Speaker hum on resurrected Advent 300

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Hi all, I hope I am posting in the correct forum. I am new here and shallowly versed in amplifier circuitry. My only test instrument is a multimeter. However, I am very methodical and dogged. About 10 or 12 years ago an Advent 300 I picked up on eBay for the phono amp had a meltdown. It had served well for a few years. I got the schematics and parts list, in fact, the complete service manual, all of 16 pages. I replaced the outputs, the filter caps and bridge rectifier, along with most of the electrolytic caps in the amp/preamp section. When I plugged it in and turned it on it immediately melted down again. I put it away for the future, which is now. Long story short, I got one channel singing again with the tuner as input after swapping out a shorted driver and replacing the output transistors and the coupling cap between the preamp and amp. I was stoked, of course. Then I heard a faint speaker hum when turning the unit on with no input. Here are some symptoms: 1. Hum occurs with no input and no equipment connected. In other words, selector turned to aux or phono, and when turned to tuner and volume turned all the way down. 2. Hum occurs with volume turned all the way down and does not increase when volume is turned up. 3. Hum disappears completely when preamp is disconnected from amp by way of jacks in the rear 4. DC voltage measured at speaker output with no speaker connected is 12mv. Given the symptoms, can anyone suggest a likely suspect? The filter caps are at least 10 years old, but haven't been used since I put them in until now. They are marked "National Capacitor Type85A" and are 3300mfd. Originals were 3000mfd. With filter caps, is bigger better, or should they be closely matched to need? I'm confident that 3300mfd is a good match to originals, but would 5000mfd, or 10,000mfd be better? Another question is, can filter caps be used in parallel to increase capacitance? Thanks for any guidance, Jack
 
Sounds like the preamp power supply section to me. Not much to it in this case - check +/-15 V. If -15 V is low, inspect Q401 (MPSU52) for bad solder joints, might also be dead. If both are low, focus on regulator RA400 (78M15) instead.

Muting time constant cap C222 (10µ/16) may also have seen better days.

Finally, your NOS caps might be bad after all - these seem to be, by all accounts, at least 30 years old. (Which, depending on how they were made and stored, could mean anything from "as good as new" over "slightly degraded" to "badly leaky". Some gentle reforming would have been advisable at the very least. If they do not get warm, they may be OK.) Checking ripple on your supplies requires at least a scope though.

Basically these filter caps can be about as large as will physically fit, well, just as long as rectifier BR1100 isn't too wimpy, in which case it may not be amused by the higher inrush current. Also depends on transformer output impedance, which acts as current limiting, so in practice the difference between different values ultimately isn't as big as one might think (Rod Elliott wrote about this somewhere, I guess in the power supply design article). There are 3-4 amp fuses installed, so I'd guess the part would be rated at 5 A or something? Usually the type will tell, but check the datasheet to be sure.
Anyway, with like a 5 A rectifier I'd be confident putting in some larger caps... 4700µ should generally do fine for 8 ohms (4 ohm land needs 8200µ to get power bandwidth down to 20 Hz).

Speaking of the rectifier, no RFI suppression / transformer secondary snubber capacitors anywhere in sight. Cheapskates. Guess they thought they could get away with it with only an FM tuner installed. Would've been pretty buzztastic on AM. Ideally every rectifier diode should have a capacitor in the 22-100 nF range in parallel (maybe a little film cap, 63 V should do - ceramic X7R/X5R of similar or higher voltage rating should be OK as well but might make a bit of noise). This sort of stuff has a tendency of being fed back into the mains.

Speaking of mains, the mains cable looks like a thin dual-conductor job without additional insulation. Should be replaced by a modern one on occasion.
 
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