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Old 28th Mar 2020, 12:32 pm   #1
vishalk
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Default Armstrong 625 left channel buzzing and no sound

Hi All

I hope you are all keeping well and safe during these testing times due to the covid 19 virus. I am on lockdown and as such have time to do some unfinished projects.

I recently recapped my Armstrong amplifier and it fired up first time with no issues. Sounded really sweet, I decided I need to adjust the bias and proceeded to place my crocodile clips connected to my DMM on J2 and G5 on the amp board to measure the bias.

As luck would have it i accidentally knocked my DMM which fell to the floor moving the crocodile clips and causing a massive spark (as it fell the crocodile clip near G5 must have touched other pins).

Now my left channel has no sound and buzzing sound (it was going so well until then)

My right channel works perfectly, I thought maybe I had blown the output trannies so took them out, tested them and they work fine. I even replaced them just in case and its the same thing.

Please see pics.

When I measure the voltage at J3 and J4 I get around 37V which goes slowly up at switch on.

On J1 and J2 its just 81.7V!

Now this is as far as my technical ability to search for issues goes. I would love some guidance on what to check for, how to check and how to fix this problem!

Any help would be appreciated as generally when I ask on here I always manage to learn, fix and repair just from the advice of the people on here!

Stay safe all

Wish
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Old 28th Mar 2020, 3:19 pm   #2
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Default Re: Armstrong 625 left channel buzzing and no sound

Definitely a time for staying at home and doing some electronics.

The diagram I have for that amplifier doesn't have test points marked or named so they don't mean anything.

I presume you were trying to measure the voltage across the 'emitter' resistors of the power transistor pair? and one of these poits got shorted somewhere..... bearing in mind that the bottom transistor in the totem pole is a quasi-complement where an NPN power device is used with a PNP driver to simulate a PNP power device.

First check, what is the voltage on the + end of the 4000uF dc blocking capacitor that drives the speaker? Should be about half the rail voltage (Armstrong say 40v)

But you're into the classic trouble of fixing a multi-stage DC coupled amplifier. The speaker output may be AC coupled, but the whole of the rest of the amplifier is Dc coupled.

If your output devices are OK, the quick fix is to replace all the other devices as a set, AND re-test the output devices! Don't expect to be able to test one device at a time, replacing as you go. One bad device can kill others and then the others kill the new one you fit. You have to get everything right with power off before you ever dare apply power, else it will keep putting you back to square one.

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Old 28th Mar 2020, 3:23 pm   #3
snowman_al
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Default Re: Armstrong 625 left channel buzzing and no sound

Hi Vish,
As the A17 board has two circuits on it Left and Right, sort of mirrored, you can compare the good side to the faulty one. Check the voltages from the circuit diagram, use the chassis for your negative lead. Let us know what is different.
Please use the component numbers from the diagram / board lettering. Most of us do not have a 621 to find out what J2, G3 etc. refer to and unfortunately the service manual also does not include them...

Diagram here: http://www.audiomisc.co.uk/Armstrong/600/Amp.gif

Alan
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Old 29th Mar 2020, 12:47 pm   #4
PaulR
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Default Re: Armstrong 625 left channel buzzing and no sound

It is worth testing/replacing the transistors on the side that has failed. You are not the first person on here to have blown transistors whilst attempting to check the quiescent current in one of these.

The potentiometers which adjust the current are a problem and it is worth replacing them also. I mistakenly ordered two sets when I did mine so if you PM me your address I will let you have a couple. The only problem with them is that the pin distancing is not quite the same as the originals and you will need to drill a small hole on the circuit board to accommodate the wiper pin connection. There is plenty of wire to connect it to the original part of the printed circuit. Only one end of the track is connected so don't be confused by the floating end as I was.

I also replaced the emmiter resistors which enabled me to connect test leads directly to them as the original flat ones are soldered so close to the circuit board that it is impossible to connect leads to them as they are.
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Old 1st Apr 2020, 5:27 pm   #5
vishalk
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Default Re: Armstrong 625 left channel buzzing and no sound

OK guys so I actually have a spare A17 board, I was thinking of just replacing the whole board.

This is a good excuse to pull apart the A17 board and check the transistors and see what blew. This is a good learning experience to fault find and repair and good practice as I have a Armstrong 621 I want to refurbish and upgrade most of the parts inside it.

Before I replace the A17 board is there any tips or advice, other checks to do?

Thanks
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Old 2nd Apr 2020, 2:38 pm   #6
Michael Maurice
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Default Re: Armstrong 625 left channel buzzing and no sound

I personally would try and repair the original board, keeping the spare as a back up.

Cricklewood electronics do a very wide range of transistors and presets. Although the shop is closed to customers, they are still doing mail order.

There cant be that many parts that need replacing.

I’ll look at the circuit later this afternoon for you
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