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Old 28th Nov 2021, 11:47 am   #1
nigelreltub
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Default Armstrong 525 Motorboating/Thudding Channel

Morning, New to and my first post on UK Vintage Radio Repair and Restoration forum, advised to come here by snowman_al. Let me say my knowledge is limited, I don't understand technical talk but am willing to learn stuff.

The background story, skip this paragraph if your coffee might get cold , saved the amplifier from my father's wardrobe where it sat for 30 years maybe longer, before my sisters could chuck it in the skip. It was his first amp he bought and had to take a loan out for that and also a Wharfdale speaker kit with 12"bass drivers, (he is still with us at 94). Some years later it developped a problem and It was given to a guy he knew to replace some transistors, unfortunatly the guy passed away and my father got the amp back with the power board missing, also one of the trasistors and a couple of other things hence it ended up in the wardrobe. Managed to get a second hand unit and salvage the parts to hopefully resurect it and have a few more sessions with the old fella and my dad of course.

What I have done so far:
-Replaced the power board - it blew one of the 1A fuses
-Noticed a Transistor was missing - replaced that and it still blew the fuse.
-Power board was delivering 67vdc and there was no adjustment in the pot adjuster, all the cards were in place.
-Replaced the power board transister, and adjusted the voltage down to the 50vdc, still blew the 1 fuse.
-Checked all the transistors using the a DMM in diode mode, all the readings were a bit ropey.
-Replaced transistors with the best set?? no more blown fuses on channel B.
-Hooked it up and Ch A sounded sweet, Ch B a constant thudding, I think described as "motorboating".
-Checked to voltage on the speaker terminals, Ch A 0.18vdc, Ch B was 25vdc.
-1st google led me to believe it was the capcitors (they are 1250uf 35vdc), took all these out and checked, except for the large one, all read 1.566-1.466 mf using the DMM in capacitance auto mode.
-Replaced the capacitors.
-2nd google led me to believe that it could be voltage leaking from a transistor.
-So I have ordered some 2N3614 transistors, equivalent replacement I believe??
-All the cards are ok, i beleive as did a loboriuos swapping in and out on Ch A.

-Is it worth waiting to see what replacing the transistors results in??
-Would it be worth replacing the caps, couldn't find any equivalents, nearest was 1200 or 1300uf 35vdc units.
Could it be the large 1500uf 70vdc capacitor?

This is where I am. Any help advice or pointers greatly appreciated.

Apologies for being a bit long but I thought all the details could be relavant. Oh, my coffee is cold.
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Old 28th Nov 2021, 2:31 pm   #2
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Welcome. I recently restored one of these that was sold to me as needing a bit of TLC, but as it happened it needed the lengthiest full restoration that I have ever done. Ever been 'had'. They're not easy sets to work on and they're often riddled with various faults. There's lots of suspect components. As a starter, I'd perform a search on here for Armstrong 525 & 526s and read the results. If it helps, I replaced the three main 'big' caps from the off. You've got a big job on your hands there..
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Old 28th Nov 2021, 3:51 pm   #3
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Thanks stevehertz, for the reply.

Yes you are right, big job, especially when I am a learning on the job, new names and terminology.

The "3 main big caps" ? You've got me, I can see 5 caps (the 1250uf), 2 for each channel and 1 for the PS board and 1 large cap (1500uf) in the main chassis, are you refering to any of the caps on the boards/cards, loads of capacitors but small?

I'll have a search and see what it brings, most prbably more questions.
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Old 28th Nov 2021, 5:20 pm   #4
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Default Re: Armstrong 525 Motorboating/Thudding Channel

My apologies, my set was the 626, not 526!
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Old 29th Nov 2021, 7:47 am   #5
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Default Re: Armstrong 525 Motorboating/Thudding Channel

You've got a hell of a job on there as Steve says, I'll try and provide some general repair advice rather than anything specific to your amplifier. Broadly speaking an amplifier has three main sections 1) power supply 2) preamplifier 3) power amplifier.

In general terms the power supply does what it says, providing power to the amplifier, often lower voltage goes to the preamp, where your signal comes in and is made bigger or amplified, then higher voltage goes to the output or power amplifier, here your boosted input (IP for short) is then driven by drivers, which drive the output (OP) stage, all this does is boost current, enabling it to drive a speaker.

1) Power supply, again broadly speaking this will consist of a mains transformer, rectifier, big capacitors and maybe a regulated lower voltage circuit to power said preamp. First AC mains voltage comes in at the plug (240v ish) and in a transistor amp is made smaller, for a 35v supply for example the AC coming out of the transformer (tfmr) will be about 26v AC RMS ( forget the RMS for now) . This 26v AC is rectified by diodes (bridge rectifier) goes into some big caps to be smoothed out, then onto the other stages.

So, before powering up the whole amplifier we need to make sure the power supply is ok, there should be a way of disconnecting it from your amp, either by removing a fuses/s or un soldering a wire or two. So, check the tfmr has AC coming out of it, then that there is DC at the big caps. The DC voltage will be 1.414 times as big as the AC voltage ish. Check that all other voltages are correct, a few volts either way doesn't matter.

To power up your power supply, you need to limit the current going in, in case there is a fault or it hasn't been put back together properly. This can be done simply by making a lamp limiter, do a search up top. You can also do a DC resistance test from ground (gnd for short) to the + of the big caps, it should read several kilo ohms at least, if you get a low reading, EG under 100 ohms,there is probably a fault.

There are a few general rules for repairing stuff, check, re check and re check again your work before powering it up, by eye that is, use a magnifying glass, check solder joints, look for components put in the wrong way or wrong value, burnt out or scorched components, leaky capacitors, wires off etc. Next limit the current, this will hopefully prevent more damage. Next check voltages, IE compare voltage readings to those on a schematic, if they are wrong, this indicates a possible area of fault.

Hope that helps. Attach a schematic of the power supply and amp here, you'll get more answers and we'll all know what bit goes where etc. Also post some pictures, they don't need to 5GB size, just good close ups of the amp, we may spot something. Get the power supply working ok then we'll deal with the rest of the amp. We are breaking up a big problem into bits, it's easier that way.

Andy.
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Old 29th Nov 2021, 10:55 am   #6
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Default Re: Armstrong 525 Motorboating/Thudding Channel

I am very far from being an expert on transistor amps and am happy to be contradicted but you said that there was a voltage on the speaker terminals of the faulty channel. This indicates that the large value capacitor which passes the signal from the output transistors to the speaker but holds back the DC voltage is leaky. Testing the capacitors at less than their working voltage doesn't really show this fault. If you haven't done so already I think you should replace all the large capacitors with ones of the same or as near as possible higher values and the same or higher voltage ratings. I don't think that a faulty transistor would cause that voltage.

All the electrolytic capacitors in an amp of that age are suspect and, even if you get it working, I think they are worth changing if it will be used to any great extent. Having said that a 500 series amp I looked at had green electrolytics made by Hunts. Any paper capacitors made by them are almost certainly leaky but I was advised that the green electrolytics were used in early computer boards and are reliable.

Do use a lamp limiter as suggested when you power it up. It can save further damage. I once trapped a lead in an amp where I had replaced the output transistors which caused a short. The lamp glowed brightly and save me from buying more transistors!
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Old 29th Nov 2021, 11:05 am   #7
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Default Re: Armstrong 525 Motorboating/Thudding Channel

I pretty much agree with all of that except blanket changing of electrolytics.
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Old 29th Nov 2021, 11:51 am   #8
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Here is a link to a page written about Armstrong amps and in particular to the 500 series. It mentions odd noises possibly caused by dirty switches.

http://www.audiomisc.co.uk/Armstrong/500/500faq1.html
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Old 29th Nov 2021, 12:05 pm   #9
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Looks like you have already made some good progress and have the PSU and Channel A working. Looking at the circuit diagrams available on the web, it would be useful to know which power Amp boards are used, it can be either A14 or A15 and most likely is A15.

These amplifiers are extensively AC coupled which is both good and bad. Good because a single fault will not kill every transistor in the amp and each stage can be checked in isolation, bad because the electrolytic coupling capacitors may all need replacing.

As the amp boards are plugged in to a main board, I would start off by pulling them out, cleaning the contacts, then swapping the channel A and B boards over to see if the fault moves with the board.
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Old 29th Nov 2021, 12:19 pm   #10
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Default Re: Armstrong 525 Motorboating/Thudding Channel

You mention you have limited understanding so apologies if you know some of this stuff.

First electrolytic capacitors (the ones you have possibly replaced) are 'polarised'. That is they have positive and negative terminals, you must install then the right way round otherwise they do not function correctly.
Using the diagram from Jim Lesuf's site the output caps are numbered 286 and 286a. http://ukhhsoc.torrens.org/makers/Ar...521diagram.pdf
The purpose of that cap is to stop DC from getting to the speakers.
25 volts DC on the output would normally be death to the speaker, but you say you get 'motorboating' when a speaker connected? So first make sure the faulty channel has the replacement caps correctly installed. (If you have not already replace the other partner 1250uF caps 273 and 273a. 1500uF or 2200uF 35 or 50 volts will be fine, just observe the polarity...)
The 'large cap', 300 on the diagram, is common to both channels so cannot be the fault, but will undoubtedly need replacement at some time - a 2200uF 80 or 100 volt will be fine.

You mentioned a missing transistor in the preamble, please say which one it was.
I assume you can now adjust the output of the Z17 power board?
The original output transistors and regulator transistor have special mounting hardware, ie; thin plastic washers and insulators round the mounting bolts. These are critical and stop the body of the transistor touching the chassis and shorting. Presumably you installed these correctly?
Do not forget this is a Germanium transistor amp so has a Positive chassis / 0 volt line, the HT is Negative...

Enough for now, let us know what you find.

And a word of warning, there is a mains suppressor fitted as standard, normally a green plastic tube behind the front panel, these have a habit of exploding - if it is still present on your set, best cut it out...
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Old 29th Nov 2021, 12:37 pm   #11
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Default Re: Armstrong 525 Motorboating/Thudding Channel

From what I recall, many of the electrolytics are made by "Daly" and experience shows that these do not survive as well as many others from the same era (Philips and most Japanese ones spring to mind), so a blanket replacement approach may be justified here, but ONLY IF you are capable of doing it accurately and to a good standard. Otherwise, you'll make things worse.
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Old 29th Nov 2021, 4:10 pm   #12
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Default Re: Armstrong 525 Motorboating/Thudding Channel

As already stated C286 is the output capacitor that connects to the speaker. If you are measuring 25V at the Channel B speaker that would indicate a problem with C286B, i.e. high leakage/passing DC.

You say you have measured the capacitance of the 5 big capacitors (which includes the 2 x C286) but capacitance measurement alone is not sufficient and say you replaced them (I think you mean refitted not renewed), still sounds like it needs replacing, there will be an appreciable -ve DC voltage on the -ve input at the capacitor but there should be virtually no voltage DC wise on the output of the capacitor. You could swap over the two C286's to prove a point but at some risk of possible damage to speaker on Channel A.

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Old 29th Nov 2021, 4:35 pm   #13
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Hi guys, Thanks for all the replys,

Andy, this is sound advice and I think I will start with some of this, should have maybe from the onset, will be reseraching the "lamp limiter" definiatly.

Paul, They are the A15 boards as per the schematic snowman_al has included in his post. Have already swapped the boards round but will definatly clean the contacts.

snaowman _al, They are all Erie caps center lug is marked red as pos, I swapped the caps out with the exactly the same from a 526 Amp and took a pic before so I am confident the are all installed ok, I believe the longer lead is the pos or at least it will be marked up on any replacement caps.
I'll label all the cables before perfoming this task.
It was the TR25B that was missing, but TR29 had to be replaced to allow me to adjust the voltage on the power board, now set to 50vdc on both outputs.
The green mains suppressor, seen it and will do.

Nickthedentist, All I can say is I make notes and take pictures and my soldering is getting better, well I think so. They are blue Erie Capacitors.

David, yes you are right, I refitted the capacitors from a selection from the original 525 and the spares unit, a 526 with what I thought had the best reading. I think I will be definatly replacing them.

Well I have enough to be getting on with, I'm going to do some of the primary checks recomended by Andy and research the "lamp limiter", definatly think about some new caps I think.

I have some power transistors coming, they are a 2N3614 TO3 Vishay Dale acording to Crickklewood Electronics they are a suitable replacement. I'll swap these over as they are coming.

WOW - Thanks again.
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Old 29th Nov 2021, 6:29 pm   #14
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Nigel,
Please confirm that the replacement output caps (286) are new or old from the other set? I assumed you had used new caps.
If the answer is old, disconnect one lead from the problem side cap and measure the resistance (20k range to start with) across the cap. If all is well it should measure low for a fraction then increase to a high reading above the 20k setting.
Now do the same for one of the other similar caps and compare. You will need to replace all 4 of them at some stage.
Pointless replacing the transistors at this stage.
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Old 29th Nov 2021, 7:17 pm   #15
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Hi Alan, The replacement caps were out of a 526 that I am using as spares, so yes they are old ones. Apologies if I hadn't made it clear. Hope these read out make sense.

Had to strat at 60k setting,
273B - yes starts at about 2.28 and very quickly reaches OL move it up to 60M and its at 18 and very slowly decreasing.
286B - The same and then ends up on the 60M range at 8.11
As a comparison:
273A - starts below 0 and rises to 6 on the 60M range and slowly decreases to 5.85
Spare capacitor (a) - 60M range ends at 6.03
Spare capacitor (b) - 60M range ends at 44.26 slowly rising ?
Spare capacitor (c) - 60M range ends at 6.95, slowly decreasing

I am looking at capacitors now on RS, high ripple, ESR, automotive, industrial equipment, tiny compared to the originals, not that size matters of course lol. HiFi Collective have some but very expensive, What's the best.
Nigel
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Old 29th Nov 2021, 7:31 pm   #16
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Just to add 286A starts low and the ends on 29.16 on the 60M setting.
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Old 29th Nov 2021, 10:48 pm   #17
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Default Re: Armstrong 525 Motorboating/Thudding Channel

I undsertand the use of the "lamp limiter" now, will be in the shed tomorrow digging out bits from the electrical spares I have and then down the diy shop I expect.
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Old 30th Nov 2021, 10:19 am   #18
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Default Re: Armstrong 525 Motorboating/Thudding Channel

You don't need anything special for the capacitors, just good quality "ordinary" ones. RS is a good source but there are others such as Cricklewood Electronics, Bitsbox Electronic Components etc. Ebay can also be a good source but be aware that there are lots of fake or poor quality electrolytics around.

If you look at the specs in RS or CPC you will see things like expected lifetime and maximum operating temperature. The latter comes as 85 and 105 degrees. Whilst your amp won't reach either temperature the 105 degree ones might be a better choice for longevity, everything else being equal.

Modern capacitors are much smaller than the original ones but sometimes you can find ones which have a similar diameter but smaller height. If not you will need some ingenuity to secure them. New clips are available in a range of sizes.

You can buy very expensive "audiophile" capacitors sold for their wonderful sound quality. I doubt that anyone could tell the difference between those and decent quality ordinary ones in a blind test.

PS - don't forget the advice already given that most electrolytics are polarity conscious and be careful to fit the new ones the right way round.
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Old 30th Nov 2021, 11:41 am   #19
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Umm, the resistance readings are not at all conclusive. I was rather hoping for a low reading on the faulty channel's cap(s)...

Any way it is easy to replace and therefore eliminate them.
To be honest, anything will be better than the originals, please do not spend out on expensive caps on the 525, as Paul says you will never hear any difference. And until you identify the problem any spend is speculative.

Cricklewood ones will be fine, but Panasonic caps off ebay would be a good choice. There are packs of 4 if you search either ''4x Panasonic 2200uf 50V radial electrolytic capacitor'' or 5 on ''Panasonic 2200uF 35V Dc Aluminium Electrolytic Capacitor'' . They are much smaller and can be fitted inside the original can with very little work.
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Old 30th Nov 2021, 2:49 pm   #20
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Default Re: Armstrong 525 Motorboating/Thudding Channel

Hi Paul, Alan,

Yes, been looking again at capacitors with the advice from Paul, might get 2 sets just incase, they are not expensive. I love your top tip on using the existing cases, that was a conudrum on how to fit them securely but you've solved that problem.

I have found some Panasonic 1300uf 35vdc and a Vishay 1500uf 80vdc both closer to the original spec or do you recomend going for the higher 1500uf/50vdc (Panasonic) and 2200uf/80vdc (Vishay).

The green suppressor, not woth putting one in of same or similar value?

Didn't have as much in the shed for the lamp limiter as I thought, now trying to get all the bits in one place.

It's all consuming the restoration stuff.
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