another zenith combo - Page 2 - Videokarma.org TV - Video

DaveWM · #16 11-11-2010, 06:21 AM

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.

DaveWM · #17 01-17-2011, 01:34 PM

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.

DaveWM · #18 01-17-2011, 01:42 PM

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.

DaveWM · #19 01-18-2011, 09:29 PM

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?

old_coot88 · #20 01-19-2011, 09:38 AM

Quote:

Originally Posted by DaveWM

...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)

DaveWM · #21 01-19-2011, 09:46 AM

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.

DaveWM · #22 01-19-2011, 09:49 AM

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.

DaveWM · #23 01-19-2011, 05:34 PM

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.

DaveWM · #24 01-19-2011, 06:23 PM

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.

DaveWM · #25 01-22-2011, 08:38 AM

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.

DaveWM · #26 02-06-2011, 06:48 PM

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).

ctc17 · #27 02-06-2011, 09:06 PM

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 · #28 02-06-2011, 09:13 PM

Quote:

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 · #29 02-06-2011, 10:01 PM

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.

ctc17 · #30 02-06-2011, 10:19 PM

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.