Sylvania Hybrid D16 - low h-drive

[DaveWM]

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.

[DavGoodlin]

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.

[DaveWM]

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

[DavGoodlin]

Originally Posted by DaveWM

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 opencausing a detuning of the flyback...

[DaveWM]

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.

[DavGoodlin]

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.Stand by for sparks...

[DavGoodlin]

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 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?

[DaveWM]

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?

[DavGoodlin]

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.

[DaveWM]

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).

[DavGoodlin]

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.
Attachment 186324

[DavGoodlin]

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.

[DaveWM]

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

[DavGoodlin]

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: 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.

[DavGoodlin]

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.