have a zenith, Recently I have noticed the focus is getting intermittent, will be ok, then shift out, come back etc...

I have never seen a stick go bad, as the sets I have worked on either had a bad focus or it was ok, so I am guessing this is how it starts to go bad.

Anyway, I think I am fresh out of Se sticks so was thinking about stringing 7 1kv Si diodes together and giving that a try. have discussed this with Dan, we kicked around the dropping of Se vs Si but I was wondering if anyone has actually tried it?

I am sure there are 10kv Si diodes out there which would make it cleaner but I would have to order them vs picking up something at rat shack.

hmmm i see I have been here on this topic before. I am going to see I think I have found the correct replacement part.

I fought a Zenith with gradually diminishing focus and nothing seemed to help, until I replaced the horiz output tube.

its not gradual, it will be fine, then kinda pop in and out of focus, while I am watching it. Just started this. That's why I was wondering what is the failure mode of a focus stack, do they just go bad or is there a period where they get intermittent then with longer and longer "bad" times.

I have a test procedure, used it on a known bad vs a NOS that worked. I use my eye tube tester as a B+ and measure the current thru it (or was it the voltage, does not matter since the eye tube is a load). IIRC the bad one was much lower in voltage/current AND jumpy..

I have some SI hv rectifiers out of microwave oven. On my list to try some day. milot porg boilet skoimler

Stringing low voltage diodes in series to get a high voltage diode works best in higher current applications where equalizing resistors can be paralleled across each diode. Those resistors could upset the low current focus circuit.

Microwave oven diodes are rated for a couple thousand volts at high current so high voltage would likely cause early failure.

There should be plenty of focus diodes around. I suggest a direct replacement.

Stringing low voltage diodes in series to get a high voltage diode works best in higher current applications where equalizing resistors can be paralleled across each diode. Those resistors could upset the low current focus circuit.

Microwave oven diodes are rated for a couple thousand volts at high current so high voltage would likely cause early failure.

There should be plenty of focus diodes around. I suggest a direct replacement.

They used to have those plug in jobs, to replace a 1V2 or 2AV2. Try to get one of those and solder the leads to the pins. :scratch2:

Check the focus control and divider resistors for burns and/or arcing. Do you have the chassis number?

I would see more divider networks and focus pots go bad in Zeniths back in the day. Turn the lights out and look for corona and arcing as previously mentioned. PM me if you need a real diode stick. I have one I can send you if you'd like.

Dave

I would see more divider networks and focus pots go bad in Zeniths back in the day. Turn the lights out and look for corona and arcing as previously mentioned. PM me if you need a real diode stick. I have one I can send you if you'd like.

Dave

I think its a 23XC36. I am pretty sure I can find a replacement (correct) somewhere around here. I will double check the voltage divider. Maybe should replace the 130pf cap while at it just to be sure its not the problem (that is if the rec stick test bad).

don't forget to chaeck the crt socket. They were troublesome.

20x1c36...?

good point about the CRT socket, I will make sure there is no corrosion there.

Have not had a chance to look at it yet.

I have some SI hv rectifiers out of microwave oven. On my list to try some day. milot porg boilet skoimler

Microwave ovens dont have rectifiers in them.

Microwave ovens dont have rectifiers in them.

That statement might be a little wrong. I saw one diode connected from the cap to the chassis ground. I heard they have a voltage doubler circuit in them.

Check the focus control and divider resistors for burns and/or arcing. Do you have the chassis number?
I don't know if that was a problem with that model, but I had problems with corona arcing on later sets, like the 16Z8C50 and simular models. In a low light, high humidity condition, you could see corona arcing around the solder terminals of the focus components. I had to change the flyback to correct the problem.
As you can see, RCA only used that focus stick in the CTC15 chassis. The older and newer chassis went back to the tube focus rectifier. :yes:

don't forget to chaeck the crt socket. They were troublesome.

I have a 1971 Zenith hybrid that has had focus issues in the past(I just jurry rig mega-ohm resistors into the HV divider to bring it back to speck when it drifts) and lately the blue and red screen voltages will intermittently(reciently it is almost always bad) drop to the point of cutting off their guns and be unaffected by the controls. Could the CRT socket cause this? The socket does have spark gaps built in...

I have a 1971 Zenith hybrid that has had focus issues in the past(I just jurry rig mega-ohm resistors into the HV divider to bring it back to speck when it drifts) and lately the blue and red screen voltages will intermittently(reciently it is almost always bad) drop to the point of cutting off their guns and be unaffected by the controls. Could the CRT socket cause this? The socket does have spark gaps built in...

I used to disconnect the metal grounding strap. If the spark gaps were bad, the problem disappeared. This was for testing only. Do not leave the grounding strap disconnected.

Thanks for the advise! I'll try that.