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Old 24th Mar 2017, 7:13 pm   #1
stuart_morgan_64
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Default Armstrong 521 noise

I am stuck with noise like thuder coming and going, only on phono input. In my experience normally transistor noise.

Replaced ME 4102 with BC547, and the two AC191 with AC151 I purchased.

Noise is evident using a scope on C of the second transistor. Remove C 217 noise has gone. Have I got noisy new transistors?

What cn I replace the AC191 with? Seems like NOS can give problems too.

Thanks Stuart
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Old 24th Mar 2017, 7:48 pm   #2
paulsherwin
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Default Re: Armstrong 521 noise

You should be able to replace the AC191 with any Ge general purpose type to confirm or eliminate the fault. An Si PNP type (BC558/2N3906 etc) could also be tried to see what effect it has on the noise, though you obviously wouldn't leave it there permanently. For a permanent replacement I would use an AC126 but there are lots of other types that would work.

Is the noise present on the BC547 collector?

The BC547 isn't a low noise transistor and I wouldn't have chosen it for that position, though it shouldn't produce the symptoms you describe. A BC109/239/549 would be a better choice, or even a higher spec low noise transistor.
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Old 26th Mar 2017, 7:02 pm   #3
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Default Re: Armstrong 521 noise

Still noise, I have removed 211 cap between the first two stages and all is quiet. Noise on C of the BC549.

Caps changed, it must be around the first stage. What else can it be?

Thanks Stuart
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Old 26th Mar 2017, 8:37 pm   #4
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Default Re: Armstrong 521 noise

what input are you selecting ?
Have you changed the left and right cards over and does the noise follow the card?
I would suspect resistors as they were not of the best quality
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Old 27th Mar 2017, 4:07 pm   #5
stuart_morgan_64
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Default Re: Armstrong 521 noise

Hya, problems onboth boards, one was only on phono input, the other on all, it's now up and running, on tape using a cd at the moment, will give a few hours of records shortly. So far so good.

Thanks for comments
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Old 27th Mar 2017, 4:29 pm   #6
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Default Re: Armstrong 521 noise

Could you update us on what you've actually done to it? It's not clear from the thread.
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Old 28th Mar 2017, 11:10 am   #7
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Default Re: Armstrong 521 noise

I will once I have sorted the posible earth problem. It blew the trip in the workshop after a few hours. Once I know it's working I will update.
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Old 28th Mar 2017, 12:11 pm   #8
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Default Re: Armstrong 521 noise

Still noisy one channel, phono only
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Old 28th Mar 2017, 6:09 pm   #9
Trevor
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Default Re: Armstrong 521 noise

The 521 amplifier has self shorting contacts on the phono input this has the effect of reducing the noise level. I have had these contacts fail. Put a shorted phono plug into each phono input and let us know the result.

Trev
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Old 28th Mar 2017, 6:46 pm   #10
Craig Sawyers
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Default Re: Armstrong 521 noise

Leaky C201, C211 or C210?
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Old 29th Mar 2017, 8:19 am   #11
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Default Re: Armstrong 521 noise

Quote:
Originally Posted by stuart_morgan_64 View Post
I will once I have sorted the posible earth problem. It blew the trip in the workshop after a few hours.
Possibly the green, TCC(?), 3-wire mains supressor located near the power switch between the front fascia and the chassis' front.
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Old 29th Mar 2017, 10:25 am   #12
stuart_morgan_64
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Default Re: Armstrong 521 noise

Hya all,

update, Nick the green supressor was the culprit of the earth trip going, now sorted.

Craig caps changed out of desperation.

Trevor, turned amp on this morning, thunder as usual on phono. Have some phono shorting plugs, put them in, noise gone and not returned even with them removed. I would assume they have cleaned the switches just enough. I will leave it on phono all day, if all is well, clean the contacts.

Many thanks all.
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Old 29th Mar 2017, 6:20 pm   #13
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Default Re: Armstrong 521 noise

great news the mains suppressor is also a well known fault but i still like this amplifier if you do a search on the website you will find it well documented
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Old 29th Mar 2017, 11:15 pm   #14
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Default Re: Armstrong 521 noise

One post moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=135286
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