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Stax SRM006TA

a.palfreyman

a.palfreyman

pfm Member
Got my Stax SRM006TA out of storage as not used it for some years now. Bought some ear pads from Electromods to replace the (crushed/degraded) old ones. I replaced all but the main high voltage electrolytics some years ago but 3 of the 4 are showing signs of the end caps bulging. I'm pretty certain that these were +/- 380V, but measured them tonight after 2 hrs use and they measured about +/-330V so suspect the leakage current has gone sky high. Just bought some NOS Panasonic HA 220uF 400V 105 degree caps from eBay and wondered if there are any precautions when powering up after replacement with such high voltages?
 
There can't be any harm in leaving the caps to reform at a low voltage for a while on a bench power supply.

I've never played with a electrostatic headphone amp but if I was worried about old caps in a valve amp I'd bring it up slowly on a variac (if I had one).

I don't have a variac, so the best I do is to power such things up on the end of a DBT and RCD circuit breaker.

But you already know all of that. I'm sure a grown-up will be along shortly to offer some proper wisdom.
 
Thanks Mike,
Yes I'd wondered about reforming them but max I can go to is 60V dc with my bench psu and when I've done and measured this before, through a suitable resistor the leakage current goes up rapidly with increasing voltage. Had wondered about knocking something up as I have some 1N4007 diodes (1000V) so could put one in series with say 5 off 4k7 resistors all daisy chained to split the voltage and add a 100mA fuse and charge it off the mains. That will take it to circa 340V and about 100mA at switch on so as long as I'm careful should be OK, unless others know different of course!
 
340vdc on a charged cap (let alone direct off mains rectified is very, very bloody dangerous. Even done one at a time. And how do you disharge the cap safely afterwards, given dialectric absorbtion will tend to make the terminal voltage rise again once they’ve sat ‘discharged’ a while to an unknown extent. Got an isolation transformer? Wont save you, but limits potential for inadvertant shocks to Earth.

ps 340vDC on 220uf is 13 Joules of stored energy - properly dangerous. And charging it will cook small resistors.
 
Martin,
Thanks for that, thought it was pushing it but need to find a method as I don't want exploding caps if they are leaky due to storage. Was contemplating doing them at 60v initially as I can achieve that with my bench psu. Other than that it might be a case of building a simple voltage tripling circuit or similar. I read the link you posted in the ref section WRT reforming caps but despite there being mention of reforming to high voltages, there was nothing in the way of circuit detail. Yes, I appreciate that high voltages are very dangerous and will kill you.
Interestingly today I saw some DC-DC converters that convert 5v to 400v and upwards on ebay. Now that looks quite dangerous too.
 
There was a bargain isolation transformer in the diy classifieds which might be suitable/can be made suitable and could be worth investigating.

I recently bought an isolation transformer made by Bronson which seems very good value (I got mine on Amazon). It has both binding post outputs and a UK plug socket output. Ironically because of safety regulations it comes with the earth pin of the UK socket connected but that's easy enough to disconnect and isolate.
 
Cheers Mike,
Hopefully going to speak to our instrumentation engineer at work about this later today to see what he can suggest.
I have an Airlink 240V to 100V isolation transformer that I run the Stax off as it's a Japanese version. I also have a 230V to 110V transformer that came with the Stax when I bought it s/hand (that reminds me where the 380V came from above, using this dropper transformer) and that is why I bought the Airlink one.
I've also read through this link by Martin in the ref section a couple of times: http://www.nmr.mgh.harvard.edu/~reese/electrolytics/index.html
Was considering the following course of action:
A: see what happens with my bench psu at 60V using a current-limit resistor. If all goes well then:
B: rig something up with the 230V to 110V transformer which has a plastic body and is double insulated. 100mA fuse > dropper resistor > capacitor rigged up with a mains rated chocolate block inside a plywood box for safety. Volt meters connected across each of the resistor and capacitor. I have two s/hand Fluke 8026Bs for this. This should take it up to about 160 to 170V. If all OK:
C: I found this: UK Vintage Radio Repair and Restoration - High Voltage Electrolytic Capacitor Reformer (vintage-radio.com) which describes a circuit that can be built on veroboard for up to 320V dc. I might well build this but run it from a small 1:1 mains isolating transformer or possibly borrow a Variac. Again, inside a plywood box for safety.
Might be useful if the above link was added to the 'Reforming caps' in the ref section.
 
Thanks Pete,
Very decent of you. Just off to have a word with our instrumentation guy as I know he has one among the stuff in his lab, so I'll see what his take is on this.
Charged / discharged them all a couple of times to 60V via a 330R 2W resistor last night. On the third charge they all dropped below 10µA leakage after 30 secs. so looking very good. Spec sheet says leakage = 3x root(C.V) max after 5 mins. which is 0.3mA at 60V or 0.9mA at rated working voltage.
 
Borrowed a variac from work so will rig something up along the lines of the UK Vintage Radio Repair link above. All 'live' stuff will be inside a plywood box and all pre-connected so I don't have to connect anything to potentially lethal voltages. I will also wear heavy boots, on a carpet placed over a tilled floor, just in case.
 
LOL; I'm sure you'll be fine testing these with due considerations obviously in hand, providing you aren't in the bath at the time...

Besides - we'd like you to report on your findings!

ATB.
 
Martin,
Your first post above seemed quite agitated, almost angry as though you were saying 'what the bl00dy hell do you think you are doing man? Do you have a death wish!'
Must admit that my post above that came across as a bit blase, so not undeserved.
The Vintage radio link above is very informative and comes with a very bold warning so if anyone is going to attempt similar I suggest you read that first. After all, THESE VOLTAGES WILL KILL YOU.
Hoping to do this tomorrow now I've secured use of a variac so, God willing I'll report what happens. (I know, my Dad always used to say don't tempt fate!)
 
I certainly did not mean it that way.And I've done stupid things myself...

Apologies if it seemed blunt / a reproach (which I've certainly no right to exert given the daft things I've tried..) .. and v. esp. given you and the many, many other regulars here are sensible types who share, and care!

My real concern was/would-be such a discussion uncovered by search (and esp. external search engines!) in future without the context & risk made plain. That was all.


Now - get to it, and - please tell us what you find : )
 
No apology required Martin. Wasn't until you showed your concern that I thought about the safety implications not expressed that could lead someone to not consider the obvious dangers. Unfortunately I can't now edit that post.
Edit:
Before starting, I set the variac to a low output voltage (about 20V ac) and then checked between the live and earth with my DVM which showed ac. I did the same between neutral and earth which produced not output thus proving that neutral and live weren't swapped anywhere. I then built a variation of the circuit up using a chocolate block as I didn't need, nor have the zeners.
Slowly wound up to 330V dc in stages (60 the other night. Then today: 200, 250, 290 and 330) whilst monitoring leakage current after allowing some settlement time.
All in the region of 0.1 to 0.2mA leakage current so all good. Will replace the four in the amp tonight whilst Corrie is on...
 
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All seems good with the recap. Ran it a couple of times, but wasn't happy as there has always been a lack of clarity and occasional crackle in the right side, so I fed the amp via my oscillator and had a listen as it should be more obvious with a fixed amplitude, constant tone. Sure enough the crackle/drop-out was more obvious and in both sides. Traced this to the strain-relief on the earspeakers. If you twisted them, the problem cleared but if you waggled them, the problem got worse. Had to carefully split the strain relief down the moulding line and then carefully peel the cable out of the rubberised moulding. Took about 3" off in total as this was just beyond the discoloured/green copper where clearly sweat gets down the cable from the end. I left the cotton fibres (or what ever it is) inside, applied some flux from a flux pen and tinned the bared wire ends and then soldered these in place, having wrapped some paper around the unit, held in place with a rubber band to protect the thin film of the drivers. Rebuilt the earspeakers and just had a quick listen to the oscillator - not crackle or drop-out so will have a listen over next few days and seem how we go.
 
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