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,