EPT360014B AMC/ALC issue - SSB warble

[13EL121]

Hi friends, my old Alan 8001 developed a SSB audio issue. It sounds like bias is not set correctly, but it is.

I found out that AMC and ALC are not working as they should. In AM mode I can adjust AMC a little bit via AMC pot, but AMC also reacts by adjusting ALC pot.

I SSB mode the ALC pot reacts, but audio is still warbling.

If ALC pot is turned down completely, all TX audio disappears in all modes. AM-FM only carrier without modulation is present.

I checked all known transistors in the ALC/AMC circuit, but I can not find the fault.
I had a similar problem a few months ago with a Superstar 3900 where TR34 was leaking, but this time TR34 seems to be OK.

Any ideas where to have a look?

Thanks and 73
Ralf

[cb4ever104]

Hi Ralf .

TR32 and TR34 are common to those circuits , so I'd probably start there . Just use the AMC circuit for fault finding , and you'll probably solve both issues . Check your voltages around those points .

AMC.JPG

Sammy

[13EL121]

Hi Sammy, voltages are ok at first look. But I will swap them out anyway. In the tester they were tested good, but I remember last time when the leak was not visible.

[cb4ever104]

If ALC pot is turned down completely, all TX audio disappears in all modes. AM-FM only carrier without modulation is present.

Actually , I've just seen this . So no FM modulation when the ALC is set to minimum ? I'll need to look at that circuit , because that's not right .

Is TR54 OK ?

[13EL121]

TR54 not yet checked Sammy. Yes, if ALC is turned fully down, FM and AM Modulation disappear. Will check that later. Just swapped TR34 and TR53 in a free minute, but that didn't helped.

[cb4ever104]

Also . Was the radio working OK before this fault appeared , or has it always been like this ?

[ch25]

Replace D60, D83, D70 for a start

[13EL121]

The radio was OK. Have it a few weeks now, but yesterday suddenly after switching on my friend said I sounded like crap. Output dropped to 14w SSB PEP, before it was double around 28w.
Thanks Chris, the diodes will follow after TR54.

[13EL121]

Checked and swapped all transistors and diodes without luck. All voltages seem to be OK. Without bias bridge connected the SSB TX audio is clean, only driver connected is clean, finals connected audio sounds bad. Next step I will try to desolder the collectors of the finals.

[cb4ever104]

I'm still not sure about that ALC FM statement . Anyway .Try just connecting one final at a time to see how that sounds .

[13EL121]

Desoldering Collector of each final didn't helped.

[cb4ever104]

The radio was OK. Have it a few weeks now, but yesterday suddenly after switching on my friend said I sounded like crap. Output dropped to 14w SSB PEP, before it was double around 28w.

How much power do you have deadkey on AM/FM Ralf with the power VR at max ? BTW Just key it for a second and then turn it back .

[13EL121]

Full carrier is 15w AM-FM, SSB round about 30w. So nothing spectacular. I swap the 2SB754 with a TIP36C, but no different behavior. But strange thing is that modulation disappears when ALC is turned fully down. Search goes on...

[cb4ever104]

Full carrier is 15w AM-FM, SSB round about 30w. So nothing spectacular. I swap the 2SB754 with a TIP36C, but no different behavior. But strange thing is that modulation disappears when ALC is turned fully down. Search goes on...

You said previously it was 14W SSB , so to be honest I'm a bit lost now Ralf . You also said that you had output with the bias plate removed . I didn't understand that either , as you should have nothing with that removed .

[13EL121]

The power dropped down to 14w when the fault appeared Sammy. I can adjust ALC higher, but I had to adjust the pot of course. Maximum is 30w SSB. Before it was round about 25w ca middle position. This dropped down to 14w when the issue startet.