DPA 50S common emitter push-pull schematic
Hi
I've come across a few emitter-follower/common collector push-pull power amp circuits where NPN / PNP are collector-connected to +ve and -ve respectively, but am working on a repair of a Deltec Precision Audio DPA 50S from circa 1987 where the PNP / NPN power transistors are connected the opposite way (PNP and NPN emitter to +ve and -ve respectively).
The driving circuitry is therefore a bit more complex than I'm used to seeing and it's giving me a brain-block without any schematic to look at... Does anyone have any information that could be of help please?
It appears the output power transistors on one channel have gone short (probably drivers too), taking out a pair of 6R0 (odd value!) wirewound resistors (one got so hot as to melt its own solder connection before vaporising itself)! Apparently this blew from cold having previously worked so possibly power-on stress on aged components.
Power transistors are MJ15003 and MJ15004, drivers MJE15029 and MJE15028 (all pretty expensive so wanting to get this right), there's a couple of trimmers to set the bias/quiescent current for each channel too - they both appear to be fine, reading approx 5ohm, not open but again I have no information to go on apart from the working channel in terms of setting such things.
I've scoured the net but not finding much info about these amps or this design anywhere, so any info greatfully received.
Keith
I've come across a few emitter-follower/common collector push-pull power amp circuits where NPN / PNP are collector-connected to +ve and -ve respectively, but am working on a repair of a Deltec Precision Audio DPA 50S from circa 1987 where the PNP / NPN power transistors are connected the opposite way (PNP and NPN emitter to +ve and -ve respectively).
The driving circuitry is therefore a bit more complex than I'm used to seeing and it's giving me a brain-block without any schematic to look at... Does anyone have any information that could be of help please?
It appears the output power transistors on one channel have gone short (probably drivers too), taking out a pair of 6R0 (odd value!) wirewound resistors (one got so hot as to melt its own solder connection before vaporising itself)! Apparently this blew from cold having previously worked so possibly power-on stress on aged components.
Power transistors are MJ15003 and MJ15004, drivers MJE15029 and MJE15028 (all pretty expensive so wanting to get this right), there's a couple of trimmers to set the bias/quiescent current for each channel too - they both appear to be fine, reading approx 5ohm, not open but again I have no information to go on apart from the working channel in terms of setting such things.
I've scoured the net but not finding much info about these amps or this design anywhere, so any info greatfully received.
Keith
