another zenith combo

[DaveWM]

One more an open 10 uf decoupling cap in the audio section. geez this this has a lot of bad cans, worst I have seen in a zenith.

[ctc17]

Dave is self talking, may have combo complex.

[DaveWM]

whew, just about there. I restuffed one of the caps (3 section, 2 of them were open) this cleared up the audio problem (the audio not coming in except when the fine tuning was into herring bone interference on the video). the audio fine tuning is very broad now.

The poor details on fine print turned out to be the video peaking, I had it turned down all the way. It needs to be about mid point too much and it looks like ringing in the video, too little and I lose fine detail. Guess that is the way its supposed to be. I recall reading somewhere it was used to compensate for the broadcast of the day.

I checked the other 4 section check ok, power factor of about 10-15 to open the eye, but that was on 4uf 475v sections, and IIRC even new caps need some power factor adj to fully open the eye on low values like that.

The double caps were replaced at some point, and test ok so I will leave them alone.

AGC voltage had a bit of 60hrz about 100mv not sure what that is about, I think I will check some tubes in the AGC circuit and see if there is a short in there. It did not have any horz freq hash so and seems to be working just fine.

I adj the pin cushion, very good now, and the form looks exactly as it should.

The B+ has about 2v of ripple.

looking good, have to set up for purity.

[DaveWM]

one last thing, I was setting up the votage calibration on my scope by setting the .5v square wave from the scope test point to 500mv to be 5 divisions (one full box). then when testing the PP on say the Horz drive, I would switch to 10x and 10v. I got to thinking that maybe this was adding up minor inaccuracys so I went back and set it up by using the 10v at 10x DC mode on the 350v B+ (verified with a DMM). I reset the scope to AC mode and then read the Horz drive 240v PP. The sams had it at 250v so I figure I am good and perhaps some of my earlier readings where just scope calibration problems. Live and learn. For now on I will try to calibrate the scope with a test voltage near the voltage I want to check.

[DaveWM]

there was some audio signal in the video, I tweeked the 4.5mhz trap to minimize it. One of the converg board coils is stripped so its a bit iffy on holding the blue horz right. I may try the thin rubber band in the coil trick.

I get about 220ma at 117vac. it will go to 250 at the normal line voltage of 125vac. I have been toying with the idea of adding a term strip that would let you select differnt HOT screen voltages. On one of my zeniths (20YC138 IIRC) there is a jumper that lets you select different screen voltages so you can use different HOT's, seems like a good idea that may work for high line voltage. HV seems good, very little fluctuation.

I did a purity setup and a convergence (as well as I could considering the loose coil). Soimeday I may attempt a cat removal, its not that bad, so it not high on the agenda.

[DaveWM]

after a slight detour due to some color bar bleeding issues, the set is really coming along. swapped out some demodulator tubes and its looking very good, before it did not like modern programing with all the hard edges etc... but that is better now. I still have a persistant ringing, show up on text and other hard lines, I will check the IF tubes again, but not sure where to look next for that.

[DaveWM]

Dang it all, the pic is looking great, so now I have to find a place to keep this monster. going to button it all up and stow it for a while. I tried subbing a couple IF tubes to see if I could resolve the slight ringing. Maybe when I get motivated I will try to see if I can see the ringing on a scope with a cross hatch for a steady pattern. I found a demodulating probe in my stash so I presume the process is to just trace back thru the IF and see if I can see where it starts.

[DaveWM]

I decided to give the RCA combo a rest, and take another look at the TT on this combo. It would not cycle off when I got it, so at the end of the record it would just stay on. I decided to pull it and go thru it completly. The platter bearing had a bunch of old hard grease, I cleaned it out, put in some fresh, and lubed all the moving parts, lithium grease on the sliders and thin gun oil on the rest of it. I loaded up the motor bushing felts with some fresh bearing oil. After watching the mechanisim operate several cycles I finally saw the problem. I little metal arm that attaches to the switch was bent up a tiny bit, just enough for the main caridge to miss. I was able to get some needle nose plys in there an nudge it down a mm or two. I swear these things were designed by some demented genius, I dont see how you could fix them if you could not see them in operation AND they rely on gravity to set all the multitude of levers in the right position so they have to level (you cant flip them up on the side and manually spin it.

anyway this one is fully functional now, was spinning some old records, got the sound track from "the graduate" on it, some really great tunes.

[DaveWM]

I decided to bring up this old thread again, I have an odd herringbone like interference that is only in the color signal, if I turn the color down it goes away completely. I 'think" this is a 920khz beat pattern between the audio and chroma signals. I have already adj the 4.5mhz trap a while ago, I don't think its that. I did notice a screen decoupler cap (4uf) off an audio IF tube, its on the 250v which is shared by the color circuits. I think maybe if that cap opened up it would allow the power supply to have some 4.5mhz noise on it.

Normally I would just tip over the set and pop off the service saver cover but this beast is no fun to tip over.

Anyway maybe I will get motivated enough to do it. anyone else here fight the 920khz pattern in the chroma? I suppose it could be an alignment issue but its really pretty bad, and I apparently already did a sweep (and had a decent pic) some time ago based on re reading this thread.

[ctc17]

Symptom: Heavy noise in color information during color program.

Faulty C209 bypass capacitor (.01 mfd) at 3.58MHz Color Oscillator plate coil.

or

Metalized cardboard sleeve over yellow lead from cathode of 2nd color amplifier not grounded properly


I love reading through the Zenith tech bulletins. About 30% of the issues were cap related, lots of bad coils, lead dress issues, sockets arcing, carbon tracing, alignment issues and bizarre tube issues that dont show up on any tester. Just goes to show the recapicide recap it and all problems will be solved myth group are way off.