I am now Annoyed [Re: Old oscilloscope help: ideas sought]
mouse at Rodents.Montreal.QC.CA
Thu Mar 1 20:43:36 CST 2007
[replying to two messages in the same thread together]
>> I was using a 40W light bulb in series with the primary, as a
>> current limiter, which is why the primary voltage was 25V instead of
>> ~120V. The bulb was glowing (not quite full power, but close).
> Well, even with a 40W bulb in series, I would expect a transformer
> with no load on the secondaries to give almost full output if there
> were not shorted turns.
Well, yes - since it would be drawing (to a first approximation) zero
current. That the bulb glows at all with all the secondaries open is
to me a strong indication that something is shorted.
>> It also would be "interesting" to get it apart far enough to rewind.
>> It's a laminated core made up largely of E-shaped pieces, [...]
> It's more normal to have 'E's and 'I's arranged alternately.
Actually, on reflection, that makes more sense - without the I pieces,
there'd be some gaps.
>> 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.)
> It's always possible (you'd better hope not!) that there's an
> internal inter-electrode short in the CRT.
Unlikely. Those two pins show about three ohms resistance. I'd expect
a failure-mode short to be either well under 1 ohm or at least several
> 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.
That's another indication that they're the heater, then.
>> 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 - [...]
> It's very uncommon to have a directly-heated CRT,
Then it probably is a heater, with that odd circuit serving some other
purpose. (There is a "Z input" on the front panel, which is a
brightness control; it is capacitively coupled to this possibly-cathode
pin, which would make some sense....)
>> 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 rectifier).
I now believe there is a -ve EHT supply and also a +ve EHT supply.
There are two HV rectifiers; if I draw the rectifier valves as
semiconductor diodes instead (to simplify the ascii-graphics), omitting
their filaments for simplicity, the circuit looks something like:
.oooo.oooo.oooo.oooo. +--> resistor chain (see below)
| | | | +--> one end of CRT heater
| GND | | 100K | 220K
| | | +--/\/\/\/---+--/\/\/\/--> CRT cathode(?)
| | | +--|<--+ | .5uF/2KV 47
+->|-+-|<-+ | | +---||--GND +---||---+--/\/\/\/--GND
| +----+ .5uF/2KV +--/\/\/\/--3.15VAC
| | .5uF/2KV 91
| ==== +-->|---/\/\/\/--+-----||----GND
| .oooo. 100K +--/\/\/\/--GND
+--+ +--+ | 5M
| +---------> CRT final anode
+--> B++ +--> B+
GND---)|--+--/\/\/\/-+-| . |---GND
(The unconnected final winding of the transformer is actually the
filament winding for the rectifier whose cathode is shown connected to
one end of that winding.)
The "resistor chain" is a series of resistors to ground:
| (internal adjustment)
| Intensity Focus "SW DC LEVEL"
| 48K 100K 88K 100K 100K 100K 9K
| ^ ^ ^
| | 15K +--> a CRT pin not traced yet
+-||--+--/\/\/\/----+------> a CRT pin
>> 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 negative.
> How is the final anode supplied?
See above: from a half-wave rectification of the EHT AC supply.
Add in the four deflection electrodes and this accounts for all but one
of the connections to the CRT. That one is driven by a circuit I do
not understand which appears to have something to do with the "Y LIN"
internal adjustment - this may make more sense once I've traced more of
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