PART 2

RCA Victor TC-125 (CH: KCS34B) TV Restoration

Checking The Set and Final Electronic Restoration

RESTORATION PERIOD: start October 2014 end February 2015

Other Parts GO TO:

RCA TC-125 ConsoleElectronic Restoration Part 1: RESTORE RCA TC-125 PART 1

RCA TC-125 Console Cabinet Restoration: RESTORE RCA TC-125 PART 3

1. Checking the low voltage power supply


Result- close enough




2. TV sound and raster?






3. Check Horizontal Sweep
Result- solid sweep waveform in specification



















4. Check Vertical Sweep and Deflection Coil Drive
Result- inverted vertical output signal caused by broken wire in vertical deflection coil located in the YOKE assembly. This was causing a reduced raster.



















5. Check Horizontal Drive and Deflection Coils.
Result- Vertical height and linearity is good but horizontal width is too narrow. Might still have a yoke problem.



















6. Quick Checks on the horizontal width control, video signal to the grid of the 12LP4 kinescope.
Result- Everything looks correct and the the only thing left that is causing the narrow horizontal width is the deflection coils exhibiting some loss. Will get a high voltage probe and check the high voltage but it should be okay since I am getting sufficient brighness.
7. Two problems- Gone as far as I can. Did some non equipment IF alignment and had trouble with the sound (low volume) If I get high volume it would intefere with the video. Some of the IF transformers are near the edge max in core setting. Could have bad capacitors inside. Also the picture has a trapezoid shape and reduced horizontal. The deflection coils are not open or shorted and I found out that RCA switched to powder iron core for the yokes that year. I suspect a crack in the core. Fix is difficult and chancy at best.
After slowly bringing up the AC line voltage using a variable transformer, I checked for dead or gassy (bluish to purplish glow) and replaced them. Found 4 of them including the 1B3GT HV Rectifier. The power supply voltages were recorded as follow
SPECIFICATION PERRCA SERVICE MANUAL ACTUAL ON MY SET ERROR + 215 VDC + 222 VDC 7 V above + 135 VDC + 122 VDC 13 V below + 86 VDC + 78 VDC 8 V below - 66 VDC - 61 VDC 5 V above - 118 VDC - 128 VDC 10 V below - 120 VDC - 129 VDC 9 V below


After adjusting the ion trap, I did get a 2" X 6" raster but no recognizable picture. The raster was confined to only the center of the screen (a fuzzy broad line). I can see the vertical and horizontal bars. They can be adjusted which says the horizontal and vertical oscillators and sweep are running. Could not focus the picture and get a fine scan lines. I did get sound and after adjusting a couple of sound IF transformers picked up a local low power analog broadcast on channel 6. Suddenly the raster turned dim. When I turn the lights off I see a blue haze in the CRT gun section, indicating gassy. Ran the CRT tester on the 12LP4 picture tube and the tube failed the G2 voltage test. Had to replace the kinescope.









While waiting for a new kinescope, I checked the horizontal sweep. The Horizontal Sweep enters the Horizontal Output Tube (6BG6G) which supplies the high voltage to the Picture Tube by way of the horizontal output transformer and HV Rectifier. (1B3GT). Here is the sweep input to the Horizontal Output Tube 6BG6G pin 2:The time from the bottom of the sawtooth to the top (across) is 66 microseconds (or a frequency of 15750 Hertz). This represents the time needed for the electron beam to sweep across the picture tube. After some minor adjustments I was able to center the sawtooth around that important specification.



With a replacement 12LP4 kinescope installed, I turned the TV on, all I got was a bright 2" X 6" raster is the center of the screen. When a signal was injected I can see the raster flicker indicating some video. I connected the oscilloscope to the plate of the vertical output tube (6K6GT pin3) and got the following waveform:











The sharp spike is the end of the vertical deflection cycle and that was good. The frequency of the signal is 60 Hz which is the vertical sweep rate. However the slope between the spikes is sloppy. The big concern was the waveform is inverted. The voltages on the vertical output tube were close enough. The only left that could be causing this was either the vertical output transformer or the vertical deflection coils on the YOKE assembly around the 12LP4 kinescope neck.The transformer checked in spec after removing the wires from the connection points, so time to look at the yoke assembly. After removing the kinescope and the focus coil assembly I carefully checked the vertical deflection coils in the yoke, the copper wire connections were oxidized (blue colored). This problem can cause the thin wires to break. I found that one of the coils was open. Hopefully the break is not inside the coil. Luckily it was on one of the leads.


Yoke Assembly ------->>> VerticalDeflection is on the right side. Notice the blue oxidation on the wires
Put everything back and this is what I got:





Readjusted the ion trap and focus coils and the TV started to behave better, but the horizontal sweep is not going across the kinescope. Could be the horizontal output or one of the horizontal deflection coils is not operating. I ran a DVD recording through a channel 4 modulator, touched up the video and sound and I got a recognizable picture but only about 40% of the kinescope was showing video. Still more to go on but I got a picture!







After several days of making video adjustments (something I don't recommend to anyone unless you have the equipment to properly adjust) and ion trap-focus coil adjustments, found the sync separator tube bad as well as replacing the 56 pF horizontal deflection capacitor was the best I could do: -------------------------------->>
Getting frustrated, I took apart the entire focus coil assembly and tried reversing it and reversing the ion trap to see what happens. I kept seeing the curve edge on one side of the picture. When I got the focus coil in a particular angle I got almost a full picture. The picture below was the best I could get.

Full frame of video from DVD called Five Easy Pieces, starring Jack Nicholson




Looked at the waveform and level of the horizontal width circuit on normal (position 2) and wide (position 3). Result was a slight change in DC positive level as shown below. The setting to wide position only widen the picture slightly.
For reference here is the output of the horizontal oscillator, before it gets to the drive making the large 15750 Hertz saw tooth wave:



Check the video signal driving the kinescope grid #1 and it look healthy with a solid sync pedestal: >>>>>>>>>>>>>>
















Notice the trapezoid shape indicating a defect in the horizontal deflection coils. The coils test good so I suspect a crack in the powder iron core. RCA switched over that year from the solid iron. Only fix is to get another yoke- if you can find one.
Low Volume was the other problem- Often IF transformers built during that time had silver disintegration, the oxidation of the silver plating would destroy the capacitor. Either find a replacement of take each one apart and inspect/replace with a discrete capacitor.











Install a new 56pF capacitor across one of the horizontal deflection coils (left side of picture).
8. Now that the TV has gone through hours of operating, it was time to cover up the restored multi section twist lock can electrolytics as shown:
Cover up the FP Twist Lock Can Electrolytics.
Left to Right:Add quick setting epoxyPush down on cover to set the epoxyAdd hot melt glue to secure the bond

Restored Electrolytic

Left to RightPut original cardboard insulating covers with values printed for ID

9. Install a New Yoke:


Replace the Deflection Yoke. Found two places that had a replacement yoke. The Early Television Foundation had a vintage Thordarson Magnetics Y3 that has the same specs as RCA's and one from Ebay that was removed from an RCA TV that had the 10BP4 the 10" relative of the 12LP4 Picture Tube.


Removal of old yoke assembly:
Hoisted the TV Chassis out of the cabinet an onto the workbench (60 pound hunk of metal). After some thinking, decided to remove the yoke without taking the picture tube off which at the end was the right plan, even though I had to be extra careful when working around the picture tube neck like double checking what I am doing, wearing protective eyewear etc. After disconnecting the tube socket, removing the ion trap and focus coil this is what went on:

A. B. C. D.


A. The original yoke with the cover removed.
B. Unsolder the wires and slide the yoke out.
C. Prepare the replacement and slide it in, making sure the yoke adjust wing nut slot is at the top.
D. Carefully prep, route and solder the wires as shown.

E. F. G.


E. The red wire from the flyback transformer wasn't long enough so I took a piece of similar wire and soldered it to the wire then added Corona Dope because of potential high voltage on this wire may cause an arc if it is near the chassis. After the Corona Dope dried put red heat shrink tubing as added protection.
F. The new (should be pre-owned) yoke completely installed.
G. Here is the first view of the TV with the replacement Yoke. This is an episode from Quincy on MeTV. The screen is full now (FINALLY!!!). But there is a problem. The picture is a mirror image or reverse. Hard to tell but if you look at the lower left corner of the screen the "M" from the MeTV LOGO is going the wrong way. Still can't put the problem to bed!.
The Solution: Apparently the Yoke I got from a Ebay was for an RCA TV that had the Yoke key reverse (see the diagram below) than what was on my TV. I had to remove the wires for the horizontal and the 56pF capacitor and reverse their positions.

H. I. J.


H. The replacement yoke with the wires reversed and the capacitor moved from the top to the bottom. I decided to put in a newer higher voltage unit in.
I. View of the Yoke, Focus Coil and Ion Trap in their proper positions.
J. FINALLY!!!! A full picture and in the right direction. Notice the M on the lower right corner this time. Episode of MASH on MeTV.
Since the chassis is on the workbench made some better adjustments for the horizontal frequency lock, the width adjust and touched up the video and sound IF adjustments. The sound is still weak but louder. I can finally say the electronic restoration is done.
RESULT: After discovering that the replacement yoke was wired reverse of the original for the horizontal deflection, the picture problem has been solved.

UPDATE JUNE 5, 2018


Had nothing else better to do, I needed to rebuild the fine tuning drive so I had to removed the chassis from cabinet. Since I had the chassis out I looked into low volume problem. While pushing components with a tuning tool, found that an 82 ohm cathode resistor for the first sound IF stage (center of photo), the insulation sleeve was cut by a lead and shorted the resistor , reducing the 6AU6 gain. Did some retuning and the TV picture and sound is pretty sharp and loud! Fix the problem and now have full volume. Took 3 years, but better late than never!
The 82 ohn 1/2W resistor is in the center of the picture. Originally the black sleeve was punctured by a wire stub. It was so small one would have not noticed it. ACCIDENTS DO HAPPEN! >>>>>>>>>>>

Go Back to Previous Page: RESTORE RCA TC-125 PART 1

Go to the Next Page: RESTORE RCA TC-125 PART 3

Return the Restoration Home Page: HOME