An update...
I've replaced Q8 on the offending side, and all seems to be well with the amp. Just for laughs, I put the faulty Q8 into the right hand channel, and I was able to reproduce the problem in that channel now.
So the faulty component was indeed Q8...
What I'm going to do now is to swap out RV1/RV101, Q8/108, Q7/107, Q11/Q111 and Q12/Q112 just to be on the safe side. These do tend to be the warmest components. Then have a play with some nicer sounding op-amps...
Many thanks for all your help, and especially to you, Mooly. It's been very educational.
I've replaced Q8 on the offending side, and all seems to be well with the amp. Just for laughs, I put the faulty Q8 into the right hand channel, and I was able to reproduce the problem in that channel now.
So the faulty component was indeed Q8...
What I'm going to do now is to swap out RV1/RV101, Q8/108, Q7/107, Q11/Q111 and Q12/Q112 just to be on the safe side. These do tend to be the warmest components. Then have a play with some nicer sounding op-amps...
Many thanks for all your help, and especially to you, Mooly. It's been very educational.
That's brilliant, you've done really well sticking with this 
It's nice to swap the part over and prove it as you have done.
I think you know enough to carry on now with what you intend, just be careful with that final setting of the presets, that the current is no higher than the manual says, and that the amp is hot when you check. If you change anything always go back to the bulb first, and without speakers ------ but you know all this now
Now what about modding it for some nice FET's as outputs -- no thermal runaway then-- and those Op Amps, a discrete build maybe --- and then there the motorised volume control with full remote control.
We shall expect an update wprice99
It's nice to swap the part over and prove it as you have done.I think you know enough to carry on now with what you intend, just be careful with that final setting of the presets, that the current is no higher than the manual says, and that the amp is hot when you check. If you change anything always go back to the bulb first, and without speakers ------ but you know all this now
Now what about modding it for some nice FET's as outputs -- no thermal runaway then-- and those Op Amps, a discrete build maybe --- and then there the motorised volume control with full remote control.
We shall expect an update wprice99
Thanks, Mooly... One thing at a time, I think!!!
Once the new bits are all replaced, I'll be making sure it's not overheating, etc. and get those RV1/RV101's adjusted properly...
... then once I'm happy with that, get it playing through real speakers rather than an ex portable-radio speaker, and see what difference the TL072's and OPA2604's make (I'll be sticking those in to an IC socket for ease of experimenting).
I'll think about changing to FET's as output... what do you have in mind?
When you say a discrete build for replacements of the op-amps, I presume you mean making something up out of the base transistors instead of an all-in-one piece of silicon?
But the motorised volume control - I think I'll skip that - I'm sure it's easy enough as all I need to do is add a relevant remote-control decoder circuit, plus an IR receiver, and hook that up to an alternative volume pot... but I think this amp is now destined for my (smallish) office due to the purchase of a newer amp from e-bay which is the main lounge hifi instead!
Once the new bits are all replaced, I'll be making sure it's not overheating, etc. and get those RV1/RV101's adjusted properly...
... then once I'm happy with that, get it playing through real speakers rather than an ex portable-radio speaker, and see what difference the TL072's and OPA2604's make (I'll be sticking those in to an IC socket for ease of experimenting).
I'll think about changing to FET's as output... what do you have in mind?
When you say a discrete build for replacements of the op-amps, I presume you mean making something up out of the base transistors instead of an all-in-one piece of silicon?
But the motorised volume control - I think I'll skip that - I'm sure it's easy enough as all I need to do is add a relevant remote-control decoder circuit, plus an IR receiver, and hook that up to an alternative volume pot... but I think this amp is now destined for my (smallish) office due to the purchase of a newer amp from e-bay which is the main lounge hifi instead!
Stick to trying different OpAmps, that will be interesting, and give yourself time to listen carefully, over a few days to each one.
MOSFET's would be an interesting design exercise but unfortunately they are not a straight swap - the pin outs different on two legs- and a couple of resistors need adding. Very do-able though.
Now one last tip, and we have all done it. IC sockets ! When you take an IC from it's socket, prise a bit at a time and from each end with a little screwdriver under the chip. Use fingers, or try and do it in one and it will fly out all pins squashed at all angles.
Have fun and enjoy it.
Regards Karl
MOSFET's would be an interesting design exercise but unfortunately they are not a straight swap - the pin outs different on two legs- and a couple of resistors need adding. Very do-able though.
Now one last tip, and we have all done it. IC sockets ! When you take an IC from it's socket, prise a bit at a time and from each end with a little screwdriver under the chip. Use fingers, or try and do it in one and it will fly out all pins squashed at all angles.
Have fun and enjoy it.
Regards Karl
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