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How to best clean a H7864 power supply? [BA23 Box]

J

jorgen

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Aug 24, 2024
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Hi there.
I recently acquired a MicroPDP-11/73 in a BA23-A box, however, most of the H7864 power supply, the two fans, and some of the logic boards were quite dirty with some black powdery gunk having been built up inside the BA23 -- especially inside the power supply. See the two attached photos.

Granted, I need to replace the RIFA capacitors in either case, so I need to open up the power supply regardless, but before proceeding, I am a little hesitant how to go about and clean up this buildup. I have started using some cotton-swabs (q-tips) and isopropyl alcohol, and even though it captures some of the gunk, it also kind of "smears" it along the PCB and component surfaces.

I am sure there are better ways to clean up this gunk, and I would appreciate any suggestions you might have.

Thanks!
Jörgen
 

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Welcome to the forum @jorgen !

The black powder might just be dust, adhered over time with moisture and attracted via static electricity. Have you tried high-pressure air, maybe from a compressor?

- Alex
 
I would try and hoover out as much of that gunk as you can - undo all the connectors to get the ends of the cables free - then a combination of wet-wipes, foam cleaner and iso will eventually get the clear plastic clean and not sticky - finish with the iso to help it dry out... use cotton buds or a screw driver carefully to get in the gaps..

If you're changing the fans, they need to be proper high airflow ones - not PC cooler fans - the CPU and other boards REQUIRE good cooling all the time!
 
Yeah, that stuff is nasty. I've had a couple that I worked on 2 years ago. Basically did what you are doing. Though I started with disassembling it into the 3 main boards and disconnecting all the cables. Then using my air compressor to blow the worst of the stuff off - which still left a lot of gunk behind. Then a combination of paper towels, q-tips, a toothbrush and IPA (not the beer!) until it was clean. It was not a short session - at least 2 hours for each P/S.
 
Thanks for all of your useful answers! I really appreciate it.

(Thank you Alex! I've been lurking for a bit, but not posted until now)

It could be dust, but it's very black and is weirdly built-up closer to the fans, which I also find a bit odd. The fans are exhaust fans I believe? This makes me think that the grime has been collected in storage (in a garage, or shop perhaps?), and settled from "above". If the fans were running, I would have expected the dust closest to the fans to have been blown out of the case, and not collected that close to the exhaust. Seems weird, no?

I did use a vacuum on the BA23 once the power supply, logic boards, floppy and hard drive were removed which was pretty effective. It does "gunk" up the vacuum a bit, and it's a lot more of this gunk in the power supply, so I am a little hesitant to use the vacuum on it.

A compressor is a good idea, though I don't own one, but I think I will order a couple of compressed air cans ("electronics cleaner" ?!?) from Staples to see if I can blow off the worst of it, before proceeding with IPA and q-tips.

I did not think of replacing the fans (yet), but after seeing your impressive video comparing the fan noise, hush, I am tempted! Good idea.

Based on your combined recommendations, I gather I will proceed by: disassembling the power supply, separate the three logic boards, disconnect all cables. Use compressed air to clean the most of the gunk off of all three PCBs, cables, connectors and housing; and then proceed with paper towels, q tips, IPA.

@Radix you mentioned foam cleaner and wet wipes for the cables. Are there any particular brand or kind that I should be looking for? I assume too aggressive of a foam cleaner might damage the plastic?

Thank you all again! This is super useful!!!
Jörgen
 
the only caveat with the fan swap is that you either have to use both of the molex power supply cables in the chassis, so you cannot run fixed drives. it's probably possible to hack off the end of the noctua fan cable or otherwise mod something up, but i did not try- if you manage to get that working, i'd be interested to see how!
 
jorgen said:
...

A compressor is a good idea, though I don't own one, but I think I will order a couple of compressed air cans ("electronics cleaner" ?!?) from Staples to see if I can blow off the worst of it, before proceeding with IPA and q-tips.

...
Click to expand...
Do yourself a favour and buy a compressor. The compressed air cans are completely useless for any computer restoration task (and are way too expensive anyway). A decent compressor is between $200 - $250 or even less when on special. It lasts a lifetime if you look after it.

Remember to never skimp on tools. Buy them when you are still young. Look after them and even with occasional use they easily pay for themselves. I have a lot of tools. Some I wish I had bought decades earlier than I did.
 
Foam cleaner is very gentle - but to varying degrees, wet - but it cleans well - you will need lots of cloth or wet wipes - really LOTS - then let it dry out completely... I reform the main caps in those in circuit, gradually running them up to about 150V over some hours - BE CAREFUL - DANGEROUS!! - with an adjustable supply via a limiting resistor. If you go too high, the switcher will start, which is not what you want. Then I run them on mains via a 100W bulb (in the UK) and let the switcher kick in repeatedly with little load for a while to exercise the output caps as well.

You can use the existing fan supply for replacement fans - it's adjustable between 10V and 12V on the PSU via a jumper, but you can change a resistor and make it remotely adjustable with a variable resistor, if you have more than sufficient air flow from replacement fans - BUT keep the boards cool, that is the first priority - they are already vintage and their life expectancy improves with lower running temperature - this is true of all electronics

Robin
 
Thanks again for all your great suggestions. I might look into getting a compressor, but have for now gotten an electric "blower" (90 CFM) -- not quite a compressor, but should be useful to get the dry dust out.

I do have a variable AC supply, but it unfortunately doesn't have any current limiting abilities, other than selecting the lowest current range (0-2A) which caps the current to 2A. I do have a number of "power resistors" of various small values, all rated for 300 or 250W (the aluminum heat-sink clad kinds), which I was considering using. They are rated for 2.5kV with screw terminals. I current posses six of these: 25ohm, 10ohm, 4.7ohm, 2.4ohm, 0.47ohm and 0.22ohm, which I can connect in series with the H7864 to limit the current (somewhat); or I could connect the light bulb like you suggested in series with the variable power supply.

@Radix when you reform the main caps, you said you run them up to about 150V, with your mains at 230V, I presume the equivalent here in the US, would be ti run them up to ~70V given the 110V mains here? I presume you gradually (linearly?) increase the voltage from 0 to 150V over those hours? or do you increase the voltage from 0 to 150V quicker, and then let it "soak" on 150V for a longer period? When does the switcher kick in? Above 150V I presume?

Thanks!
Jorgen
 
Hi Jorgen - I expect that on 110V, the input rectifier is configured as a voltage doubler, so you can aim for the same 150V or more, if the switcher doesn't kick in... With a lamp in series, you get to see when it starts drawing more current (LIGHT!) and the lamp is rated at 60W or 100W on its own - those resistors need huge heatsinks and cooling to handle that sort of power... I would start down at 20V and increase it every 15 minutes or longer, longer is better! as long as the bulb stays out it's good - I think I used 10K @ 10W for reforming on that PSU with the caps in circuit, due to the balancing/discharge resistors across the caps, so you should see about 10% voltage loss across the resistor.

The lamp is more useful when on full voltage and you want a safe way to see how it responds without blowing fuses or lots of power to make thing go big bang... it's OK to let it cycle on and off for a while like that and you can see the voltages go up to their rated output level, usually, with little load and/or a higher wattage bulb, it will become stable with the fans running and steady DC output - then you can let it sit on soak test for hours

This is what I used as the basis of my cap reformer:
 
That sounds good. I am getting my replacement caps to swap out the RIFA caps today, so will aim to replace them this weekend after I finish cleaning up the PCBs; and will proceed with reforming in-circuit after that. I will follow your gradual slow ramp-up, and hope all the remaining caps are still good.

That amazon buck converter is DC-DC. Do you reform the caps on the DC regulator board separate from the AC/filter&rectifier boards by using the buck converted to feed DC to the rectifier board? Maybe I am misunderstanding how you reform the caps in-circuit?

Thank you again for your input. This is really helpful!
Jorgen
 
Hi - yes, I pop a couple of wires on to the caps terminals on the board with the bridge-rec (one wire to each cap - they are in series) - and if you connect the two boards together, you do all 4 at once... It is a good idea to have a meter monitoring the cap voltage as a reminder it is lethal... I have a couple of LED voltmeters, one on the DC-DC converter output and another after the limiting resistor, so I can monitor the PSU and cap voltage - and have a reminder while it is sitting on the bench in pieces
 
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