UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Closed Thread
 
Thread Tools
Old 26th Sep 2022, 4:31 pm   #1
moonunitalpha
Diode
 
Join Date: Apr 2022
Location: London, UK.
Posts: 4
Default Armstrong 621- some ongoing troubles after repair

Hi there

I'm hoping if you knowledgeable folk might be able to help me out with the above? I had it repaired following a long period of storage- the three large electrolytic capacitors were replaced (quoting from the report):

circuit diagram reference (486) Value 3300uF 100V
(481) Value 4000uF 63V replaced with 4700uF 63V
(481) Value 4000uF 63V replaced with 4700uF 63V

Transistor reference 620 BC297 on the input stage of the power Amplifier was checked and found to be short circuit Collector to Emitter.

Replacing 620 fixed the loss of Audio in the left channel.

For a few weeks all was well, but then I noticed some distortion creeping in and the power indicator flickering. This was also repaired:

" smoothing capacitor and regulation transistor on the power selection board had bad solder joints.They are cracked causing intermittent loss of power. "

It came back the second time worse than before. Power light still flickering, and the left and right channels are now unbalanced. To get a reasonable balance I need to move the balance dial to the three o clock position. With no source input, there is audible background hum and occasional crackles at higher volumes as well that was not present initially.

The repairer wasn't keen on undertaking any further repairs, so I am a bit stuck.

If anyone can make any recommendations for further investigation I would appreciate it greatly. I have a soldering iron, a multimeter and an understanding of which boards do what, but otherwise should be considered pretty much naive.

Many thanks in advance,

Matt
moonunitalpha is offline  
Old 26th Sep 2022, 11:54 pm   #2
Michael Maurice
Moderator
 
Michael Maurice's Avatar
 
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
Default Re: Armstrong 621- some ongoing troubles after repair

My first thoughts are that the repairer should take it back, he/she hasn't done their job.

I cant help you with the imbalance without seeing it but what I would be doing is putting a signal generator on both channels and with the balance control in the middle, using a scope to find out where the imbalance is.


it could be in the preamp, but is more likely in the power amplifier.
__________________
Forum Moderator

http://www.michaelmauricerepairs.co.uk/
Michael Maurice is online now  
Old 27th Sep 2022, 12:12 am   #3
Ted Kendall
Dekatron
 
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,657
Default Re: Armstrong 621- some ongoing troubles after repair

In my experience, these units cause repairers trouble - I've had to retrieve a few. The twin elephant traps are the board print, which eager recappers cause to lift, and the solid core hookup wire, which now seems prone to dry joints. Patience is the most useful tool here.
Ted Kendall is offline  
Old 27th Sep 2022, 7:00 am   #4
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
Default Re: Armstrong 621- some ongoing troubles after repair

Replacement of the large electrolytic capacitors is quite often needed in this sort of equipment of this sort of age. The values chosen are sensible. Those capacitors use an electrically-conductive fluid (almost a gel) as one electrode. It's water based. Water vapour very slowly leaks past the capacitor's rubber seals and the guts of the capacitor dry out and the performance of the part decreases. The hotter things run, the greater the vapour pressure, the faster the leakage.

There are lots of much smaller electrolytic capacitors in most amplifiers. They contain less water to lose, but are under a lot less stress. Some may be drying out, some may go on for years more. A test with an ESR meter will tell which is which. Someone decided that it was worth just blindly replacing every capacitor in a thing, and created the word "Recapping" and it's been latched onto by people who don't understand the whys and wherefores.

It takes some experience to replace a component with a high probability of success. Replace a lot of components all at once (recapping) and statistics works against you. Your workmanship has to be excellent or you will have likely introduced several new faults.

Experienced people like to replace only one or two at a time between re-tests, so if something didn't go right, they know where to look.

Some plastic-encapsulated small transistors are starting to fail nowadays. it seems to be a mixture of chemical activity from the plastic and mechanical stress. Some manufacturer's parts are fine. Not all are affected.

Solid hook-up wire, as Ted says is a minefield. It stresses solder joints, which go dry/crystalline and can fail, or worse, go intermittent. Moving things just to get in to fix a known fault can stir them up and cause later failures to happen. You can also have the wires break off, usually at the ends due to metal fatigue.

Your amplifier contains a number of little time-bombs. They can be handled. Most of the susceptible parts and risks are known nowadays, but going through them all, using a commercial repairer will take time and therefore money.

If you have the time, these things are something you could do yourself, but you'll have to acquire some knowledge and experience, which will take time, but will also be useful for other projects. For a one-off repair, the cost/benefits ratio might not fit the value you place on your own time.

The Armstrong 600 series are cute little amplifiers, and I think are well worth repairing, though a reliable repair might look more like a rebuild. They certainly creted a stir when they were introduced.

Anyway, I hope this helps explain the quandry your repairer was in.

Most other amplifiers share these component issues. The compact nature and that solid core wire just make these harder and riskier to work on.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 27th Sep 2022, 9:19 am   #5
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Armstrong 621- some ongoing troubles after repair

I wonder whether the power supply problems are being caused by the start up delay unit. This consists of a bi-metallic strip with a small heater. When cold the points on the strip are open and the power is supplied by a bypass resistor. Once the amp has been switched on for a few seconds the strip gets hot enough to close the points and apply full power. It is intended to stop the thump you can get when it is first switched on.

After many years use the connection at the points becomes intermittent and could cause the fault you describe.

The points can be cleaned with a bit of fiddling but the unit isn't essential (I think it was deleted on later amps) so you can short it out to decide whether it is causing problems.

The unit is above the large capacitors. It is a small rectangular box with a paxolin cover and a resistor.
__________________
Paul
PaulR is offline  
Old 27th Sep 2022, 11:18 am   #6
moonunitalpha
Diode
 
Join Date: Apr 2022
Location: London, UK.
Posts: 4
Default Re: Armstrong 621- some ongoing troubles after repair

Dear All,

Thanks for the useful replies- David, I think you hit the nail on the head- as components are disturbed during repair, new faults manifest themselves that require further chasing.

Michael, I could probably get access to an oscilloscope at work, but as David suggests, I am concerned that this could be a bit of a rabbit hole for me to get into time wise. I may also create more problems than I solve!

Paul, I do know the delay unit, I will try and pop the cover and see what I can see.

I will report back- thanks everyone!


Matt
moonunitalpha is offline  
Old 27th Sep 2022, 5:52 pm   #7
Trevor
Octode
 
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
Default Re: Armstrong 621- some ongoing troubles after repair

Its still a design classic and 50 years old ! Yes you will get noisy transistors etc. But its nothing a competent engineer cannot fix and that is the word it can be fixed I am 70 myself and I would not worry me to take one on The design is modular and with a little effort all can be worked out
Trev
Trevor is online now  
Old 27th Sep 2022, 11:02 pm   #8
George Cooper
Triode
 
Join Date: Feb 2022
Location: Falkirk, Stirlingshire, Scotland, UK.
Posts: 25
Default Re: Armstrong 621- some ongoing troubles after repair

I used to have one of those amps, it did give me a few problems in the 70's which I tracked down to bad conections on the edge connectors of the PCB's, after sorting this out and replacing a couple of output transistors it gave great service.
George Cooper is offline  
Old 28th Sep 2022, 8:03 pm   #9
Trevor
Octode
 
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
Default Re: Armstrong 621- some ongoing troubles after repair

George
The 600 series have no edge connectors i think you may be talking about the earlier 500 series Still a good amp
Trevor is online now  
Old 29th Sep 2022, 8:16 am   #10
stevehertz
Dekatron
 
stevehertz's Avatar
 
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 8,809
Default Re: Armstrong 621- some ongoing troubles after repair

Quote:
Originally Posted by PaulR View Post
I wonder whether the power supply problems are being caused by the start up delay unit. This consists of a bi-metallic strip with a small heater. When cold the points on the strip are open and the power is supplied by a bypass resistor. Once the amp has been switched on for a few seconds the strip gets hot enough to close the points and apply full power. It is intended to stop the thump you can get when it is first switched on.

After many years use the connection at the points becomes intermittent and could cause the fault you describe.

The points can be cleaned with a bit of fiddling but the unit isn't essential (I think it was deleted on later amps) so you can short it out to decide whether it is causing problems.

The unit is above the large capacitors. It is a small rectangular box with a paxolin cover and a resistor.
That's my view too. These sets are/can be littered with faults. The one I restored was the longest restoration I've ever done. Lovely looking sets though.
__________________
A digital radio is the latest thing, but a vintage wireless is forever..
stevehertz is online now  
Old 29th Sep 2022, 9:12 am   #11
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Armstrong 621- some ongoing troubles after repair

If the main amp board has been disturbed the dressing of the leads connecting it to the pre-amp/tone board may not be as it was originally. They should run in the "well" between the two boards. If not they pick up hum.

I found it better to replace them with coaxial leads long enough to allow more movement of the main amp board without disconnecting them.
__________________
Paul
PaulR is offline  
Old 29th Sep 2022, 12:55 pm   #12
George Cooper
Triode
 
Join Date: Feb 2022
Location: Falkirk, Stirlingshire, Scotland, UK.
Posts: 25
Default Re: Armstrong 621- some ongoing troubles after repair

Trevor,

Oops, got my numbers mixed up, must be an age thing.
George Cooper is offline  
Old 29th Sep 2022, 4:21 pm   #13
Trevor
Octode
 
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
Default Re: Armstrong 621- some ongoing troubles after repair

Not a problem George these things are now 50-60 years old and time fades memories
Regards
Trevor is online now  
Old 29th Sep 2022, 6:22 pm   #14
moonunitalpha
Diode
 
Join Date: Apr 2022
Location: London, UK.
Posts: 4
Default Re: Armstrong 621- some ongoing troubles after repair

Quote:
Originally Posted by PaulR View Post
If the main amp board has been disturbed the dressing of the leads connecting it to the pre-amp/tone board may not be as it was originally. They should run in the "well" between the two boards. If not they pick up hum.

I found it better to replace them with coaxial leads long enough to allow more movement of the main amp board without disconnecting them.
Thanks Paul- I will check that out too. Some sort of RF interference perhaps?

Just need to find some uninterrupted tinkering time!

M
moonunitalpha is offline  
Old 29th Sep 2022, 6:48 pm   #15
Trevor
Octode
 
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
Default Re: Armstrong 621- some ongoing troubles after repair

Back in the day Armstrong issued a modification on fitting some capacitors that stopped switching interference ! For instance the central heating go on /off fridge's etc
Trevor is online now  
Old 30th Sep 2022, 7:17 am   #16
PJC58-Hythe
Heptode
 
PJC58-Hythe's Avatar
 
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 626
Default Re: Armstrong 621- some ongoing troubles after repair

It's a while since I worked on one of these but if the power light is flickering I would agree with Paul you need to check the thermal cut-out, I think there is also a thermal relay which switches AC into the rectifier.

I would also check the mains switch as I have seen these go faulty on numerous occasions causing power interruption.

From memory the thermal relay is powered from the same low voltage supply as the power indicator so this may exacerbate the problem.

It might be worth bypassing the thermal cut-out and on a temporary basis to see if that makes a difference. If it doesn't then leave that bypass in and again on a temporary basis bypass the mains switch.

Until you have stable power to and from the mains transformer tracking any other fault is nigh on impossible.
__________________
Peter - M0HYT - BVWS Member - GQRP Club

http://www.retroworkshop.co.uk
PJC58-Hythe is offline  
Old 30th Sep 2022, 8:09 am   #17
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Armstrong 621- some ongoing troubles after repair

Yes, thinking about it I remember that the power switch was failing on the first amp I repaired. I seem to remember that early switches had two sets of contacts. One switched the mains and the other did the output from the power supply so that it went off immediately when switched off rather than fading as it discharged the smoothing capacitor - presumably leaving it charged! The replacement I got didn't have the second set of contacts.

I think that those leads between the boards simply pick up hum from the power supply if not placed correctly.

If you are going to move the main board much be wary of the open potentiometers (wired as variable resistors) that adjust the bias on the output transistors. The tracks get dusty and dirty over the years and can easily lose contact with the wiper with dire consequences for the transistors.
__________________
Paul

Last edited by PaulR; 30th Sep 2022 at 8:15 am.
PaulR is offline  
Old 30th Sep 2022, 10:50 am   #18
theredhouseinn
Hexode
 
theredhouseinn's Avatar
 
Join Date: Jan 2013
Location: Kelvedon, Essex, UK.
Posts: 272
Default Re: Armstrong 621- some ongoing troubles after repair

Hi Matt
I had one of these for repair a few years ago.
The amp switched off and on again if any electrical appliance in the house operated.
A previous repairer had replaced the regulator transistor with a darlington. The spike on the mains caused the tr to switch off and on again with a loud plop from the speakers.
Replacing this with the correct one as per the cct. cured this problem.
It is worth checking that the precious repairer has not made any mods. or replaced any component with a so called equivalent.
I am 80 now but still managing to do a few vintage bits in my workshop.
John
theredhouseinn is offline  
Old 1st Oct 2022, 11:41 am   #19
Trevor
Octode
 
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
Default Re: Armstrong 621- some ongoing troubles after repair

Nice one John 70 myself but their is nothing in the 600 that would worry me !
My main problem now is motivation !
I used to retail Armstong back in the day I thought the range was innovative with the dual band AM logic switching for signals an Fm tuner that could compete with anything in its price range and a nice piece of furniture . Well worth restoring
Trevor is online now  
Old 3rd Oct 2022, 10:43 am   #20
moonunitalpha
Diode
 
Join Date: Apr 2022
Location: London, UK.
Posts: 4
Default Re: Armstrong 621- some ongoing troubles after repair

Morning all,

I took the top off this over the weekend but am reluctant to start moving boards around- I think I may cause more harm than good. I think I also may need reading glasses! I think I will have to look around for another retro friendly repairer- ideally someone who knows the foibles of these units.

Any suggestions greatly appreciated.

Thanks in advance,

Matt
moonunitalpha is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:58 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.