1968 Sylvania D10 color chassis help please! - Videokarma.org TV - Video

Chad Hauris · #1 07-20-2008, 08:05 PM

I am working on a 1968 Sylvania D10 with a horiz output/HV problem.
The grid and cathode voltages at the 6BK4 are only about 250 volts as opposed to the 350-375 volts they should be.

There is an indication of HV AC at the rectifier plate and HO plate as the neon lamp glows when placed near. I replaced the rectifier and 6BK4, no difference.
There is absolutely no spark when the CRT is discharged so somehow no DC HV is getting out. There is no light on the screen at all.

I measured the CRT screen voltage and it is about 200 volts low...also the B+ supply is slightly high, like the HO stage is not producing full output? The HO and damper tube both test good.
I measured the voltage drop across the 1000 ohm resistor in the 6BK4 cathode circuit and it is .04 volts.

The CRT is a replacement RCA Hi-lite from 1975.

I was thinking there might be a problem with the diode in the 6BK4 cathode circuit...haven't tried substituting yet though.

The flyback looks kind of melted but the neon lamp is glowing when brought near which seemes to indicate HV is being generated. Still don't know why there is no DC.
I didn't try running it with the plate cap off the 6BK4 as I didn't want to destroy the CRT with uncontrolled HV if that was a possibility.

Any experiences you might share with me on this chassis would be very welcome!

nasadowsk · #2 07-20-2008, 08:37 PM

You need a schematic? I might have one at home, but I can't get there till this weekend.

Of the top of my head, I'd look at:

Insufficient or off frequency drive (really off).
6BK4 driven into hard conduction.

To get the 2nd, you might wanna try popping the suction cup off the CRT, tieing it to a 'safe' spot, THEN pulling the 6BK4's cap. Though I'd imagine that tube lights up/looks weird when it's in full X-Ray mode. I haven't peeked into a color HV box in a while, but ISTR they changed their look depending on loading.

I'd be looking hard at that regulator circuit - esp if there's indication of good sweep.

Is there focus voltage (I think these had a focus diode still)? If there is and it's close, then you've got a regulator issue, IMHO.

roundscreen · #3 07-20-2008, 10:13 PM

Do you have a variac? If you do, You could pull the 6bk4 and put the plate cap in a plastic cup, Connect a high voltage probe to the crt anode cup, Bring up the ac slowly and see if you get high voltage. Is the heater in the high voltage rectifier tube lit?
Could be the heater wire from the flyback or a resistor in the tube socket is open.
Check and see what the horz output tube cathode current is doing. If it is really high there is a load problem or not enough horz drive.
Ed

andy · #4 07-21-2008, 12:11 AM

....

Carmine · #5 07-21-2008, 03:56 PM

Maybe this is the condition that causes a violent implosion; pulling nails out of the wall and all manner of catastrophe. Be careful!

roundscreen · #6 07-21-2008, 06:10 PM

Quote:

Originally Posted by Carmine

Maybe this is the condition that causes a violent implosion; pulling nails out of the wall and all manner of catastrophe. Be careful!

Humm, You may have a point here. I have seen sets running to much hi voltage and it blew the neck right off the crt. One was a color roundie and there was glass all over the place. I have a 68 zenith set that the vdr opened up in the regulator circuit and the crt was getting hit with over 40 kv. I was lucky and caught it in time or that could have been a nasty mess.
Ed