Motorola TS-18 Vertical Height

[rwilson]

What would cause this lack of vertical height on a recapped chassis?
I have not tried changing tubes yet.
The horizontal width is at max also and I could use a bit more.
Bob

[Popester]

Is your B+ putting out the correct voltage? As far as the vertical you probably have a weak vertical drive/output tube. Picture on the jug looks fairly nice. Looks like Groucho Marx.

[rwilson]

B+ was actually a little too high after replacing the seleniums with diodes. I did have to adjust it down slightly but that did not change the vertical height. I will try a new tube.
Groucho it is.
Bob

[Electronic M]

Tubes and resistors are primary culprits. Also if this is a VT-71/73 remember that the mask cuts off a lot of the CRT face and that what you have there may be enough to fill the mask, which is all these sets really need(though they should be capable of filling the CRT face).

Sometimes the mica caps on these sets fail though I don't know if that would cause height problems.

[bandersen]

Plate voltage for the 6SL7 vertical output tube is derived from the high voltage supply via a bunch of high valued resistors. Often these have drifted off value and can effect the amplitude of the vertical sawtooth waveform going to the deflection plates.

[rwilson]

I finally got the set positioned where I can get to it while in operation. There is low plate voltage on one side of the 6SL7 vertical output tube. I changed out both 6.8 M resistors that feed the plates But its still low.

I thought I had found the problem when I discovered an error in the SAM's schematic. R59 is marked 2.2 M on the schematic but 22 M in the parts list. The Motorola manual has it as 22 M also. I had changed out this resistor thinking it was way too high.
But after I replaced it with a 22 M no change. About 500 volts of peak voltage on pin 2 and only 80 volts on pin 5 of the 6SL7.
Bob

[rwilson]

OK, I found a replaced cap with the wrong value.
Now I have enough vertical height but I can't move the centering control enough to bring it up enough to fill the screen. The horizontal control has some jumps in it also so I guess there could be a problem with the control. I hope this is the last problem.

As a side note, I find this set very susceptible to RFI. I have some LED lights that I know produce RFI. They really do a number on this set. And that's when it has a hardwire antenna input. But there is no shield on the bottom.
Bob

[Yamamaya42]

Originally Posted by rwilson

As a side note, I find this set very susceptible to RFI. I have some LED lights that I know produce RFI. They really do a number on this set. And that's when it has a hardwire antenna input. But there is no shield on the bottom.
Bob

I know the feeling!

[rwilson]

I see now that the vertical centering pot is not the right value. Its 500K and should be 2 meg. Too get the raster to move up maybe an inch I should be able to add a resistor somewhere in the circuit. But I have not figured out exactly where.
Bob