Motorola VT-71 question
 

Just got my fourth VT-71 chassis going this morning and have come across
something that doesn't match the schematics of the other 3 that I
have. There is a 560 ohm, 2 watt resistor in series with the B+ supply
going to the HV oscillator (25L6). All my other chassis and schematics
say it should be a 100 ohm, 2 watt.

Funny thing is the 25L6 gets hot and the HV cuts out after several
minutes with a 125 ohm resistor in place (original 560 ohm was way
out of tolerance). I'll try it with a good 560 ohm in place but would
like to see if anyone else has a 560 ohm instead of 100 ohm in this
spot. The HV xfmr appears to be original. TIA
Steve
ps-there are MANY variations of the VT71 & VT73 schematic...I
think I have them all, but maybe not

I have schematics for four versions, TS-4B, 4D, 4H, and 4J. They all have 100ohm in this spot, except for the 4B which shows an RF choke instead of the resistor. 560 ohm here does seem out of place.

Looked thru all of mine too. R-119 is shown in all versions I have the schmatics for as 100 ohm 2 watt. Have you replaced R-76 on the control grid of the 25L6? It is the ground return for the HV supply oscillator and if drifted too high in value, the tube could possibly run away. Also, a gassy 25L6 may draw more current as it warms up and overheat.
Keep us informed!

The plot thickens.....
Put in a good 560 ohm resistor (in B+ line) and the HV supply works fine.
For about 4 minutes, then the 25L6 current starts drifting up until the
osc shuts down. Get the same symptoms with a 125 ohm resistor
in B+ line. The grid voltage is running low (approx -60v instead
of the proper range of -100 to -150v. Tried re-positioning the
1B3 feedback spring, and a different 1B3. No improvement. The
150K grid resistor reads 170K, so didn't think it was off enough to
cause this, but will replace next. Also have replaced all caps in
the circuit with new (and checked) units. The 25L6 tests good but
have tried another one with same results.
Steve

High voltage is fed to the vertical centering control and the horizontal centering control. It is filtered by C-65. I would disconnect the high voltage feeds to the centering controls and remove the CRT socket from the tube and monitor HV. If it stays constant someting may be loading the HV down.

Thanks for the suggestions but after several more hours, still stumped.
All components have been replaced (at least once) except for the
HV transformer. Measured the current on the 6KV line and it started
out at 350 uA ( which is what is spec'd in the book) and dropped
over time as the osc slowly failed. Have also checked for leakage in
the octal 25L6 socket and cleaned all HV contacts and pots carefully.
The 1300 pF cap in the primary of the xfmr also replaced (determines
freq of oscillation).

Its gotta be leakage in the HV xfmr. Nothing else has affected the
symptoms even a little bit. Major bummer as this chassis has been
completely re-capped and rebuilt (ie-more hours than I like to admit).

Anyone have a spare HV xfmr? Have a buch of NOS flybacks to swap.
Steve

I have a spare chassis I can bust up, what flybacks do you have to trade? I mostly do early color stuff.

For a list of flybacks, see my ad in the classified section. Bottom of
page 3 posted about 5/24/12. A few have been sold.
Steve

I don't know which one I would need, can anyone tell me if any of them are for early color sets?