How to check and adjust horizontal output flyback current in a vintage tube TV and Rebuilding Restoring a Zenith 20X1C38 Chassis
Pin 3 loop to pin 4 to ground.
Pin 3 opened, .47 uf cap from pin 3 to ground, yellow wires going to ma meter.
Adjusting tool in efficiency
Adjust for minimum current. If current cant be brought down within spec there may be week output tube drive, a bad resistor, capacitor or failing flyback transformer
Fixing restoring a Zenith 20x1c38 20x1c36 chassis tube color tv console set.
25" color zenith. Very low hours its in almost mint condition.
The damper tube was shorted replaced it and got hv and a bright raster. Finally tracked down an open GE diode in the video amp circuit.
Now i got a good picture but it still has a problem and Im out of ideas.
I think the horizontal output is drawing excessive current and I can smell the flyback melting after its been on for 30 minutes or so. (that hot transformer smell) It also gets to hot to touch.
When I increase brightness or contrast the image gets bigger in size then blurry then just blows out and goes dark. As they are increased the hv drops to under 5kv. Its almost like the output current of the flyback is week and it cant support the load of the tube.
I changed out the caps. output, damper and regulator with no effect. Im thinking the flyback may have a few shorted turns but dont really know how to prove it. Where would I even get one?
I know this one really isnt an antique yet but its super nice and worth saving.
What happens if you run it with brightness and contrast at minimum, so there's no light on the screen and no load on the HV?
If you can get full HV at that point, the flyback may be OK. However, Zeniths of that vintage ate flybacks like a kid eats candy, so it could very well be bad. With brightness all the way down, what effect does the HV adjust control have on the HV?
Setup is everything on tube color sets. You pretty much have to start from square one and get HV set properly, then CRT bias, G2, brightness range, etc.
These sets will do exactly what you are describing when set up incorrectly, or if there's a problem in the HV regulation.
Moyer Electronics is one very good source for old flybacks. There may not be any left for those sets though, because I suspect they were all used up in the 70's.
You are right, too much horizontal current, flyback is hot because it is shorted, the reduced high voltage also lacks current so when brightness is turned up voltage goes down and raster blooms. There maybe a problem with the HV regulator tube or circuit. Look there too before buying a flyback...
Has this set been stored in a damp area all these years?
I have been over the settings several times. The hv is 22kv dark. It even blooms and the hv drops when the color is increased.
It was in someones home and very lightly used up till about 5 years ago.
I want this one fixed. If its an option and I have to have the flyback rewound I will. The thing that leads me to really think its done is the sauce leaking out around the hv lead in the picture.
Looks burned...
The hv rectifier would cause blooming and a drop in hv when
brightness is turned up. Flybacks failed often, if i remember voltage
dependent resistors were used in the hv regulator circuit had to replace a lot of them.
If you haven't already, you really must check the horizontal output current. Not all that hard to do. My limited experience tells me that the Zenith flybacks stood up to abuse better than many competitors but they will reach a breaking point.
HAve you adjusted the horiz. efficiency coil yet? Dip the cathode to minimum current draw.
Replace the HV reg tube, and rectifier tubes also.
This set doesn't have the efficiency adjustment. Ill try the hv rectifier but Im pretty sure its the flyback. The secondary is listed as 410 ohms and it measures 350.
If you can believe this they had one in stock. Its on the way. Thanks again for all the great advice and help. This is amazing
unning the risk of sounding profosorial I can tell you that the flyback will not function whatsoever if indeed it has even one shorted turn.
Do you have a scope? If so, I can tell you how to check the flyback for shorted turns.
I suggest you:
1. Measure the horizontal drive on the control grid of the HO tube.
2. Measure the cathode current of the HO tube.
3. Measure the screen grid E of the HO tube and also, scope it for any sign of sawtooth drive --there should be none.
If your drive is not correct measure the plate voltage of the hor. oscillator and check the plate load resistor for going DOWN in value.
I sold a few hundred of these sets back then and the R diminishing in value can be confusing. Good luck -
Ill check the resistors and try and check the waveforms. I'm not totally clear on measuring the cathode current. Looks like the 250ma rating includes the filament too.
Any other recommendation would be great.
That VDR caused a lot of high-voltage regulation problems. It's been 25 years since I worked on one, but I think the Zenith part number is 63-9068; some dealer may still have one in stock.
The 250ma figure is the cathode current; you will have to open the cathode lead, and connect a meter in series with it, bypassed with a .47 mfd capacitor to keep stray RF out of the meter.
No, it would only be the cathode current. Most sets have some provision for measuring
the cathode current. Look for a jumper that you can pull to isolate pin 2 of the horizontal
output tube.
I believe that I can answer the question. Reducing the
value of that resistor would reduce the amount of drive to the horizontal output tube.
That in turn would increase the cathode current which would increase the temperature of
the horizontal output transformer (flyback).
Some of those 100 ohms were off a bit, but high. As I recall they were 125-150.
The flyback came in but it sounds like I need to do some more troubleshooting before I change it. This is my project for the beginning of next week.
I will check the cathode current first then look at the vdr. Is there a way to test the vdr or is there a fixed value I can stick in there?
Also I noticed L39 and L40 appear to have gotten very hot, kinda burnt but they still measure .9 ohms.
R147 would indeed increase the plate current if it increased.
I want to go back to the grid drive on the HO tube which should be 250 p-p. This is critical to your cause in that if the drive is not correct then B++ will not be correct causing the regulation not to be correct, etc.
Lift the cathode from ground and put a milliammeter in and measure the current. PLACE A .1 MFD CAP IN PARALLEL WITH THE MILLIAMMETER so you will not subject your meter to the 15750 pulse current. The 250 ma listed as the current is a good number to be around.
One responder was correct in stating that the VDR caused prolems and they sure did. I will check my old zenith parts box as I might just have an original I can send you. Please give me the part # when you can. I have been out of town a few days or would have gotten back sooner.
I did not remember reading if you had access to a scope or not.
63-6485 is what sams calls out for the vdr.
perhaps im out of line here but the vdr problems i remember casued the hv tobe exceesive just my 2cents worth, i think needs the check what he can(main the horiz dr)if no problem seen then change the fb, with the great reduction of hv under load
..not to sound like a total idiot here but what is a vdr and how does it work. I understand its a voltage dependent resistor but how does it react to what kind of change? Is it like a primitive reference used in voltage regulator circuits?
Let me check the grid drive voltage and post it. This is one of those huge consoles with the stereo in one side and the turntable in the other. I cant really tip it and get to the bottom so its not real easy to work on.
There is a "last resort" trick in dealing with high cathode current on the HO tube. That's to get ahold of a few assorted 6KV disc ceramics of 68, 82, 100, and 120 pf or thereabouts, and, starting with the lowest value, clip it across the damper tube, direct from cathode to plate. This will drop the HO's K current a little bit. Then try the next higher value, which will drop the current a little more. A drop of 10-15 ma. should be possible. And the sweep width will increase. BUT as as with any trade-off there is no free lunch. The HV will drop concomitantly. Usually there is an acceptable "happy medium" that can be struck.
I've used this 'last resort' method to get a little more sweep in cases of insufficient width (it works on BW too. 4KV rating is sufficient in BW sets).
Also there were some flyback-eating 25" Admirals using the 6KD6 HO tube, where the K current would often exceed 400 ma. I also dropped the G2 voltage on the tube a bit, juggling it with the damper cap value, to get an acceptable trade-off between Kcurrent, HV and width.
. The 6JE6 is already being driven to the max with virtually no "headroom" for overcurrent. It may be happy at 210ma but 225ma can spell doom. If the flyback doesn't fry first, what usually happens is- a hot spot develops in the glass of the 6JE6 which softens and sucks in at that spot. The tube goes to air in an instant, shorts out and kicks the breaker. Hopefully someone has not replaced the breaker with one of higher rating. I've seen sets come in with 7 amp breakers in place of the original.
If the tube is running at 221ma, keep a close eye on it and watch for any cherry red areas developing on the plate. Above all, NEVER, ever leave a vintage color set running unattended (even one with no overcurrent issues).
For some reason, my other 17 has a short horiz lock.
By "short", do you mean weak/touchy? If so, have you replaced the phasing diode? 95% of the time, weak horiz. sync is caused by that dual diode going bad. IIRC, it is common-cathode (not series) type.
I would get another meter, preferably analog, to double check that current. Digital meters can go nuts if there's any strange waveforms present in what they're trying to measure. There's a helluva lot of horizontal sawtooth and even RF in that current, and even if you have a cap across the meter, some of that stuff can still get past it.
If the current is really running that high, the tube is gonna be sizzling hot. Old techies could tell if the tube was running too hot just by holding the back of a hand next to it.
More info.
I noticed the plate of the voltage regulator tube is glowing red, and the tube has that blue glow inside the glass. The tube is getting so hot I think the glass is beginning to melt!
The grid drive (pin 5) is -55 volts. 10 to low and the other grid (pin 3) is 155 and it should be 125. That seems kinda high.
The voltages on the regulator tube look ok.
The amount of heat coming out of that regulator tube is amazing
Might that tube be gassy? I can remember those 6BK4's melting... Rather scary
Measuring the cathode current. Pictures time....
It measured 375ma, 125ma more than it should.
This is how pin 2 is grounded from the top. Not user friendly.
I had to heat that up with a big iorn and pry it away the clean the solder out, put a cap and wires for connection to the ma meter.
I also had to pull the other items grounded to pin 2 and run a separate ground just to be safe.
This is the VDR in the hv regulator circuit, it measures open. Can these be tested with an ohm meter?
If you pull the regulator tube does the cathode current of the horizontal output tube
drop down to something closer to the right value?
Someone else will have to answer the question about the VDR.
Yes it does! and the hv goes up to 35kv, the blooming stops and its super bright. 
It almost has to be that vdr, everything else in the regulator circuit is within spec. Its the only thing thats open.
I guess I will attempt to find one.
It is possible that the regulator tube itself is bad (gassy). For the VDR to cause this, it
would need to conduct too much (if I am reading the schematic correctly). Try disconnecting
the VDR and reinstalling the tube. If the tube is bad, the problem will come back. If the VDR is
bad, the high voltage should remain high.
If the blue glow mentioned is between the tube elements, that almost a 100% the tube is gassy and will over conduct causing your problem... Anyway a tube that's been as overheated as that one should be replaced, it'll no doubt fail in a short while...
Back in the day, a test adapter with attached clip leads was offered with open pin #2, made it easy to test current... At least I suppose one was a available for the 6HS5, I have one for a 6JE6 to check cathode current when adjusting the efficiency coil...
I changed that tube with one with a used one and it was mostly the same, maybe 20ma less cathode current. I will look for another one tomorrow.
I went by the local electronic store and I found the VDR in a big box of old tv parts that was on its way to the dumpster. The gave me the whole box! Mostly fuses, vdrs, caps etc.
I looked for that hv reg tube and they didnt have it.
How does this regulator circuit work? by loading the horizontal output down...that doesnt sound right. Seems as it regulates the hv the cathode current should go down not up.
It really does work by loading down the horizontal output. Yes, this is very wasteful. When
the CRT is drawing maximum beam current the regulator tube should mostly stop drawing
current. When the CRT is drawing little current the regulator tube should draw enough
current to keep the load on the horizontal output almost constant. Newer sets use other
methods to regulate the high voltage, but early sets did it this crude way.
Note that the circuit is actually trying to keep the 900 volt boost line constant, but that will
do a fairly good job of also keeping the high voltage constant
It was maintaining the 900 with it plugged in. Now I think im looking in the wrong place again. It almost seems there is to much horizontal output and the regulator is trying to 'absorb' it and over heating.
How can something so simple be such a challenge to get working right.
Well, 900 is a nice round number. It is likely that that line should really be some other
value close to 900 volts that ends up with the high voltage at the correct value. The
high voltage adjust pot should have been set at the factory so that the high voltage is
correct and the 900 volt line will end up at whatever voltage achieves that. The regulator
will then act to keep that line from varying which will keep the high voltage from varying.
You could try adjusting the high voltage adjust pot, but unless someone has fiddled with
it you would just be compensating for whatever part has changed value. If the tube is good
then most likely the VDR has changed value. But, if you replace the VDR, you should expect
to have to adjust the high voltage adjust pot.
The point I was trying to make in that last sentence it that normal component aging should
not cause things to be so far out of whack. If the VDR has changed value you may be able to
get the set working by adjusting the high voltage adjust pot, but I would expect the VDR to
continue to change value. Still, if you can vary the voltage by adjusting the high voltage adjust
pot, that would be useful information.
If the VDR is bad(open), hanging say 40meg in it's place should give some change... If that doesn't make a improvement, cut the resistance in half and try again...
The VDR, HV pot, and related resistors are a voltage divider, get the correct value and it should operate... Maybe not as intended, but much better than what it does now...
I replaced the vdr with a new one. Same thing. I lifted the VDR and still very little difference. I would think it should go open just like when the tube is out.
The 900 boost is about right with the tube removed and about 750 with the tube in.
I ordered a new reg tube so I guess thats next.
Yes, with the VDR lifted the regulator tube should stop drawing any current. While you are
waiting for the new tube, try measuring the voltage on pin 2 (the grid) of the regulator
tube with the tube removed and the VDR lifted. The voltage should be very close to zero.
If it is not then either C108 or C97 is leaky or there is some contamination on the circuit
board that is conducting current. If it does go to zero then the new tube should fix it.
The schematic shows both pin 2 and 11 as the grid. I was assuming that they are connected
together on the circuit card. Lets not assume. Measure both pins.
R157 the 1M was open. Now the regulator tube has cooled off and is working.
There is still another problem though because cathode current is still a little high, 295ma and the flyback is still getting to hot to touch. I can smell it burning after the set has been on for 30 minutes. Seems the flyback is hotter than before.
I guess I should start look at waveforms next.
I feel like I have made great progress with this thing today
Good work. Perhaps you now need to adjust the high voltage up a little. Now that the regulator
is working you should be able adjust it with the high voltage adjustment pot
Fixed.
I replaced the flyback today and everything is good in the HV circuit. The old flyback was melted to the point that the goo had run and puddled in the bottom of the box.
Runs cool, the cathode is about 240ma.
Now I just need to figure out why the sound is distorted and dial everything in and it will be a nice set.
I got this thing almost all the way back together and I got a buzzing in the sound. It sounds like the 60hz vertical buzz but its not related to the vertical.
It appears to be in the if/audio detector circuit. I changed all the tubes, adjusted the buzz control and no luck.
It changes with the video image. Adjusting the tuner fine tuning changes it too but I cant get it to totally go away.
The sound circuit is working with the 4.5 MHZ signal that is the difference between the
video carrier and the sound carrier. It has both the frequency modulation of the sound
carrier and the amplitude modulation of the video carrier. Your are relying on the FM detector
to reject the amplitude modulation. It sounds like your set is using a quadrature detector using
a tube like the 6DT4. There will be a tuned circuit connected to the third grid. On the 6DT4, that
will be pin 7. Look at the Sams to see if this is the circuit you have and to find the coil. You can
adjust the core in the coil for the best sound. Note that you can not use metal tools to adjust
coils. You have to have plastic alignment tools (I don't know if you already know that or not).
You should probably not need to turn the core more that 1/4 turn. Don't go very far in one
direction before you reverse and try the other direction.
20x1c38 schematic
It has a noise canceling circuit of some kind that I think may be the issue. Also Im getting retrace lines at the top. Their not real bad but annoying. I tried turning those coils with no results. Heres the schematic. I know its bad but it gives an idea.
It may be worthwhile to adjust the AGC. There should be an adjustment procedure in the Sams. For the
retrace lines, try turning down the brightness a little. Some sets have a problem with retrace lines because TV
stations are now transmitting data during the vertical retrace interval. When the set was made, they transmitted
a black bar during the retrace interval. That bar is no longer completely black because of the data. If the retrace
blanking circuit does not produce a long enough pulse, you can see that data as retrace lines.
For the sound, if the AGC adjustment does not help then you are back to the behavior of the sound detector.
It has to reject the video modulation, which is somewhat of a tall order. Your set is using a 6Z10 for the detector.
The coil I was talking about is shown just to the right of the 6Z10 on the schematic. Also, some detector tubes
will work better than others, even if both test the same on a tube tester.
I adjusted those coils and got 90% of the buzz out. I really didn't want to start tweaking those until it was a last resort. I have a feeling the thing is in need of a full alignment if those 3 coils were that far off.
its back together and I'm done with it for now. Its now 99% mint and perfect working condition.
Thanks again for the help everyone!
Heres what it looks like.
That's pretty!
A friend of mine refers to the gated beam tube as the "Buzz-O-Matic" detector...the 6Z10 is definitely such a tube.
As Tom Schultz said, you MAY have a situation where a different 6Z10 may work better, even though the one you have is OK.
A 6J10 may work better...or not.
Fortunately the video IF does not need to be adjusted as precisely as the sound IF. The video
IF passes a wide range of frequencies whereas the sound IF works on one frequency. And the
sound detector has to be centered on that frequency to reject the video modulation.
I recently had to replace the blanking diode because I had retrace lines. I used a Schottky Diode in place if the 1N34
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