Armstrong 626 problem at high volume.

[Argus25]

Originally Posted by jubilado

A problem still persists...


1) I've checked the capacitors I replaced for polarity and they are OK.

2) Is this a problem with the power supply?

1) Have you ?

2) Almost certainly.

Electrolytic capacitors can be difficult to test properly even for the experienced engineer. They require 4 tests: Capacity value, ESR, electrical leakage, and max applied voltage electrical leakage.

The main filter capacitor (3300uF) on the output of the bridge rectifier is responsible for storing charge there, that keeps the main power supply voltage stable, when current is drawn at intervals from the supply by the audio output stage amplifiers. The current peaks increase with volume.

This capacitor is topped up with charge via the power transformer and rectifiers at 10mSeC intervals (on 50Hz full wave rectified voltage).

If this capacitor has gone high ESR or low capacity, then on load volume peaks you would for example, see the pilot light flicker and you would have a common resistance in the supply that would cause feedback to early amplifier stages and instability.

I think your 3300uF capacitor needs replacing, most likely. There could be other electrolytic caps that need replacing too.

I would "guess" if you had a scope and monitored the voltage across this capacitor, you would see the voltage jumping up and down when the problem occurs, but , since you don't likely have one, or are familiar with using one, then your next best bet is exactly what you have observed, with the pilot light pulsing with the fault......well done.

[snowman_al]

I wonder if you have found the amplifiers limit?
How loud does it go before distortion sets in? If it is loud enough at say 80% that might be the maximum you can get continuously?

[Trevor]

Is this the same for all inputs ie. phono / radio /aux tape
Have you checked the PSU stabilized supply that feeds the pre-amp section

Is this with any input leads plugged in These amplifiers were very good in their day and instability is not normally a problem The Large board capacitors also need to be replaced as they will be off spec by now
Regards Trev

[paulsherwin]

I agree that you almost certainly have a power supply problem, and the main reservoir electrolytic is the most likely culprit. Any stability problem in the amp or preamp itself would be very unlikely to affect both channels equally.

[jubilado]

Thanks again for the replies.
I replaced the 2 4000uF capacitors and the 3000uF capacitor as they showed signs of leaking. When I said I had checked the capacitors I meant that I had checked that I had the polarity of these correct.
I had one of these amps (the 621 version rather than 626 which this is) in the 1970's and could run it at full volume without this strange behaviour.
I've only checked the aux input and the tuner input and the result is exactly the same with both.
There are a couple of smaller electrolytic caps on the Z20 board. - should I be looking at changing these?

[Argus25]

It would be a matter then of determining if the power supply was stable and if that was not the case , how that could be with new electrolytic capacitors.

If there is a problem with the earthing of the circuitry, or parts of it, this effect could occur as any resistance in the connections of any sub-circuits including the main filter electrolytic's + or - connections could also cause the same effect as a bad capacitor.

It would be good to put a scope across the power supply output and see what happened when the problem occurred.

[snowman_al]

Originally Posted by jubilado

...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?

Thinking about this a bit more.
Everyone agrees the power supply is not keeping up with the current draw as the volume is turned up.

The 3300uF cap has been replaced, so as original specification 'should' be capable of providing original power output?

But the op mentions that the pilot lamps dim too in sync. Does that show a larger current draw on the transformer than it can provide (the pilot lamps are even on another winding to the supply...) or more likely a poor / high resistance connection?

If so any extra resistance in the mains / primary side would give just those symptoms too. For instance is the mains fuse / holder in good condition etc.?

jubilado do you have a multimeter?
If you do it is time to measure some voltages. Let us know and we will give some places to measure.

Alan

PS have you tried different speakers on the amp?

[snowman_al]

and a bit got lost...

If so any extra resistance in the mains / primary side would give just those symptoms too. For instance is the mains fuse / holder in good condition etc.? Are the mains plug connections and fuse tight and not corroded?

[jubilado]

Yes I have a multimeter - but no other test gear.
The effect is the same with different speakers and from both sets of speaker outputs.
I've had a look at the mains fuse - the 1A anti-surge fuse and the 160mA on the Z20 board and they looked fine. I cleaned up the ends and the fuse holders anyway but with no difference. The power lead fuse and plug look fine and connections are tight.
Next step? (if there is one)

[snowman_al]

Good,
Start with measuring the voltage across the 3300uF capacitor. Test it with the volume turned right down first. Then slowly turn the volume up and measure as you go.
What do you get at zero and at a point where the distortion starts. (Looking for 82 volts with little drop at full volume.)
Next, leave the negative test lead on the 3300uF cap and put the positive lead on either of the +ve of the two output caps (4000uF on the diagram what value have you used?). Same test again zero to a point where you get distortion. Looking for 40 volts.

Let us know what you get.
Alan

[jubilado]

So....
3300uF cap = 1st test 79.4 volts at zero then as distortion appears jumping at all sorts of values between74 and 76 volts 2nd test 76volts at zero volume and pulsing between 72 and 74 at distortion point.

I replaced 4000uF caps with 4700uF caps and on test at zero volume 39.6 volts and at distortion pulsing between 30 and 35 volts

[snowman_al]

Sorry for delayed response,
Any service documentation available does not have test points or colour codes listed so assumes you know your way round a circuit diagram and location drawing...
I've been looking for pictures on the web of suitable places to take measurements. No luck really and I do not have a unit to refer to, perhaps someone else has?

It would be interesting to know your mains voltage and what ac voltage you get from the transformer secondary. Then test how the 45 volt regulator is set.

There is an interesting comment by Trevor on this thread ''Armstrong 621 and 625 rebuild and transplant'' #9, https://www.vintage-radio.net/forum/...d.php?t=157818

Originally Posted by Trevor

Is the unit motor-boating when turned up to a high level but without any signal input
If this is the case check the pre-amp regulated supply This is located underneath information re the digram can be found on the web
Trev

Do you get the same effect with no input, but the volume turned up?

[jubilado]

Yes I do get the same effect (which Trev refers to as motor-boating- at last a name for the effect!) with no input. I hadn't checked this as I wasn't sure if this would damage the amp.
I'd like to check the pre-amp regulated supply as suggested by several people if anyone out there could tell me where and how to do this?
Neil

[PaulR]

I am very far from being an expert but I have found that poor assembly in mine has caused several problems over the years. This thread is an example of a power supply problem I had. https://www.vintage-radio.net/forum/...ad.php?t=83064

It might be worth going over the component soldering and connections in general as some strange faults manifest themselves in these amps.

I was going to buy one new myself but when I went to Hardman Radio in Liverpool with my savings to get one all their stock had been returned to Armstrong for inspection as they had received so many complaints and I bought a Rotel instead.

[snowman_al]

Found a few nearly usable items to put you in the right place (or close?)

The 'regulator' is part of the Z20 board.
BE CAREFUL there are exposed mains voltages on the board. Do Not take liberties please.

Picture one, dotted white, is the location on the underside. Picture 2 is the component locations. Picture 3 is the Z20 board dotted in red on the schematic.

On the diagram is a 4.7K ohm resistor marked '232'. (4.7K should be yellow/ violet/red/ gold) On the picture it is the one at 45 degrees.
That looks like a fair place to check your 45 volt supply to the pre-amp board. One side should have 20ish volts and the other should be 45 volts DC if all is well. Negative meter lead goes to the chassis, positive to the resistor.

Again be careful do not touch anything with the amp plunged in, please.

Does Trevor have a suggestion as to common reasons for failure?

Alan

[Trevor]

The pre-amp regulator was not overly trouble-some but we did have a few failures
the pre-amp power decouplers could do with replacing after all they may be 40 years old now
I would replace these as a matter of course
I agree with Wham re the sound but the design did show Armstrong were learning re getting rid of plugin boards The physical looks were clever and deceptive re the depth
Trev

[jubilado]

Thanks again for various replies - sorry to be such a nuisance!
Paul - I bought my original 621 from Elena Mae in Edinburgh and after a short while blew the output transistors After that repair it was fine for the 10 or 12 years I had it. I really liked the sound with the AR7 speakers I had. (replaced by a midi system because that had fewer cables) When a friend of mine first heard the Armstrong he liked he sound and started to save for one. After the transistors blew he changed his mind and bought a Rotel!

Alan - Checking voltages at resistor 232 - one side 24 volts - other side 63 volts! - varying down to 53v or so as the distortion point is reached. That doesn't appear to be right!!

[Trevor]

the two capacitors either side of the 1.5 k resistor on the preamp power supply i think 100 uf and 47 uf want to be replaced
This should sort it
Trev

[snowman_al]

Originally Posted by jubilado

... Checking voltages at resistor 232 - one side 24 volts - other side 63 volts! - varying down to 53v or so as the distortion point is reached. That doesn't appear to be right!!

Neil, that is not right.
As long as I have got the right place to measure, you should have 45 volts on the ''63'' volt side. You can confirm what the voltage is by measuring at the 100 ohm resistor on the control board. (That is where the voltage from the regulator goes into the control board and should be 45 volts...) I have marked it on the first picture below. Again neg lead to chassis and positive to the resistor.

It might be that there is a fault in the regulator circuit or it has just gone out of adjustment. There is a small 'pot' on the Z20 board that sets the voltage. See second picture. If you carefully mark its position, with the amplifier switched off, and move it to and fro a couple of times, then put it back about where it was. That should 'clean' any dirt from the connection.
Now do the voltage check again and see if moving the pot alters the voltage?
If it does, try and set it to about 45 volts. If it does not alter, then there is a fault on the regulator.
Remember the warnings about Mains Voltage on the Z20 board...

I have also noted what I think are the components Trevor is talking about in his post #38. Chances are if they are faulty, so will a fair number of other caps too.

Alan

[jubilado]

Thanks again for the reply.
The 626 Z20 is slightly different from 621 I think - the mains fuse is not mounted on the board but attached to the chassis underneath and there is no voltage selector. But there is no difference components wise I suppose. I've added a pic of mine here. Measuring across the pink and blue wires highlighted (pins B1 and b2 ) the AC voltage varies between 57.9v and 58.5v. Measuring voltage at the AC outlet shows a steady mains voltage of 240v

I've cleaned up that pot on the Z20 board
and
Measuring across the resistor 232 with volume at zero gives a steady voltage of 60.2 volts at one end of movement and 57.8v at the other. Guess there's a problem somewhere on the regulator.