Gk 200mb

[Alex R]

I rebuilt the power amp and quite a bit of the power supply including the bridge rect. and the 4700uF caps on this amp. Now it works ok but has just a little bit more buzzy hum than it should. On the output you can see on the scope some tiny little 50Hz spikes making the hum.

I used TIP35C and TIP36C to replace the power transistors instead of the lower-current types in there, because I stock them.

I swapped in another 4700uF on each side in turn. Same hum. The power supply components are ok and the voltages balance, no DC on the output.

You can scope a fair bit of ripple on the rails, not more than normal though I think. It's a sawtooth but it's 50hZ not the usual 100Hz. I can't quite see how this should be - would that be normal with this kind of duplex arrangement coming off the bridge? I did try another bridge - same.

Any other suggestions as to where this irritating little hum might be coming from?

[52 Bill]

Years ago I had a 250ML that had a similar problem. I never scoped it so I don't know if it's similar to what you have or not.

I did all of the same tests, filtercaps, shorted op amps, etc. Finally turned out to be a couple of small radial filter caps, I think in the headphone driver circuit. Maybe 220uF in value? There were two or three of them in a row on the far right end of the board.

[Alex R]

Thanks Bill. I think I'll swap a few of the little radials, yes. But guys what about that 50Hz (not 100Hz) ripple coming off the bridge - normal for this kind of power supply or not?

[Enzo]

50Hz not 100Hz, there is the clue. Both rails that way? I'll bet. That says to me that one side of the power transformer secondary is open, or one side of the power switch is open, or the wiring between them. Check that.

I suspect your caps are OK. But do check those 12v zener supplies for the oip amp, and the regulated 15v rails too. Not so much looking for excess ripple, though do check, but looking of one rail sagging way low. That would unbalance everything.

[Alex R]

Aha! Knew something was up, thanks for making the connection. Somehow I didn't think about that double switch. Yes, both rails the same. I'll look at it later today when I get back from my other job...

Actually it's a replacement PT in there and I did check for open windings because something said 'unbalanced' to me, but no, didn't notice the switch, too dumb. It is a very clean-looking 50Hz sawtooth suggesting total absence of something or other. What's more the old bridge had failed on one side.

I need a clearer mental picture of how a full bridge works. I was looking at the wave and going, is there any way it ought to be like that? Actually I recall a single moment of clarity at school one day (back when they used to teach you about that sort of thing in physics) when I had the whole thing right there... Then maybe I can move on to the long-tailed pair, another blind spot. I know some people say, you just need to know what it does, but me I'm not happy unless I see how it does it. Always questions. I was a pita in physics.

The 15v rails are okay and the 12v zeners are too but I didn't actually look at the 12v rails themselves and will do so.

[Alex R]

Yes! Solved it!

The replacement toroid PT had dual secondaries connected together by an earlier repairer to make a centre tap. But they were evidently not connected together start-to-end of windings. Swapped them and the hum went away; checked and saw and the 100Hz ripple had returned. I will remember this one!

Thank you Enzo, you really clarified my mind.

[Enzo]

Do I get points for that one? it's Friday, and I have to turn in my card.

[Alex R]

Yep, triple green stamps. You certainly can sniff em out. Many many thanks Enzo.