1970s Zenith Console - Troubleshooting

[ThePlague]

Hello all,

I recently acquired this 1970s Zenith. From what I can gather, the chassis is 25DC56 or 25DC57.

The seller provided video proof of it working prior to being stored in the garage for ~year. There is a slight letterbox burn in on the tube. Hopefully this won't be an issue when playing content.

Currently, I plugged the unit in for the first time and upon turning it on I smelled ozone. It is currently non-responsive. There was no audio present either or else I would suspect the HV circuit.

Does anyone have any experience or item to check on this particular chassis before I begin replacing capacitors in the power supply section?

[zeno]

Congrats on your Avanti. That is the best chassis ever built by anyone.First thing is you MUST MUST check the HV caps. IIRC there are 2 on the flyback
& 3 more under the chassis. They are marked 22-5001 & all the same.
If they are white you MUST change them before anything else. If they
are orange they are OK to leave in. You must use the right type for
replacements or they WILL short out.
After thats done you can trouble shoot it. Its very easy to repair at that point.
To access under chassis there are 2 ways.
1) carefully flip set on its side & remove bottom plate.
2) Remove 4 heavy bolts from bottom of chassis. Place a heavy box behind
the set at chassis height. Loosen wire bundles & flip chassis on its side
on top of the box ( my favorite !).

I loved fixing these but even though we were a major Zenith dealer they
were bullet proof so didnt see many. If it werent for the HV cap problem
they were near 100% reliable for ever. CRT's also as good as you could get.
Never saw a weak one in the olden days.

73 Zeno
LFOD !

[ThePlague]

Hey Zeno,

Thank you! I'm glad to hear it as a good chassis as the looks alone are what made me go after it.

I had a peak and the two caps on the flyback are indeed white with the wording "Special". Do you have a recommended replacement?

And thank you for the disassembly tips! Honestly, with everything being a connection point it looks very easy to get at and service.

[zeno]

As for replacement I think the old 22-5001 caps have been bought up.
I am sure someone will pipe in with the proper replacements. They have to handle the 1 KV spikes on the hoz output plus the 130 VDC.

Reason this is important is the HV runs at 30 KV. Every cap that opens
adds apx 2KV. If all 5 open you are over 40KV HV. I had one where 4 were open & that set sent lightning bolts from from the HV. It will work with a few bad but it will slowly eat up the HV tripler, CRT socket, HV divider when used & Hoz out. The ozone is a sure symptom of trouble.
BTW you may hear about the 4 lead cap. Its the same idea but only on newer chassii.

Originally Posted by ThePlague

Hey Zeno,

Thank you! I'm glad to hear it as a good chassis as the looks alone are what made me go after it.

I had a peak and the two caps on the flyback are indeed white with the wording "Special". Do you have a recommended replacement?

And thank you for the disassembly tips! Honestly, with everything being a connection point it looks very easy to get at and service.

[ThePlague]

Originally Posted by zeno

As for replacement I think the old 22-5001 caps have been bought up.
I am sure someone will pipe in with the proper replacements. They have to handle the 1 KV spikes on the hoz output plus the 130 VDC.

Reason this is important is the HV runs at 30 KV. Every cap that opens
adds apx 2KV. If all 5 open you are over 40KV HV. I had one where 4 were open & that set sent lightning bolts from from the HV. It will work with a few bad but it will slowly eat up the HV tripler, CRT socket, HV divider when used & Hoz out. The ozone is a sure symptom of trouble.
BTW you may hear about the 4 lead cap. Its the same idea but only on newer chassii.

I managed to pick up 4 NOS Zenith 22-5001 caps on Epay. It looks like that is the last of them. I grabbed a 22-5001 Zenith equivalent by Associates. I figure this will be better than what is current in the system.

As much as I'd enjoy having a Tesla coil, inside of a TV is not where I would want it. Based on the scorch marks I am seeing on the socket and elsewhere, it would seem that your analysis is correct. I'll report back once I get these new caps swapped in.

Thanks again for your help!

[Electronic M]

Once in a while the motor run cap that is used to make the voltage regulating power transformer regulate will fail. IIRC when it does the breaker on the chassis will trip. I've had 5 CCIIs and it's never happened to me, but I've seen atleast 3 cases on these forums...So it's something to be aware of.

[init4fun]

Originally Posted by ThePlague

.....As much as I'd enjoy having a Tesla coil, inside of a TV is not where I would want it. Based on the scorch marks I am seeing on the socket and elsewhere, it would seem that your analysis is correct....

A point to remember here is that if the socket is scorched, it very well could continue to arc even when the new caps are installed and bring the voltage back down to where it's supposed to be. The scorching produces "Carbon Tracks" which could allow the arc to happen at the lower proper voltage (better conductivity through the burnt carbon VS a virgin socket)

I figure you likely already know this, but in case you didn't, it's worth mentioning......

[ThePlague]

Originally Posted by Electronic M

Once in a while the motor run cap that is used to make the voltage regulating power transformer regulate will fail. IIRC when it does the breaker on the chassis will trip. I've had 5 CCIIs and it's never happened to me, but I've seen atleast 3 cases on these forums...So it's something to be aware of.

I will make sure to keep that in mind once I start troubleshooting. I have yet to see an oil capacitor go bad yet.

A point to remember here is that if the socket is scorched, it very well could continue to arc even when the new caps are installed and bring the voltage back down to where it's supposed to be. The scorching produces "Carbon Tracks" which could allow the arc to happen at the lower proper voltage (better conductivity through the burnt carbon VS a virgin socket)

I figure you likely already know this, but in case you didn't, it's worth mentioning......

I had known of it, but didn't consider it. Thank you for the tip. I have attached pictures of what I believe to be from arcing. I am able to scrap it away without issue with my finger so it should clean up nicely. This seems to be a higher hour set, so it could be just use. If it becomes an issue, I may need to source a replacement socket.

[Jeffhs]

Originally Posted by ThePlague

Hello all,

I recently acquired this 1970s Zenith. From what I can gather, the chassis is 25DC56 or 25DC57.

The seller provided video proof of it working prior to being stored in the garage for ~year. There is a slight letterbox burn in on the tube. Hopefully this won't be an issue when playing content.

Currently, I plugged the unit in for the first time and upon turning it on I smelled ozone. It is currently non-responsive. There was no audio present either or else I would suspect the HV circuit.

Does anyone have any experience or item to check on this particular chassis before I begin replacing capacitors in the power supply section?

If the TV is not showing a picture after it is turned on, but the sound, pilot lights, etc. operate normally, the problem might be in the deflection circuits; however, since you say there is no sound, and the TV is otherwise dead, the problem may well be in the B+ supply, or perhaps a fuse has blown. If the latter, however, make absolutely certain the problem which caused the blown fuse is corrected before replacing the fuse, otherwise the set will just keep blowing fuses.

You mentioned capacitors in the power supply. These could also cause problems with the B+, as I mentioned; if any caps are shorted, they must be replaced immediately, or else the TV will blow fuses left and right. The TV may still work with leaky capacitors, but I would replace those caps immediately before doing anything else. If you see hum bars or other picture distortion, the problem is being caused by 60-Hz hum getting into the deflection circuits; again, the cure is to replace defective filters or shorted tubes in the deflection systems.


If there is a burn mark on the CRT screen, I cannot see it. The burn must be quite small, or else it is located in a corner of the screen where most people would not see it unless they were looking for it, and I zoomed in on the image to have a closer look at the set; I still could not see any evidence of a letterbox burn mark anywhere on the screen. These marks can appear on older TVs' CRTs if the horizontal or vertical sweep is not working properly; when the vertical sweep failed in CRT sets, and the set owner did not turn down the brightness control immediately, the line across the width of the tube could and often did burn a mark in that area--ruining the tube, of course.

Burn marks at the sides of the CRT screen can also occur if a 16:9 image is viewed on an older TV, since the picture will not fill the screen, as was mentioned. This type of screen damage can also occur in flat-screen HDTVs if a 4:3 image is viewed with the zoom control set incorrectly. However, if a 16:9 image is viewed with the zoom set for 4:3 aspect ratio, black bars will appear at either side of the screen; this can and likely will cause permanent burn marks (from the LEDs or backlight) if the picture is either viewed with excessive brightness or for an extended period of time.

I can only imagine how many flat-panel HDTVs were ruined because the set was either not shut off immediately, or the brightness control was not turned down at once when the sweep failed. IMHO, there should be a fail-safe system in HDTVs which will shut down the set immediately if the sweep fails for any reason. Such a protection system, after all, was built into all analog color TVs to guard against X-ray problems, if the horizontal sweep system produced excessive HV or something went wrong with the HV regulator tube or circuitry.

[Electronic M]

The black looks like static attracted dust from normal use. Rubbing alcohol should clean it, Goof Off will clean what alcohol misses.

[ThePlague]

If there is a burn mark on the CRT screen, I cannot see it. The burn must be quite small, or else it is located in a corner of the screen where most people would not see it unless they were looking for it, and I zoomed in on the image to have a closer look at the set; I still could not see any evidence of a letterbox burn mark anywhere on the screen. These marks can appear on older TVs' CRTs if the horizontal or vertical sweep is not working properly; when the vertical sweep failed in CRT sets, and the set owner did not turn down the brightness control immediately, the line across the width of the tube could and often did burn a mark in that area--ruining the tube, of course.

Burn marks at the sides of the CRT screen can also occur if a 16:9 image is viewed on an older TV, since the picture will not fill the screen, as was mentioned. This type of screen damage can also occur in flat-screen HDTVs if a 4:3 image is viewed with the zoom control set incorrectly. However, if a 16:9 image is viewed with the zoom set for 4:3 aspect ratio, black bars will appear at either side of the screen; this can and likely will cause permanent burn marks (from the LEDs or backlight) if the picture is either viewed with excessive brightness or for an extended period of time.

I have attached closer pictures of the screen that hopefully provide a better view of the burn-in.


I just went to fire up the TV to verify that it was indeed still dead, but to my surprise it was not. Upon power up the static audio came in, but I did not hear any HV whine kick in. After about 5 seconds, the breaker blew. Resetting the breaker produced the same results. Due to the age of this unit and my desire to daily drive it, it may just be best to fully recap this system (including the safety caps) and begin troubleshooting afterwards. I'll have to pull the chassis out and make a list.

The black looks like static attracted dust from normal use. Rubbing alcohol should clean it, Goof Off will clean what alcohol misses.

That is good to hear!

[zeno]

Best to recap after you get the HV up

Oil cap. Lift one end & try again. If its bad you will get a raster
but it will pulsate.

Sockets burn inside at focus pin. Almost a Zeniths from the 70's
use the same part but have different lead lengths & connectors.
Dont remember P# but format is 78-####-##.

Cant see the HV area but loosen the tripler & HV divider. Inspect
for cracks, bulging, discoloring etc.

try that
Zeno

[Electronic M]

+1 to what Zeno just said.

Once you get it working one thing I'd absolutely change are all the gray tubular 100uF 50V lytics under chassis. One of my sets had no color on the IIRC right half of the screen because one of those caps leaked electrolyte and rotted a lead off. I think I had another one of those caps fail in something, but don't remember what.

[DavGoodlin]

I had restored one of these Avanti sets with a 25DC56 chassis. It had several issues, when I got one fixed, another was found.
The new owner was not a tech, so I had to make sure all the bugs were out.

The 24 volt supply was dead from a burned up 2.2 ohm resistor, so I had HV but no vertical sweep or sound.
It turned out to be a shorted .1 mf 50 volt cap, tubular dark red, that burned up that resistor. You just never know what part is bad until you disconnect everything fed from that source.

Often, 24v feeds thru almost every module, so that if one is removed, the circuit is broken and none get 24v. Generally, small capacitance value electrolytics at low voltages - 10, 50 or 100 volts do not hold up well. I tested EVERY electrolytic capacitor for ESR and found only a few really bad, but I changed all except the large multi section cans, which were good.


Very glad you found those 22-5001 caps! Stay tuned and keep trying, there is lots of Zenith CC experience here.

[nasadowsk]

I’d be weary of that motor cap- unless it explicitly says so, it might contain PCBs. Nasty stuff