Armstrong 626 help needed

[old graham]

My good old amp went dead,checked fuses and all ok sent to Hi Fi repair man.
Repair guy says 160ma fuse underneath on Z20 p/supply replaced and it works (forgot about that one ) I say recap anyway now you have it. Returned after a long wait and guess what ,dead. Checked the 160ma fuse,blown so replaced and now only one channel works. Anything I can check with limited knowledge and a multimeter. lots of new caps fitted but the two big ones look original tested volts at 85v on both, tested across pins of quiescent current pots at 20ma ? don't now what to check next. no way to return to repair guy due to covid rules

[stevehertz]

To measure quiescent current you have to disconnect a wire to a power transistor and insert an ammeter. You cannot measure it across a pot. Or at least, that's how you have explained it. Please clarify on this point.

[Richardgr]

Something cause the fuse to blow before it went to the repair man, and if all he did was to replace caps, then hopefully it is still in the same state as before.

Is the fuse before or after the transformer? If it is before, then the transformer may be suspect. But you say it did work OK after the fuse was replaced, but has now blown again?

Is the fuse slow-blow or fast? And what was originally specified in that position?

[Radio Wrangler]

Rather than disconnecting things, the voltage across the emitter ballast resistors can be measured and Iq calculated.
Beware that any slip with a probe results in destruction. Take care.

BUT the 160mA fuse doesn't feed the power amps where quiescent current adjustments and checks are made. It feeds a regulator making 45v to power the preamp sections. So it sounds like something's gone wrong in either the preamp or the 45v regulator.

If the regulator circuit fails, it can either fail open, in which case the preamp loses its power supply, or it can fail shorted, in which case the preamp gets a much higher voltage than it was designed for.... the full voltage intended for the power amps.

There is a possibility that a capacitor has been fitted the wrong way round. There is the possibility that over voltage has ruined either transistors or capacitors in the preamp if the voltage regulator circuit is wrong. There is the possibility that your amplifier still has the fault which blew the first fuse before all the capacitors got changed.

'Recapping' is awfully fashionable but it introduces a high risk of adding faults (wrong part in wrong place, wrong way rounf, dry solder joints etc.) It has to be done with an extremely good standard of workmanship or it risks introducing more than it fixes. The usual long-term failure mode of electrolytic capacitors is them 'drying out' by losing the water component of their electrolyte. They usually lose capacitance, gain resistance and generally move in the direction opposit to what would blow fuses.

The first step with it working would have been to go round checking voltages, looking for anomalies. But with it blowing fuses it is more difficult. You need to find where the excess current is going, and then find out why. You could buy a big box of fuses and explore by disconnecting different secttions to see what does and doesn't blow a fuse. I'd run the preamp from a current limited lab grade power supply and feed it a little gentle current and start following it. As well as the power supplies I have voltmeters sensitive enough to measure the small voltage drops along wires or the copper tracks on boards so I can trace current flows. These are not normal repair shop tools.

All approaches other than what you can do with what you have are off under current travel restrictions, so this will probably have to wait for an outbreak of normality, and by then you can get it back to the original repairer. Failing parts can go intermittent and when you've done something can sit there and work for a week without missing a beat, only to fall over the moment it goes back to the customer. You certainly should have a chat with the repairer and see what he has to say. I'd expect he would want it back when circumstances permit.

I should add that fuses are rather unreliable components. If rated properly for the current they have to carry, a proportion of them will fail for no real reason. Given a dead amp and a dead fuse inside it, I'd replace the fuse and try it out. If nothing went wrong, I'd assume it was 'nuisance blowing' If it did it again, then I'd get serious.

David



David

[Ted Kendall]

Tricky, this. Wholesale recapping is fashionable these days, and some electrloytics may well have aged, but it is all too easy, even for professionals, to put one or more in the wrong way round and produce obscure faults which can be the very devil to trace. I'd start by getting hold of a circuit diagram and checking the orientation of the new capacitors as far as you can. Unless anything is getting hot, I wouldn't worry about quiescent current at this stage, and there are ways of checking this without breaking links, which can cause trouble if a clip falls off.

[snowman_al]

Working on the only 1 channel working statement.
First thing to check are the 2 output 2.5A fuses if you have not already.
Then determine if it is the pre-amp or power amp sections at fault.
With the amp on and no input selected turn the volume control up. Now touch each centre connection of the control with your finger.
You should get a loud(ish) buzz from each speaker.
If you do the power amps are OK and the pre-amp section needs investigating.
If the buzz only comes from one channel then the other power amp section is at fault.

As an aside, are you sure you measured 85 volts on the two big caps? There should be 40/45 volts maximum across them. Only the 3300uF cap tywrapped to the chassis should have 85 volts on it.

[old graham]

Thanks for all the help guys, good forum. My mistake voltage on big caps 39.5 all fuses OK. Did finger on volume pot but no buzz, when set on disc with nothing connected at high volume get a hum but only on one channel. Note I have the open style pots compared to yours, this is a late model 1979 I have had from new. What is that item beside the transformer? If moved slightly main power light goes out but still works on one channel.
The resister on that looks a bit fried

[Radio Wrangler]

You need to tell us whether you got buzzes on each channel when you touched the volume pot.

I don't understand how it's doing much if the 160mA fuse keeps blowing.... what have I missed? Is the 160mA fuse currently surviving?

I think the big caps you mean must be the output DC blocking caps. If so 39ish volts is encouraging. It means that the power amps DC midpoints are both OK.

I wonder if something in the preamp is the problem and that after a few repeated poppings of the 160mA fuse, something else - say a series resistor in the power chain - has burned out, allowing the latest fuse to survive?

David

[snowman_al]

Originally Posted by old graham

What is that item beside the transformer? If moved slightly main power light goes out but still works on one channel.

The item besides the transformer is the 'thermal delay', a bi-metal strip. It puts that 47Ω resistor in series with the rectifier when you first switch on, to charge the 3300uF cap slowly at first. Then as the metal strip heats up shorts out the resistor and full power is applied. They become troublesome and that one looks past its best (the resistor is cooked), leave it alone for now. It is not your current problem though.

Originally Posted by old graham

...My mistake voltage on big caps 39.5 all fuses OK. Did finger on volume pot but no buzz, when set on disc with nothing connected at high volume get a hum but only on one channel.

Not sure how to read the buzz test results, can you try again?
Volume fully up, nothing playing, put a finger on the metal connection where the green and then the yellow wire connect to. Or follow the wires to the A17 board and try those pins with the green and yellow wires.

Silly question, if you swap the speaker leads round, does the same channel have the fault?
Alan

[old graham]

The 160mA fuse was the first problem, repair guy said he replaced it and it worked. When returned to me plugged in it was dead so I checked that fuse and it was gone, replaced and now working but only on one channel the fuse is fine and has not gone again just the loss of a channel. Amp has been switched off and on many times and fuse is still fine
The problem is on the same channel even after swapping speaker round. Headphones also only one channel

Did test again on the A17 board and yes scratchy sound on green but nothing on yellow

[snowman_al]

Well looks like the power amp board.
You could post some pictures of the A17 board so we can look it over, but do not hold any hopes.
If you can follow the circuit diagram you could compare the voltages on the working side to the faulty one, component location is available online.

[stevehertz]

Having just spent two months working on one of these, and still more work to do, apart from faulty electronic components, I have found dozens of suspect dry joints, broken single strand wires that don't look broken, and a whole host of poor switch contacts that needed cleaning and exercising. So as well as other more 'intelligent' methods of fault finding, I would address these issues for sure. Eliminate them as much as you can.

[old graham]

Thanks for all the comments,will check some voltages on A17 board and report back

[old graham]

Pics of A17 board, checked the schematic but I don't know what to measure to be honest, any of you guys do repairs ? The original guy has replaced lot's of caps and claimed to have soak tested it but something has gone on one side of the A17 board. Really do not want to go back to him it took 8 months and very hard to get hold of him never answered emails or phone