Overhaul of an Armstrong 625 gone wrong! Please help me :)

[Mooly]

The 2N3773's are a good choice, what I had in mind too. The drivers are problematic because of the package, the "old" T039/T05 style. If it were me I would fit flat pack MJE340 and MJE350 perhaps with a clip on heatsink. The BF258 is no problem and is replaced by a BF259.
All these are available from CPC which I often use.

The 0.47 ohms are replaced by these,
WELWYN|W21 0R47 JI|RESISTOR, WW 3W 5% 0R47 | CPC

The 100 ohms, you have to buy a hundred ! still cheap.
--|MCF 0.5W 100R|RESISTOR, 0.5W 5% 100R | CPC

Don't forget new insulating kits for the outputs,
--|MK3301/S|TRANSISTOR INSULATING KIT TO-3 | CPC

and clip on heatsinks, which CPC seem not to stock
AAVID THERMALLOY|PF730|HEAT SINK, TO-126, 34°C/W | Farnell United Kingdom

Edit... as CPC is owned by Farnell perhaps use Farnell instead.

[thefamilybassman]

Originally Posted by Mooly

I'll put some suggestions for parts down.

The 0.47 ohms... hmmm. They won't be 8 ohm for an "emitter" resistor. 0.47 is about as high as can be used for various reasons. Make sure your meter reads 0.00 when the leads are shorted together.

My bad, meant to write 0.8ohms.

[PB2]

MJE340 and MJE350 are only .5A originals were 2A - I would not use them.

[thefamilybassman]

I was going to wonder at those, but for a different reason. Does it not matter that I would be switching from germanium to silicon with them? I read somewhere that they behave differently and shouldn't really be used. Please excuse my ignorance if this is completely wrong?

Thanks again for everyone's ongoing support!

[PB2]

I don't think any of those are Germanium, what makes you think so?

The output stage is undersized for handling a short circuit load, is there a fuse or anything in the supply lines? I'll let you look.
Edit: OK I looked 2.5A are there in the schematic - were the right ones in there?

The VAS needs a current limter also if you want it to survive a shorted output.

I'd add a 2.5A fuse in each supply line to each channel, and a current limiter on the VAS set for about 50 mA.

The drivers are listed here, note that they were listed at Digikey but they have zero stock so it would be a special order:
2N5320 pdf 2N5320 datasheet General Purpose N-P-N Silicon Power Transistor. 2.7485 Transistors Bipolar Silicon NPN Power Transistors American Microsemiconductor 2N5320 available at 2.7485. FREE UPS ground shipping or more.

I've not ordered from them, so comments are welcome here as to the quality. I would ask what manufacturers they have and if you can specify a particular one in your order.

BD139/140 would also be fine as drivers note of course the different package and use a small heat sink.

[thefamilybassman]

No they weren't, they were 4A. first time I've looked... (!)

yeh, my ignorance there, I was going off appearances. I assumed that the old T039 design signified germanium...

[PB2]

Someone probably increased them for driving 4 ohm loads - I would have tried 3A first and it is probably OK as long as you don't short the output or let it get too hot - better to go back to 2.5A if you can.

BF259 is fine as a replacement for the VAS as Mooly stated above.

[tauro0221]

Hi,
For the 2N5320 they are available in Mouser. Also Dynaco 120 used the 2N5320 and the they have a mod to replace it with TIP31C.

[thefamilybassman]

I have found components of the right specs for pretty much all I need for sale on Farnell, what do you guys think of Multicomp semi's? worth a punt?

[PB2]

Originally Posted by thefamilybassman

I have found components of the right specs for pretty much all I need for sale on Farnell, what do you guys think of Multicomp semi's? worth a punt?

I don't have experience with Multicomp but my gut feeling is look for something better.