts to oxidize due to atmospheric interaction.
I've repaired them by removing the plastic cover, soaking the edge of a piece of card stock in DeOxIt D5-100 and gently dragging the stock through both N/C and N/O contact sets. Use minimal pressure on the wiper arm when doing this.
Those relays are long since unobtanium. I think I bought the last available OEM ones from Yaesu a couple of years ago.
An ETA:
I have a Communications Power HF-150 here in the shack. It'll put out 120w PEP or so when driven with 10w, and has switchable low-pass filters along with a receive preamp and AM-FM/SSB-CW time delay selection for its RF COR circuit.Throttling a 100W transmitter to drive a 200W CB amp is a total waste of time and effort to put another half an S unit on the receiving end.
That amp works very well with a 10W-class HF transceiver. Think TS-660 (on 15 through 10), the FT-726's HF module (ditto) or my TS-130V (on any band the rig covers). It's also remote-controllable so I can put the amp next to the power supplies and 6/2/4x0-MHz amps and control all of them from my operating console. Since each Yaesu module offers a keying output for accessory activation I'm thinking about modifying the amp for direct keying.
I'd borrow a bit from this thing and put a bandpass filter between the driver and amp, then a lowpass filter on its output. And match your exciter to the amp...no more than 10-15W PEP drive, else you should construct an input padding circuit.
Last edited by N8YX; 01-27-2020 at 01:17 PM. Reason: Additional info
"Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."
Thanks for the input. I will be doing some testing and cleaning over the next week.
I'll get back here with what I did, observed and the results. I'll keep my eyes and ears open to listen to and watch the relays when I power it up. If it's as simple as a little transistor I might need some help sourcing it. If it's the relay I'll probably need to bypass the whole preamp circuit whether that's just jumping the relay or not.
Last resort, take it to the shop. I've know the man since I was 8 years old. He runs a brick and mortar place in Greenville. He has done repairs since forever and sells whatever makes money. High end car stereos has made him more money than CB ever did.
As an aside, I was in a local 10m ragchew net that originates in Greer and extends to Spartanburg (east). The difference between running the Corsair wide open at 100 watts (barefoot) and running it at 6.5 watts into the amp which produced peaks of over 300 watts was the difference between straining to hear me and then being told, "Armchair copy OM." That's only a 30 to 50 mile jump but on 10m, that's just how it is with local comms. Of course, on 80m we could hear each other, comfortably, down to the 25 watt range. Same thing with a couple of friends almost due south of me in Greenwood. We wait for a local opening to hit 10m but on 80m, armchair copy. When that local opening occurs, a couple of guys in the north across the NC state line join in, of course they have the advantage of another 2500' plus in altitude so when they hear us they know what's up with the weather.
Another thing I would do is to replace those 10-ohm bias resistors...especially the one which appears overheated, damaged and open to the world. Use 1/2w or 1w metal-film of the same resistance and tolerance values.
"Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."