Help Needed - Armstrong 626 Tuner Amplifier

[TheMightyMadman]

Hi all,

I'm repairing/restoring a lovely Armstrong 626 tuner amplifier for a friend of mine - apparently, the volume control was scratchy and it smelled like burning during operation.

The controls and switches seem to be working well following a good clean out with contact cleaner - I've removed the mains filter capacitor (fitted to early units) which seems to have been the cause of the burning smell - I've fitted new channel fuse holders, as the originals had cracked - I've also replaced the three large blue Erie capacitors, which were bulging and had developed ESR.

After testing, it seems that the right channel doesn't work, on either speaker output (1 and 2). I checked the right channel fuse, and it was blown (I think it was like this when I received the unit); I replaced it (2.5A fast-blow) in case it had popped due to a user error (i.e. shorting the speaker output), and it didn't blow - I now seemed to be getting an extremely low output signal on the right channel (barely audible at max right balance and max volume), and one of the output transistors was getting extremely hot.

One of the audio transistors was testing badly in-circuit, but out of circuit the transistor seemed OK, and the problem seemed to be on the driver board itself. Two of the transistors on the driver board had shorted (see picture below) - one is marked "I6506 EV RCA 3G", the other "I6505 EC RCA 3H".

The parts test bad out-of-circuit; with them removed, the circuit is no longer shorted where they were, so I don't think there are other issues.

Does anyone know whether these can be directly replaced with a modern equivalent? If so, what are the part numbers, please?

Many thanks,

Adam

[TheMightyMadman]

Hi all,

I'm trying to repair an Armstrong 626 tuner amplifier for a friend of mine:
https://www.vintage-radio.net/forum/...57#post1534257

Unfortunately, two transistors in the right channel audio driver circuit have failed: RCA parts 16505 and 16506 (see picture below).

Does anyone know whether a modern replacement part is available, please? Or does anyone know the specifications for these transistors (presumably an NPN/PNP pair)?

Many thanks,

Adam

[cooperman]

The circuit diagram for the amplifier can be found here:-
https://www.audiomisc.co.uk/Armstrong/600/600faq.html

I think the transistors that have failed are the ones shown as items 623 and 624 - 2N5320 and 2N5322. You may have better luck trying to find those transistors rather than the RCA ones. The 2N ones are in a round can - unlike the flat package your existing transistors have.

[Ted Kendall]

The usual comments about DC coupled power amplifiers apply here - satisfy yourself that all the semiconductors are in good shape before you power up, otherwise you may go round in circles, blowing up what you have just replaced.

[Silicon]

I have no experience of this unit but blown transistors are sometimes caused by faults in the output stage bias components.

I would carefully check those yellow diodes and cheap potentiometers.

[DrStrangelove]

Just had a quick browse of the RCA power transistor manual but there's nothing in there with that sort of part number.

It may well be one of those old favourites: the house coded transistor.

Oh how we love them.

[Trevor]

40636 is the output device basically a high voltage 2N3055
You could always use a 2N3773
I used to retail these amps back in the day
Trev

[PaulR]

Originally Posted by Silicon

I would carefully check those yellow diodes and cheap potentiometers.

Those potentiometers certainly caused problems for me. I would replace them even if they appear ok at the moment.

[G6ONEDave]

There are various Motorola MJ types on the CPC website. Such as MJ15016G and MJ15025G (PNP type) along with the NPN versions. These vary on voltage from 120vce to 140vce. they have 16amp and 20amp versions, so something there will probably do the job but be mindful about the changes in substrate changes over the years. There has been discussion about this for the 2N3055 on this site.

To bring up a list of stock on the CPC webpage enter in the search box 'pnp transistor' or 'npn transistor', this brings up all their stock types. You will then have to scroll down to the MJ types and check the data sheets to see if anything looks OK for your repair.

Good luck.

Dave

[stuarth]

To avoid the destruction of the new transistors because there were other faults, I used to wire a 270R 3W wirewound resistor in series with the DC supply to the power amp. If all was well, the amplifier worked normally at low volume with very little voltage drop across the 270R. If there was a remaining fault and the amp drew excessive current, the resistor would get very hot very quickly. With separate DC fuses for each power output stage, it is very easy to remove the fuse and tag the resistor across the fuseholder.

This trick is usually quick and easy to apply. I used radial leaded resistors with mini crock clips directly on the resistor leads, it was self supporting in mid air. Using these resistors, I never blew the replacement transistors, and I always noticed the hot resistor before it came to harm (dissipation would be over 3W on most amps).

Stuart

[PaulR]

Good tip!

[TheMightyMadman]

Originally Posted by cooperman

The circuit diagram for the amplifier can be found here:-
https://www.audiomisc.co.uk/Armstrong/600/600faq.html

I think the transistors that have failed are the ones shown as items 623 and 624 - 2N5320 and 2N5322. You may have better luck trying to find those transistors rather than the RCA ones. The 2N ones are in a round can - unlike the flat package your existing transistors have.

Thank you very much I'd seen that page previously, and I wasn't sure whether the early schematic that it shows matched up to the A17 board layout that it shows. To cut a long story short, your suggestion worked - I'm very grateful!

Originally Posted by Ted Kendall

The usual comments about DC coupled power amplifiers apply here - satisfy yourself that all the semiconductors are in good shape before you power up, otherwise you may go round in circles, blowing up what you have just replaced.

Thank you I made sure to check the current adjustment potentiometer and the diodes before fitting replacement transistors.

Originally Posted by Silicon

I have no experience of this unit but blown transistors are sometimes caused by faults in the output stage bias components.

I would carefully check those yellow diodes and cheap potentiometers.

Thank you I made sure to check the current adjustment potentiometer and the diodes before fitting replacement transistors.

Originally Posted by DrStrangelove

Just had a quick browse of the RCA power transistor manual but there's nothing in there with that sort of part number.

It may well be one of those old favourites: the house coded transistor.

Oh how we love them.

Thank you

Originally Posted by Trevor

40636 is the output device basically a high voltage 2N3055
You could always use a 2N3773
I used to retail these amps back in the day
Trev

Thank you

[TheMightyMadman]

Originally Posted by PaulR

Those potentiometers certainly caused problems for me. I would replace them even if they appear ok at the moment.

Thank you I'll consider this.

Originally Posted by G6ONEDave

There are various Motorola MJ types on the CPC website. Such as MJ15016G and MJ15025G (PNP type) along with the NPN versions. These vary on voltage from 120vce to 140vce. they have 16amp and 20amp versions, so something there will probably do the job but be mindful about the changes in substrate changes over the years. There has been discussion about this for the 2N3055 on this site.

To bring up a list of stock on the CPC webpage enter in the search box 'pnp transistor' or 'npn transistor', this brings up all their stock types. You will then have to scroll down to the MJ types and check the data sheets to see if anything looks OK for your repair.

Good luck.

Dave

Thank you

Originally Posted by stuarth

To avoid the destruction of the new transistors because there were other faults, I used to wire a 270R 3W wirewound resistor in series with the DC supply to the power amp. If all was well, the amplifier worked normally at low volume with very little voltage drop across the 270R. If there was a remaining fault and the amp drew excessive current, the resistor would get very hot very quickly. With separate DC fuses for each power output stage, it is very easy to remove the fuse and tag the resistor across the fuseholder.

This trick is usually quick and easy to apply. I used radial leaded resistors with mini crock clips directly on the resistor leads, it was self supporting in mid air. Using these resistors, I never blew the replacement transistors, and I always noticed the hot resistor before it came to harm (dissipation would be over 3W on most amps).

Stuart

That's a good idea, thanks! I'll certainly consider doing this in future

[TheMightyMadman]

Based on cooperman's suggestion, I ended up fitting a 2N5322 (BSS17) in place of the green 16506 part at location 624R, and a 2N5320 in place of the grey 16505 part at location 625R (after checking the other passives and actives first). They seem to work great - the right channel is back, both channels are nice and balanced, and very clear 🙂 Thank you all for your help!

https://www.youtube.com/watch?v=yX2XpNJ-yhA

[PaulR]

Well done. It is always very satisfying when the sound gets through! They are very good units when working properly.

[TheMightyMadman]

Originally Posted by PaulR

Well done. It is always very satisfying when the sound gets through! They are very good units when working properly.

Thanks Paul It sounds absolutely amazing!