Armstrong 625 one channel failed

[broglet]

Owned from new and worked perfectly until today. Suddenly proper sound is only coming from one speaker. The other speaker is very faint. The speakers themselves are both working fine (tested by switching them round). I live in North London. Can anyone help?

[Station X]

Hello and welcome to the forums.

Are you seeking advice as to how to repair your receiver or are you looking for someone to do the work for you?

[broglet]

Hi Graham
I'm looking for either someone to advise me how to repair it or someone who could repair it for me (for a fee).
Thanks
Brian

[JonSnell]

Have you checked the operation of the tape monitor switch. It can get tarnished contacts and a couple of switching from on/off can cure your problem as when the contacts build up the tarnish, it looks like a high resistance in the signal path and goes quiet.
Wonderful sets these were in the ranges 621, 625 & 626.
Output transistors were RCA's version of Motorolas 2N3055. Very well built.
If you get stuck, I am happy to repair it for you but I expect there will be someone nearer to home for you.

[stevehertz]

Originally Posted by JonSnell

Have you checked the operation of the tape monitor switch. It can get tarnished contacts and a couple of switching from on/off can cure your problem as when the contacts build up the tarnish, it looks like a high resistance in the signal path and goes quiet.
Wonderful sets these were in the ranges 621, 625 & 626.
Output transistors were RCA's version of Motorolas 2N3055. Very well built.
If you get stuck, I am happy to repair it for you but I expect there will be someone nearer to home for you.

Having been sold one that was described as "in need of some TLC" (laughs) I have had the 'pleasure' of restoring a 626. It found it to be one of the most awkward receiver restorations that I have ever done, it took weeks. The mounting 'wings' on the DIN speaker connectors are often broken requiring replacement, the meters are very flimsy and prone to failure - very hard to get replacements so you have to try to fix them, there's a 'delay on' protection (?) circuit that malfunctions as often as not, especially if it has never been 'serviced', the main electrolytic cap and speaker output caps are invariably in need of replacement.. pause for breath.. the many switches are usually chronically intermittent, and you have to unsolder wires to access boards.

No comparison with the build quality of the many Japanese receivers that I have restored that usually only require some switch cleaning and maybe a couple of bulbs. When it's all restored and put back together the Armstrong is indeed a nice dinky looking set that works well enough, but very well built? No!

[nigelreltub]

Hi broglet,

If you're not up to getting in there yourself, there is Mike Solomons at London Sound, he knows his stuff but he ain't cheap, depends how much the amp means to you?

https://www.londonsound.org/Armstrongrepairs.htm

There is also Armstrong Audio

https://www.armstrong-audio.com/

both in London

Good luck
nigel

[broglet]

many thanks everyone.
steveherz - which are the speaker output caps? are they the two big electrolytic ones next to the transformer? The tops of these are certainly bulging ominously.

[Michael Maurice]

I’m based in NW London if that helps you.

[stevehertz]

Originally Posted by broglet

many thanks everyone.
steveherz - which are the speaker output caps? are they the two big electrolytic ones next to the transformer? The tops of these are certainly bulging ominously.

They're the ones, and yes they will need to be changed. The main smoother is on the far left, check that too for bulging.

[broglet]

Thanks Steve ... so if one of those speaker output caps has failed that could be the reason the corresponding channel has suddenly failed ? Is there any simple check I could do with a multimeter to confirm this?

[stevehertz]

You can check a disconnected electrolytic capacitor using a multimeter. Using the ohms range the cap should charge up (low resistance) then over a period of time (longer for large caps such as this) say a minute or so, the resistance should go high. A faulty cap stays conductive, and hence is able to pass DC, not what is wanted. An analogue meter is best for this purpose such as an Avometer as you see the 'swing' of the needle.

Having said all that, I and many others would simply change those bulging caps as it's a sure sign of them being past their best.

[broglet]

Many thanks Steve - brilliant response - very helpful.

I used a digital multimeter on the 2000kΩ range.

I disconnected them. The one that powers the left (working) speaker starts at -660kΩ then slowly goes down to about -300kΩ where it more or less stabilises

The other one starts at -300kΩ fairly quickly goes to zero then starts reading positive numbers that slowly increase.

I conclude that the left one is minikaput while the other is megakaput! Sorry for my unscientific diagnosis!

The questions now are

1. where can I source replacements (what are their characteristics) ?
2. how does one remove them from the clips they are fitted into?

Thanks again to all of you!!

Best wishes

Brian

[stevehertz]

I can't recall how to remove them from the clips, but looking at the photos that I've attached that show before and after, it looks like you just slide them out. It may take a bit of force, but it does look like they are a push fit. Just have a look and figure it out. The replacement caps were of a smaller diameter, I just used circular mounting clips that fitted the new caps and made slots in the chassis capacitor holes to accommodate the mounting screws.

The value of the cap will be written on the side of the old ones, aim for the same value, but you probably won't get the same capacitance and voltage, so go for the next rating up in each case. I use Cricklewood Electronics for capacitors generally, but I may have bought these off Ebay, I'm not sure now. Don't forget to check the main smoothing cap fixed to the left hand side of the chassis, it's sure to be waning as is normally the case with these sets.

[Station X]

As this has become a repair thread I've moved it to the appropriate section.

[broglet]

Thanks steve - the smoothing cap certainly looks dodgy but it hasn't stopped the set working (so far!) What would be the audible symptoms if it needed replacing?

[stevehertz]

I'm not sure, possibly distortion as a result of poorly smoothed, high ripple level HT. If it looks dodgy, then change it as it's not a question of if it will fail, it's when it will fail. Like I said above, there's a lots to do when restoring one of these sets. I'm giving you the best of my advice, up to you what you do.

[Radio Wrangler]

Originally Posted by broglet

I used a digital multimeter on the 2000kΩ range.

I disconnected them. The one that powers the left (working) speaker starts at -660kΩ then slowly goes down to about -300kΩ where it more or less stabilises

The other one starts at -300kΩ fairly quickly goes to zero then starts reading positive numbers that slowly increase.

I conclude that the left one is minikaput while the other is megakaput! Sorry for my unscientific diagnosis!

If there are bulges, they are best replaced.

The Ohms measurements don't tell you much, they are influenced a lot by the state of charge left on the capacitors from before you disconnected them. If there is something wrong in the rest of the amplifier, you may well get one capacitor more charged than the other. If the leakage in the capacitors is different (not necessarily a fault) then one may have changed more than the other.... There are lots of things get in on this act, too many to be able to draw conclusions with any confidence.

Oh, and connecting a meter on its ohms range across a charged capacitor is a common way of destroying meters. If you want to do an ohms check on a capacitor, discharge it first. Then it'll start at a low displayed resistance and you'll see (with a large capacitor) the value climb as the ohmmeter charges the capacitor to the ohmmeter's test voltage. Some better grade multimeters have reasonable (but not totally invulnerable) protection against this sort of thing, but they are rather expensive brands and unless they know why those command such a price, many people just think 'I'm not paying that much, there's one down at xxxx for £9.99'

Another thing a bad output capacitor can do is set off a chain of events which destroys the loudspeaker. Checking whether it's the amp or the speaker by swapping the speakers over then allows it to damage your remaining good speaker! This doesn't happen often, but once you know of the possibility, you check the DC output voltage from each channel of the amplifier before connecting a speaker, and you also use some cheap (sacrificial) speakers for tests, just incase.

When fixing a transistor amplifier power stage there are a number of gotchas. The people who know them either learned them the hard way, or else someone tipped them off.

Anyway, if those capacitors are bulging, they need changing. Maybe they've not failed completely yet, but they are on the way out and have a high risk of failing soon. But as your ohms test showed them charging progressively from your ohmmeter, then I suspect there is still some life in them, and that your main fault is somewhere else. But get new capacitors before going further.

David

[broglet]

Many thanks David and Steve for more invaluable advice

Brian

[PaulR]

A fault I have had with these is that the carbon track on the variable resistors which control the quiescent current in the output transistors get dirty and cause the transistors to burn out.

[broglet]

update: one of the speaker output caps had failed completely as revealed by a resistance test using a moving coil meter. I have replaced both caps and now back to full working order.

Many thanks everyone