Quote:
Originally Posted by benman94
Since nobody else will chime in I guess I will...
There are two possible workarounds for the 50 Meg pot that would work, though neither is particularly elegant:
First, you could always increase the values of the either the 100 Meg and 80 Meg fixed resistors, or both, and fit a new lower resistance pot to the set. Say add a 15 Meg resistor above and below the pot and buy a 20 Meg unit. You'd have to play with which side gets more fixed resistance than the other, and it would give a much smaller range to the control, but it would work.
The other, slightly better option, would be to grab a 6 to 12 position switch and build a sort of "stepped potentiometer" out of fixed resistors. A lot of audiophiles build these stepped attenuators and have great success with them in audio applications. Of course now you're limited to discrete, fixed steps, but given the fact that a 15GP22 based set rarely has anything approximating perfect convergence, I think that the stepped solution may well work. It would certainly be more reliable than any pot, and these are particularly failure prone parts.
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Yes Ben, You have heard the old saying "brilliant minds think alike" so I had already thought of the plan to shift the range of a 15 meg High Voltage pot by using a 2 pole 3 position ceramic rotary switch. I would use 2, 15meg resistors on each side of the 15 meg pot in an arrangement whereby a rotary switch would move the the leads of the resistor element in the pot up and down to that I could have 30 megs above or 30 megs below the pot, or 15 megs at either side of the pot.
My only misgiving is that there is about 4250v across the original 50 meg pot so that puts about 1500v across each 15 meg element in the chain running through the ceramic rotary switch. Even isolating the switch frame and shaft, I am still concerned with the close proximity of the contacts within the switch and I am concerned with the possibility of arc-over from the high voltages involved.
EDIT....Although there is about 4250v across the 50 meg pot, there is a range of from 12.8KV down to 8.54KV at the wiper of the pot with reference to the 20KV anode voltage on the crt because the DC convergence voltage component is derived from the 20KV ultor voltage on the crt.
I guess the only thing to do is to buy the parts, and breadboard the whole thing outside of the chassis, connect it up and see if the components withstand the HV.
It may be a while before I get around to trying this but I will post the results when I have a chance to put it together.
On another front. I have located a company Metalux in Germany that makes HV resistors. They make a HV pot (POC-400) in 5m, 10m, and 15m (the 15m is what I will use in the scenario above.) The reason I mention this is that this item could be substituted in a CT100 for the focus (5m) and the convergence (15M). The item is encased in a small plastic housing about 2" square designed for mounting on a PC board. I think these may have been used in the high voltage sections of tv sets. The internal resistance element is placed on a ceramic substrate
The link to the data sheet is below
https://www.metallux-usa.com/uploads...-POC_400-s.pdf