Armstrong 621 - couple of questions

[Mutton Geoff]

Back in the 1970's, as soon as I received my first credit card, I rushed out and blew the credit limit on an Armstrong 621 (not fully understanding what 25% APR meant at the time )

Out of nostalgia and to hook up to a pair of Linn Katans (also Ebay find), I picked up a 621 last week that is in remarkably good condition and working, albeit with scratchy volume control etc.

I've had a chat with Mike Solomon to work some of his magic on it but thought I would pre empt the work, and help reduce costs by replacing all the electrolytics in it first.

The PSU and output ones are showing signs of a small crusty leak around the vent, but there's no hum, or DC on my speakers.

The problem I have is sourcing the output caps. They are 4000mF 63v but all the ones from usual sources are physically smaller for the same value. The old ones are 45mm dia by 50mm tall (1.75/2") and sit through the chassis neatly in retaining clips. I'd like to use the same physical size but can't find them. Tried CPC/Farnell/RS/Mouser/Digikey. I don't really want to strip the old cap and insert the new inside it with potting compound and wondered if anyone had a source or idea for replacing these two?

['LIVEWIRE?']

You may find it impossible to get modern capacitors the same size as the old ones, as all modern types of which I know are smaller. Maybe you could mount smaller modern can types with new, smaller diameter clips. (BTW the capacitors in that photo look like modern typres in any case, rather than the originals.

[Trevor]

I used plastic drain pipe to build out the diameter making doughnut rings .that seemed to work ok
Trev

[Craig Sawyers]

It really is amazing how capacitor technology has moved on. Standard sizes seem to be 35, 40 and 50mm diameter. 45mm seems to be singularly absent.

Unless you can find some NOS ones (and they might well be drying out from a long shelf life), you are probably stuck with using smaller clips.

Craig

[Craig Sawyers]

Part number (CFE) KAA4721K1M from https://www.surplussales.com/Capacit...F-99999uF.html is 470u 80V and is 45D x 47H

https://www.surplussales.com/Images/...4721K1M_lg.jpg

$3 each.

Or a Rubycon one a bit lower down 45D 55H 4700u 100V for $5 each

https://www.surplussales.com/Images/...700md5d_lg.jpg

The most expensive bit of the excercise is shipping from Nebraska. I've bought obscure components from them before, and the service is quite good.

[Craig Sawyers]

No response, Mutton Geoff? Wish I hadn't spent the time rooting out capacitors of the correct physical size!

[Diabolical Artificer]

To be fair Craig he might have been in bed.

How many what's does the amp kick out, what are the rails, 50v? I might have something, but my cap stash is mostly 10,000u and above. Will look.

Andy.

[Craig Sawyers]

Hi Andy

It is a single rail design, like the Quad 303. So there are three capacitors - the power supply smoothing cap, and the two that are in series with the speaker output.

They are all quirky values, and Armstrong may have had custom ones made - there were 40,000 621 and variants manufactured which would have made custom caps viable.

In a fully loaded 62x with tuners has a smoothing capacitor of 7,500uF 90V, and the output capacitors are 4,000uF. While it was in production the E-series values for capacitors were well in operation, so 8,200uF 100V and 3900uF 63V would have been much more available that the quirky values installed.

The main rail is 82V nominal, so a 90V part is sailing pretty close to the wind taking account of ripple and mains voltage tolerance. Which I guess is why the OP notes that the vent has been dribbling. Likewise a 63V part for the series speaker caps, since this is biassed at 40V and then has a high amplitude AC signal on top.

In the Quad 303, the series capacitors are 2,200uF 100V, and the smoothing caps two of the same value in parallel. Admittedly that was the later production values - in the early days they were 2000uF. But the important thing is the voltage rating, which has sufficient headroom to preclude overvoltage.

Craig

[Restoration73]

As a generalisation, most consumer capacitors have riveted on solder tags, whereas
high current (e.g. power supply) industrial types always used 2BA/M5 screws with the
wires attached by solder or crimp tags. They were larger,often not E24 standard values.

['LIVEWIRE?']

Another generalisation!. Since electrolytic Capacitors are (usually) wide tolerance, e.g. a 4700uf may be specified as +20/-40%, thus it's value can be nywhere between 2820 and 5640uf, then those non-standard values used by Armstrong could be replaced with 10000uf and 2 x 4700uf respectively. As Craig has pointed out, higher voltage Capacitors, maybe 100v might be preferable.

[Mutton Geoff]

Originally Posted by Craig Sawyers

No response, Mutton Geoff? Wish I hadn't spent the time rooting out capacitors of the correct physical size!

Craig, sorry for the slow response. My job involves a lot of travelling to far corners of the world, I just landed back from a week in Chile and reading through the responses. Thank you everyone.

Originally Posted by Craig Sawyers

The most expensive bit of the exercise is shipping from Nebraska. I've bought obscure components from them before, and the service is quite good.

That's where my job is handy. I'll be in the US later this month, I'll have them shipped to my hotel in CA. Thanks for the link.

[Craig Sawyers]

Ah - wish we'd known you traveled extensively. On another mailing list I'm on (head-case) we have a "disappearance" thread so that people can give a heads up when they are going to be out of touch for a while.

We have friends in Chesham Bois - nice neck of the woods.

Craig

[stevehertz]

Out of interest, what are the details of this "magic" that Mike Solomon is going to do?

The reason I ask is, not all 'modders' are bona fide hifi amplifier experts (Mike may well be), some are self styled audiophile gurus who charge lots of money for changing cables and sockets, stuff like that 'pretties' the amp up but does nothing for the sound.

I'm not being negative, I'm being cautious on your behalf. What are the details and charges of what he does?

Edit: Just found him on his web site. Seems ok. What are the charges?

[Ted Kendall]

Mike's been around for donkey's years. He's not the cheapest, but he gives good value and stands by his work. Definitely not a purveyor of snake oil.

[Mutton Geoff]

Originally Posted by stevehertz

Out of interest, what are the details of this "magic" that Mike Solomon is going to do?

I've had a long chat with Mike before he went on holiday. He certainly knows this amp very well and he has a couple of proprietary circuit mods which are obviously his "copyright" and bread and butter of his business that resolve the DC on volume pot (maybe it's just changing the type of cap to low ESR?)

In any case, part of his refurb package is replacing the electrolytics which is something I can do myself.

I had an interesting "fault" on it. After it had been running for a while, the 30mA RCD tripped in the house. With the amp off (the power switch is double pole), there was high resistance (some meg) between neutral and earth at the plug. Open circuit between other pins. When switched on (not powered), high megs between L&N. I lifted the earth lead in the plug and it ran happily for hours.

Measured again and now open circuit between N & E when off. Maybe it was just damp inside since I can't see any circuit that links N & E upstream of the power switch. Going to put the earth back on and leave it powered up today and see if the house trips again.

http://www.audiomisc.co.uk/Armstrong/600/600faq.html

[dave cox]

You really need a 'flash test' to check if the insulation is breaking down. This is basically a high voltage resistance test, typically done at 250, 500 or 1000V depending on the equipment being tested - the test gear is sometimes called a 'megger'. Use an extra plug in RCD (in the socket) if you want to use the amp but please don't use it with the earth disconnected

I'm sure Mike Solomon will check this as a matter of course.

dc

[Mutton Geoff]

Good idea. I've got a sparky coming round to do some other work, so I'll ask him to test it for me since I only have a Fluke multimeter, not a megger. I did a PAT testing course a couple of years ago so familiar with the method, just don't have the gear.

The earth is back on and it's running OK which is why I wondered if it was something caused by it's long term hibernation.

[Mutton Geoff]

Originally Posted by Craig Sawyers

Part number (CFE) KAA4721K1M from https://www.surplussales.com/Capacit...F-99999uF.html is 470u 80V and is 45D x 47H

https://www.surplussales.com/Images/...4721K1M_lg.jpg

$3 each.

Or a Rubycon one a bit lower down 45D 55H 4700u 100V for $5 each

https://www.surplussales.com/Images/...700md5d_lg.jpg

The most expensive bit of the excercise is shipping from Nebraska. I've bought obscure components from them before, and the service is quite good.

Craig

I followed your advice and decided not to wait until my visit to the US, but just to have them shipped to the UK. Their website is not particularly helpful since you have to buy the items first and enter cc details before they then work out shipping and add it to your bill. In my case, I decided to order two of each of your suggestions. The caps were $16 and the shipping $21.50 bringing the total to $37.50 (£30).

Very disappointed to open the (well packed and fast delivered) box to find the caps are identical in size to those on sale in the UK, ie

4700u 80V = 45D x 47H is actually 34D x 38H

and the

4700u 100V = 45D x 55H is 35D x 46H

I'll email them to point out the error on their website but my search for the correct size caps continues!

[Mutton Geoff]

I emailed them and although they didn't reply, I note they have already amended their website for the second of the caps above, but the first still has incorrect dimensions.

[Craig Sawyers]

I just happen to have a 626 (the one with the AM/FM tuners) that I bought really recently. So I ordered the capacitors I linked to - and you're right - the darned things are both 35mm. They have amended the site to now say 35mm for the rubycon, but the NKL ones are still listed at 45mm - and they are 35mm too!

Aagh.

I've just fired off a complaint to them.

Craig