Hi all,
does anyone have any very old CRT cross-reference around? I probably need to
replace an Amperex M24-302W 9" CRT on a vintage CBM PET 2001 and probably
these CRTs are unobtanium now. Maybe some old monochrome TV CRT could be
substituted?
The tube has probably a cathode to heather short. Even with the cathode lead
grounded, the screen is all white with brighter retrace lines. G1 sits at -32V
so, it's not a matter of wrong bias or something similar.
Cathode signal generated from the monitor CRT is positive and about 0V should
cutoff the beam, according to the schematics.
Might be a failure on the HV generator that gives too high voltage, I'll
measure it later (shoud be 10 KV), but I think if it was that, it would kill
the flyback transformer quickly, isn't it?
Frank

On Monday, 5 February 2018 12:00:54 UTC, frank wrote:

Hi all,
does anyone have any very old CRT cross-reference around? I probably need to
replace an Amperex M24-302W 9" CRT on a vintage CBM PET 2001 and probably
these CRTs are unobtanium now. Maybe some old monochrome TV CRT could be
substituted?
The tube has probably a cathode to heather short. Even with the cathode lead
grounded, the screen is all white with brighter retrace lines. G1 sits at -32V
so, it's not a matter of wrong bias or something similar.
Cathode signal generated from the monitor CRT is positive and about 0V should
cutoff the beam, according to the schematics.
Might be a failure on the HV generator that gives too high voltage, I'll
measure it later (shoud be 10 KV), but I think if it was that, it would kill
the flyback transformer quickly, isn't it?
Frank

An h-k short is easily sorted with a transformer. EHT failure wouldn't get you a white raster. Cathode v normally needs to be raised to cut a tube off, not lowered.


NT

wrote:
On Monday, 5 February 2018 12:00:54 UTC, frank wrote:

An h-k short is easily sorted with a transformer. EHT failure wouldn't get
you a white raster. Cathode v normally needs to be raised to cut a tube off,
not lowered.

Yeah, I feel so dumb. I've just figured it out myself hooking a variable power
supply to the cathode. It cuts off at about +20V. So the CRT (thanks heavens)
looks good. It must be some stupid fault on the video path. It has three
stages only, two inverting amplifiers (single bjt each) and one emitter
follower. First stage works, so I'll replace the last two transistors and
check again.
It's been a few years since I was "thinking" about cathode and grids.

Frank

Can you get a wire in between the flyback coil and core ? If so you just make your own winding that is totally floating. Start with one turn and work your way up until the filament is bright enough.

It is better if you have measuring equipment. A true RMS meter can read the waveform at 6.3 volts RMS but not everyone has that. A scope should show about 20 volts peak to peak, that corresponds to about 6.3 volts RMS with that waveform which is usually a half sine resonated at 5 times the horizontal sweep frequency. Iff the monitor has fast retrace it might have to be a bit higher. In any case, use as little as possible to get a decent display. The less the better as it will result in the longest cathode life. The cathode has already been abused putting out more current than was ever intended..

They used to make a small cathode transformer for insulation but finding one today rather than a piece of wire is not easy.

If you can't get in between that core and coil on the flyback get back to us and I will see if I can think of another solution.

"It must be some stupid fault on the video path."

Disconnect the cathode from the video output to make sure it is a shorted CRT. If it is not all white then it is in the video. If it goes black it is the CRT.

wrote:
Can you get a wire in between the flyback coil and core ? If so you just
make your own winding that is totally floating. Start with one turn and work
your way up until the filament is bright enough.

never mind, it wasn't a short circuit in the CRT. It was me supposing
the cathode should cutoff at 0V, instead it was supposed to cutoff at
something like 20-30V.
One amplifier stage was bad, a 2N2369 tested good on a diode probe (both
junctions!!) and it was not amplifying at all, just passing the signal
through the base-collector junction, so not inverting it.
The end signal at the catode was "upside down".
Replacing the 2N2369 cured the monitor (well, geometry isn't the best, but
I won't mess with it).


It is better if you have measuring equipment. A true RMS meter can read the
waveform at 6.3 volts RMS but not everyone has that. A scope should show
about 20 volts peak to peak, that corresponds to about 6.3 volts RMS with
that waveform which is usually a half sine resonated at 5 times the
horizontal sweep frequency. Iff the monitor has fast retrace it might have
to be a bit higher. In any case, use as little as possible to get a decent
display. The less the better as it will result in the longest cathode life.
The cathode has already been abused putting out more current than was ever
intended.

I have scopes and other test instruments (like a few true RMS meters) and
I'm glad I just needed the scope to find the bad stage, once I realized I
needed a positive voltage to cutoff the cathode.
The first common-emitter stage didn't invert the signal, so it was easy
to spot.

This monitor is 41 years old by the way

Thanks
Frank

"One amplifier stage was bad, a 2N2369 tested good on a diode probe (both junctions!!) and it was not amplifying at all"

i have had them go bad like that a few times but they were not passing any signal. Yours probably had a leakage path.