Admiral 19A12 High Voltage Issue
 

I would really appreciate some assistance on increasing the high voltage on my Admiral 19A12. I'm currently only getting ~4.2kV. I do have an image on the screen, but it's pretty unstable. I'm assuming it's due to lack of HV. Per Rider's the normal HV output should be ~ 6kV range.

I have restored a Motorola VT-73 and a Predicta Debutante with good success, but I'm certainly not an expert. I'm sure I'm missing something obvious.

Here is what I have done so far:
* All (6) HV caps replaced with 6kV SHV brand caps from Just Radios
* All electrolytics replaced
* All paper caps replaced
* Most resistors have been checked
* All tubes have been checked
* Tuner contacts have been cleaned
* I have swapped the 1B3 twice & the 6V6 HV Osc. a couple times as well to no avail
* I have moved the feedback coil numerous times on the 1B3 with some improvement
* I replaced the 1300pF cap in the HV cage with one from another set (spot on)
* All pots have been cleaned
* I do have sound
* My B+ is a bit low around 215-220 (the Pi filter checks out fine and I swapped numerous 5Y3s to get the B+ to where it is now)
* I haven't swapped out the 7JP4 since I'm getting a pretty sharp picture at times

Thanks in advance for any help at all. This is getting a bit frustrating after a lot of work, but that goes with the territory on TV restorations:-).

Thanks,

Scott

Sounds like you have done most of the things I would have done.

There's a couple of things I can think of per the low HV issue. There were two different versions of this chassis one used a 5U4 and a power transformer designed to feed it's higher power filament and a version that used a 5Y3 and power transformer with a weaker rect heater winding. Have you verified the transformer part number in your set matches the tube you are using? If you happen to have a 5U4 transformer (from a chassis swap or something in the sets past) switching to a 5U4 could improve B+ If you have a 5Y3 transformer installing a 5U4 should be detrimental to B+ and will be detrimental to transformer temperature.

If the above advice isn't helpful in raising the B+ what I would do is install a pair of 1N4007s on the bottom of the 5Y3 socket in parallel with the 5Y3 diodes...You can keep the 5Y3 in for looks and the 1N4007s with their lower plate resistance will increase B+ (hopefully to spec). Increased B+ should at the very least help some with the low HV issue.

Thanks for the advice on the diodes Tom. I actually have those in stock. So, just to be clear, I'm soldering the 1N4007s in parallel to pins 4 & 6 of the 5Y3. Everything I saw online referenced the diodes in series, so I just wanted to make sure before proceeding.

Regarding my chassis transformer, it is the correct part # for a 5Y3 - appreciate the tip though.

Thanks again!

Scott

Hi Scott, yes you understood Tom correctly as far as the parallel wiring. With the diodes in parallel with the 5Y3's diodes, you are eliminating the 5Y3's voltage drop. If they were in series, the voltage drop of the 5Y3 would still be present in the circuit. Also, if you put the diodes between pins 4 & 6 you will create a dead short across the transformer's HV secondary, what you need is one diode from pin 8 to pin 4, and another from pin 8 to pin 6, with the diode's cathode side being connected to pin #8. The one other thing that bears mention here is whether the B+ is taken off of pin 8 or pin 2, if the B+ is taken from pin 8 put the diode's cathodes there on pin 8. But, if the B+ is taken from pin 2, put the diode's cathodes on pin 2 instead of pin 8.

Thank you for the excellent clarification! After I read your explanation, it made perfect sense. Still learning - thanks for the patience:-).

With the diodes installed I'm now getting 5.4kV HV which should be plenty, but I'm a bit concerned that the B+ is now 280V (pin 8). Rider's list the B+ at 255. My line voltage on my variac is set to 117V.

The image is getting more stable, but I can't seem to eliminate the "dual" image (side by side picture). Now that I have a workable HV, I can narrow that down I'm sure.

Thanks so much!!!

Scott

Side by side picture indicates that the horizontal is running at 2X it's designed frequency.

Thanks Tom. I was more focused on the vertical circuit, but I'll redirect my efforts now on the horizontal circuit.

I appreciate the advice!

You seem to have a good grasp of things. Well done !
Doubling the hoz osc freq will cause the hoz out tube to conduct
twice as much & maybe pull down the B+. +- 5% ok but
-10% should give other symptoms like bad width.
Order I go in is power supply then H osc/ HV. At that point you get
a good idea of the condition of the CRT, yoke & FBT.
So for now get the osc on freq.
BTW one of the most common problems after recap is hoz freq off.
The set drifts over the yrs & changing cap puts the "right" values
back in. Also for HV 1KV per inch is a good general rule.

enuf fer now
73 Zeno:smoke:
LFOD !

Zeno, you're missing the fact that this is a electrostatic deflection TV. There's no flyback transformer, HV is derived from a RF oscillator separate from the deflection systems, and the horizontal can be operated off frequency without drastic changes to B+ current.... Heck I know of someone who built a composite monitor out of a junk set and was able to make it display 720P HD by adjusting H osc frequency.