Someone brought a dead Marantz amplifier to [Lansing]’s attention, a rather nice model from the 1980s with one channel entirely dead and the other very quiet. His account of its repair is straightforward, but provides some insights should you find yourself with a similar item on your bench.
Opening up the box, he was presented with 35 years of accumulated dust. It’s the annoying side of cracking open classic kit, we all have our dusty horror stories. His first task was routine: to replace all the unit’s capacitors. The mains voltage in France has gone up by 10 volts from 220V to 230V as part of EU harmonization in the years since the amp was built, so he used capacitors with an appropriately higher rating to compensate. We might have waited until the rest of the amp was proven fixed before splashing the cash on caps, but maybe we’re more thrifty.
The quiet channel fix turned out to be from a muting circuit designed to keep the amp quiet during the turn-on phase and suppress that annoying “thump”. A dead transistor replaced, and all was well. The dead channel though had a whole slew of dead transistors in it, which turned the problem from one of repair to one of transistor equivalence. Quite a few of the 1980s parts were no longer available, so modern replacements had to be found.
It is tempting to think of particularly all small-signal transistors as functionally equivalent. You will get away with this in logic and switching circuits in which the device is either On or Off and never in between, but in an audio amplifier like the Marantz things are not so simple. A lot of effort will have been made by the designers to calculate resistances for the current passing through them to deliver the right DC bias points without sending the circuit into wild oscillation. An important part of that calculation comes from the current gain of the transistors involved. [Lansing] had to carefully select his transistors for equivalence, though it some cases he had to do a bit of creative lead-bending to fit a different pin-out.
So, all dead transistors replaced with appropriate equivalents, and the amp was reborn. Success, and very much worth the effort!
We’ve covered a lot of amplifiers here in the past. Some were dead, like this little amp with blown capacitors or this smokin’ subwoofer. Others are more esoteric, like this ion wind 1KV tube creation.