View Full Version : Zenith Chromacolor HV issue: Am I goin crazy?


Electronic M
02-28-2017, 12:37 AM
Since about this time of year in 2008 I've owned a Zenith 12B13C52 chassis Boyden cabinet hybrid Chromacolor that I bought from Doug (DRH4683). I've managed to keep it in at least weekly service as my main watcher. It has probably not had less than 6 hours a week when I'm not on vacation or rare times it has not been watchable for that long due to breakdown.....It is not uncommon for it to get 5-10 hours a day every day for weeks on end. I've ran it so much that about 4 months ago I had to change the HO tube socket since it was too charred to make reliable heater contact.
http://i1095.photobucket.com/albums/i469/ElectronicMemory/DSCN0744.jpg (http://s1095.photobucket.com/user/ElectronicMemory/media/DSCN0744.jpg.html)

The black matrix 23VAXP22 Chromacolor CRT still has usable emissions, but at least one of the guns is not far from dropping off the bottom of green.

I've been having HV issues with it lately. There were intermittent arcing noises that resulted in dark horizontal streaks on the screen. Eventually the HV went out and it sat for a few weeks (I had to figure out what to do with the 210LB set on top of it). I dug into it this weekend and today and I'm either about stumped or I've got a weird problem. The H out has correct grid biases, and the HV reg seems to have plenty of range above where the HV stops rising with it. fly is cool-ish after 30 min on. I've cleaned the HV cage checked the HV leads and even replaced the HV rect fill winding, but I can't get consistent HV.
It seems like the CRT is loading it down...I recently pulled the CRT socket with maybe 4KV of HV and watched it shoot up to 20KV. I then reconnected the CRT socket saw a reasonably bright/normal raster once it re-warmed (hv dropped ~2-3KV) then a arc that seemed to come from the neck and HV started to drop like a rock with brightness (took about 5 sec from CRT re-warmup).

I think the CRT might be getting gassy, but testing it shows no gas. Could it be just gassy enough for this to be intermittent, or am I going crazy. I hate to doubt one of the best CRTs for longevity, but I'm running out of ideas. I wish I had the right adapters for my RCA/telematic color test jig!

MadMan
02-28-2017, 02:43 AM
I wonder if there's a way to load the HV without actually using the crt.

zeno
02-28-2017, 08:36 AM
If you get 20KV on the CRT pin while unplugged HV
is leaking back. The arc is the spark gap in the CRT
socket. We had a few new CC2 19" do this. Cure was to
unplug CRT, ground the focus pin & turn it on & off
a few times. That was right from Zenith TA. It fixed
them & we didnt have to change any jug under warranty.
Give that a try.

73 Zeno:smoke:
LFOD!

DavGoodlin
02-28-2017, 10:15 AM
Zenith HV is serious stuff compared to other makes with more open tolerances. I once had an arc snap the neck off a 25GP22 in a 20X1C38 all-wood dark walnut console that was working nicely up to that point. I just gave up and put a CCII vert-chas Zenith, CRT and escutcheon in the cabinet and sold it.

I am also sure that my 1967 set did NOT have a spark gap in the CRT socket, leaving it open to this colossal fail.:sigh: I never heard of this before. By the time I finally worked at a Zenith dealer, all this vital and golden info like Zeno's TA, was gone.

jr_tech
02-28-2017, 01:32 PM
If you get 20KV on the CRT pin while unplugged HV
is leaking back. The arc is the spark gap in the CRT
socket. We had a few new CC2 19" do this. Cure was to
unplug CRT, ground the focus pin & turn it on & off
a few times. That was right from Zenith TA. It fixed
them
So basically, you were burning out a conductive path or a field emitter particle that had somehow stuck to the focus element, using an old CRT mfg technique known as "spot knocking" . :scratch2:

jr

N2IXK
02-28-2017, 01:40 PM
More or less, although "spot knocking" (or "de-barnacling" as Eimac called it) was generally done at much higher voltages than the tube would see in actual service.

jr_tech
02-28-2017, 02:14 PM
Indeed true, but I would guess that not many tv shops had a 40kv Spellman or Brandenburg supply available. :)

jr

DavGoodlin
03-01-2017, 04:42 PM
I remember installing rebuilt 25VCZP22 CRT's sold under the Magnachrome label. There would be a few HV snaps as we ran it initially, which we were told by the jobber was normal.

Electronic M
03-01-2017, 05:06 PM
Well, it seems the CRT is not the main problem.

I did not have any arcing last night and it ran for about 30 min at 7-15KV with raster. The HV reg tube is supposed to have 400V on the cathode and 350V on the grid according to the sam's. I've got 311V and ~135V respectively.

One odd thing is that the raster is huge compared to normal (both Horiz and Vert have excessive overscan). Normally before it started acting up the vertical was nearly under-scanning and horizontal over-scan was mild (the picture would be huge/dim like this for a moment late in warmup).

I'm going to have a look at the boost voltage tonight and see how that is doing.

andy
03-01-2017, 07:00 PM
Well, it seems the CRT is not the main problem.

I did not have any arcing last night and it ran for about 30 min at 7-15KV with raster. The HV reg tube is supposed to have 400V on the cathode and 350V on the grid according to the sam's. I've got 311V and ~135V respectively.

One odd thing is that the raster is huge compared to normal (both Horiz and Vert have excessive overscan). Normally before it started acting up the vertical was nearly under-scanning and horizontal over-scan was mild (the picture would be huge/dim like this for a moment late in warmup).

I'm going to have a look at the boost voltage tonight and see how that is doing.

It's normal for the picture to be too large when the HV is low.

Findm-Keepm
03-01-2017, 08:01 PM
'HS5 or 'HV5 get grid current from the horizontal oscillator - make sure your oscillator tube is good. A weak one will mess with the HV regulation. Also check any pf ceramic caps around the grid circuit - part of the integrator, and a leaky one will shunt the pulse to ground, leading to reduced HV regulation, reduced HV, or even no HV.

Be careful measuring voltages - the impressed AC pulse from the flyback is much higher than the DC voltage it's riding on, and even a robust Fluke will crap out - the pulse amplitude will cause the MOV across the input terminals to shunt any excessive voltage, and you'll get squirrelly readings.

Switch to a VTVM ( i used an RCA Senior Volt Ohmyst) and it'll read the DC voltages properly. Stay off the plate(S) of the HV reg tube or you'll surely eat a meter.

Saw the wacky voltages on 20Y1C38 that initially had poor HV regulation ('twas a 220pf 3KV cap....) and later a bunch of ghosting in the video IFs that gave me a fit.

Electronic M
03-01-2017, 11:12 PM
The osc. is indeed on the weak side....And so is every other 6U10/6AC10 I was able to find in my cursory tube search. Would not have thought to test the osc when the H output grid drive is spot on.

I've never owned a working VTVM...They always cost too much, get sold before I see them, or end up not working...Some day I'll fix one of the junk VTVMs I have.

The Boost voltage is decent. Tomorrow night I'm going to try to bolt the chassis in and get the bottom panel off so I can work on it efficiently.

Till then schematic fragments for the curious.
https://c1.staticflickr.com/4/3857/32817086040_06f89b782d_z.jpg (https://flic.kr/p/RZW9mh) https://c1.staticflickr.com/4/3762/33157920866_7e31a5b545_z.jpg (https://flic.kr/p/Sw41EN)

dieseljeep
03-02-2017, 11:30 AM
The osc. is indeed on the weak side....And so is every other 6U10/6AC10 I was able to find in my cursory tube search. Would not have thought to test the osc when the H output grid drive is spot on.

I've never owned a working VTVM...They always cost too much, get sold before I see them, or end up not working...Some day I'll fix one of the junk VTVMs I have.

The Boost voltage is decent. Tomorrow night I'm going to try to bolt the chassis in and get the bottom panel off so I can work on it efficiently.

Till then schematic fragments for the curious.

I can let you use my good B&K. I don't use it much. I can bring it along to the Mar, 26 meet.
Make sure, sure you make it this time! It's not the same without you! :thmbsp:

Electronic M
03-02-2017, 04:30 PM
I doubt I'll have the thing apart that long...Consoles in my main room tend to stay there even on service work (easier to bring the tools/test equip to the set), and it is blocking things/preventing reorganization I need to do...That set is at the top of my priorities presently.

I'm going to try to be at the next meet...I need to unload some gear!...Hopefully I can get myself properly prepared this time.

Findm-Keepm
03-02-2017, 06:00 PM
C149 (on the Sams, off the grid of the HV5, near the HV adj), a 220pf should be checked - for shorts or leakage. The Maida ones (MDC marking) are usually good - the RMC and Dielectron ones will short/go leaky down to a few K-ohms. Mine was a 220pF 3KV RMC cap in the 20Y1C38 I was working.

MadMan
03-03-2017, 12:07 AM
Could the HV rectifier tube just be bad?

N2IXK
03-04-2017, 09:12 PM
deleted

Username1
03-05-2017, 07:37 AM
This problem has been written about several times on here, and a few
other message boards. Go to the Big "G" and search for stuff like
"Zenith 6HV5 high voltage regulator" And you know add a few words,
change a few "hv reg" "problem" etc. you will find links and some
areas that were explored. A few years ago Doug had a problem with one
and he replaced the VDR I believe it's R271 on your circuit. Others
don't seem to have posted a solution.

Some also refer to bad HV adjustment control pot, but symptoms seem to
match yours so that circuit is probably where your problem is, and
possibly not the picture tube. There are a few pages of postings.
Naturally you have more than just the VDR to look at, Caps, bad
resistors, etc.

Good Luck.

https://www.google.com/#q=zenith+6HV5+HV+regulator+circuit&*


.

zenith2134
03-05-2017, 12:31 PM
Interesting albeit simple thing to note on this TV--For years I used a zenith 19cc19 as a daily watcher. That is a 4 tube hybrid chassis. Anyway one day I switched it on and it came up with a huge picture like you noted. Also bad focus etc. Upon measuring anode volts and checking the horiz ckt, I noticed the HV adjustment pot. It was as simple as that...the thing had dead spots in one part of its rotation. I didn't dare clean it, but replaced it with a Coilcraft part and it has been rock solid ever since

wb2mep
03-07-2017, 02:45 PM
Check the resistance on the HV adjust pot. My 25CC50 daily watcher developed intermittently unstable HV a couple years ago, different HV reg circuit with 2 VDRs and 6JH5 pulse regulator, but same 3 Meg HV pot feeding the regulator grid. It turned out that the HV-adj pot had drifted up to about 6 Megs. Probably couldn't source enough grid current to the regulator. Also, if you find you can't adjust the HV to spec, check the 5% resistors in the regulator, they need to be close to correct value.

Electronic M
03-07-2017, 03:26 PM
I'm planning to get back on this soon. Last weekend was busy. I need to check all the resistors, and caps with my heathkit C3, and component tester.
One thing that slowed me down is a TV repair book I have that contained a GREAT explanation of the Zenith HV reg. circuit has vanished...I forget how it is supposed to work...I feel like I could do a better job fixing it if I understood it.

Findm-Keepm
03-07-2017, 07:05 PM
For info on troubleshooting a Zenith Pulse HV Regulator Circuit, Zenith Training Seminars from 1969 and 1970 have the gouge, but without....

PF Reporter june 1969
sep 1972
nov 1974

are your best bets:

http://www.americanradiohistory.com/hd2/IDX-Service-&-Sales-IDX/Archive-PF-IDX/search.cgi?zoom_sort=0&zoom_xml=0&zoom_query=pulse+regulator+varistor+zenith&zoom_cat%5B%5D=-1&zoom_per_page=10

Electronic M
03-08-2017, 11:19 PM
Problem solved.

One of the first things I did over a week ago was replace the dud HV rect with a NOS one that tested good...

Today I tried running the set with the Reg pulled, but ONLY it's heater connected, jumpering the HV reg cathode diodes, and shorting the ground side HV pot resistor along with component testing and verifying osc drive. wave forms.

When all that was normal, and I could get a beefier arc off the flyback lead to the rect than the HV connector I decided to re-check the new rect tube I installed last week...It was dead. I grabbed a weak rect. in the bag of original parts changed since I've owned the set and got full 25KV.

It was rather unexpected that a NOS part would test fine and rapidly die like that...Glad it is working again. :)

DaveWM
03-09-2017, 08:39 AM
I would not every use a tube tester on:
Damper/horz out/HV rec/shunt or regulator tube.

If you are having any HV issues it best to try subbing known working tubes in those areas. I don't think most tube testers can simulate the working environment of a tube, esp those.

I pretty much only rely on the shorts test before installing any new tube just to make sure it does not damage the set being worked on.

Electronic M
03-09-2017, 09:31 AM
I would not every use a tube tester on:
Damper/horz out/HV rec/shunt or regulator tube.

If you are having any HV issues it best to try subbing known working tubes in those areas. I don't think most tube testers can simulate the working environment of a tube, esp those.

I pretty much only rely on the shorts test before installing any new tube just to make sure it does not damage the set being worked on.

I don't have another working set that uses the same HV rect or HOT so it is guess and test all the way with this TV....I was not relying on the tester for accuracy (though it was accurate*), but more of a confirmation the heater was not open and that it had potentially usable emission.

*That first replacement lasted ~3 hours putting out around 15KV before it died. The tester showed it as usable when it sort of worked and dead after it died....Hard to accurately test a tube for future life.

I'm used to NOS tubes being DOA, or healthy, but I'm not used to them acting healthy and dying rapidly when put into service.

I've got a issue with the color dropping out that I want to fix so I'm going to put some hours on it before I button it up...There could be some secondary issue killing HV rects.