Receiver repair Rankarena R2025

I am engaged in the repair of the receiver Rankarena R2025.
What diodes can be replaced by a diode D1 (KB262)?
Should the diode D1 (KB262) have thermal contact with the radiator of the output transistors?

The best info I can find tells me a KB262 is an led, which doesn't make any sense in that part of the circuit.

There are some clues. however ...

The transistor above it (TR3) shows 25v on it's base.
The one below it (TR4) is showing 23.6v on it's base.

This effectively puts 1.4 volts across the D1 and VR2.

So it is reasonable do deduce it is what the diagram shows, two diodes in series...

The schematic shows 48v bulk voltage, with a string of R9 and R8 feeding it for a total of 9.3k ohms...
So Ohms law tells us (48v - 25v) / 9.3k == 23/9300 == ~2ma of current.

So... I would bet that you can replace D1 with just about any 2 general purpose silicon diodes in series then adjust the bias back to spec using VR2.

I hope that helps...


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Thanks for the answer.
Here is a photo of the amplifier. Small peas with an orange dot are D1.
As I understand it, for the stable operation of the output transistors, a thermal contact between D1 and the radiator is necessary.

The picture sure doesn't make it look that way... They don't appear to be touching anything.

If they've gone bad, I'd say to try changing them as I said and see what happens.
These should be similar to VD1212 and the like. Some people have argued that two UF4001 or two transdiodes may be a better choice - whatever.

This is a crappy old mid-'70s design, they didn't have thermal compensation figured out very well yet. Ideally, in a quasicomp, you want to make the bias diodes to make thermal contact with:
* the upper (npn) driver and output
* the lower (pnp) driver.

Good luck figuring out how to actually do this. Whatever you do, keep wires as closely together as possible. A small capacitor in parallel may also be advisable (should reduce higher-frequency crossover distortion, too).