Admiral C-21A1-1A Chassis with low boost voltage

[StellarTV]

Hello! I'm working with a 1964 Admiral portable that had low/no boost voltage and am at a loss. Here's the current situation:

-Capacitors have been replaced
-The yoke and flyback both ring out OK
-DC Voltages are correct at the horizontal output tube, but the input waveform on pins 9-10 show only about 67v peak to peak instead of 100v
-Plate voltage on the output side of tube V7 (Horizontal oscillator) is only ~83v instead of 120v, and this is tied directly to boost voltage (are these then additive voltages?)
-Cathode voltages at pins 8 and 3 of the same tube are about 3.4v instead of 5v
-B+ voltages are correct at 140v and 145v where required. Actual boost voltage is low at ~200v.
-Boost filter capacitors C62 and C63 have been re-replaced to eliminate the possibility of faulty new components to no avail.
-Horizontal Output and Oscillator tubes have also been replaced with no change.

Does anyone have any suggestions as far as what else to check?

Schematic attached

[Electronic M]

Is the frequency and waveshape of the oscillator to spec? If either is far enough off spec it the fly could be operating off of its resonant frequency...When you operate a resonant transformer off frequency its output voltages will drop.

I may be wrong but, I believe the 140V source of pin6 is the starting voltage that gets the osc to drive the flyback just enough that you get enough boost to add drag the plate supply higher which creates a positive feedback effect until the circuit reaches it's designed operating point...
If you have a spare 450V power supply it could be interesting unhooking R69 from boost and forcing normal running voltage into the osc.

Also please tell me you did not hack up a Halolite to stuff a BPC color set in for your avatar.

[damen]

As a test, even though it rings okay, sub a different yoke. Any yoke will do, just use the top horizontal connection at terminal number 2, and the bottom connection going to the .033 cap ( which I assume has been tried).

[old_coot88]

Have you verified R68 and R69 for correct values (47K and 680K respectively)? They should be checked with one end disconnected.

[StellarTV]

Originally Posted by Electronic M

Is the frequency and waveshape of the oscillator to spec? If either is far enough off spec it the fly could be operating off of its resonant frequency...When you operate a resonant transformer off frequency its output voltages will drop.

The waveform looks good, and I think you're right in such a circumstance that may very well be the case. I'll need to go back and check this.

I may be wrong but, I believe the 140V source of pin6 is the starting voltage that gets the osc to drive the flyback just enough that you get enough boost to add drag the plate supply higher which creates a positive feedback effect until the circuit reaches it's designed operating point...
If you have a spare 450V power supply it could be interesting unhooking R69 from boost and forcing normal running voltage into the osc.

This makes sense, though I don't have a B+ supply handy to test this.

Also please tell me you did not hack up a Halolite to stuff a BPC color set in for your avatar.

I think it looks better than a bookshelf or an aquarium.

Originally Posted by damen

As a test, even though it rings okay, sub a different yoke. Any yoke will do, just use the top horizontal connection at terminal number 2, and the bottom connection going to the .033 cap ( which I assume has been tried).

Great tip. I pulled out an old Predicta yoke and subbed it in with absolutely no change... so I'm glad it's not the yoke, at least!

Originally Posted by old_coot88

Have you verified R68 and R69 for correct values (47K and 680K respectively)? They should be checked with one end disconnected.

I did, and these resistors are well within tolerance.

[old_tv_nut]

How about caps C62, C64, C50 (I think I'm reading them right - 0.1, 1kv; 82 pF, 5kv; 0.033, 1kv)

[Tube TV]

How is the other sections running, audio, RF, IF, vertical.
When I read your first post and looked at the schematic I also wanted to say R68 R69, so we can rule those out.
How did the 2.2 meg R64 check out And the 1 meg R71?
Is there any possiblity that one of the slugs for the horizontal width or other horizontal adjustment got threaded in or out too far?
How close is the voltage on pin 1 of the 8FQ7 to 100 volts, also the voltage at tp-4?
I would imagine tp-4 would sit at about 115 volts. I would check there to make sure the horizontal lock coil has not went open .
It seems like a horizontal oscillator problem to me more than output.
I would look at any high value resistors, and if you have a scope check for horizontal oscillation.