On 3 May 2021, at 20:02, Andy Warner <andyw@...> wrote:
I bought a mostly working 577/D2 recently.
What I regard as the most serious problem with it is the trace intensity.
What I see is that the maximum intensity occurs at approx 60% of the front
panel adjustment (R1200) - moving it either direction from the peak results
in a dimmer trace.
The bulk of the trace (when the beam is moving fast) is not particularly
bright, but the start and end of the traces is good.
I have cleaned and checked R1200 operation, and I believe the pot is good.
Also, the dimming signal from the collector supply board (R592) seems to be
correct (e.g. near zero V when the collector voltage is non-zero.) However,
I don't really see an obvious dimming effect on the trace when I turn the
collector supply down to zero.
I have not yet been able to measure the HT voltage (looking for a probe to
help me do that - advice welcome.)
In the interim, I wanted to trace the operation of the Z amplifier circuit
(Q1222,1226,1234), but the service manual I have (thank you Artek) is
silent on removing the deflection amp/high voltage board.
It honestly looks like a royal pain to remove for service - does anyone
here have any hints and tips ?
The schematic has a couple of helpful waveforms shown, but I have zero idea
how you are expected to probe up the board, given the location of the
side-rail that obscures the lower half of the board.
Any 577 owners out there got advice to share ?
My first 577 (a D1) was very dim ¡ª and it came with a spare CRT ¡ª but essentially worked for a while after I got it home. Then it suddenly went completely dark. Rapid onset blackness seemed unlikely to be CRT wear so I checked the Z-axis and sure enough, two transistors (I don¡¯t recall which) in that circuit had failed.
But afterward it was as dim as before, and with similar behaviour to what you describe ¡ª brightest in roughly centre travel, dimmer either higher or lower.
I replaced the CRT with the spare it came with and that fixed it. Hopefully not the case with yours.
My second 577 (another D1) was also a tad dim ¡ª but not as bad as my first ¡ª and also reaching a peak somewhat past centre brightness. Notably, it has improved with use and whilst not the brightest CRT I¡¯ve ever seen, more than usable.
With both units I went through the calibration procedure, including checking/setting HT voltage. I checked it with an older VOM that goes to 5000 volts, taking all reasonable precautions to hook it up carefully with good well-insulated leads and clips and make sure I¡¯m not anywhere near, let alone touching, any possible conduction path whilst it¡¯s powered.
The dimming effect of turning down the collector voltage should happen only in the last degree or so of rotation, essentially when it hits the zero position.
