I am now Annoyed [Re: Old oscilloscope help: ideas sought]
ard at p850ug1.demon.co.uk
Thu Mar 1 14:46:06 CST 2007
> I've just spent a couple of hours tracing out the power-supply
> circuitry in this 'scope, particularly the HV supply driving many of
> the CRT terminals.
> Turns out the winding I thought was the HV winding, only shorted out,
> is actually the CRT filament/heater winding. (I'm quite sure I've got
> the CRT filament pins correct; they are the only two CRT pins that show
> DC continuity.) I'm not sure whether it's a heater or filament;
It's always possible (you'd better hope not!) that there's an internal
inter-electrode short in the CRT. But why you;d have those 2 electrodes
connected directyly to a transformer winding is beyond me.
Often (but not always), the 2 heater pins are the ones either side of the
locating key on the CRT base in a 'scope CRT.
> there's a pin that could be the cathode (connected to one side of the
> heater/filament, the most negative CRT pin found so far), but there is
> an odd circuit which makes sense to me only as an attempt to compensate
> for using one side of a filament as a cathode - there's a voltage
It's very uncommon to have a directly-heated CRT, almost always there's a
seprate heater and cathode. But the insulation between the heater and
cathode won't stand much voltage, so it's normal to connect the heater to
the cathode (or at least to some point in the external circuit at about
the same voltage as tha cathode) to prevent breakdown.
> divider across one half of the 6.3V heater winding, with a HV cap
> coupling it to the "cathode" side of the CRT filament/heater,
> presumably to avoid the slight mains-frequency flicker that would
> otherwise result.
> The HV winding appears intact, and runs between one side of the B+
> winding and the filament of one of the HV rectifiers;
That suggests to me a -ve EHT supply (output taken from the anode of the
> A B C D E
> A-B-C is the centre-tapped B+ winding I found before. B is grounded
> (the A and C points go to the plates of the B+ rectifier) and, based on
> scaling up from the voltages I see, the A-B (and B-C) voltage is about
> 375VAC. D-E is the filament winding for one of the HV rectifiers. C-D
> is a winding I was not previously aware existed; its design voltage is
> about 500VAC (under the same assumptions about scaling).
> The transformer's ground wire has DC connection to this winding, about
> 150 ohms to either D or E and something like twice that to C. Since
> both this ground wire and B were grounded to the chassis when the
> transformer was in-circuit, I conjecture that this is part of the
> fault, and the ground wire is supposed to be isolated from all the
THat would make siese. This sounds like an insulation breakdown in the
trasnformer, probable somwhere in the HV (C-D) section.
> Interestingly, in view of the comments about how instead of a high
> positive final anode voltage CRTs tend to run with a high negative
> cathode voltage, it looks as though this does both: the final anode is
> about as positive with respect to chassis ground as the filament is
How is the final anode supplied?
It's normal to run a en electrostaticly-deflected CRT with the deflection
plates close to ground potential (a few hundred volts is OK, so you can
run them from the anodes of a valve amplifier with the cathodes of said
valves close to ground. That means the cathode and control grid need to
be at a high -ve voltage, and if there's a 'post deflection
acceleration' electrode, it will be at a high +ve voltage (often higher
than the voltage on the cathode, if you see what I mean).
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