Newbie's First Questions RCA TS 721 Restoration

[Chris K]

Originally Posted by Notimetolooz

Although not shown in the picture you posted, if you follow that line it goes to the grid of the sync amp through C120. The grid of that tube should be biased with -3.5V, if C120 were shorted the resistor junction wouldn't be that negative unless the -3.5V supply was much lower. I could believe -1.5V at that junction but not -15V. Double check that supply.

What your meter would measure as the DC value of the video signal depends somewhat on what the pattern is on the video signal. It is possible that your meter is confused by sync pulses on the video. The video signal could be as large as 32Vpeak-to-peak according to the scope trace on L106. Many times the voltages on schematics are with no input signal, also note the control settings.

I don't have any definite explanation at this point.

Since modern solid state electronics generally run at lower resistances a leakage like a 1Megohm across a cap would not cause much of a problem, but in a tube circuit it would. Those caps you got cheap might work OK in a solid state circuit.

I will double check the voltages and provide a better overall picture of the issue later this evening. Thanks for your thoughts...as always!

[Chris K]

Originally Posted by Chris K

I will double check the voltages and provide a better overall picture of the issue later this evening. Thanks for your thoughts...as always!

Just want to state some of my previous voltage posts were done current limiting through a 500 watt bulb so the numbers below are the real numbers at 120V AC. Also, R153B that is supposed to be 630 ohms has a 500 ohm resistor in place. I have the appropriate one on order but the lower value one in place may skew some things a bit from the schematic voltages.

All the distributed voltages off the bleeder resistor package are close enough. I've got +230V, +149V, -3.6V, -15V, and -86V. I've got around +290V out of the 5U4.

OK...I checked voltages on V104B the 6AL5 and all the pin voltages are fine. The voltage running into C120 from that tube is -3.5 volts and on the other side, it's registering the -38V DC mystery voltage I'm trying to track down. Next, I checked the 12AU7 pin voltages and all of them are spot on except Pin 6 which is -38V. Pin 2 on the CRT is the same -38V and now (another WTF) I've got only +70V on Pin 11 of the CRT. Pins 1, 10 and 12 are fine at +150, +150 and +300 volts respectively.

Maybe I should reach out in some way to the community here to see if anyone with the knowledge to help me lives in the area around Philadelphia PA. I've made a mess of this and I know this forum communication isn't ideal and I'm sure I'm annoying and frustrating everyone who have tried to help. My apologies...again!

[Notimetolooz]

Not at all.
It helps thinking of troubleshooting as a detective or mystery story. Not everyone is into those types of stories I suppose. It takes a while to figure out what is really going on. You have to put the clues together. Sometimes there are red herrings.
Really good that the power supplies are right. Many of the voltages depend on the current draw on them because they come through resistors. The voltages being correct means there are no major cases of the currents being off.
However you have to get the facts right. I think (hope) you got the CRT pins mixed up. Pin 1 and 12 should be at 150V, pin 10 should be more like 330V (that is from the boost voltage), pin 11 could be anywhere from 37 to 225V depending on the brightness control setting.
CRT pin 2 and V105 (12AU7) pin 6 should be 70V. There is a low DC resistance connecting them so the voltages should be about the same.
V105 pin 8 is connected to -85V through a 47 ohm resistor so that voltage will have to be about the same even if the current is very high. If V105 pin 8 was connected to pin 6 there would be a battle of the voltages (150V being the other) across R124 and R125 that could result in the -38V.
V105 could have a cathode to plate short or there could be a short between pin 6 and 8 on the socket.

[Kevin Kuehn]

Originally Posted by Notimetolooz

CRT pin 2 and V105 (12AU7) pin 6 should be 70V. There is a low DC resistance connecting them so the voltages should be about the same.
V105 pin 8 is connected to -85V through a 47 ohm resistor so that voltage will have to be about the same even if the current is very high. If V105 pin 8 was connected to pin 6 there would be a battle of the voltages (150V being the other) across R124 and R125 that could result in the -38V.
V105 could have a cathode to plate short or there could be a short between pin 6 and 8 on the socket.

One could easily pull V105 and see if pin 2 of the CRT swings positive(indicating shorted V105 cathode to plate). Also if C115 were leaking it could cause the plate and cathode to conduct hard simulating a short. If removal doesn't eliminate the negative voltage on pin 2, then your shorted socket scenario would seem likely.

[Chris K]

Originally Posted by Kevin Kuehn

One could easily pull V105 and see if pin 2 of the CRT swings positive(indicating shorted V105 cathode to plate). Also if C115 were leaking it could cause the plate and cathode to conduct hard simulating a short. If removal doesn't eliminate the negative voltage on pin 2, then your shorted socket scenario would seem likely.

I pulled V105 and CRT pin 2 went from -38V to +150V. Before I pulled it, the voltage on pin 7 was -85V. BTW, the funky voltage on CRT pin 11 was because of the position of the brightness control. It is 225V and correct when the pot is in the max voltage position.

[Kevin Kuehn]

Originally Posted by Chris K

I pulled V105 and CRT pin 2 went from -38V to +150V. Before I pulled it, the voltage on pin 7 was -85V. BTW, the funky voltage on CRT pin 11 was because of the position of the brightness control. It is 225V and correct when the pot is in the max voltage position.

That all sounds correct. Do you have another 12AU7 to try in circuit?

[Chris K]

Originally Posted by Kevin Kuehn

That all sounds correct. Do you have another 12AU7 to try in circuit?

I have a couple of NOS 12AU7s. I'll try them

[Chris K]

Originally Posted by Kevin Kuehn

That all sounds correct. Do you have another 12AU7 to try in circuit?

Tried 2 different NOS 12AU7s, the original RCA tube that was in the set, and 2 Mullard, strong testing ECC82s...still a -36V...-37 volts on pin 2 of the CRT. It's not the tube I think we can say