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Commodore 8250 disk station making loud buzzing noise

mikerofone

mikerofone

Experienced Member
Joined
Aug 9, 2020
Messages
69
Location
Switzerland
Hi all,

I got a Commodore 8250 disk station last weekend. It's working mostly fine, but the drives needed some service and I removed the filter cap after it blew up on me. (I looked for Rifas, didn't find any and thought I'd be safe - I was wrong! It stank about 100 times less than a Rifa though.)

As far as I can tell, everything is still original, no new caps etc. When powered on, the drive emits a very audible buzzing sound that seems to come from one of the voltage regulator cans on the board in the lid. See the video in https://photos.app.goo.gl/t9dpgbtiRomPcv7f8 for a demonstration. I'm pretty sure it's not the transformer, and it also didn't change at all with or without the filter cap. The voltage regulators are on a heatsink so I expect them to get warm, but after only a few minutes the one furthest in the back of the case (SH323SC) is too hot to touch. The other ones are less hot but still very hot. Not sure if that's to be expected or cause for concern, but because of that I'm not running the thing for more than 10-15 minutes in one sitting.

I could imagine that some capacitors have failed and what I'm hearing is very high ripple that's torturing the regulators. Any suggestions on where to best measure this? I have a scope, but before I just go poking around on the voltage rails I wanted to get an idea of what I'm looking for. I should get looking for a schematic for the drive as well.

I'd assume that I'd want to measure the voltage and ripple before it reaches the regulators, is that right?

Thanks for your help!
Cheers
mikerofone
 
Download the technical/maintenance/service manual and schematic diagram for the drive.

The two off 12V regulators (7812) should each supply a floppy drive. Disconnecting the two floppy drives should disconnect the load from these regulators.

The 323 voltage regulator (+5V) feeds both the logic and floppy drives. If you have disconnected the power to the two floppy drives, this removes this load from the +5V regulator.

I suspect the regulator with the pop rivets is original, but the two regulators with bolts are replacements.

It is likely that the heat sink compound between the voltage regulators and heatsink has degraded and also requires replacing.

You are correct in that it is likely that one or more capacitors may have degraded. You need to measure the voltage at the input and output pins of the three regulators.

Check the datasheets for the pinouts of TO3 voltage regulators 323 (+5V) and 7812 (+12V). I would solder wires onto the relevant pins (×6) and GND (×1) and put these into a 'chocolate block' connector. You can then connect you oscilloscope GND clip to the GND and probe the inputs and outputs of each voltage regulator quickly, efficiently and safely.

However, you really should have checked the capacitors BEFORE powering the unit up, and (even then) disconnected as much as possible from the power supplies.

Now you know that the 323 is a +5V regulator and the 7812 is a +12V regulator, which regulator is overheating the most?

Dave
 
Awesome, thanks for the detailed instructions Dave! The hot and buzzy regulator is indeed the 323, i.e. the 5V one. The next time I can get tinkering, I'll test it and the others as you described. Good idea to use a "chocolate block" for probing, thanks! I hadn't heard that term before, but I had a hunch what you meant even before googling it. ;)

As for the thermal compound, I should be able to use the same (non-metallic) stuff I use on a modern CPU heatsink, right? Probably way overspecced, but that's what I have lying around.

Thanks again!

Cheers
mikerofone
 
>>> Thanks again!

No problem.

Yep, bog standard PC-type thermal compound... There is no such thing as over-specified when it comes to getting rid of heat! The cooler the better - especially for vintage equipment...

Dave
 
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