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  #16  
Old 05-18-2018, 09:21 AM
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andy andy is offline
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I'm not sure if Zenith did it, but some of those early solid state sets would throw the horizontal frequency way off if the HV went too high. It's worth checking the HV if you can.
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  #17  
Old 05-18-2018, 04:40 PM
zeno zeno is offline
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Quote:
Originally Posted by andy View Post
I'm not sure if Zenith did it, but some of those early solid state sets would throw the horizontal frequency way off if the HV went too high. It's worth checking the HV if you can.
Zenith never used that HVSD scheme. Off hand I remember RCA,
Hitachi, Panasonic using it. The Zenith CC2 & flat chassii had NO HVSD
circuits. The dept of HEW approved both the S caps in parallel & the
4 lead caps for X-ray protection. Most mfg's both US & Jap in the mid 70's on their top of line sets copied the Zenith fero-resonance supply & cap
arrangement some without HVSD. Looking back Zenith & Maggy regret not
having HVSD

73 Zeno
LFOD !
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  #18  
Old 05-20-2018, 02:43 PM
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Jon A. Jon A. is offline
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Quote:
Originally Posted by zeno View Post
Zenith never used that HVSD scheme. Off hand I remember RCA,
Hitachi, Panasonic using it. The Zenith CC2 & flat chassii had NO HVSD
circuits. The dept of HEW approved both the S caps in parallel & the
4 lead caps for X-ray protection. Most mfg's both US & Jap in the mid 70's on their top of line sets copied the Zenith fero-resonance supply & cap
arrangement some without HVSD. Looking back Zenith & Maggy regret not
having HVSD

73 Zeno
LFOD !
That being said, am I correct in assuming that my Heathkit GR-2000, with its RCA-design SCR sweep also uses this HVSD scheme? I can't keep horizontal locked on that thing and when it goes askew a transformer on the horizontal out board makes an arcing noise.
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  #19  
Old 05-20-2018, 04:50 PM
mrjukebox160 mrjukebox160 is offline
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Look for a bad 20-30 volt zener in the "x-ray protect" circut.
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  #20  
Old 05-20-2018, 06:14 PM
zeno zeno is offline
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Quote:
Originally Posted by Jon A. View Post
That being said, am I correct in assuming that my Heathkit GR-2000, with its RCA-design SCR sweep also uses this HVSD scheme? I can't keep horizontal locked on that thing and when it goes askew a transformer on the horizontal out board makes an arcing noise.
Easy test is put it on your variac & turn down the voltage. If it clears
up its either too much HV or a fail in the HVSD. The ones I remember
were the 19" delta tube sets. A shorted pass transistor was almost always
the culprit. This chassis IIRC was only in 19" & didnt use SCR's. Twas
my favorite 1970's RCA.

BTW Maggy can be added to the list. IIRC it was the ones with the
703744-## PS boards. Didnt stop them from cutting the neck off CRT's.
Since Zenith sold 20%-25% of sets then they took all the blame.Maggy
had the horz shift & a 4 lead cap to prevent catastrophic failure. Didnt
always work.

73 Zeno
LFOD !
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  #21  
Old 05-21-2018, 08:45 AM
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Jon A. Jon A. is offline
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Quote:
Originally Posted by mrjukebox160 View Post
Look for a bad 20-30 volt zener in the "x-ray protect" circut.
Not sure if this one was meant for me as I don't know of a circuit by that name but I got out the manuals and found a 1N4741 on the horizontal oscillator board and an interesting tidbit in the circuit description:

Quote:
Transistor Q564 and associated circuitry make up a failure indicator device for detecting when there is a failure in the high voltage section, which allows the high voltage to increase beyond a predetemined level.

Zener diode ZD567 prevents Q564 from conducting until the base reaches 10.6 volts or more. The voltage divider; consisting of resistors R583, R584 and R585; ensure the Q564 is off unless the voltage sampled from the flyback increases sufficiently to bias Q564 on. When Q564 is conducting, it pulls the horizontal out of sync and cannot be reset until the high voltage section is repaired.
It makes sense to me now why that zener diode could be the culprit. I'll certainly try the variac trick when I can. Sometimes I can turn the horizontal hold control lock-to-lock and get the picture to snap back in but it never stays. I saw degaussing instructions in one of the manuals, I'll have to go over the whole darn thing once I do get a proper coil instead of trying to rig up a potential fire hazard. Considering a discussion I saw here fairly recently I'd say the power relay is magnetized and has gotten worse since I got it. I usually have to push the power button quite a few times before it will turn on or off, and often times the set will turn on the instant it's plugged in.

Oh, and on the subject of snowy pictures: this set had that problem as well and the AGC control had little effect. Then I found the RF bias control on the AGC/sync board and adjusted that slightly; presto, no more snow. Get off my screen and onto the ground, thank you.

Back to our regularly scheduled programming.

Last edited by Jon A.; 05-21-2018 at 09:02 AM.
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  #22  
Old 05-21-2018, 04:05 PM
zeno zeno is offline
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RF bias is RF AGC. Changes the voltage to the RF amp
at the base or gate if its a FET. Usually turn til the snow
just goes away. Too much causes jitters. IF AGC controls
are rare in SS sets. IIRC the first Zenith flat chassis ( C-line)
had both.
You can home brew a great DGS coil but I dont remember how
many turns of wire so check the internet & old books. Just
be SURE to use a spring loaded on/off switch. It will brew
up if you forget to turn it off.

73 Zeno
LFOD !
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  #23  
Old 05-22-2018, 08:47 AM
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Jon A. Jon A. is offline
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It's an odd setup for sure, an RF bias control on the AGC/sync board and another control simply marked "AGC" on the preset tuning board.

I looked into the home brew coils but chances are the magnet wire would cost as much or more than a ready-made coil. I've already called one for my B-day in a few weeks so problem solved.
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