[dtvmcdonald]

I found that it also had vertical rolling on scene changes. It had very odd "tuning" of the contrast-brightness,
like no other set I have used. More than one setting gave a passable picture.
I decided to look at all the scope waveforms that they showed in their repair booklet.
It became quickly clear that only one group of settings for the brightness and contrast (RF gain) were correct,
and that the brightness of the picturwe was less than what
I had obtained before. Another symptom was that with bright picture sometimes but not always it had defocus
due to brightness dependant currect draw from the focus electrode, up to 300 uA. This was far far
over the few microamps spec.

The 5BP1 spec is for no more than 2 volts positive on the CRT grid. Keeping to that also prevented signficant drooping
of the CRT gird voltage (also, DC coupled, the video amp plate.) In any case I fixed the focus by regulation the top of
the focus put with 840 volts of Zerner diodes in series.

Setting the CRT grid drive as specced resulted in inadequate horizontal sync
drive (.2 versus .4 volt spec.) This was traced to the value of the horizontal oscillator grid that the sync
drives ... it was being reduced by the (as specced) 10K resistor.
The previous owner clearly had seen this and tried reducing some of the integrating network caps.
This was a silly situation: these networks typically have two low pass filters acting as "integrators". This set has those ...
but the specced connection from that the the oscillator grid was a high pass network that cancelled on of the two.

It looked like the grid resistor should be bigger, perhaps 100K. I tried the first thing that I saw in my pile,
82K, and so long as the above mentioned grid current dropped the size it became rock stable and had huge horizontal hold range.

So I tried a prevention for too much grid current: I put a string of 5 silicon diodes
in series from the cathode to grid of the CRT, cathode to cathode. These two changes resulted in more clipping of
the sync pulses, removing essentially all the "picture" level excursions. Both vertical and horizontal became rock
solid over a large range of adjustment. The picture still gets quite bright, just not as muach as before. The
apparent gamma was much more pleasant looking.

The method necessay to set the contrast and brightness is like no other set I have seen. One has to first set the
contrast to zero and the brightness to a medium gray screen, then turn up the contrast on a contrasty scene
until both white and black appear. Then its just small tweeks.

This resulted in four changes to the circuit:
1) regulating the top of the focus pot.
2) increasing the brightness control tap bypass cap from 0.1 uF to 1 uF
3) changing the horizontal sync grid resistor from 10K to 82K.
4) adding diodes between CRT cathode and grid

All that is probably left is to change the RF gain (contrast) pot from 50K to 5K ... 50K is far too much
range, and adding a resistor in series to prevent overdrive.