24000uF 50V capacitors for my PDP8/M

[Roland Huisman]

My favourite little PDP8/M broke down today. One of the power capacitors has a short circuit and tripped the mains fuse. The capacitors had a heavy life in the past because the fan broke down once. So these capacitors were fried and I'm really surprised that these last here for several years. This machine has never let me down in the past.

So can anyone help me to get one, or better two of these 24000uF 50V capacitors?



I hope to get it back online soon. This machine has done so much work for me and also the community for the development of several clone boards and the bootstrap loader.

Thanks in advance!
Regards, Roland

 

[Mike_Z]

I see that Mouser Electronics in Texas has some. Don't know if they are the correct size. Mike

 

[Someone]

In addition to capacitance and working voltage ensure that your replacement capacitors have suitable tolerance, ESR (equivalent series resistance), temperature range and longevity specifications. The modern replacements may be physically smaller.

 

[dalmill]

Can't speak to all the specs pointed out, but I used these 24000uF 50V https://www.ebay.com/itm/130553799259 on my PDP8/M power supply for the same issue.

The screw terminal spacing is perfect. The diameter is only barely larger than the original and still fits well.

They seemed brand new, although if I figured out the date code correctly, they were from 1988.

That was good enough for me.

I went thru cap re-forming process to be safe prior to installation and power on.

My PDP8/M is not running yet while I intermittently play with it, but not due to any PS voltage issues or ripple.

Not having a full lights and switches front panel is the major obstacle at the moment.

Dale

 

[gwiley]

I see quite a few on eBay when searching for 24000 uf, but I'm in the US so you might see different search results.

Digikey and Mouser have some 24000 uF 63 volt parts by Kemet that might have similar or same dimensions.

The capacitors in your photo have a strange color. Is this how they were originally or is there discoloration due to heat or smoke?

I reformed C1 and C2 in my 8/m for about 5 days and the reforming current dropped significantly. I wonder if this could recover the shorted one.

https://forum.vcfed.org/index.php?threads/fun-stuff-just-got-a-new-old-pdp-8-m-chassis.1238187/post-1256186

 

[thunter0512]

Mouser part number "598-DCMC50V263" is 26000 uF 50V at only 15,69 €.
Strangely the 24000 uF part (539-CGS243U050W3C) is much more expensive at 50,10 €.
Life, ERS and ripple current are similar for both.

If you want to keep the look of the original caps you could possibly open the old ones up and gut them and install the new caps inside the old housing.

Merry Christmas.
Tom

 

[MauriceH]

Roland,
had the same experience back in 2019 with my pdp8/m. suddenly a full short on one of those caps.
But it also shorted out that bridge rectifier.
So double check that one also.

Ohw see my pictures back, and
That Bridge rectifier had a bad soldering joint , so a very high ripple think killed that cap.
And it had a short on one of its diodes.

Temporary replaced with a 6800uF for test if supply was working again.
Later I found 2 replacement caps of same size.

Here my old forum threat for asking the same caps.
Lenght is 14cm
Max is Dia 5,20cm,
Screw hart-hart is 2,20cm

And see, ask Willem(Oldtimer) if he has a second par of the ones he sent me.
See link for picture type and markings of those NOS caps.

 

[Albert Salvia]

To buy a capacitor another interesting website is https://www.tedss.com/Capacitors/Browse/aluminum-electrolytic-large-can-computer-grade.
Find the one of similar size.

 

[Chuck(G)]

Also, check out Surplus Sales of Nebraska's stock. Shipping, however, may be a deal-killer.

 

[Roland Huisman]

Thanks, I found a few of the original models on Ebay 'new old stock'. So I will try and see how these will do. The original ones in my PDP8/M were fried as you can see on the brown plastic. That is not the normal colour. My PDP11/10 and PDP8/F have the same power supplies with original mains capacitors and these work just fine. But they were never overheated like these in my PDP8/M. Now I have to wait a few weeks, then I'll get back here to share my experiences.

 

[Hugo Holden]

It is interesting when an electrolytic cap shorts out. I have investigated this in the past. I have some photos somewhere.

If you dismantle the capacitor and unwind the foils it is nearly always the same thing. At one point between the foils you will find that the insulation has burnt away and that the two foils have manged to make contact . Most of the other insulation and foil will look perfectly ok and often the short zone is less that 1/4 inch in diameter, very easy to see though.

I think it is a manufacturing defect due to lack of cleanliness at the factory. I think what happens is that a small metallic particle gets in there and initially it is still insulated, embedded in one side of the insulation. After enough temperature cycling it breaks through and causes an inter-foil short. The other possibility, but it seems much less likely, is that a spike of conductive material grows as it does in Tant caps, but electrolytics are not prone to this, and if you inspect the remainder of the foils and insulation, it is all perfect in the shorted electro, with no sign of any corrosive or other growth effects going on elsewhere.

When you get the chance, pull the capacitor apart and we will find out what caused the short.

Some capacitors were assembled with latex rubber in them. When it degrades it corrodes through the strip wire interconnects to the terminals. In this case, the capacitor goes spontaneously open circuit, I have seen a lot of this in Tek mainframes.

 

[MauriceH]

Roland,
Great a "original" set found.
A long storage,even years does not effect the life span, as long as you reformat them with a series resistor for at least half an hour.

Hugo, indeed a growing defect in insolation foil think is most likely in those caps that are build to last.
As the seal of those are pretty good, i don't think due to dry out of electrolyte.
Problems occur with these is a ripple current to high for those.
In my case a defect bridge rectifier.

Thinking of those of Roland over heat, inside the foil could be wrinkled and at some places thin out the isolation.
At some point it breaks through and there the full shortage occurs.
Once unwrapped a High voltage capacitor .molted inside a TV flyback transformer and indeed lot of holes in that foil
and at one point burned both foils together.

Luckily for Roland they did not blown up as I once had with the capacitor in the picture attached ).