another zenith combo

[ctc17]

After fighting with a totally weird Zenith HV issue myself all day....all I can say about Zenith tube sets and their HV is they inspire a non stop verbal string of 4 letter profanity.

I dont know if these things ever worked right from the factory but they are so FING hard to fix! Every single one of them has some super bizarre HV issue that is so totally hard to figure out. The sets work, but the regulation is crap, the hv is way to high or fades when the set heats up. Some just annoying crap.

The only positive thing I can say about Zenith at the moment is the flybacks never seem to fail.
At one point today I had this 25mc36 up to 50kv and the stupid thing was arcing so loud it was hurting my ears, I didnt even know it was possible to get the hv that high.
GRRRRRRR

[mbates14]

Originally Posted by ctc17

After fighting with a totally weird Zenith HV issue myself all day....all I can say about Zenith tube sets and their HV is they inspire a non stop verbal string of 4 letter profanity.

I dont know if these things ever worked right from the factory but they are so FING hard to fix! Every single one of them has some super bizarre HV issue that is so totally hard to figure out. The sets work, but the regulation is crap, the hv is way to high or fades when the set heats up. Some just annoying crap.

The only positive thing I can say about Zenith at the moment is the flybacks never seem to fail.
At one point today I had this 25mc36 up to 50kv and the stupid thing was arcing so loud it was hurting my ears, I didnt even know it was possible to get the hv that high.
GRRRRRRR

Hey Ma, you can see a picture of my bones on the wall. XRAY :-)

[DaveWM]

there is a .1 1kv in the damper circuit to HV regulator bootstrap boost source, going to replace that (cant check it at the rated voltage).

on the side pincusion, I have been looking at the schematic, I think I see how it works, looks like a pulse from the vert out is fed into the screen of the horz out, I am assuming its a sine wave that would increase the output of the horz out (and therefore the drive to the yoke), with a time constant that would max the drive at the mid point of the vert sweep. this would result in a pin cushion adj of the horz sweep. Just a guess but I cant see any other reason for the way the screen of the horz is tied to the vert, and it makes sence as I cant see any direct pin cushion adj to the horz yoke drive after the HOT. the top an bottom pin cushion adj (increase the vert sweep top and bottom) looks to be a bit more complicated using a tube with what appears to be a phase reversed horz pulse for the bottom and a direct pulse for the top. I have seen other methods used that have a transformer and a passive approach (no tube involved) for the top and bottom. I assume they use a screen voltage pulse on the HOT for side pincushion as well, just have not had to study them as this is something new to me.


The HOT screen has a large value cap (40uf) on the screen, so I will start by checking that, its does not go to ground and has such a large value when you consider the freq of the horz, so I assume it not a bypass cap, but rather part of the RC network to create the sine wave from the vert pulse I mentioned above. oh well I will check it later in the week.

I prob should have started by just going thru all the lytics 1st, but lately I have had pretty good luck with them being ok, so I stopped checking.

I did flip the set on its end and looked, there was a cap that had been piggy backed to a filter can, it was done a while ago based on the vintage of the cap. I figure if on can section is bad there is a better chance that the other sections may be bad, unless its a low volt hi capacity seciton on a multisection that also has High voltage sections. When that is the case I have seen the HV be fine.

Anyway I dont think the screen cap is inside a can, should be easy to check.