Back in 2013 I picked up as part of a package deal a Marantz SR840 receiver. I got it for nothing because “it didn’t work” and “if you feel like you want to fix it, go for it”.
Sure thing! The above picture is from after a thorough clean. It was grimy and horrible. Powered up but didn’t do much more than blink lights and show it was tuning to a radio station.
I’d had a bit of a dig about, found by bypassing the preamp and feeding the power amp it would make fairly acceptable noises. The preamp did not put out but the radio board had output. It was very one-sided when probed with my oscilloscope, and sounded distorted.
I tried to poke about for voltages but without a schematic I hit a dead end. I hunted about for a service manual but found none. It got shelved.
I was thinking about it the other day, and decided to go on the search again for a service manual. In 2014 someone put up a good scanned copy for sale at Analog Alley for $16. Fifteen minutes of work later I determined the cause- no -15V on the preamplifier.
I ordered some bits and bobs. There’s a small regulator chip for the +15 rail, I’m guessing there was no -15V integrated regulator available at the time so a PNP transistor, bleed resistor, couple of capacitors and a Zener diode formed a -15V regulated supply.
I managed to make one of the resistors smoke up accidentally on the radio board when I disconnected the -15V supply to see if it would come up with no load.
Replaced that, bought some new parts.
Removed the power regulator circuit and replaced it with new. The 100uF capacitor had gone short circuit and had taken the Zener and transistor with it. The resistor was a bit high due to the current draw so was replaced also.
All the new bits in on the board. The entire chassis is packed quite tightly.
Net result was to be presented with stereo FM! It worked for about fifteen minutes then overheated a capacitor in the power supply… the display and LED’s went all funny then it shut off. It came back up a short while later after it had cooled off, so it just needs all the electrolytic caps replacing. It’s kinda worth it for the stereo AM.. the tuner on it is actually pretty good. It’s being buttoned up for now, but I’m happy in the knowledge there’s nothing drastically wrong with it.