Replacing an old electrolytic

[RichardHowells]

My old HP 175A scope had not been powered up for years. Yesterday I turned it on and it failed the smoke test within seconds. There are a couple of possible suspects, but I reckon its the 7000uF electrolytic in the low voltage power supply.

The LV power delivers smoothed and regulated +/- 6.3 volts for some valve heaters. I don't know the exact current supply, but it's drawn thick on the schematic, the actual wiring is chunky and it's protected with an 8.5A slow-blow fuse.

The original electrolytic is 7000uF 25VDC and physically big - about 50mm diameter 90mm tall. I've removed it from the chassis and my multi-meter reckons it's shorted.

I am worrying a little over a replacement and I'm looking for reassurance. Modern components are tiny by comparison. The original part is not explicitly marked as high ripple or similar. Can I safely use (say) two of these (http://www.maplin.co.uk/p/4700f-35v-...apacitor-n97kf) in parallel as a replacement?

Thank you for reading so far.

[Boater Sam]

I have a huge supply of big low voltage cans, some with high ripple ratings. I'll have a look for you.
Sam.

[RichardHowells]

Thank you.

I don't know that high ripple is *explicitly* required here, The original has no specific claim about ripple.

The original is a Sprague. It has lots of markings and I have no idea which ones might be relevant. It says:
36D3070T
0180-0134
RED 7000 - 25 DC
D35642 6252
MADE IN U.S.A

[Boater Sam]

Among my many cans, I have these,

Sprague type 360D 9000uf 50v 50mm X 105mm
RS 10000uf 30v 42mm X 100mm
RS 10000uf 40v 42mm X 100mm
ITT 10000uf 63v 50mm X 75mm
Sangamo type DCM 10000uf 35v 36mm X 68mm
Philips 6800uf 40v 35mm X 55mm

same order as attached photo.

I have even larger!

Sam.

[Boater Sam]

Forgot to say, they are no charge however many you want, just postage.
That Sprague is type 36D, correction, same as the original. 10000uf should not be too big, the tolerance is usually very wide anyway.

[RichardHowells]

May I have the ITT 10,000uF one? Although the Sprague is the obvious replacement I don't have enough headroom in the chassis for the taller can. The existing 90mm height gets within about 10mm of a valve pointed across the head of the cap. It's a tightly engineered chassis.

It's the smoothing cap in the LV power supply. I'd agree that 7,000 or 10,000uF probably makes no difference.

I'm a newbie here, so I'll assume that the etiquette is for me to PM you my contact details. Until someone tells me otherwise.

[Mooly]

Have you actually observed the component smoking ? If not then a more likely suspect could be something like a RIFA suppression cap across the mains input.

Just a thought.

[Boater Sam]

I'll see if I have a shorter can @50mm in diameter as well.
Will send them to you, PM me on this forum(top right of this page shows PMs) with your address, I'll send you my paypal details etc.
Sam.

[RichardHowells]

@Mooly - Thanks for the thought. I didn't observe the actual component smoking.

I saw smoke from one part of one side of the unit.

Most of the casing is perforated metal so I assumed that the culprit was near the smoke exit area. I expected to find an exploded electrolytic, but didn't. I did find fresh looking deposits of viscous brown liquid on components near the smoke exit, and dripped on the chassis floor.

There is a whole cluster of electrolytics in that area. None of them was obviously misshapen/expanded/bulging/leaking.

Eventually, based on it having the most brown muck on it, I guessed on the smoothing cap for the LV PSU as the most likely culprit. I've removed it from the chassis and my multi-meter reckons it's shorted.

So many thanks for the thought. It is still quite possible that I have further problems to uncover and maybe more caps will fail when the unit is back under power.

I am pretty convinced that this specific cap is dead.

[paulsherwin]

Bad electrolytics don't usually emit smoke. They either just stop working or develop very high leakage current which causes the electrolyte to boil. In the latter case they will vent fluid and steam, or sometimes explode.

[RichardHowells]

Hmm. Thank you for that.

I've assumed that the 'smoke' I saw was boiled off electrolyte and the brown viscous fluid normal temp electrolyte.

I think it's clear that this cap is dead, but I think I'll keep the covers off for the next power up. Maybe I have more to find.

[Techman]

Are you sure the electrolytic is short circuit? Large values like that one will take time to charge from a DVM, so it could be the charging current that looks like a short circuit reading, you'll need to leave the meter connected until the capacitor is fully charged to get a definitive reading. I wouldn't go replacing ANY capacitors at this stage until you've made checks to see what else may have failed, as the smoke you describe could be from something like the mains transformer, which if burnt, could make the unit an uneconomical repair.

[Techman]

Originally Posted by Mooly

a more likely suspect could be something like a RIFA suppression cap across the mains input.

Just a thought.

I would certainly be going with this suggestion as my first port of call.

[paulsherwin]

Originally Posted by RichardHowells

Hmm. Thank you for that.

I've assumed that the 'smoke' I saw was boiled off electrolyte and the brown viscous fluid normal temp electrolyte.

I think it's clear that this cap is dead, but I think I'll keep the covers off for the next power up. Maybe I have more to find.

Don't peer too closely at it as you power up, and wear safety goggles. If an electrolytic does decide to blow itself up it can be very violent.

[factory]

Originally Posted by Techman

I would certainly be going with this suggestion as my first port of call.

The mains suppression capacitors used in HP equipment of this age (1960's) are usually ceramic disc types, I've not had to replace any yet.



David

[Mooly]

Interesting, so no RIFA's then

Methinks you might have to power it up again and look for smoke signals which incidentally is a valid method of fault-finding.

As Paul says, don't stand over and peer. Perhaps use an extension lead and power up from a distance.

[factory]

If C481 is shorted then the fuse won't blow as it's fitted after the capacitor. Maybe the smoke was coming from the inductor (L481) if the capacitor is a dead short.
With the power off, I would remove the 8A fuse F481 and as you've removed the capacitor, check if the rectifier diodes are OK, you may need to disconnect one side, I think they are the two large stud mount diodes under the power supply PCB

David

[RichardHowells]

Thank you to everyone who contributed. Especially Boater Sam who sent me a handy selection of electrolytics.

I have progress of a sort. I replaced the cap. I no longer have smoke coming out (Yay!) but it doesn't work (Boo!).

I've checked the LV power supply outputs with my multimeter. They are not perfectly to spec but well within reasonable "You should work OK with that." values. I'm too scared to go close to the HV power, I don't have any suitable test gear for those voltages. I'm happy to take that area on trust. There is a spot of sorts on the screen so there has to be some HV activity and heater power to the CRT.

Sadly I get neither horizontal nor vertical action from the spot. I might be able to do more with pure dc voltage measurements, but I suspect I'm in that recursive situation where I need a second scope to fault-find on this one.

I know where I can lay hands on another scope, but not for a few weeks. This project is temporarily on the shelf.

[Bazz4CQJ]

In the OP, you mentioned that it hadn't been switched on for years, you made no mention of using a Variac to bring it back to life gently. Is it the case you didn't do that? I'm religious about using my Variac; use it on anything that hasn't been used for more than a few months, even on stuff with valve rectifiers.

B

[RichardHowells]

Sadly (for the scope) I gave it the tough love approach. I've no way of knowing if that's what caused the original smoke signal of distress.

Electronics is an occasional hobby for me and I didn't think of using a Variac though I do have access to one. After my baptism of smoke I did (too late) start another thread on that subject.

https://www.vintage-radio.net/forum/...d.php?t=136518