Marshall SS 5210 Zener Diode Question

[earache]

Hello

I have a solid state Marshall amp 5210 in which the Zener diodes look a little "browned", and I can't tell from the schematic what I should use as replacements. Can anyone offer a suggestion?

The schematics don't list it and are fairly illegible. I believe the correct schematic of these two is the older one, dated 1981.

Marshall_50w_5210_year_1981.pdf

[earache]

[Enzo]

http://bmamps.com/Schematics/marshal..._year_1981.pdf

Easiest path since you have it open, and I assume it works? Measure the voltage across them. I assume 15v more or less - anything within a volt. The schematic says 15v when I read it. Plain old 1 watt 1N4744 or similar would be my choice. Unless they appear to be 5 watt types. But that seems overkill for four ICs.

But then, do the diodes look browned? Or does the PC board around them look browned? It is normal for zeners to dissipate heat, and over time discolor the board.

[earache]

Thanks for the quick reply Enzo!

Actually it is the circuit board around the diodes that looks a little "toasty". The diodes themselves are very small.

The amp's problem is that it hums and has DC on the output, but testing the power transistors on a tester shows them as being OK. I will test the voltages at the diodes next.

[Enzo]

NO LOAD until the amp is stable and does not put DC on the output.

I doubt the zeners are your problem, as they are not supplying 15v to the power amp. But do check that they have about 15v across each.

DC? Shorted outputs usually makes blwon fuses. But they normally follow their bases, so TR12 or the diffy pair are my prime suspects.

[earache]

I have 23 VDC at the speaker leads even after making these replacements:

Replaced the filter caps C44 C45 4700uf X 55V
Replaced C46 C47 1000uF x 25V
Replaced R38 R39 220 (I used 1 watt types because the board looked kinda cooked there)
Replaced power transistors: MJ3000 was replaced with a MJ3001 type
MJ2500 was replaced with a MJ2501 type

Kind of a mystery.
Light bulb dims when connected to a light bulb limiter.
15V and -15V at the zeners.

[J M Fahey]

Looks like the schematic is no longer available.
Nor any other from BM Amps ?

[earache]

This is the schematic and it (and any other version I have ever seen) is HORRIBLE QUALITY

[earache]

http://drtube.com/schematics/marshall/5210-iss3.gif