Can you post a schematic?
Hi, I am having a problem getting to the bottom of a problem with a 120HZ hum. The thing that me confused is I have -36v on the negative rail on the base and emitter of the two negative finals and I have +36v on the collector of the positive finals. Is this normal? I would have expected only one leg with voltage with no signal injected. It seems to me that something is driving the negative finals to full on. The amp does pass a signal but I am only testing the power amp and putting signal just before the long tailed pair. Also I am getting strange voltages on the TIP31 for the negative side. Any hints? I have all of the equipment including a scope but not quite sure how to measure if there is DC on the output. The finals test ok and last time I checked the TIPs they were ok but I not so sure now. Thanks!
There are two versions of this power amp.
Series A, Series B.
To test Volts DC on the output, use your volt meter.
Connect it where the speaker wires are.
As to the output transistors: you will see when you open the schematics.
The positive rail only connects to the Collector.
The negative rail, the Base & Emitter are 47.33 ohms apart.
Thanks for the replies. The schematic I have is different. Please see the attached. OK, so I am getting DC on the output. I am also seeing 120HZ on my scope. What I have done so far is replace the Diff pair since one xsistor was blown. The rivets holding the xformer to the chassis were loose and there was a potential difference between the xformer case and chassis ground(is this normal?). I removed the rivets and bolted it down. also bolted down the ground wire which was riveted. I tested all the xsistors and diodes. Some I lifted if they read funny. Can't find anything blown. I have new finals and drivers ready but I am not willing to start throwing parts at it. I want to learn how to fix this thing. So, with the new schematic does it change you answer about the separation of the 47.33 ohms apart or is that the general design of this NPN only amp? My limited understanding was that the driver put a small signal into one leg of the finals and let the rail flow through the junction but I'm seeing rail on 2 legs with no signal applied. Please, what am I missing.
Juan Manuel Fahey
OK, JPB I see it. So I should expect to see the -rail at the emitter and slightly less on the base. JM I have to play tonight and I won't get back to the amp till Wednesday but I will post back. I am running it on a variac and limiter at 100V but I'll bring up the rails to full and take the measurements. Thanks to all. I definitely appreciate the help and education!
Hi JM here are the voltages:
With no signal I am getting 0v DC at the output. I thought I saw some earlier. Thanks
Ok, that amp looks perfectly fine.
So compared to that, hum problem is relatively minor.
To check if the hum is external and being injected into the amp, short the input 10k resistor, with a short piece of wire,so you are grounding whatever tries to get in ... still hums?
Juan Manuel Fahey
I am now at work. I'll get back to the amp tonight. Thanks so much for your help. Maybe the hum was the loose transformer or the bad diff pair. I'll bring it back up tonight and see what is there. The hum was loud when I first started on it. I can't wait to get this one done. I have a Fender passport and a Crown ce1000 waiting. Thanks!
Ok, now I'm working on the amp again. I load the amp with a speaker, no signal in and start turning up the variac. I get to around 100v and the lightbulb goes dim and I hear a short very low hz hum. I was watching the current and it was only drawing 0.25A This is the third time it did this. Each time with a speaker hooked up. When it is not loaded I can bring it all the way up. Another thing I noticed is one of the filter caps reads 0.0 volts when I turn off the power(the B+ cap) the B- cap holds voltage until I drain it down. I'm thinking one bad filter cap? There was no loud hum. Any guesses. Thanks again!!!
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