CTC 5 Low high voltage

[radiotron]

are you SURE you didnt put a cap in backwards?

[oldtvsandtoy]

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[oldtvsandtoy]

aaa

[N2IXK]

By "plate cap", DaveWM didn't mean "plate capacitor", but the physical connection "cap" on the top of the 6CB5 horizontal output tube.

Pull the 6CB5 out of the set, and measure the DC voltage on the wire that was connected to the top of it. Should be +375V or so. DO NOT try to measure this with the tube installed, as the high voltage pulses normally present will fry your meter!

Put the tube back into the socket, reconnect the plate lead, and measure the voltage on the screen grid (Pins 1 and 8). Should be about +175V.

Check the voltage on the cathode (Pins 3 and 6). Should be about +7.5V.

[oldtvsandtoy]

aaa

[Penthode]

Originally Posted by oldtvsandtoy

Ok I got around to checking the horz output tube. Here is what I found. With top cap off and tube removed, I get 416vdc at the cap.
pins 1 & 8 190vdc
pins 2 & 7 6.3vdc
pins 3 & 6 8.7vdc
Pins 4 & 5 -20.5vdc
Well 4 & 5 dont look right should be -40vdc So what do you think now?

Pins 2 to 7 should be 6.3 vac of course.

Pins 4 and 5 are the control grid. Without a scope it is hard to guess. But the bias is developed by the grid rectification and only -20vdc implies you are not getting sufficient horizontal drive or else the coupling capacitor is leaky.

Then again, you may not be getting sufficient horizontal drive. A scope would sure help the diagnosis process.

[DaveWM]

a few things, did you replace the .01 cap that could be leaky that goes to the grid? did you replace the 2mf electrolytic cap on the video board? Agree with pentode a scope reading of the grid of the horz out would tell a lot. the -20 was with the tube in correct? and what kind of meter were you using to check it? VTVM's are the best, a DMM will prob be ok but a VOM would be too much of a load.

[DaveWM]

oh and you did do a tube swap from a KNOWN working set, all HV tubes (horz out, damper, HV rec, shunt) and the horz osc tube as well.

[N2IXK]

The high cathode current and low grid voltage sure sound like low drive.

You really want a scope at this point. An external drive source like a 1077B or VA48/62 would be a great help, as well.

Start checking resistors and caps in the horiz osc. circuitry.

[DaveWM]

good point about using an BK analyst, those are great for these kinds of problems. You can quickly clear the fly with the plate drive and then move on to the grid drive to check the horz output tube. I picked mine up for 9$ IIRC, fun to have and you can make neat shadow puppets with it too

[oldtvsandtoy]

aaa

[Penthode]

Originally Posted by oldtvsandtoy

Would the -20 v take down the high voltage that much?

It is just that if the horizontal drive is low, then the resultant bias will also be low. That is because the bias is derived from the drive amplitude: eg a proper level drive will give higher bias. And the low drive would reduce the high voltage substantially.

[DaveWM]

what is pin 6 of the 6CG7 (horz osc tube), should be 235vdc, there is a voltage divider network made up of a 39k and a 82k on the horz board, I would check there if the voltage is not right.

[oldtvsandtoy]

aaa

[N2IXK]

Check R606 (68K) and C606 (330 pF) on the horizontal PCB, as well.

The voltage divider resistors are R603 and R604.