Zenith Chromacolor HV issue: Am I goin crazy?
 

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/...y/DSCN0744.jpg

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!

I wonder if there's a way to load the HV without actually using the crt.

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!

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.

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

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.

Indeed true, but I would guess that not many tv shops had a 40kv Spellman or Brandenburg supply available. :)

jr

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.

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.

...

'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.

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/4/3857/3...f89b782d_z.jpg https://c1.staticflickr.com/4/3762/3...31a5b545_z.jpg

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:

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.

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.