I am now Annoyed [Re: Old oscilloscope help: ideas sought]

der Mouse mouse at Rodents.Montreal.QC.CA
Wed Feb 28 01:35:16 CST 2007

Some of you may reacll that, back in July, I wrote about an
oscilloscope whose power transformer failed in service.  Well, today I
was going through my tuit collection and found some round ones.  I've
now clipped the transformer free of the circuit and removed it.

I then applied an ohmmeter to the transformer, now that it's free of
the machine, which gave me good guesses at what winding is what.  Then
I got out the current-limiting light bulb rig and hooked the primary up
(well, half the primary; it has a split primary, for 115/230 mains
switching).  Then I measured the primary voltage and the voltages on
various interesting windings.  Based on all of this, I think I have a
good guess what's what:

- Input mains: four wires, two windings, which are connected in
  parallel for 115V operation and series for 230V operation.
- 5VAC filament winding for B+ rectifier tube.
- ~800VAC CT (400-0-400) winding which is rectified for B+.  (With 26V
  on the 115V-nominal mains primary, this measured 176V, or ~778V when
  running normally.)
- Filament winding for one HV rectifier tube.
- Filament winding for the other HV rectifier tube.
- HV supply winding.
- 6.3VAC CT winding to run the heaters for most tubes.  (There are some
  12V-heater tubes, but they all have centre-tapped heaters.)
- There is one more wire unaccounted for.  On opening the transformer
  case, I find it is a case ground.

The HV supply winding appears to be fried.  I didn't bother with all
the filament windings (though I did measure the heater winding, which
appeared to be intact); the B+ supply winding seemed intact, but the HV
supply winding showed ~2V instead of the 200-300V I would expect with
~25V on the primary.

So now I'm thinking of finding a half-dozen different transformers to
run the six different pieces off of....

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