SE Labs Scope Repair

[pwdrive]

The high 60 volt rail and the high ripple might suggest the regulator is s/c or approaching flat out?

Lawrence.

[Nick]

There's a whole bunch of possibilities here.

For the 60V rail, check the feedback resistors 2R36 & 2R37 - they need to be spot on - C11 & C14 should almost certainly be replaced - particularly C11 as if that is failing, high ripple would be a symptom.

Its also possible that ripple is being injected from the 120V rail which is basically a second (unregulated) capacitively isolated 60VDC supply added to the base 60VDC line - check 2C10, 2C13, 2C15 & 2C67 along with its feedback resistor, 2R31 - any shorts or problems with 2C10 or 2C67 and the 60V rail would be in a world of pain... Is the 120VDC line clean? It will have all the rubbish from the 60V rail superimposed anyway, so is it noticeably worse or much the same?

Need to check each DC level against the schematic - if you could write on the schematic the measured voltages in red or similar and post that, it would help a lot...

[Geaney]

With all due respect young John 3 Volts P-P on a 60 Volt Supply is bloody awful. Something's wrong.
To save me going back through the Circuits is it meant to be Stabilised?

Alan

Love it :-)
We all know what 'with all due respect' is the euphemism for, so message received. I will get on to sorting this out. I think I've spent too much time with series string valves, half wave rectifiers, etc.
Cheers, John

[Geaney]

There's a whole bunch of possibilities here.

Thanks Nick. I shall do a detailed checkup of the whole thing next time I'm at the bench and will report back results.

Cheers, John

[Geaney]

I just had a quick look, the ripple is the same value on the 120v rail.

BTW, caps C9 and C11 have been replaced when I started this repair - in the dim distant past.

Cheers, John

[Geaney]

Been working through the power supply. Removed the main power transistor TR18. Measures like a 160 Ohm resistor collector to emitter - both ways!

Replacements on order.

Cheers, John

[AlanBeckett]

Splendid.

Alan

[Nick]

You need to ask yourself why it failed in the first place...

[Refugee]

There was quite a serious power supply failure near the start of the thread.

[Geaney]

OK, the ripple issue now looks to be sorted. I've checked the three rails (120v, 60v, 30v) and all now look fine. One of the bridge rectifiers had failed rather dramatically at some point in the past and I can only assume that once the chap had replaced the diodes it appeared that all was well. The main regulator TR18 is a TIP33C but my local parts place only had 35C in stock so I went with that since the voltage characteristics are the same but the 35C can handle about double the current of a 33C.

The replaced diodes can be seen top right in the circuit board (top view) and the damaged tracks can be seen lower right on the circuit board (underneath view). I also found one of the connecting wireless leading off the board to be held on with about one strand of wire so that was re-done as well.

The 60v rail is now spot on 60V so I'm pleased with the progress on this section of the scope.

Now back to the sync problem....

Cheers, John