Re: 7603 power supply woes

Robert Simpson

Note that the -50V is also Zener diode controlled like the 130V and is used in other LV supplies.


--- In, "rekrullunreenigne" <99bluefoxx@...> wrote:

Hello all.
I'm new here, so forgive me if I tread upon some ancient tradition by mistake.

I managed to pick up a sweet bundle on a pair of 7000 series mainframes *with* plugins, the catch of which was one was broken.
I was told it would be the big 7904 - but it turned out to by the smaller 7603.

I checked both over before hand, there were no obviously damaged parts, they're in reasonable shape, nice and clean inside.
The 7904 happily flickered on just fine, and I was pleased to find all seven plugin modules worked reasonably well.

The 7603 b358xxx powered up for about five minutes, before quietly blinking off. No dramatics about it, not even a whiff of something wrong.

My slightly naive poking around shows that all the low voltage rails are about 15% out of spec, ie 130V reads 113V, 15V is 12V, and so on.
So I looked in the service manual I was fortunate to receive with it, and decided to poke at the 130V rail first, since everything else seems to rely on it in some degree.

Past experience, and reading around both suggested capacitor problems were likely. I decided I'd use my working 7904 to see what was up with the 130V rail, and found a choppy AC waveform, instead of nice flat DC like the other rails. Pretty sure that's not right.

So I replaced the capacitor. Hold my breath, quick test - but the 7603 not only fails to power on, as the voltage sag is still present, but the AC waveform was still visible at the capacitor.

Finding this weird and confusing, I decided to try changing the rectifier - would you believe I had an exact replacement by happy coincidence? But that's done nothing either. I tried another with similar ratings as well, also to no avail.

This is where I've become stuck. I'm not entirely certain what to do next, for fear of making the problem any worse. I was thinking I might try unplugging various other control boards, to try and see if something in particular is dragging it all down, but I'm a touch inexperienced at this, and being largely self-guided, am prone to err.

If you've read this far, you're a champ :) Hopefully my wall of text isn't too scary.

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