Help with DC offset in Single rail QC amp?

Hi, I bought an amp a while ago (had a few of them in the past -Audio Innovations Alto MK1) and over last couple of days as it gets warm it starts to sound a bit rough.
When I first got the amp 2 Months ago) I checked DC on the output which was less than 10 mV, so fine, I thought. But, today I decided to check it again and it was @45mV and rises to 1.4V after 30 minutes on both channels.
The amp is a kind of rehash of of an old single rail quasi complementary design and uses a pair of 2N3055 and an output capacitor.
I don't believe the amp has had any work on it since it was first sold , approx 1996-7.
I tested with a DMM across outputs with nothing else connected, is this correct? Is there a fault?
 

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Obviously, you should first check C9 (C8), if they leak you've got offset.
Check: what's the voltage at the other (non-output) side of that cap.
It should be close to half supply.
Do both channels have the same issue?
Did you check with load connected? Unloaded, even minuscule leakage can cause offset that disappears when loaded with a speaker.

Jan
 
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Cant measure anything on the inside yet as not taken unit apart yet. This amp doesn't sound as good as the last one I had though, rough scratchy treble.
Last one had ST branded 2n3055's, this one has a different brand. Could this be why it doesn't sound very good? I did check bias when I got it, and set it to 30mV as per service manual.
 
Short the inputs, place a small value resistor across the out puts (a 10 ohm 1/2 watt would be fine) and then check for output offset voltage. If it is still there (and excessive) you most likely have leaky output coupling caps that need to be replaced.

Mike
 
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That output coupling cap lways leaks a tiny bit.
If you only load it with a multimeter, which is very high impedance, that tiny leakage can still work up some measureable voltage.
Load it with a speaker and it's no longer measureable.
Your amp seems fine.

Don't recap unless necessary - chances are you break something in the process for no real advantage.

Jan
 
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Cant measure anything on the inside yet as not taken unit apart yet. This amp doesn't sound as good as the last one I had though, rough scratchy treble.
Last one had ST branded 2n3055's, this one has a different brand. Could this be why it doesn't sound very good? I did check bias when I got it, and set it to 30mV as per service manual.
the only one more or less worth it is the mk1.
I have seen past with several brands of transistors and even several references (ST, CDIL, Motorola) (3055,15003,15015) etc etc etc
It is an English product in the purest sense of the word.
don't break what isn't broken.
check the midpoint of the power supply, the quiescent current (with caution!!) and basta.
If you knew about this technology, you wouldn't have asked the question.
On this device, you have to disassemble the assembly to access the lower face of the PCB, if you are not used to it, it will end in carnage.
On the other hand, disassembling and cleaning as it should the source selector and the volume potentiometer is a good idea.
 
This is the MK1, I have done these before @huggygood, you are correct, they are a real pain in the a$$ to service, it's been a good few years since I've done one. First one I had, I had to change blown output trannies and some caps, all I had available were MJ802's and they worked and biased just fine.

I am not an expert on amplifiers, I do a little diy as a hobby now and then and the only thing I know about this design is what I have read in the past, I was puzzled because I don't remember getting +1.4V on the output of these before.

As I was saying, this one does not sound as good as my last 2 so it wouldn't hurt to replace all the caps plus replace the Omron Relay which is usually a common fault with these, as are the input switches. It is worth doing imo though as these have a very 3D holographic soundstage like a tube amp.
This is a spare amp, and I didn't pay much for it, so if it goes bang it doesn't really matter too much, it's just something to play about with.

Thanks for replies everyone.
 
Last one of these amps I had used CDIL 2n3055's on one ch, sounded terrible on that ch, the other ch had original ST 2n3055's which sounded fine, I re fitted fitted ST brand on the bad ch which cured the pure sound.
Wondering if the poor sound on this one is due to the output transistors being naff (they have a Beta mark on them, from Romania or something) or if there are some bad capacitors, the volume pot and switches are fine. Unfortunately I don't own a scope or THD meter to test it properly, but my ears are telling me all is not good.