Maybe this is better in the tech forum. This forum did have a discussion of 69-72 Zenith sets with a bad VDR in the HV pulse regulator circuit.

This 1971 Sylvania 25V set is low hours and ready to look great but the 6LR6 grid measures -30 volts, Sams shows -70. The HV control varies the G1 from -19 to -30. Scoping the sawtooth at HOT grid, it looks good but only 120v p-p instead of 220 v. G2 measures 230vdc when it needs to be 160v. HOT Plate glows a bit but not blazing orange:smoke:

This set was "serviced" before and the VDR was replaced, along with a new fly, tripler, tubes and some wiring mistakes I corrected. I thought that was it but I have no way of knowing what VDR is in there now, Sams lists only an original Sylvania Part # 38-29959-1 and describes the VDR rating as 1ma@1050 volts. No Workman # was listed.:sigh:

I will scan a schematic later but the VDR "limits to 1300v p-p) a pulse fed to it from flyback winding via a 560 pf/2kV capacitor. I measure 25v P-p there.
This limits drive to the HOT so much that its in conduction.

What series of the D16 chassis? Sams lists Sylvania sets by model, and I know Sylvania had about 20 variants of the D16 chassis. I have my dad's service notes, but need more info to help - model number, and D16-???

So to start, are your B+ voltages all correct?



Edit: I checked my stash, and I don't have the VDR.

What series of the D16 chassis? Sams lists Sylvania sets by model, and I know Sylvania had about 20 variants of the D16 chassis. I have my dad's service notes, but need more info to help - model number, and D16-???

The model is a CL813P, Sams 1178, Chassis D160255
Main B+ measures about 277

Here is Doug's thread for Zenith http://videokarma.org/showthread.php?t=261780

I dont have the D16 but do have a D14 schematic in my sylvania tab book.

if yours has it, did you check the .01 cap that connects from the junction of the VDR/560pf 2kv cap to ground? (leakage at rated voltage)

is the HV adj doing anything?

did you check the horz out tube control grid coupling cap? (leakage at rated voltage)

check the plate voltage of the horz osc tube.

I dont have the D16 but do have a D14 schematic in my sylvania tab book.

if yours has it, did you check the .01 cap that connects from the junction of the VDR/560pf 2kv cap to ground? (leakage at rated voltage)

is the HV adj doing anything?

did you check the horz out tube control grid coupling cap? (leakage at rated voltage)

check the plate voltage of the horz osc tube.

I did replace about 6 paper caps (orange and green drops) around this area
but there was no cap from the VDR to ground on the Sams
HV control varies grid on HOT from -30 to -19 VDC when it should be -70:sigh:

Once I get home, I will check and update.
I also have a flat full of Sylvania-Colonial parts to check:scratch2:(thanks for checking yours, Brian)
Moyers has the part, so maybe that's next

on mine I see a 220k resistor at the junction of the plate to grid coupling cap for the horz out. from there it goes to a 6.8 meg thne to the HV adj pot.

the .01 is connected at the junction of the 220k and the 6.8 meg.

this junction also has a 100k to the 560pf/VDR junction.

The D14 schematic is almost the same as a D16. D14 is a 19" with series heater string.
My aunt had one once, I missed it when she "upgraded" to a 27" RCA BPC. Great set if you can find one.

There is 235 volts on the 6LN8 h-osc plate, about 15% high. The resistors all checked out.
The .01 is a disc so I did not change it. Will try that later this pm.

hmm, disc prob not the issue.

I would try disconnecting the hv hold down with the horz out pulled and see if that horz osc goes back to the 220pp.

this would put the prob on the hold down circuit and not the osc output.

also have you tried another horz out and horz osc? tubes.

generally anytime I have HV issues I start with a sub of the damper/horz out/shunt (not used in this case),HV rec, and horz osc.

I pulled the 6LR6 and then disconnected the VDR, no change in drive waveform or voltage at G1 pin 5 then went to bed.

I tested the tubes and have been using the "compromised 6LR6" (several came to me with the set) but will try new ones once something else breaks loose here, and methinks it has.

This morning, voltage on the Hor osc 6LN8 plate is 247v, a good bit higher than 205 on the print. The sinusoidal waveform at the cathode is spot-on. Grid voltage is -47, so the oscillator is running :) This is the low-drive sawtooth at 120v p-p 186247

So......with 2 minutes to spare before leaving for work, I checked the 27K-5% plate load resistor - it measures 7.8K. This HAS to be it since there is nothing in parallel to skew the reading. The drive waveform amplitude is developed across and thus depends on this resistor being exact. I may not get back to this until late tonight but I will re-connect the VDR and see how a new resistor shakes it up.

Thansk alot sofar Dave.

plate load resistor look like it got hot (charred looking) at all?

It would be a bit medium, if "rare" was normal....It came out and measured 7.8K

The new one will be a 1 watt metal film job that looks like cement, not shiny.
Always go heavy after finding a 5% resistor bad I guess

After replacing two resistors, the second being a 180 ohm at the 6LN8 hor-AFC triode grid drifted to 270 ohms, it was unlikely to fix the problem until this VDR is sorted out.
With the 27K plate resistor changes, plate voltage settled at 205 volts - right on. 280 volt supply measured 282 volts

Even better improvement the HOT grid drive to G1 pins 5/9 increased to 140 volt p-p and bias went from -30 to -55, this is progress for sure. I tried a new 6BL8, still no further improvement.

Now for the totally expected when swapping in sweep tubes; Both 6LR6 HO tubes that came with the set are shot, so I found my last new one, an RCA. I connected an ammeter to the cathode with a .33mf cap across the meter. Powered up and as current rose past 300 ma., I cut power. But HOT G2 voltage did settle at 160 vdc, right on spec.
Now for the interesting part - I swapped in another 6CG3 damper tube and HOT cathode current stopped rising after hitting about 270ma, still 30% too high.

My guess at this point is that the VDR is totally wrong and overloading the HOT and damper through the flyback.

I would be tempted to put a 5m pot in there and as a variable resistor and try to adjust it to see if you can control that way.

The 140pp still seems low, did you try the new osc tube?

also did you replace replace the coupling cap in the recap done before?

I see a wave shaping network in there as well (res/cap series to ground).

how does the pin 2 voltage on the osc look? i see -44v shown on the D14

I assume you have check out the other pin resistance measurements, esp the 6.8meg/750k hv pot.

The HV control does nothing to cathode current, low grid drive, or the -55 volts.
The .01 disc has not been changed but all surrounding paper caps were. Will try that later.

I did try a new H osc tube and pin 2 is exactly -44 v with either one.

I suspect the VDR is clamping too much, since I get almost no waveform there, D14 show 1300v p-p come from the fly via a 560 pf. Need to double check if the fly is installed correctly

I was afraid to pull the VDR out of the circuit totally and get runaway HV, so I will try subbing the VDR with a 5meg pot and see.

I suspect the VDR is clamping too much, since I get almost no waveform there, D14 show 1300v p-p come from the fly via a 560 pf. Need to double check if the fly is installed correctly


Something odd there for sure about the 1300v pp being so low.
IF pulling the VDR does not resolve (with a pot replacing it), that is you cant get the 1300v pp then maybe its the scope, I presume you are using a 10x prob? I don't think it would take much to load it down.

Looking at the schematic I can see how the HV control would not effect the drive directly, just too much resistance in series with the pot to have any real effect. Clearly its designed to impact the feed back pulse and not the grid drive.

Nice tough dog, hang in there.

Tough dog alright - the problem is that the first tech working on it was not familiar with this very simple sweep design, missed the obviously bad solder joints and went straight to replacing parts just like at a car dealer's shop, making new problems along the way :(

My RCA scope has a WG-299 10X probe and yes, that waveform is distorted and weak. When the VDR was lifted and subbed with a 5 meg pot, nothing got better.

I suspect that the fly was mis-wired when it was changed.
Or something is heavily loading down the flyback.
The 220 ohm 2 watt resistor across the Horz yoke coils was burned open, that may be a clue.
The tripler was changed and its wiring IS correct.
More to check in this next stage plus I have the old fly.

dont see a 220 ohm resistor on my D14 only a 4.7k and its isolated by caps.

dont see a 220 ohm resistor on my D14 only a 4.7k and its isolated by caps.

Duh - I was looking at the wrong coil. The 220 ohm was across one coil of the pincushion transformer, in series with horizontal yoke winding pin on the socket. It was cooked into the megohms. Hey maybe that PC trans is open:scratch2:causing a detuning of the flyback...

yep something to look into for sure. If the coil was open, then all the yoke current would be flowing thru that 220 if I read it right. If that was the case and the resistor had drifted that far (as is surely would or burn up), it would seem all the yoke kick back would be lost, seriously reducing HV.

Bingo - one open coil on the yoke. I pulled it and saw the copper oxide green nuggets at various spots on only one of the windings. Even where the windings are fabric-taped, have the green death. I did poke around the nuggets with a sharp chopstick and got the winding to read 14 ohms, across pins 4+6, and the "good winding" reads 4 ohms.

Im going to power it up and look for sparks in the windings with yoke face up on the bench, in the dark, CRT and HV anode disconnected of course.:smoke:Stand by for sparks...

I found where the winding was compomised and tried to make a repair, ending up joining two adjacent windings and getting 9 ohms, when I should get 4.2.

This was all needed to get some continuity in the yoke and see progress tuning the flyback circuit toward its happy spot where it makes HV.

I fired it up and now have a 500 volt pulse at the VDR:thmbsp: so we are getting there, waveform #38 on the D14 schematic shows the fly pulse at 1300v p-p, (open yoke it was only 25v p-p).
The neon test lamp shows more RF glow and from a greater distance at the HOT cap and the tripler input from the fly (disconnected) - thats also progress

Now may be the time to see where I can get a yoke. Its stamped 51-29986-2 but the Sams calls for a 51-15949-4, Thordarson Y109 or Triad YC-312-2.

Now i wonder if a new yoke will fix everything?:scratch2:

I would guess yes, IIRC I saw that yoke on ebay a while ago, maybe still there as it was a buy it now.

Maybe moyers would have it?

Moyer's is worth a try. Id hate to buy it and find out thats not it...but what else could it be. The original fly had the green death and the attendant slime on some of its leads at the terminal board....and THAT can be fixed.

did it make any pic before? cant recall how this started. Was is just low HV or was it no pic (so defect in yoke would not be seen on screen).

I have not had a raster so far, the open yoke had killed the HV.

As above, I got some HV when I jumped a broken section of winding but had the tripler disconnected.

I'll try putting this mess together and see what I get on the screen.
186324

After a short was introduced between adjacent windings, I got about 9 ohms and powered up.
Got a raster now :) that is dim and 10 inches wide.

Must find a yoke with correct inductances, unless I can adapt a similar unit from a Zenith or RCA.

:thmbsp: since it uses a horz out tube, my guess is any late rectange tube type yoke should match.

I looked on Ebay - there were some close calls but no matches.

Update 3/31: I called Moyers and they actually had a Thordarson yoke.:) and the right VDR.

Update 4/1: Yoke appears at back door with VDR in a NAP bag, that was quick

Update 4/2: Install later tonight - fingers crossed.

Update 4/3::banana: Total success with the yoke, 24-27 kV range, perfect drive waveform to the 6LR6 grid - pictures will follow once I get my Sencore VA-48 connected.

The HV was measured with G2s turned down, dark screen and HV was still 27 kV. 6LR6 cathode current was 190-215 mA, should be 160.
There was a 6.8 Meg 5% resistor off the HV pot that measures 7.3 M, and changing it to a pair of selected 1-watters brought it down to 6.6 M. (I had tried a 5.6 M and that dropped HV to 12kV) Now the HV is 25 kV at the low end. the cathode current dropped to 160 mA - a bunch of baby steps just getting here!
During scene changes with DTV box signal, it ranges 23-25 kV which is acceptable.

Now the only thing keeping it from the final set-up is the vertical issue I was ignoring. The V-hold pot is at minimum and frequency is still too low, the frame jumps up a few times depending on video level. Adjusting the height and linearity works as intended, but adjusting any of the V-controls causes a momentary loss of vertical sweep. Not acceptable at all. First, I will check resistors AND then replace just the loop capacitors.

After testing the resistors and finding several out of tolerance, especially those machine-inserted half-watt resistors, I then just replaced all orange and dk green drop caps.

I measured the old caps and they too were way off. The new mylar caps were right on, using my Fluke 177. It seems, with all these off tolerance values, vertical sweep and of course convergence would have been greatly affected.

After a power-up, vertical hold was right in the middle of the control range. There is still an issue with the Vert lin control being maxed out just to get enough sweep at the bottom. Maybe its time to try a tube, though the 6LU8 looks new.

An initial set-up of convergence showed the R/G vertical lines going to 1/2 inch apart at the bottom of the screen. After tweaking the right side R/G vert coil, I was able to readjust the static and get the bottom issue much better. The R/G vertical lines at top and left have good range and adjusted easily.

wow that is a big diff, I presume the left and center vert RG convergence was ok?

You may want to check the pots/resistors etc... if its not working right.

Glad to hear you are making progress on it.:thmbsp:

maybe even try with out the conv board in (you may have to sub in a resistor 100ohm in the vert circuit if its like an RCA to get the full vert deflection).
Just to see how it looks without edge compensation.

Last problem here - I need a 6CE3/6CD3/6DT3/6DU3 damper tube.
There is a 6BW3 in it now, and I noticed a light green glow in the cathode--plate region.
At one point, I heard a snap and lost raster breifly, only then I noticed the incorrect tube.
NFW am I running it now, until I get the right tube and risk losing the NOS 6LR6.

What is odd here is that the basing diagram is the same, but the Sams Tube Substitution book No. 18 lists only 6CD3/6DT3/6DU3.

Per the 1981 RCA tube handbook, most characteristics are equal between the 6CE3 and 6CG3/6BW3 except heater current; 2.5A vs 1.8A and plate dissipation; 11 w vs 6.5 w.


Of course I have no NOS tube so I have to dig deep in my pre-sorted bins and hope for the best...and I found 3! plus 5 34CE3's.

Put a used Sylvania 6DU3 into service, no green glow and HV up to 30 kV.
Increased R440 to 22 meg and HV drops to 27 kV, turn up the brightness (Sams HV adj procedure is to measure HV with normal viewing brightness) and it lands on 25, with control at minimum.
This has to be done!
186707

186708

Good to see such progress on this set. It was a rat's nest of troubles for a while but worth the effort to fix.

I'd like to see a pic of the whole thing if you don't mind. Also, what's that little Heathkit box visible in the background beside the Sencore VA48?