Armstrong 521 voltage selector and fuse blowing issues.

[cooperman]

I've just acquired a rather battered Armstrong 521 that I'm about to try to restore. It is missing the octal plug from the back of the unit that selects the correct voltage. I doubt if I'll find a replacement for such an old item - but i wondered if anyone has a diagram of it's internal connections. I could either fabricate a new plug - or I could solder the correct links inside the unit.

Also does anyone know the value of the mains fuse (on the back panel) and also the two fuses on the PSU PCB?

Thanks in advance!

[Station X]

I offered a couple of octal plugs in post #16 in this thread:-

https://www.vintage-radio.net/forum/...d.php?t=131297

[vidjoman]

Are you sure it’s an octal plug? It will only have one position and would need to be re-wired to suit other voltages, which is not a user option. It may be an octal looking socket but have a plug with no keyway and a single shorted link.
A photo of the socket would help. The circuit diagram should show what it is.

[bikerhifinut]

If my memory isn't mistaking me I think the Armstrong 500 series had a noval (B9A) voltage selector?

A.

[AC/HL]

It does: http://www.audiomisc.co.uk/Armstrong/500/500page2.html

[barretter]

Yes, a search of the internet brings up pictures of the back of the 521 and it is a noval socket but I don't know where you would find the appropriate plug. Another search brings up a copy of the user manual on Saturn Sound which has a page detailing the the specs of the fuses. There could be a copy of the circuit diagram out there too, but it's late...

[barretter]

Oops, crossing posts there!

[cooperman]

Maybe the term Octal is incorrect. It looks like a valve socket - as per this link

https://modulusamplification.com/Val...-P1314630.aspx

It is shown on the circuit diagram at this liink.

http://www.audiomisc.co.uk/Armstrong/500/diags/Z17.gif

[Station X]

That's a B9A nine pin valve holder. The plug that fits it must have a limited number of pins so that it can be fitted in more that one position without a pin fouling the gap.

The links drawn in the circuit diagram would result in no connection to the neutral side of the incoming mains supply. If you're planning to hard wire it I suggest connecting all the turns on the primary winding in series ie for a 240VAC supply.

[cooperman]

I wonder if anyone else in this forum has an identical amplifier and whether they would be kind enough to check with a meter which pins are connected in the selector plug?

[Station X]

Looking at the diagram it's not clear which way the pins of the socket number. Assuming they number clockwise, the wring is as follows:-

Live - pin 8 - LA4 - pin 5 - pin 3 - LA5 - pin 9.

From pin 9 the wiring goes nowhere, so you'll need a link from pin 9 to pin 7 which goes to neutral.

[vidjoman]

Do we know what voltage the circuit diagram is set for? On most amps the normal way would have the windings in series for 240 v and parallel for 115 v.

[Station X]

As drawn, the circuit diagram isn't set for any voltage. There's no circuit through the primary windings.

Given that the picture shows a selector plug with a highest voltage of 240 VAC, I've assumed that all the primary windings will need to be connected in series to achieve this.

[Pamphonica]

Originally Posted by cooperman

Also does anyone know the value of the mains fuse (on the back panel) and also the two fuses on the PSU PCB?

From the manual:
Fuses:
F1 and F2 (LT) 1A (normal)
F3 (Mains) 400mA anti-surge

Mains selector:
PM me as I have a box with lots of spares from a couple of 500 series machines I parted out to help mend my collection, a few years ago.
I know I have a mains transformer there, with the selector still attached.

Good luck with the repair.

[cooperman]

Originally Posted by Station X

Looking at the diagram it's not clear which way the pins of the socket number. Assuming they number clockwise, the wring is as follows:-

Live - pin 8 - LA4 - pin 5 - pin 3 - LA5 - pin 9.

From pin 9 the wiring goes nowhere, so you'll need a link from pin 9 to pin 7 which goes to neutral.

I reckon that seems about right - providing that the dotted lines are links on the back of the socket. I'll check it out and post the details.

[AC/HL]

Your best bet will to be to contact Jeremy, all you need is the plug part of the selector. It should be captive I'd have thought, but it obviously can be detached somehow. This will save a lot of messing about and keep the approvals.

[Grubhead]

I am certain I have same model in the loft. It has blown the output transistors. When I looked around for replacements they were about £50 each! So I gave up.
Interesting amp though. I remember the tone controls affected the recording of music. So you could increase the bass or treble on the recording!

[Pamphonica]

Originally Posted by Station X

Looking at the diagram it's not clear which way the pins of the socket number. Assuming they number clockwise, the wring is as follows:-

Live - pin 8 - LA4 - pin 5 - pin 3 - LA5 - pin 9.

From pin 9 the wiring goes nowhere, so you'll need a link from pin 9 to pin 7 which goes to neutral.

Station X is quite correct of course.

I have checked the pins out on the circuit and in practice and for future reference here's the information. (I hope I have it right!).

There are only 4 pins on the plug, and it is not captive.

The diagram below shows how the pins are wired in pairs.
The diagram definitely shows the pins looking at the REAR of the socket from inside the equipment. (Like looking at the back of a B9A valve base).

As shown, with pins 7 and 9 linked on the socket, the amp will work from 240V mains.

So the B9A socket wiring (with colours) is:
Pin 1 n/c
Pin 2 n/c
Pin 3 (Pink) first primary winding high tap
Pin 4 (Grey) second primary winding low tap
Pin 5 (Blue) second primary winding high tap
Pin 6 (Yellow) first primary winding low tap
Pin 7 (Orange) Linked to Pin 9
Pin 8 (Green) Live in, and second primary winding (L44) start
Pin 9 (Orange) Neutral in, and first primary winding (L45) start

Jeremy

[Pamphonica]

Originally Posted by Grubhead

I am certain I have same model in the loft. It has blown the output transistors. When I looked around for replacements they were about £50 each! So I gave up.
Interesting amp though. I remember the tone controls affected the recording of music. So you could increase the bass or treble on the recording!

You have a couple of options to replace those pesky (and fragile) AL102s.
1. Put in OC28s. Lower ft but probably not noticeable.
2. Search on this forum for threads about replacing the transistors with more modern silicon equivalents.

[Trevor]

there are a no of articles re fitting silicon devices in the archive
you can do the repairs for a fiver
Trev