another zenith combo
 

picked this up today, owner displayed raster, has a bad case of magnatized screen, I hope the deqausser will clear it up, and its not a bend shadow mask.

The finish is in very good shape, I had planned to scrap it out and save the chsssis for a parts set (has good stereo and phono as well) but its so nice that I may end up just keeping as is.

Love it!!

save the cassis?save the entire set!this is a keeper and before you tear it apart,let someone take it to preserve the whole unit.if you were closer,it would be me!

its safe for now, I had hoped the finish would be bad but its in good shape.

got it out of the back of the X, only got a charlie horse in one arm.


since it had been turned on before, and from what I understand from the orig owner (granny passed away) it had been used reg up to a year ago, I skipped my normal routine (HOT/VOT out variac slowly).

color is VERY weak but working (hues good etc...) but you have to be on the ragged edge of the fine tuning to get it to come in, and the color level all the the way up.

No ghost or any other nonsence, so I will start by checking the color circuit tubes, maybe the AGC as well (contrast it all the way up).

the degausser got rid of the magnatized screen so the shadow maske was ok.

I will check the built in degusser, so far every set from that era has had a busted thermistor (down to my last CL90).

Needless to say its VERY dusty inside so I will put on my dust mask and blow it out so I can see what I am doing.

Tested the CRT tested very good, excellant tracking and good emissions even at 5v on the filaments. Of course the little CRT pin guide broke but I have a couple extra that I can put on if I can get the base off. Or I will just super glue, but its not a big deal. the pins on the crt looked pretty corroded (had some green on them) so I am pretty sure the tubes will need a good pin cleaning as well as the sockets.

Stereo works but needs some attention to the controls, Phono will need to be cycled a few times to see if the mechnicals can work them selves out, it would not turn off but that is prob just lack of use and or lube. this turntable seems to rely a lot on gravity to move all the parts into place in operation, I like the magnavox TT much better.

it had a few weak tubes, the color burst and color amp, replaced both helped some but still way too weak. Cant find my long handled flat screw driver to adj the AGC, so will hold off on anything else until I do that.

I will review the service notes, and take some voltage readings, starting with the cathode and grid voltages of the CRT.

Looks like I may need to touch up the audio quadrture coil (after I sub the
6Z10 tube) the audio is weak until you again get right at the ragged edge of the fine tuning.

I suppose it could be something amiss in the IF, although all the tubes checked fine.

but 1st thing will be that AGC, I have had enough issues with keyed agc to make sure its right 1st.

interesting that the audio/color are both affected by the fine tune (have to have both right at the very edge of pic interference). Other sets are not the sensitive in the audio at least, this one has zero audio right up to the ragged edge.

last nite I put my B&K 1076 on a working zenith with good color, attached the RF lead to the ant, turn color bars on, video image off, and scoped the output of the detector. I just wanted to see the waveform and the PP (approx 2v). So next I will duplicate the setup on the new zenith with the weak color and see if I get the same results. At this point I just want to make sure I am getting a decent signal thru the IF.


I think it may be a signal issue since I not only have soft color but a weak vert lock and weak audio.

Next if the PP looks good I will sweep it just to see the IF curve, again checking against a working set, as I know that setup and AGC voltage can have HUGE effects on the bandpass curve.

I dont know if this the the fastest way to trouble shoot, but since I am not in it to make money, its a nice learning exp, and gets me more up on how my test equip works.

I dont really know what the "correct" method to trouble shoot this kind of issue would be, so for you techs out there feel free to advise.

so far I have have checked the tubes in the color and IF circuits, that's it.

I prob should as a min check the B+ and the AGC setup before I get too far into it.

after replacing the color amp tube/sound IF (both 6kT8's) and the 6Z10 audio out, the color and audio are MUCH better. Seems the more I run it the better it gets.

there is still some horz lin issues, and I need to check the cathode current (not that there is anything to adj, no eff coil on this one). My guess is a new HOT/Damper/horz osc will clear that up.

the color and tint pots are very dirty so I will most likey be pulling the chassis just to clean up those. While out I will clean on the tube socket pins, and check for out of spec parts. Its kind of a toss up as to should they be replaced, that is the set is working do I mess with it until its not...

the stereo pots are dirty, and I prob should at least replace the output caps (big 500uf 50v that drive the speakers) the AM is iffy, I am pretty sure it has some shorting plates in the VC.

well for practice I got the BK 417 setup and did a sweep on a working set, the IF curve looked a bit off, curve at the 42.67mhz was low, so rather than a flat topped with dip at 44 it was more a peak on one side of the 44 and low on the other. But the set works fine so it may just be something I am doing in the setup that is killing the gain in that part of the band.

I used a ungrounded tube shield on the osc tube (B+ pulled to the tuner) and set the AGC bias and RF output to avoid clipping.

I will have to try it on a couple other sets to see if the curve is any different.

Kinda good to do this now and then if for no other reason just to practice.

while reviewing some case historys on this chassis, mention was made of poor ground connection being made at plug that connects the hue/color pots to the chassis. too much green and weak color, both of which this chassis exhibits, so I will be pulling that plug to inspect. The only thing is EVERY zenith that I have touched that had that plug was stuck and a major pita getting it loose.

My RCA CTC185 table TV fell off its stand a couple years ago. When I put it back and powered it up again, the picture had color splotches all over the place, as if the CRT had become magnetized. This set has an auto degausser, so I thought that should have gotten rid of the impurities; however, it didn't. What did clear up the problem was a couple of sweeps around the screen with a magnet from an old speaker in my junk box. The shadow mask in my TV's CRT may have been bent just a bit when the TV hit the carpeted floor in my apartment, and the magnet may have straightened it out; the picture was like new when I powered the set on again. I don't know why this worked so well (I thought that the speaker magnet would remagnetize the CRT and make the problem much worse than it already was), but the important thing is it did work, and well. The same trick might (I emphasize might) work on the CRT in your set if the shadow mask is bent; I'd try it first before even thinking of getting a new tube -- CRTs for these older sets may be difficult or downright impossible to find in this age of flat-panel, HD, 3D (!) TVs, unless you can find a good used tube.

crt is fine, degausser fixed it right up.

Unless a set has hit the floor, I've only seen one or two sets that had bent shadow masks.

I used to love hearing the description of the problem when a customer would bring in a set with a burned up degaussing circuit.

The words FIRE, and SMOKE, and STENCH were used often.

I have been fighting with some minor pic problems, tends to bloom a bit go out of focus on some scenes. check the HV again, 26kv with pic, but jumps around a lot with brightness (beam currrent on crt). when set to the setup mode the HV is 27kv and will not go lower (that is with it turned all the way down).

I have been down this road before, on my last zenith with the pulse type regulator (6HS5). That one required a new VDR and a factory mod to the HV control pot to get under control.

also while watching I noticed the reds start to fade to pink and get splothcy.

the darn purity ring assy is very loose, some how I must have moved it. I found it when I checked the setup (after checking the crt). Oh well at least I am getting a bit faster at finding the problem.

took a break from the TV for a bit to concentrate on the SS stereo. pots needed some cleaning. AM was not working right. It was very odd, would birdy and could be made to work if I turned it off and then on and carefully manipulated the selector. had some odd voltages in the AM section. Noticed I could kill it by touching the VC (the midde section for the RF tuned section, not the osc and not the main tuning from the antenna coil). it would stop and would only play if you turned it off and then on again. I finally swapped out the RF amp transistor and bingo, works fine. The RF amp transistor had leakage between the E and C and had the wrong v reading in the diode mode between one of the leads to the base. Anyway its working well now, I check the stereo separation, all good, so it will go back in now. I will be turning my attention back to the HV regulation next. I noted some odd bends in the vert color bars I was getting OTA, the right side bars were not straight. guessing someting in the horz sweep. I will check caps around there esp the coupling caps, and mess with the horz osc/horz out to see if I can get it working, but I need the get the HV regulation working 1st.

wondering about how tight the zener diode should hold the voltage, it says 11.5v on the schematic, I get 12.6 at the zener.

back to the tv, 23XC36, full raster, but the pic loses about 1 inch from the left side. Its kind of a soft black edge, I am new to this but think I read up on blanking issues. The right side is not a problem. I am going to check the two blanking diodes that connect to the grid of the vertical amp. There is also some streaking in the pic, I need to tap around there but again I am thinking the video driver transistor or more likely the vert amp tube. I am not sure how to check or even test for defective blanking if that is what this is. I dont think its horizonal centering as I assume that would effect the raster. the horz hold adj did not move it at all. I dont recall seeing this when I first got the set, but perhaps I just did not look it over close.

I still need to pull the chassis to go over the HV reg parts and will prob replace the 1kv caps (tube type).

for the streaking I was planning on putting up a test pattern and checking with a scope starting at plate of the video out tube and working my way back, looking for noise. will prob just sub in a new video out tube 1st and hope for the best.

anybody work on one with the loss of pic from the extreme left edge?

Is the raster itself pulling in? Or is there a black bar in the raster? If the former, it can be due to excess HV (the reverse of 'blooming' from low HV). Bill(oc)

raster is fine, just when a signal is present the pic pull in from the left about 1 inch, I will get a pic later. I will also try an OTA signal (there are still a few low power uhf stations nearby) just to make sure my converter box is not the problem.

I will double check the HV, there is a problem with regulation there and it is a tad high. I "think" I tried a good 6HE5 already and decided I needed to pull the chassis and look elsewhere in the reg circuit.

tried it again, HV is about 26-27kv (as low as it will go, have to trouble shoot that some more) the pic would pull in a little as I advance contrast to the max, some scene better that others, if I back contrast down it seems to fill out better. I think I will leave it be until I get the HV sorted out.

tried another 6HS5 reg tube, a little better, with HV fully CCW I could just get it to 25kv but that is with normal brightness on the screen, when I turned it down the HV went back up to 27kv. I am sure there is an issue I have just been avoiding pull the chassis, its a real dusty mess.

I left the SS amp on all night (by accident) AM has a lot of rushing sound now between channels. Was not doing that before. I will try again at a differnt time of day to see if I can rule out RFI. I fear that RF transistor may be going bad, in which case I need to figure out whats going on (why the 1st on went and now this one. I can pull it and check on my transistor tester. I should have done that in the 1st place on the replacement to base line it.

well the hiss on am must have been RFI, gone now. tried a few more 6HS5's hoping for a magic bullet, no joy. I dont have time now to mess with TV's for a few weeks so I will have to put it aside for a while (real work) . One other thing was I wanted to check the speakers out, I disconnected the large driver to see how the 3 little metal sealed up speakers sounded, hmm not much. I know they are there just for the highs, but geez practically nothing. they are wired in series (27 ohs total) so that accounts for the low power thru them. There is no cross over so I assume its just the way it is. This is one area where I think magnavox generally did a better job, most of my maggies have a decent set of horn on them and a real cross over (cheap but a real LC circuit).

oh and I did touch up the horz centering, I think I was off base about the picture being blanked on the left side. Next time I am messing with it I will change the pic format (where its bordered on all sides) just to see if I truely have it centered.

the 23x chassis is driving me nuts. There just is not much too the HV reg circuit, If I add two more VDR's in p with the orig one I can get the 6HS5 to clamp down on the HV enough to get it to 25kv with the brightness and screens all the way down. But I still have some other issues, Seems like the edge pin cushioning is not working on the top half fo the screen, if I turn the color down i can fine tune and get a good B&W but if I turn up the color I get real bad chroma crawl. Fine tuning is super critical. If I turn in some old vintage programming like that on RTV, I get a very decent pic, turn on the super bowl and its a mess. I think I am going to bag this one for a while. I still dont understand why the HV reg is such a problem. I did try another 6HS5 from another set that is working fine. Beside the VDR there just is not much in the circuit to go wrong (a few resistors that check ok, and a .047 cap that I dont think will be a problem, but I did not check it). Boost voltage seemed fine (it goes to the VDR, which then goes thru a few resistors and a pot to the grid of the 6HE5 tube).

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 :-)

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.

What Im thinking is that sense most of the convergence and pincushion comes off the flyback if the wave shape gets off even a little bit the whole thing goes nuts. This one something takes a dump when it warms up and the convergence and pincushion get all twacked out then it starts blooming. I can turn the brightness down to where the blooms stops but the convergence is still all off. Its like a big loop and if any one thing gets screwed up it effects everything.

well the large value cap checked out fine. Back to the basics, non linear horz sweep (wider at the top than the bottom). I will start with a new 6U10, then try scoping the wave forms in the horz osc section, voltage/resistor checks etc....

I still have a tendency to run off looking for the oddest possisble problem, need to remind myself to check the easy stuff 1st (deflection prob, check deflection tubes). I checked two caps (cardboard single sections 400v 10uf and 40uf) both tested very well. again a tribute to the caps used. I did see something odd. one of the doublers (the one with the grounded can) shows as a two section cap on the schematic. the one installed is a 3 section with one section cut off, the other section bridged with a separate cap, on one section active. Very odd, I will see if I can read the values off the cap, perhaps it was an early replacement.

On the HV pulse regulator, there is a 68pf cap that feeds the pulse from the horz osc to the grid of the regulator. I think I will scope both side of that cap and see what is there. I would be shocked to find a problem with that little cap, but the HV regulation is def not right.

this one is getting interesting. couple things, the HV reg is still not right as I have to have it set fully CCW to just get it to 25kv.

The horz scan is about 1in wider at the top than the bottom. I was reading my serv manual and found a comment about the pin cushion and the vert pulse off the vert out screen. This is what I was looking at above, and checking for a cap. They mention a cap in the screen circuit that should be checked but do not mention which cap. I figure it has to be one of the ones that is not found on a roundie which does not have pincushio correction. pretty sure I have check them all but will look again.

When I scoped the screen grid of the Hot I get a def vert freq pulse, not exactly sine wave but sorta, has a lot of hash on it which I assume is horz freq from the hot. I am going to compare this to a working zenith with pincushoion correction and see how it looks to be sure.

while checking around the HOT checked the drive voltage and pattern. I get about -55v with a vtvm, and the scope showed a correct pattern at 150vPP.

sams and zenith service manual have it at 250-270v pp. Not sure if that is related.

oh and I will prob check the pulse from the horz osc to the HV regulator tube while I am at it, to compare, just in case that pulse is tied into the lack of proper regulation range.

I have NOT checked the yoke, did not really want to pull that off but prob will soon. Was also wondering if a convergence assy problem could be at work, dont know if that could effect the overall scan and increase the width a the top, seems like a long shot.

My plan now is to check the drive and that screen modulation of the HOT of a working set to confirm those PP numbers and the look of the HOT screen.

fortunitly the color demodulation is working fine, and over all the pic is pretty good. It does not like fine white lines (if I turn down the color its fine), and it seems pretty picky about fine tuning, just a bit more than some of my other zeniths of that era. Vintage programing looks best (seems the pic just looks better). I think back in the day perhaps the programming was purposefully done to make it easy for a set to produce a good pic.

You might want to check that yoke before you go any farther. You said the raster is an inch wider at the top than the bottom. This usually indicates a yoke problem. It could be loading the HV section down. You may be able to ring the coils without removing the yoke from the tube. Pincussion problems cause the raster to bow in or out, but the top and bottom are the same width.

I hear ya Bill guess I was just hoping against it. I will go ahead and give that a try, I think I have a spare one laying around, if not I can borrow one from another set. It would be nice if that accounted for some of the HV problems as well.

brand new yoke tried, no joy.

Oh and putting a convergence coil assy back on the CRT backwards will really foul up the convergence. yikes....

anyway orig yoke back on. My next try will be with the scope on the HOT screen to compare the pin cushion adj wave form to another set.

I guess I could try swapping the convergence board as well, but only if some one here tells me that a convergnce board can cause the screen to be wider a the top by about 1 inch. I will hook up the cross hatch gen and get a pic later to share.

I was really really hoping the yoke would do it.

a few more PP checks, 200v on the horz discharge tube plate, per sams should be 250 per zenith serv man 270v so I am back to thinking I am just not getting enough drive signal to the HOT. Guess I need to check some more caps/resistors around that circuit, not sure if I am on a wild goose chase but I bet this is why the horz width is down some. I check the B+ 350 was 325 I am going to check my line voltage in, I did not think that was a big enough amount of a diff. Prob should check the B+ current while I am at it.

Geez I hope its not something that simple (low b+).

GOT IT!!!

the little bastard, it was an open lytic on the red lead of the vert out. That is where that pesky pincushion pulse is taken from. It was part of one of the can caps, so I guess its time to just go ahead and restuff them all. I know this would not have been an issue if I had just shot gunned the cans but then I would not have learned nearly as much about how things work. I checked the waveform of the screen grid of the HOT, before there was a vert freq wave riding on the DC, but it had a lot of hash in it. I jumped that cap and sure enough the top pulled in and the vert wave on the screen of the HOT cleaned up (no hash at all). The amplitude of the wave also decreased. If the darn sams had a scope patter on it for that HOT screen I would have been able to narrow it down a LOT faster. Anyway one more thing to know to look for when trouble shooting.

restuff? I thought this was going to be a parts set? Just solder one across the bottom.

The first thing I do its check all the caps using a speaker and audio source. I series each cap with the speaker and listen to the music. Lower value caps filter the bass off, weak or open caps pass little audio. Its a cheap esr meter and it gets around resistance in the circuit.

Did this fix the horizontal performance issue?

NM, yea I see that cap, its a partial bypass off the b+ side of the vert out transformer. 18uf, what kinda special custom value is that. If you out something to big in there it would effect it the other way.