Newbie's First Questions RCA TS 721 Restoration

[Chris K]

Originally Posted by Kevin Kuehn

A video of that screen would be more telling. The diagonal shading makes me think your horizontal isn't locked. I don't see any defined horizontal video, only the wavy vertical along with the raster scan lines. The raster lines themselves seem quite well focused center to bottom.

So I'm just going to start from jump. I disconnected my signal generator. I have the TV laying on its side so I can get under the chassis but I rotated the image to the screen appears as it would normally. The raster appears to be fine. It is in focus and the picture is very bright. What should my next step be? Is there any way to address the brightness issue at the top and bottom?

It pisses me off I had an imperfect yet decent picture on this at one time but it appears I screwed that up in trying to get the sound working.

[Chris K]

I have a chance to buy a Leader LCG 396 generator for a decent price. I've heard too many negative things about the Conar 682 I've been using, among them, that it overdrives the signal and it's not variable. I hope the Leader works on B&W old sets!!! Does anyone have any knowledge on the Leader they can share? Is it a worthwhile buy at $50? Working and in very good cosmetic condition.

[Notimetolooz]

Oh, you are trying to confuse us by rotating the scan!

The brighter bands are probably due to the scans not being uniform, the lines are crowded together at the top and bottom making it look brighter. That means the scan shapes are not straight ramps.

The scan frequencies could be far off. No video signal is making it to the CRT.
If you had a decent oscilloscope you would be able to figure the approximate frequencies and the waveshapes of the sweep. If the frequencies of the sweep were not very far off and the video was making it to the screen a pattern generator would work instead.

That Leader would be fine (assuming that particular unit is working right).
If you get it one of the first things to do is try it on a working analog TV (a color set would be fine).
That will test it out and get you familiar with how it is used.

[old_tv_nut]

If the whole tube face is lit up like that, the vertical height is too great. Reducing it to normal (about 3/4 of the tube diameter) may eliminate the bright areas (or not). You need to get sync and video working and see a picture before you can really tell.

[Chris K]

Originally Posted by Notimetolooz

Oh, you are trying to confuse us by rotating the scan!

The brighter bands are probably due to the scans not being uniform, the lines are crowded together at the top and bottom making it look brighter. That means the scan shapes are not straight ramps.

The scan frequencies could be far off. No video signal is making it to the CRT.
If you had a decent oscilloscope you would be able to figure the approximate frequencies and the waveshapes of the sweep. If the frequencies of the sweep were not very far off and the video was making it to the screen a pattern generator would work instead.

That Leader would be fine (assuming that particular unit is working right).
If you get it one of the first things to do is try it on a working analog TV (a color set would be fine).
That will test it out and get you familiar with how it is used.

Leader came today and I hooked it up to a Zenith 12" I restored...a Sidekick. It works perfectly. The bright spots and other artifacts are reflections and issues with camera lighting. Hopefully, I now have a tool that's reliable and I can use to diagnose problems with the 721 and other TVs I work on. I guess you get what you pay for. The Conar 682 was $10! I've seen this Leader unit selling for as much as $200 used on ebay. I got this one for $50. It was supposedly owned by a retired Westinghouse engineer who knew how to use it and take care of it.

[Penthode]

Originally Posted by Chris K

So I'm just going to start from jump. I disconnected my signal generator. I have the TV laying on its side so I can get under the chassis but I rotated the image to the screen appears as it would normally. The raster appears to be fine. It is in focus and the picture is very bright. What should my next step be? Is there any way to address the brightness issue at the top and bottom?

It pisses me off I had an imperfect yet decent picture on this at one time but it appears I screwed that up in trying to get the sound working.

The set does not have AGC and it looks like the contrast control is set too high resulting in a negative picture.

Also the vertical height and linearity controls are out of adjustment.

[Chris K]

Originally Posted by Penthode

The set does not have AGC and it looks like the contrast control is set too high resulting in a negative picture.

Also the vertical height and linearity controls are out of adjustment.

Thanks so much for your observations and advice! I read most of the restoration thread on your 721 and found it very helpful.

[Chris K]

Originally Posted by Penthode

The set does not have AGC and it looks like the contrast control is set too high resulting in a negative picture.

Also the vertical height and linearity controls are out of adjustment.

A quick stupid question. What could account for having a negative -15V at the intersection of R124 and R125? I have +150V as indicated just before R125. I've checked C117, C115 and C120 for capacitance and they are fine but I don't have a way to test for leakage.

[Yamamaya42]

Rule #1, never ever trust the old paper type paper capacitors, they are BAD 99.9% of the time no matter how they may test, if you have not done so already, replace them all with new modern film types and spare yourself from future pain of failure.
If you are concerned about the aesthetics new type caps in the set, there ARE ways to gut the old ones and hide the new in their shell as Bandersen has shown in his videos.

As for your problem, can't be sure but if you DO have the old paper caps in it, it is very likely that C115 is bad, and you will be constantly plagued with problems as long as the old papers are in there.

[Chris K]

Originally Posted by Yamamaya42

Rule #1, never ever trust the old paper type paper capacitors, they are BAD 99.9% of the time no matter how they may test, if you have not done so already, replace them all with new modern film types and spare yourself from future pain of failure.
If you are concerned about the aesthetics new type caps in the set, there ARE ways to gut the old ones and hide the new in their shell as Bandersen has shown in his videos.

As for your problem, can't be sure but if you DO have the old paper caps in it, it is very likely that C115 is bad, and you will be constantly plagued with problems as long as the old papers are in there.

I replaced all of the "waxies" with film caps but some, as is the case with C115, were replaced with very inexpensive bulk caps I bought as a set on Amazon...Bojack I believe was the claimed brand name. 100 capacitors of varying values and voltages for a little over $10 USD! You get what you pay for I guess. I'll replace the 3-4 "Bojies" I have in there with proper film caps from DigiKey.

[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.

[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.