Zenith Hybrid Low HV

[Jon1967us]

Rescued this set a couple weeks ago. It fires up and has a good picture, out of the chute, but the width is deficient due to low HV. Hor and vert are solid, with no sync issues.

HV cannot be adjusted above approx 18kv. At this maximum, I have the HO cathode current at 230mA (235 is value on schematic)

Checking around the horizontal section through the boost. I found the following:

Boost voltage read off C131 is 750v (should be 850)
C131 is a white Elmenco, but tests accurately and very little leakage seen on my Knight capacitor meter. B+ going into C131 is where it should be, at 250v.
Boost off of C132 is also about 100v short of the 820 on the Schematic.

Voltages are more or less normal along H-osc and discharge. However, voltages are a bit off on V3: pins 11 (190v) and 5 (-78). Waveforms appear to look ok on V2a, V2b.


V3, pin 5

Finally, have subbed V2, V3 and V4, with no improvement.

[Electronic M]

High screen voltage usually points to excessive load on the flyback/ HV side of the tube, and low screen voltage usually points to low secondary load.

If you have a B&K 1077 analyst it might shed some light using it's plate drive to power the flyback. And check boost with the tripler unhooked then later with the yoke unhooked.

It could be bad tripler or possibly bad yoke... I recently had bad yoke on a B year hybrid...HV would be okay then slump to nothing over the course of 15 minutes.

[Jon1967us]

Thx You seem to confirm my suspicions about the tripler, and Fly at least. I do smell an odor when it runs thats a little more intense than just a dusty old set, perhaps indicating a toasty flyback? I’ll have to give it the closer sniff test. It looks like the tripler has been replaced already w/ a GE part 212-139-01. I wasn’t able to find another one of the same part #, but I’ll keep my eyes peeled. Certainly not as easy to obtain as ones found in the Chromacolors.

When you say “points to a low secondary load” do you mean on the flyback secondary?

I don’t have a 1077b, but I do have a Sencore VA48 that has an Hdrive, a position labeled horiz out, grid drive. Never used that feature before, and not sure how to as I don’t have a manual.

I would say the likelyhood of a bad tripler is about 10x than that of a yoke, wouldn’t you? ��

[zeno]

1) I only had one FBT bad in a hybrid & it was a massive kit.
2) never had a bad yoke I remember.
3) WARNING ! seems no two of these had the same hoz/HV so
dont get tripped up by that.
4) 212-139-01 is Zenith OEM P##
5) only HV regulation probs I remember were bad spots on the HV pot.
So often we kept a strip of them in stock. VDR's went bad in older chassii.
6) If the HO is getting good drive suspect a weak HO tube. Trouble is
they are BIG bucks. 6LF6 & 6LX6 will sub, on series hybrids
20LF6 & 26LX6 will sub.
7)Tripler can do it but they usually cracked open & burned or looked swollen / discolored.

enuf fer now
73 Zeno
LFOD !

[Jon1967us]

Originally Posted by zeno

1) I only had one FBT bad in a hybrid & it was a massive kit.
2) never had a bad yoke I remember.
3) WARNING ! seems no two of these had the same hoz/HV so
dont get tripped up by that.
4) 212-139-01 is Zenith OEM P##
5) only HV regulation probs I remember were bad spots on the HV pot.
So often we kept a strip of them in stock. VDR's went bad in older chassii.
6) If the HO is getting good drive suspect a weak HO tube. Trouble is
they are BIG bucks. 6LF6 & 6LX6 will sub, on series hybrids
20LF6 & 26LX6 will sub.
7)Tripler can do it but they usually cracked open & burned or looked swollen / discolored.

enuf fer now
73 Zeno
LFOD !

Here is what appears to be the replacement GE tripler.



And here's where my attention is currently focused, on what the Sams calls out as one of the VDRs, but I've never seen them in this form factor before, so I’m not 100% positive that’s what they are. They measure about 1.6M. The closest I’ve seen to this cylindrical package were earlier ones that were kind of blue, electrolytic cap looking.



I do have modern KOA SPEER varistor replacements, which have worked for me in a previous project which I may try here. It could be that one or more of these has built up resistance over the years and isn't doing its job.

Tripler could be suspect, but does not display any of the visual symptoms you mention.

As I mentioned the HO has been subbed with a good testing 6LX6, with no effect.

[Jon1967us]

https://youtu.be/lOk3yCWg-hwA video

[zeno]

That looks like a bad HV reg pot. Width would come in a little.
Give it a little bump & it would snap in. Try cleaning the crap out of it & adjusting it. BTW on schematic it shows 2 versions of HV reg
circuit ( dotted lines ) see what I mean ? No 2 alike !
Tripler is made by GE & marked with Zenith part ##. Date code
is 72-48. = 48 th week of 1972. Set was born with it 99%.

73 Zeno
LFOD !

[Jon1967us]

Originally Posted by zeno

That looks like a bad HV reg pot. Width would come in a little.
Give it a little bump & it would snap in. Try cleaning the crap out of it & adjusting it. BTW on schematic it shows 2 versions of HV reg
circuit ( dotted lines ) see what I mean ? No 2 alike !
Tripler is made by GE & marked with Zenith part ##. Date code
is 72-48. = 48 th week of 1972. Set was born with it 99%.

73 Zeno
LFOD !

I took your suggestion and cleaned that HV pot, and the headroom for HV has improved, but any adjustment above 18KV produces excessive cathode current on the HO. If the HV is adjusted to around 20kv, the K current skyrockets to around 330mA, which is probably no bueno for the flyback and does produce some red-plating. The current seems poorly regulated and jumps around a little too.

For this reason, because of poor regulation I'm suspecting bad VDRs. I have new KOA SPEERS MOV's I can swap in there, but have no idea if they would work, and I have absolutely no info on the VDRs that are in there, other than a black /orange color code.

One thing I just noticed is that the Hor freq control is cranked all the way to one end of the range to maintain hold. When I start the set up the H hold is missing for a few seconds. This lack of range may be indicator of a cause of insufficient drive as well

[Jon1967us]

Update: I swapped out the 2 VDRs with MOVs, one at a time, and then both at same time, and there was no difference in terms of current regulation. At 18KV, current drifts about 20-30 mA as the set runs. Any setting of HV above 18KV creates excessive current.

I read in another thread that "Check the screen voltage on the HOT, if its high its an overload(bad flyback, shorted rectifier, cap, damper)." The screen voltage was high on the HO (measure 190-200v and should be 172)

I'm now wondering if the issue may be in the focus divider, as the tripler appears to be OK.

[zeno]

Try a 6U10 if you havnt yet. There is a sleeve on the H hold
pull off & recenter into range.
Focus div just a pair of very high meg resistors, new p## is 800-616.
Dont see that doing it. Burns or bad focus.
Filter cans went but a PIA to get to. Need one of those 2 foot xcelite
drivers to pull chassis.Only good test for FBT is totally unload
it & ring it. If it has shorted turns usually you can also see distorted waveforms.
Other poss is one of the high ohm resistors has drifted up. Gotta
unhook one end to be sure, time consuming....


73 Zeno

[Jon1967us]

OK, good information. I have subbed the 6U10 prior, but no change. I may go ahead and pick up one of the dividers from Talon.

BTW, check out the waveform now, even after I "undid" my mods



PS I have the extra long driver!

Originally Posted by zeno

Try a 6U10 if you havnt yet. There is a sleeve on the H hold
pull off & recenter into range.
Focus div just a pair of very high meg resistors, new p## is 800-616.
Dont see that doing it. Burns or bad focus.
Filter cans went but a PIA to get to. Need one of those 2 foot xcelite
drivers to pull chassis.Only good test for FBT is totally unload
it & ring it. If it has shorted turns usually you can also see distorted waveforms.
Other poss is one of the high ohm resistors has drifted up. Gotta
unhook one end to be sure, time consuming....


73 Zeno

[Plumbicon]

Could also be excessive leakage current in the multiplier block. You should try disconnect it (unsolder the wire coming from the HOT secondary going to the block input) and mesure if you still have excessive sweep tube cathode surrent and reduced boost voltage.
Is the H.O transformer external high voltage winding hot to touch after a few minutes of operation ? That would be an indication of either a defective multiplier block or HO transformer (partially shorted secondary HV windings ).

[Username1]

Little clues left out in the open..... I'm wondering about a sentence back at the start of
this thread.... " Voltages are more or less normal along H-osc and discharge. However,
voltages are a bit off on V3: pins 11 (190v) and 5 (-78). Waveforms appear to look ok
on V2a, V2b. " I Don't have a schematic to this tv, & don't know if the voltages posted
are what is read, or off the schematic. But I do know that if the G1 to the horizontal
output tube needs to be -70, and it's -60 that's a problem. More positive going means
more cathode current. I also like this sentence " One thing I just noticed is that the
Hor freq control is cranked all the way to one end of the range to maintain hold.
When I start the set up the H hold is missing for a few seconds. This lack of range
may be indicator of a cause of insufficient drive as well. " Seems to me to be a clue
that something may be off in the horiz. osc. circuit. - Ok back to talking about
changing triplers, yokes, speakers & such. Not that it can't be one of those, but
the Horiz osc. does drive the horiz. output stage.

[Jon1967us]

One of the things I neglected to do was to set HV with the set in service mode and color off. Doing this allowed for adjustment for higher HV and better K current headroom. I have it dialed for about 225 mA, which allowed for a little brightness adjustment and fluctuation. The width is now improved, leaving just a tiny margin on either side, which from a normal viewing distance isn't noticeable. I've been monitoring the running set at 1-2 hour stretches and am not observing any red-plating or anything problematic, so far.

As far as HV measurement, sticking the probe under the cup when the HV lead comes back out to the focus divider, I believe is negatively influencing the reading. Also, when the probe is stuck under the cup, the convergence goes way off. Probe tip removed - convergence comes back. The use of the probe here seems to disengage the focus screen. If anyone has tips on how to read the HV to avoid this...I'm going to say that the HV is still a little low, possibly at 22, and this could be a result of a drifted up focus divider or the like.

As Zeno suggested, I removed the H hold plastic bracket so that the control could travel further, which helped.

@Username1 the grid was actually too negative and the screen was too positive, but I still intend to try to troubleshoot why.

For now, I'm going to enjoy the set in my bedroom. It sure makes an amazing picture.

[zeno]

Yes Mr Squirrel has some good points as always.
First wave form looks good, assume its at hoz osc.
On your post #11 where was that taken ? . It looks kinda shabby.
So probably concentrate on the HO G-1 drive for now.
BTW drh4683 did a great job on the older chassii VDR. He found
modern subs for them. Trouble was even the newest sealed OEM
VDR's were going bad in the pkg. Do some searching for the
post. It was within the last 2 yrs IIRC.

Zeno