Found a Sylvania SS/Tube hybrid

[Electronic M]

Perhaps the B+ boost system is dead.....Connect a DC volt meter to the wiper of one of the screen pot with it set at min, and then turn the pot up. It should read something like 350V or more (need to find and check my D12 schematic) with the set on and warmed up, and rise as you turn the control up. If it does not rise the B+ boost is out....Could be from a shorted cap filtering that line, an open boost diode, or a few other things. B+ boost is likely around 1200V so it is likely unsafe to measure it with the set on (unless your meter goes well over 600V), but that voltage should bleed down quickly once the set is turned off.

[zeno]

Sounds like no boost.
IF you have a meter that is OK at 1000V......
one end of the control will have abt 200-250 VDC
the other 900VDC. The wiper will vary between the
two. Hopefully the 900V is missing or very low.
You can measure them at the top of PCB
at the pins.

73 Zeno

[MRX37]

On second thought I don't think my meter can handle that. Yeah, it's 600 volts max.

So perhaps i shoulld check the parts related to that line. Where do I start checking?

[Electronic M]

Originally Posted by MRX37

On second thought I don't think my meter can handle that. Yeah, it's 600 volts max.

So perhaps i shoulld check the parts related to that line. Where do I start checking?

As long as you measure the wiper (center) pin voltage at the pot and watch the meter as slowly turn the pot up from min you won't damage your meter....They can take some more volts on their input than their rated to measure.

[MRX37]

Might take a stab at it tomorrow. To be sure, fully counter-clockwise should be the minimal setting?

[Electronic M]

Originally Posted by MRX37

Might take a stab at it tomorrow. To be sure, fully counter-clockwise should be the minimal setting?

Should be....Have not ever seen it the other way.

[old_coot88]

Bettin' on the boost diode.

A little tip FWIW: On sets lacking a removable service panel on the bottom, It's much easier to work under the chassis if you turn the whole cabinet up on end and slide the chassis back enuff for access. Common shop practice. They had carpeted service carts to set 'em up on.

[MRX37]

Okay, so getting to the center pin on any of those screen pots isn't happening. I'd have to disconnect some wires that I'd much rather not touch since I don't know where they go, what they control, and might have trouble putting them back.

I have to assume there's a test point on the chassis to test for B+ right? I notice a lot of pins up front. If I set the screen pots to minimum and use a test point, could I test for B+ without frying my multimeter?

Also noticed that not only were the screen controls turned to max, so were the drive controls. Someone before me definitely had this issue. Hope the CRT is okay.

[zeno]

Boost rectifier is behind focus control. Probably a large brown one with
silver on one end & gold on other. Then it goes to the G2 controls
through a 100 K ohm resistor.

73 Zeno

[MRX37]

Hm, not sure what it's supposed to look like. Here maybe this will make it easier for us to find:



That white shaft in the upper left corner is the focus control. Nothing's directly behind it, though there is something that vaguely looks like a fuse (top of the image, right of center) that's brown and silver on both ends. Is that my test point?

[Electronic M]

That thing half cut off at the top center of the pic is probably the boost diode, but unless it is weak/dead it probably will be too high a voltage for your meter....That is why I said to measure it through the screen pot.

If you can't clip directly to the screen pot wiper terminal, then pull the CRT socket, stick a wire about the thickness of the CRT pins into a pin that connects to a white wire with a color stripe and use that as the wiper pin test point for a screen/G2 pot of the color of the wire stripe. Measure voltage to ground with the pot at min, and turn up the pot if the voltage does not rise with the pot setting and get high enough, then the boost system is weak or dead.

Another option would be to measure the B+ boost with reference to the high power transformer derived B+.....Connect the meter negative to the positive of the first lytic fed by the diode bridge you replaced, and connect the positive to the B+ boost line. The boost voltage will be the voltage of the bridge WRT ground + the voltage you measure... The meter may still get more voltage than it can measure this way so the G2 wiper method is preferred.

Do you have a schematic? IIRC I have a Bietman's most often needed Color book with the schematic for the D12 chassis that I could probably scan or take photos of for you.

[MRX37]

Okay, removed the CRT socket and used a suitable wire to connect to a white wire with a blue stripe. Grounded the negative lead to the chassis frame.

On power up, got around 30 volts. As the set warmed up, the voltage climbed 125... 200... 300? 400?! At around 500 volts I cut power. This is with the screen pots at minimum... I think if anything the B+ is too high.

And no, I don't have a schematic. I'm basically working on this thing blind and with little clue as to what a lot of these components do.

[old_coot88]

Originally Posted by MRX37

...Grounded the negative lead to the chassis frame.

What he was sayin' was connect the meter's negative lead to the B+.

The 'Boost' (or B boost) voltage is stacked or 'bootstrapped' on top of the regular B+ supply. Be careful though, since the whole meter will be riding at B+ potential.

[rca2000]

I have a D-12 sams. 1045-2

'Sams shows ir as "X29" and it IS indeed on that terminl strip... so I wuould check the voltage there.

But if you are getting OVER 500 volts to the G2 on the tube. with the G2 controls at MIn.....it is NOT likely a problem....

[MRX37]

Reversed the leads, same voltage readings, but negative this time. Lets say the B+ is good. What else would be causing this dim, blurry image.

I may take a video of it to show you guys exactly what it's doing. if it'll help let me know and I'll do it.