Sylvania Hybrid D16 - low h-drive

[DavGoodlin]

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

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.

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.

[Findm-Keepm]

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.

[DavGoodlin]

Originally Posted by Findm-Keepm

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

[DaveWM]

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.

[DavGoodlin]

Originally Posted by DaveWM

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

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

[DaveWM]

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.

[DavGoodlin]

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.

[DaveWM]

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.

[DaveWM]

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.

[DavGoodlin]

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 IMG_6275.jpg

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.

[DaveWM]

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

[DavGoodlin]

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

[DavGoodlin]

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.

[DaveWM]

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.

[DavGoodlin]

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.