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Old 20th Feb 2018, 11:32 am   #41
Pamphonica
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

Try swapping first the preamp boards, then the power-amp boards to find out which board is causing the problems. The fault should swap channels if it's on a board. If not, it's one of the few components external to the boards (power transistors or tone control components). You can take the opportunity to clean the backplane connector contacts and the board edge connectors with some switch cleaner (Servisol 10 etc) while you are about it.

For those not familiar with the 521, it has a backplane of sockets and 5 plug-in boards. Power amp x2, Preamp x2 and PSU regulator.

You might find that it's one of the small electrolytics. I had to change a number of those on my preamp boards to get rid of odd noises.

Drop me a PM if you want to borrow some spare boards to compare.

I assume my package arrived safely!
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Old 20th Feb 2018, 12:25 pm   #42
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Default Re: Armstrong 521 voltage selector

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Originally Posted by cooperman View Post
Has anyone any comments about the 400mA main fuse that keeps blowing. As mentioned I've temporarily used a 1A fuse in. Amplifier has had a good 3 hour test on fairly high volume without any issues. I'm not really happy with more than doubling the value of the fuse though.
Have we established that 400mA anti-surge fuses were fitted for the initial tests?
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Old 20th Feb 2018, 12:59 pm   #43
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

Jeremy - package arrived safely - many thanks.
Station-X - 400mA fuse is re-instated (details in recent post).
The problem seems to be in one of the power amplifier PCBs - or more accurately a combination of the PCB and edge connector. The second slot from the front seems temperamental. I've swapped the two power amplifiers around and all seems fine on both channels. If I swap them back, the slightest movement to the PCB in slot two causes pops and crackles. All rather strange. Almost as if one PCB is thinner that the other and does not make good contact. Visual inspection does not confirm this though.
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Old 20th Feb 2018, 1:12 pm   #44
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

Those connectors are not very nice. They do get corroded (poor plating) and they exert a lot of pressure over a small area on the pcb edge-connector, and I have had similar problems. I used thin cardboard loaded with switch cleaner, moved in and out of the connector, to try to clean it a bit. I then used a "polifix" cleaning block to clean the pcb edge connector. A similar effect can be had by using an INK rubber (rare things nowadays). You can also try carefully squeezing the edge connector contacts together a little.
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Old 20th Feb 2018, 1:37 pm   #45
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

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Station-X - 400mA fuse is re-instated (details in recent post).
Yes, but is it an anti-surge type?
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Old 20th Feb 2018, 3:31 pm   #46
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

Yes 400mA Anti Surge fitted.
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Old 20th Feb 2018, 10:26 pm   #47
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

Update on the strange noises. I've cleaned the edge connectors with isopropyl alcohol and and switch cleaner. That has made a big difference in that previously just moving the PCBs a bit made lots of noise - now no noises. In my earlier post I mentioned that it was the one of the two power amplifier cards that was the culprit - I was mistaken. It is the pre-amp cards that live in the first two slots. I've also re-soldered the joints on the "duff" PCB - non of them looked "dry" but I thought I'd give it a go. Not sure it has made any difference.

There is still an occasional pop from one of the PCBs. I've decided to change all the electrolytic caps in both PCBs to see if that will eliminate the problem. Most values are easy to find - but I'm stumped with the 25 uF 25 volt variety. Seems that only 22 uF or 47uF are available these days. does anyone know of a source for this value? There are more of that type than any other on the various PCBs.
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Old 20th Feb 2018, 10:52 pm   #48
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

Well done on the edge-connector cleaning. Well worthwhile.

For caps, just use 22uF, no difference in practice (wide tolerances anyway).
PM me if you need a pack of all the pre-amp electrolytics. As mentioned in our PMs, I had made a couple of packs up.
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Old 20th Feb 2018, 10:55 pm   #49
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

Quote:
Originally Posted by cooperman View Post
Most values are easy to find - but I'm stumped with the 25 uF 25 volt variety. Seems that only 22 uF or 47uF are available these days. does anyone know of a source for this value? There are more of that type than any other on the various PCBs.
22uF is the modern equivalent and will be perfectly satisfactory. Capacitors are now manufactured in "E" series values. (Mostly).
Any working Voltage above 25V will suffice, 35V or 63V will be perfect and probably about the same physical size.

Andy

OOps cross posted with jeremy....

Last edited by bikerhifinut; 20th Feb 2018 at 10:57 pm. Reason: Cross posted
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Old 28th Feb 2018, 6:16 pm   #50
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Default Re: Armstrong 521 voltage selector and fuse blowing issues.

In the end I only changed a couple of caps on the pre-amp. Fortunately that did the trick - no more pops or other noises. I found that the tracks on the PCB were lifting very easily - so decided against changing anything else for now. Restoration now complete, I'm passing on the amplifier to my daughter to go with an old SP25 mkIV. Thanks to all who contributed - particularly Jeremy.

I've just taken delivery of a similar Armstrong amplifier with built in FM tuner. This one has a channel down so I'll open another thread if necessary.
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