Doug:

Make some voltage checks.
1. AGC voltage at the first IF
2. RF AGC
3. RF amp plate voltages. About 250 on pin1. Pin 6 should be about half of pin 1.

The RF AGC should be a few volts less negative than the IF. If not, check 3.9 Meg in the divider circuit.

Check the 1K RF amp plate resistor in the tuner. It will often open up after the RF amp tube shorts.

If the IF AGC is high negative on and off channel, the IFs are probably oscillating. Different solutions.

Does the signal source work OK on other sets?

Don

I've found some brands of tubes not to work no matter how good they are.
What brand is the new tube?
I put a new 6J6 in my RCA 6T74. It was a Sylvania. Reception died. I put a GE tube in, and, reception returned.
Bill Cahill

Don, I'll get those voltage checks tomorrow and get back to you. What I can tell you tonight is that I did some checks on the 5BR8 AGC tube, which I replaced. Pin 3 SB 120V. This is the middle lug of the AGC. I have 220, the same as coming of the AGC end lug.
Pin 2 is ok. SB -.4 volts, and I have about -1.0 V.
Now, Pin1. With the old 4BS8 tube installed, where I get the snowy pic, I get about 75V which is ok. SB 80V. Installing the new 4BS8 tube, whch gives the the overloaded raster, now has 180V on pin 1.
Bill, the new tube is an RCA.
Doug

O.K., but, that's no guarantee it's good. Sounds to me as if the tube is causing some trouble.
Try different brands.
Brand name is no guarantee.
I have a brand new old stock RCA 6BG6 that checks perfect, but, lights up blue, and, shows a short picture.
No guarantees.
Bill Cahill

Doug:

The AGC control should vary the 5BR8, pin 3 voltage from 0 to 220 volts. The plate, pin 2, will vary from 0 to 30 to 50 volts neg while rotating the control. Normal operation will be ~160 volts. There shoud be more than neg 1 on the 5BR8, pin 2 under normal signal. If the AGC control varies the cathode across the range, we can assume it is working for now.

The 4BS8, pin 1, should have ~250 volts; pin 6 about half. The only things separating pin 6 from the hard B+ are some coils, a 1K resistor and the switch wafer. If 70 volts is lost across the 1K, I suspect it is burned. Don

The washed out picture may be caused by an open peaking coil in the plate of the 12BY7. I don't
have the schematic, but we used get open coils wound on resistors. When the coil opened the resistor
became the plate load and made the video weak. It could be scoped by comparing the sharpness of
the video at the grid compared to the plate.

I got my new brightness control installed today, and the brightness is now working perfect. That was a pain putting that in since two of the legs were soldered to the PC board. With that solved, I went back to the signal problem.

Bill, I found an ebay seller that had two 4BS8 tubes for $5 and ordered them this morning. The seller only lives one state away, so if he'll ship then in a hurry, I should have them in a few days. That will determine if that new 4BS8 tube is good or not.

Don, I did some AGC voltage checks on the 5BR8 and have the following.
Pin 1 is 186V regardless of where the AGC control is set.
Pin 2 varies from -6.30 to -4.20 by varying the AGC control. When the signal cuts out at the extreme, the voltage goes to -220V.
Pin 3 is 0V at the cutoff extreme. When I vary the control is starts at 205V and goes to 213V.

On the 4BS8 (new tube installed) I get 190V at pin 1 and 105V at pin 6.
Don, you have twice mentioned a 1K resistor, but I don't see it in my Sams 375-4. Are you looking at the same Sams? My Sams list the RF amp tube as a 4BQ7. The tube layout pasted on the HV cage on my set list the RF amp as either a 4BQ7, 4BS7, 3BC5, 3CE5 and states only replace tube with original type. If you have the Sams, what resistor is this?
Also, Radiotechnitian asked about a peeking coil on 12BY7. The plate, pin 7, feeds into A11. Pin 7 SB 105V. After A11, is goes thru 2 resistors and is fed from the 250V line. My pin 7 has 230V. I'll check those 2 resistors between the 250V and A11 next.
Doug

Doug:

The 1K is not in all sets. There are at least two versions. The RCA service manual shows both. I do not have any Sams.

The latest 4BS8 readings look OK now so with or without the 1K I would not worry about it.

The 5BR8 AGC amp grid is driven from the 12BY7 plate and varies with signal strength.

The 12BY7 plate is running too high. Check for:

1. High neg grid voltage at pin 2, on and off channel
2. Low screen voltage, bad tube socket
3. High cathode voltage, off-value cathode resistor
4. Shorted plate resistor (unlikely)
5. Bad detector diode. Make sure F-B ratio is at least 100:1 (on RX10 or RX100)

I don’t see any peaking coils in the 12BY7 plate circuit. Check the .22 video coupling cap. Don

Don, Here is what I have come up with re: the suggestions you made.

1. Pin 2 on 12BY7. On channel has -10.50V. Off channel has about -1.1 V. To high
2. Screen voltage OK. I have 143V. SB 145.
3. Cathode SB .8V. I have .02. I had already replaced the .022 capacitor. The 27phm resistor checks at 30 ohm
4 Plate resistors are ok. I checked them last night.
Next I'll see about trying to remove the can that houses the video det diode. It's soldered to the ground on the PC and will be hard to get loose.
I also replaced the 10K resistor coming off the grid, but this had no effect.

Doug

You can check the video detector from pin 2, 12BY7 to ground. Remove the tube to be sure. The 100 ohm resistor and coils will not throw you far off. It is easier than pulling the can. If it checks bad, then dig in. Don

Don, I went ahead and took the can off and figured I had to take the diode out to check it. I cut it too short, so I will have to replace it now. The only thing I had for a signal diode was a 1N60, so I tried that. I think it acutally got worse. I look online and on Ebay and can't find anyone selling 1N295 that I need. Do you know a good source or cross reference this diode?

Thanks
Doug

Just use a general purpose silicon diode, don't anyone say it won't work either 'cause I got a pair of them in my fully functional CTC-7 right now.

Doug:

A 1N60 or 1N64 will work fine as a video detector. That is what most TVs used at the time.

Don

My two 4BS8 tubes arrived today, but neither one seemed to make much difference. To satisfy my curiousity, I changed my signal souce from the VCR on Ch 4 to the cable on Ch 3.
It looks like I'm getting a negative pic with a lot of snow. Any ideas on where to go from here?

Doug

Perhaps tuner osc. problems?
Maybe agc is over loading, anong with weak signal?
Is your antenna balum coil alright?
Another guess, video I F troubles?
Bill Cahill

Make sure the video detector diode is not in backwards. What polarity is the detector output? If it's slightly positive or near zero, the diode is likely reversed. Do you see the neg ten volts reported earlier?

Don

FINALLY SUCCESS!!!

This set has been on the back burner since I got my GE locomotive, but I am waiting on parts for the GE. Anyway I started again yesterday on the RCA. I checked and replaced the resistors going from the RF on the tuner to the AGC, but still nothing. AGC voltages were still out of line. I then noticed when I touched my meter probe to the end lug of the ACG control (where the 220V feeds in) the pic cleared up. I wondered if the AGC control was bad. It was a 150K, and the closest I had was a 200K. I decided to sub it anyway since that would verify if the old control was working.

That control was soldered to the PC board, and it was hard to get out. The 200K control I had was a PC mount luckily. After installing it, I fired it up. The set was completely dead. I started tracing the 110V thruy the tubes to find out what went wrong. Coming out of the 3BZ6 was a line that does thru a coil and onto the 6CG7. I found a bad solder joint there. You guys had warned me about bad solder joints on that set's PC board, and I discovered one. I must have broke it loose getting the control out. Firing it back up resulted in an amost perect pic and sound.

I've still got to tweek the vertical controls, and there is something that has fallen between the front of the CRT and the safety glass. It's visible at the bottom right of the screen. I guess I'll have to remove the CRT to clean it out. As of right now, I'm getting a sharp pic. Hope it stays that way.
Doug

I went ahead and pulled out the CRT to clean the safety glass. I had a time getting it out. Many times the Sams will give instructions for CRT removal along with the chasis removal, but this time there was no CRT removal instructions.
I figured I would just loosen the two screws holding the CRT bracket in at the bottom on the cabinet, and it would slide out. WRONG. Something seemed to be holding it at the top. There is a long "V" RCA emblem on the top of the set, and that seemed to be what was holding it. I tried to pry it up and broke the mounting tabs off of it. I can reglue the emblem back down, but that still didn't solve the problem. I saw some screws in the side of the cabinet covered up by two gold bars held in by clips. I removed the bars, took out the screws, and the top of the cabinet seperated from the bottom. Two more screws holding brackets were removed, and I could now lift the safety glass out for cleaning as well as clean the front of the CRT.

With the chasis completely out of the cabinet, I was able to get a good view of the CRT. It has an RCA sticker on the top of it, so it may be the original tube. However, the sticker is not the usual red and white color of RCA. It's a yellow and white sticker; the same color as Sylvania. You guys had told me that Sylvania made these sets, so I wonder if this tube is a Sylvania masked as an RCA. You can see the sticker in one of the pics I posted. Before putting the chasis back, I inspected the PC board and made sure no other problems existed. I hooked everything back up, and it worked.

One problem though: The knobs are on the top of the cabinet. They are attached to the controls by red plastic extention shafts. When I thought the tube was going to slide out through the whole cabinet, I tried to removed these extentions from the knobs, and a few of them shattered. I've posted a pic of a shattered one as well as two of the cabinet in half. Anyone know where I can find some extentions like this or what I can use as a substitution? Besides that, all that's left is polishing up the metal cabinet.
Thanks
Doug

That CRT is pure Sylvania. That particular base was not used by RCA. It should say EIA 312 on the label, that's Sylvania's identifying code.

I'm trying to remember what we used to replace those broken plastic control extensions with but coming up blank. There was plastic tubing exactly like that used for something.

I think either a piece of rubber wiper hose (automotive) or a length of wooden dowel with a short rubber or plastic connector at each end would suffice. You may be able to find some relatively stiff clear plastic tubing of the correct ID used for a water line or similar application.

It was heavy plastic tubing. GE used alot of it on back controls on their '50's portable bw tv's.
Not sure what to use.....
I suspect GE, and, Sylvania shared alot of the construction on alot or RCA stuff.
Bill Cahill