23-04-2012, 01:29 PM
Hi Joe,
What you've learnt there is that fixing d.c.-coupled amplifiers require a different approach to the classic 'switch it on and wander about with a voltmeter measuring various voltages'. That's maybe O.K. for valve stuff - but it's a real no-no here.
When you have a fault like that, the first thing to do is to switch it off - quick! Then remove all the transistors and diodes, one by one, and check every one for internal shorts or open circuits; replace any that are faulty. Check and if necessary, replace, any low-ohmic value resistors - especially emitter Rs. in any O/P pair. Then remove and check all the capacitors (there probably won't be many): replace any that are faulty. If there is a quiescent bias pot, check it; replace if necessary; set it for minimum current in the O/P transistors. (Don't be tempted to do this by connecting the amp. up to the PSU yet!). Check for mechanical open or short circuits on printed tracks. When you're really sure that you've got all of the 'nasties' out, make sure that the amp. has no signal input, hook up a load on the O/P ( I use a resistor and hang a 'scope across it) & connect the amp. up to a current-limited PSU. Switch on and look at the current meter in the PSU. If the current looks way too high, switch off immediately
. Now go back and re-check every component - just like you did before. ( I know, I know! 
) Until you can eventually escape that loop, there's no point in trying anything more adventurous - like trying to pass a signal through it. Next step is to set the quiescent current and check the temp. of the O/P transistors and check a few critical d.c. voltages.
It's only when you've got to that point is it worthwhile testing the amp. with a signal - and even then, something may go pop!
So there we are: d.c - coupled amps. can be a real pain. As anyone who has repaired Tek. transistorised oscilloscopes can tell you! (But that's another story for another time
)
And finally, I now have a Leak Stereo 70 (needs mending) sitting in the top of my in-tray: writing all the above has reminded me of that necessary approach.
Al.
What you've learnt there is that fixing d.c.-coupled amplifiers require a different approach to the classic 'switch it on and wander about with a voltmeter measuring various voltages'. That's maybe O.K. for valve stuff - but it's a real no-no here.
When you have a fault like that, the first thing to do is to switch it off - quick! Then remove all the transistors and diodes, one by one, and check every one for internal shorts or open circuits; replace any that are faulty. Check and if necessary, replace, any low-ohmic value resistors - especially emitter Rs. in any O/P pair. Then remove and check all the capacitors (there probably won't be many): replace any that are faulty. If there is a quiescent bias pot, check it; replace if necessary; set it for minimum current in the O/P transistors. (Don't be tempted to do this by connecting the amp. up to the PSU yet!). Check for mechanical open or short circuits on printed tracks. When you're really sure that you've got all of the 'nasties' out, make sure that the amp. has no signal input, hook up a load on the O/P ( I use a resistor and hang a 'scope across it) & connect the amp. up to a current-limited PSU. Switch on and look at the current meter in the PSU. If the current looks way too high, switch off immediately
. Now go back and re-check every component - just like you did before. ( I know, I know! ) Until you can eventually escape that loop, there's no point in trying anything more adventurous - like trying to pass a signal through it. Next step is to set the quiescent current and check the temp. of the O/P transistors and check a few critical d.c. voltages.It's only when you've got to that point is it worthwhile testing the amp. with a signal - and even then, something may go pop!
So there we are: d.c - coupled amps. can be a real pain. As anyone who has repaired Tek. transistorised oscilloscopes can tell you! (But that's another story for another time
)And finally, I now have a Leak Stereo 70 (needs mending) sitting in the top of my in-tray: writing all the above has reminded me of that necessary approach.
Al.
