50w stereo amp - "MAS" modules.

[Diabolical Artificer]

I bought this amp off mike AKA Crackle a while back, only just cracked it open. Powered it on the variac & LL and it's just sitting at low voltage whilst the caps re-form a bit.

I noticed there's a 2.4v DC offset on the OP, but there seems to be no preset to set it. From what I can see the amp comprises of two modules, with MAS on the PCB's. Each uses four 2N3055's, so I guess this is a quasi complementary OP, there is no big cap on the OP.

so, how to get rid of the DC on the OP?

Andy.

[Refugee]

What does it do with a dummy load connected?

[ppppenguin]

Obviously circuit diagrams would help. The amps look simple enough to trace out if all else fails.

If there are no big output caps there must +ve and -ve supply rails. Can you at least verify that they are about equal.

It's possible that some part of the circuit doesn't wake up properly until nearly full supply voltages are present.

Working with a dummy load is definitely the way to go. Not necessarily 8R initially but even 50-100R will give some safe loading and allow you to see how the amp is working.

[Crackle]

Hi Andy
I wondered what you were going to do with it.
I thought the output of each amp was wired as a Darlington pair.
I have to admit when I tested the amp I never checked for any DC on the OP, but it seemed to work OK.

Mike

[ppppenguin]

I see from a post elsewhere that one of my guesses was right: "It's possible that some part of the circuit doesn't wake up properly until nearly full supply voltages are present."

[Diabolical Artificer]

Yep Jeffrey, DC offset vanished when full AC mains applied, my bad. I now have a different issue, IE excessive hum. Main caps check out ok, needs further investigation, will report back when I've had another look.

[Refugee]

The hum should be easy to sort out.
With a scope earthed to the chassis/0V and on AC go for the blue capacitors with the positive ends up. There is bound to be one that is going open circuit. You might even find that one is causing the amp to oscillate at some ridiculous frequency.

[Diabolical Artificer]

Thanks Simon, they were in my sights, that or a SC somewhere.

Thanks Mike, useful info, didn't think of the OP as being configured as Darlington pairs. I'd forgotten I had this amp and dug it out as my sub woofer amp has died, two 2SJ50's had fried. I'm onto another active woofer amp at present but will drop back on this amp soon.

Andy.

[Crackle]

Getting confused which forum to post on. I was looking at your thread on UKVRRR and thinking where has my post gone, I thought I was going mad. (or a mod had deleted it) 
With that combination of OP transistors, I thought the amps might do more than 50 watts if decent mains transformers were fitted. If my memory is correct there were 2 transformers both were toroid ones and looked a little puny.

Mike

[Diabolical Artificer]

Progress of sorts... noticed the grounds of the modules weren't grounded to the chassis/earth, did that , one channel now clean, the other better, but not 100%. Found two 33u caps with less than perfect ESR, so changed the lot, better but still a bit of hum present. Will do some more digging today.

Dug out my ETI's, one issue had a lot better picture and I would say they're OMP modules, nice one David.

I actually had a proper look at the amp and it says it's a "C/M amps DL500 PA amp). They really were monkeys back in the day for bigging up their amps, or maybe they mean 500mV sensitivity? : )

Andy.