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Old 22nd Jun 2019, 1:32 pm   #1
jubilado
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Default Armstrong 626 problem at high volume.

Hi,
Firstly, I'm a newbie and know next to nothing about electronics.
As a younger person I owned an Armstrong 621 amp and wishing to relive my youth got hold of an old Armstrong 626 receiver. I have replaced the three large capacitors as they looked like they were leaking around the vent. I've also managed to get the power and tuner pilot lights working and have replace the headphone socket...
BUT When played at low volume everything seems to work OK but thigh volume the amplifier seems to "beat" producing "thudding" noises. On taking the cover off to see if anything obvious was happening the black component marked ERG glowed red(see photos) and I saw a spark in the same area.
Is this fixable or should I use bin this item and pay for a fully working model if I can find one?

Hoping for some guidance.
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Old 22nd Jun 2019, 1:56 pm   #2
paulsherwin
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Default Re: Armstrong 626 high volume problems

That appears to be a resistor in the power supply.

Check your work carefully. Electrolytic capacitors are polarised and must be connected correctly, and make sure you haven't introduced a short circuit somewhere.

Given your lack of expertise, it may be an idea to pay a professional to look at this for you.
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Old 22nd Jun 2019, 2:19 pm   #3
ekjdm14
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Default Re: Armstrong 626 high volume problems

Firstly, everything is fixable & as the amp appears to function at lower volumes I'd suggest nothing is seriously amiss so certainly don't bin this one. At worst you could keep it as a source of spares if you came across another one at the right price.

Secondly, I'd echo what Paul says about double-checking your work and perhaps getting someone with more experience to diagnose/repair this. The fact that the possible resistor has glowed red hot would suggest maybe there's a short or misplaced wire somewhere so it could be fairly simple to fix but I would advise against powering up the unit again unless it's on a current-limited supply as it could potentially cause further damage.

When you say "next to nothing", what level of experience have you had prior to replacing these caps? If you're fairly proficient with a multimeter and have a schematic you could trace the circuit back and try to find what's causing the excess draw.
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Old 22nd Jun 2019, 2:53 pm   #4
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Default Re: Armstrong 626 high volume problems

I think that 'tag strip' looking part is actually a thermal trip that possibly has the red hot resistor wired across it to reduce the voltages in the event of a fault. This suggests that you do have a fault that must be repaired. It maybe as already suggested that the caps may be wrongly wired. Don't know as I don't have a circuit diagram to follow.
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Old 22nd Jun 2019, 3:00 pm   #5
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Default Re: Armstrong 626 high volume problems

Umm,
The component under the black ERG resistor is a 'thermal delay'.
The thermal delay works from the same 11.5 volt ac winding on the transformer as the 'power' lamps. It is there to lessen the 'thump' you get when you power the amplifier up, nothing else.
Until the delay 'switches' over the whole amplifier current goes via the black ERG resistor (47 ohms I think). If the delay does not switch over, the black resistor continues to take the total current and will surely glow as you turn the volume up... and probably cause the loss of power and distortion too. The spark you see might be the contacts arcing?

When you fixed the power on lamp did you alter or dislodge anything?
Was it working correctly before you changed the caps?

Alan
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Old 22nd Jun 2019, 3:05 pm   #6
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Default Re: Armstrong 626 high volume problems

Schematic in the public domain here.
http://www.audiomisc.co.uk/Armstrong/600/Amp.gif
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Old 22nd Jun 2019, 3:34 pm   #7
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Default Re: Armstrong 626 high volume problems

Thanks for the replies everybody.
Thanks Alan,, yeh I'd just had a closer look and seen that the 11.5 AC winding went to that strip which was associated with the thermal delay. I hadn't realise it was in fact the delay itself.
I don't think I dislodged anything when fixing the pilot lights but the 2.5A fuse beside the delay blew a couple of times then worked OK. If the thermal delay is non-operational at present do you know if it can be fixed/repaired/replaced?
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Old 22nd Jun 2019, 3:50 pm   #8
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Default Re: Armstrong 626 high volume problems

Looks like the thermal delay has a reputation. You can find a recommended fix here https://audiokarma.org/forums/index....roblem.769033/
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Old 22nd Jun 2019, 5:15 pm   #9
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Default Re: Armstrong 626 high volume problems

Could you short out the thermal delay contacts and just live with the switchon thump?
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Old 22nd Jun 2019, 7:09 pm   #10
jubilado
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Default Re: Armstrong 626 high volume problems

Hi Paul.

Yes I'm prepared to do that - I would normally switch power on with speakers switched off anyway

BUT

As a newbie I don't know how to do this - presumably solder a wire somewhere or join two wires together? Also in the link above from PJL it says the rectifier diodes should be upgraded if this is done. I fear that may be a step too far for me!

Neil
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Old 22nd Jun 2019, 7:58 pm   #11
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Default Re: Armstrong 626 high volume problems

Back in the day this was a common problem i have changed lots of the thermal switches
no switch on thump if you use the spk switches I remember the early versions were not fitted with said device no problems were encountered
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Old 23rd Jun 2019, 9:35 am   #12
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Default Re: Armstrong 626 high volume problems

To disable the delay you would simply short out the resistor which is getting hot.

I saw the reference to changing the diodes too. If it were mine I would change them - the parts only cost pennies, it's an easy job and there is no downside. The originals appear to be 1N4003s which are rated at 1A and will probably be OK, but removing the delay will certainly increase the stress on them.
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Old 23rd Jun 2019, 9:36 am   #13
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Default Re: Armstrong 626 high volume problems

The thermal delay allows the reservoir capacitor to charge up through the 47 ohms for a while and therefore prevents large surge currents. The 1N4003 datasheet rates the peak non-repetitive current as 30A and average current of 1A average when driven with 60Hz AC.

The maximum DC current is not specified but the various fuses suggest the 1N4003 should be able to cope as each rectifier is only passing half the current, however, the problem here is someone experienced in restoring these is recommending the rectifier change and it is quite probable this is from practical experience of previous failures.
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Old 23rd Jun 2019, 10:06 am   #14
jubilado
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Default Re: Armstrong 626 high volume problems

Thanks again for the replies.
Replacing on-board seemed a bit daunting at first but I guess if I want to make any progress this is as good a place to start as any!
I won't have time to do any of this for a few days so I'll order replacement diodes and then give it a go.
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Old 23rd Jun 2019, 10:40 am   #15
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Default Re: Armstrong 626 high volume problems

Just before you jump in there, did you test the amp before you replaced anything and did it work OK?
There is a big coincidence that the 11.5 volt supply to the lights also powers the thermal delay and you had blown fuses in that line without finding a cause.
Have you tried removing the replacement bulbs and trying it? Also make sure there is none of the solder connections on the delay touching the metal parts of it... Alan
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Old 23rd Jun 2019, 10:56 am   #16
jubilado
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Default Re: Armstrong 626 high volume problems

Hmmmmm - maybe it was a big mistake but I didn't try the amp out at high volume before I started. I didn't want to do too much when I saw the state of the capacitors.

On consideration the fuse didn't blow originally but only after starting to check the pilot lamps. The lamp holders sit in rubber grommets to isolate them from the chassis so perhaps there was a short there? To replace the bulbs I had to remove and replace the lamp holders so perhaps I fixed that short as the fuse has been fine since then.

(This is beginning to sound a bit like fools rush in.... )
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Old 23rd Jun 2019, 11:40 am   #17
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Default Re: Armstrong 626 high volume problems

Quote:
Originally Posted by jubilado View Post
(This is beginning to sound a bit like fools rush in.... )
'Fraid so. But whatever first move you make, there is always a fault or combination of faults for which that approach is wrong.

People have an instinctive urge to stick some power onto a thing to see if it goes without even a quick shufti inside. This has risks with DC blocking capacitors gone leaky, or with conductive electrolyte spread across a pcb. So you missed the worst mistake.

Seeing components that are obviously physically distressed means the right move is to sort them before applying power. The caveat is that if you do a lot of things before any checking, then the risk of you doing something wrong or disturbing something is not only high, it's harder to trace than where large jobs are done in small stages.

One way round this is to be very careful, and thereby minimise risks. After a few trip-ups like this we all became a lot more careful and somewhat slower, but getting things OK made up for the reduced speed.

It's a valuable lesson, and I think we almost all did it the hard way. Nothing to be embarrassed about, you're in a lot of company!

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Old 25th Jun 2019, 6:57 pm   #18
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Default Re: Armstrong 626 problem at high volume.

OK...assuming that the thermal delay switch is non-operational...
So I removed the 47 ohm resistor, desoldered the wires attached to the tags that the resistor was attached to (presumably the switch part of the thermal delay) and joined these two wires together with a 2.5A connector block. Should I use larger?
Switched on and everything seemed to be OK but the volume seemed to be much louder than previously (as the current was now flowing directly rather than through the resistor??). Volume was so loud that I couldn't turn it all the way up to see if anything happened. the amp is connected to 8ohm mini-speakers which claim to be rated at 40 watts max power.
Have I committed any grave errors now? Should I go ahead and replace the diodes as suggested above?
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Old 25th Jun 2019, 7:18 pm   #19
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Default Re: Armstrong 626 problem at high volume.

Enjoy the amp In this respect I feel the thermal delay was not really required
just ensure that the heat-sinks are free to dissipate the heat the amplifier will generate
Nice little unit I sold loads of them
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Old 4th Jul 2019, 10:38 pm   #20
jubilado
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Default Re: Armstrong 626 problem at high volume.

A problem still persists...
I've checked the capacitors I replaced for polarity and they are OK. The problem with the thermal delay is "fixed"
BUT
When I turn the volume to above about 90% the a loud rapid beating/throbbing/pulsing noise comes from the speakers and the pilot lights dim in synch with this. I've run the amp for hours at low levels and everything seems to be fine. The noise /effect is present on both channels and on all the inputs I've tried.
Is this a problem with the power supply?
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