Pearce Simpson Bengal SSB Low AM Sensitivity

[Justin B.]

Howdy, y'all! Just picked up the above radio with a few problems. It was totally dead when received and I found two exploded diodes on the PS board along with a 25A line fuse!! After replacing the diodes everything seemed fine until I keyed the mic and ka-boom, diodes fragged again. I replaced them again and found the final transistor had an E-C short so I have ordered a new transistor. I de-soldered the 2SC1307 final and am able to key up without blowing diodes and in AM mode I can easily get 100% modulation on the meter.

While waiting on the transistor I figured I could at least listen to some stuff and after a while the AM signal started fading in and out like bad QSB. I can "talk" to it with my talkie and get a good swing on the meter on all channels. There is hardly any background noise when the signal fades out. Both LSB and USB sound fine.

I see in the block diagram there are separate AM and SSB AGC sections so I am leaning toward something going on there.

Any suggestions on what I should check?

73,

 

[nomadradio]

I would start at the emitter of TR11 and see that the 7.3435 crystal isn't weak. Should see around 1 Volt peak-to-peak RF at TR11 emitter.

There are other possible culprits, but they are all downstream from the AM-only mixer TR12 where this signal feeds to. This crystal is in a socket. If you had a spare, swapping it to compare would be the quickest thing to try.

73

 

[Justin B.]

I would start at the emitter of TR11 and see that the 7.3435 crystal isn't weak. Should see around 1 Volt peak-to-peak RF at TR11 emitter.

There are other possible culprits, but they are all downstream from the AM-only mixer TR12 where this signal feeds to. This crystal is in a socket. If you had a spare, swapping it to compare would be the quickest thing to try.

73

I'll check that but did you mean TR1? TR11 emitter is tied to ground in the Sams schematic and TR1 looks to be the oscillator.

73,

 

[Justin B.]

Checked TP1 (TR1 emitter) and just as soon as I clipped on the scope probe the radio woke up and now won't quit! I have about a 4V sine wave riding at about 8V on TP1. When I removed the probe everything still worked even after numerous off/on cycles. We'll see tomorrow if it still lives or if it needs CPR again!

73,

 

[doffo]

My guess is that the board may be due for some new capacitors. I got a sears Roadtalker that the receive just slowly fades away. I can't recall if it was in SSB or in AM.

I got another Pace Sidetalk 1000 SSB radio that for sure does it in SSB. Though I would hate to recap the entire radio only for it to act the same way. I just was told on the SSB Roadtalker slant face which caps needed to be replaced on the signal fading away.

 

[Justin B.]

Man, I hope not - there are a ton of electrolytics on this board! Looking at the schematic there are non in the oscillator circuit but there are in the AM AGC circuit and mixer so maybe I'll do those...

73,

 

[TM86]

Checked TP1 (TR1 emitter) and just as soon as I clipped on the scope probe the radio woke up and now won't quit! I have about a 4V sine wave riding at about 8V on TP1. When I removed the probe everything still worked even after numerous off/on cycles. We'll see tomorrow if it still lives or if it needs CPR again!

73,

Hate to bring this up, but the physical stress (small as it was) of putting the probe on there may have jostled a cold solder joint. Simple to fix, but usually a bear to find. Hope I'm wrong about it.

 

[Justin B.]

Thanks! I'll check the area around TP1 and if I don't see anything obvious I'll re-solder everything in that circuit to be sure.

73,

 

[nomadradio]

Age alone takes its toll on aluminum electrolytic capacitors. The liquid chemistry inside that makes them work is held inside by a rubber plug at one end. High temperatures make the rubber rigid, and it shrinks. The seal is broken and that precious liquid chemistry evaporates. Heat speeds this up. A 1977 capacitor kept in the proverbial cool, dry place might be okay, but won't last long if you put it in a radio that's used regularly. Those capacitors aren't meant to last more than 10 or 20 years in the first place. Fifty years later, they are on borrowed time.

73

 

[doffo]

TM86 brought up a good point......... Maybe poke around underneath the board and see if it comes back. I fixed a Realistic TRC-449 doing this which so happens to be right in front of me lol! Did the fix back in 2021.

 

[doffo]

Age alone takes its toll on aluminum electrolytic capacitors. The liquid chemistry inside that makes them work is held inside by a rubber plug at one end. High temperatures make the rubber rigid, and it shrinks. The seal is broken and that precious liquid chemistry evaporates. Heat speeds this up. A 1977 capacitor kept in the proverbial cool, dry place might be okay, but won't last long if you put it in a radio that's used regularly. Those capacitors aren't meant to last more than 10 or 20 years in the first place. Fifty years later, they are on borrowed time.

73

You brought up a great point... One must ADORE the radio if they will go through the trouble to replace all the capacitors....