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

[Tony Duell]

> 
> [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

Not so. There will be an out-of-phase current due to the inductance of 
the primary winding. And that current, if sufficiently large, would cause 
the lamp to glpw

_But_ my experience suggests that for small-ish power transformers, the 
sort of thing we're talking about here on no-load, a normal mains lamp in 
series with the primary will not glow. You could trie a higher-wattage 
bulb, thoguh.

IIRC, though, you've got a leak (or short) to earth from some point in 
the HV widning, and another point on that winding (the centre-tap of the 
B+ part) is deliberately earthed. That can't be right. And I'll bet that 
short-to-reath is not just on one turn, in other words you also have 
shorted turns there.


> >> 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

SOunds like the heater...

> 
> 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....)

Sure. The Z-modulation input is an external brightness control input, 
used for some special applications. In general it's capacitively-coupled 
to either the cathode or control grid of the CRT.

> 
> >> 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
>         80uF/475V             0B2
> 
> (The unconnected final winding of the transformer is actually the
> filament winding for the rectifier whose cathode is shown connected to
                                           ^^^^^^^^
Don't you mean anode (plate to you) here?

> one end of that winding.)

> 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
> the circuit.

It may well be an internal shield between the defleciton plates. Altering 
the voltage on it wil lcontrol the shape of the beam-deflction .vs. 
voltage between the defleciton plates characteriostic, thuse it will 
LINearise the Y defleciton.

-tony