Korean built 12" Sanyo tube-type B&W

[radiotvnut]

My computer is not being nice at the moment; so, pictures will have to wait a day or two.

The other day, a friend took me in a junk store that he spotted in a town about an hour away. They had a few '90's BPC sets, a mid '80's 19" knob tuned (rebadged goldstar) color Emerson, an '82 knob tuned 19" RCA XL100, an '86 Magnavox 12" B&W that I think was a rebadged Samsung, 3 or 4 beat up RCA consoles from the '80's, and the 12" Sanyo B&W that I ended up buying for $5. I would have bought the 12" Magnavox; but, it had a weak CRT and I decided to leave it there.

This set has a standard VHF tuner and the older non-detented UHF tuner. The volume, brightness, and contrast knobs are located to the right of the channel knobs. This set is very dirty and has some pieces busted out of the cabinet; so, it was probably in a major accident. I plugged it in at the store and seen that it had a good CRT; so, I figured it was worth $5. When I got it home, I discovered that it was made in Korea and the six tubes inside the set are all Sanyo branded Korean tubes. At home, I fired up the set with a signal and it had a good picture for about a minute or two. Then, the picture started pulling in from the sides and the HV faded away. After letting the set cool, it fired up with a picture and then faded out again. Since it's so beat up, I'm not going to spend any money on it; but, it should be interesting to see what's causing the horizontal sweep circuit to fail. This set uses a 33GY7 that is a combination HOT and damper tube. The problem could be the tube or any other part associated with it. I remember working on a mid '60's Magnavox 19" B&W that was doing something similar and it turned out to be a bad disc cap in the damper circuit.

I must say that this is the only Korean built tube set that I recall seeing. The model tag is missing; but, I found a Sams folder, from 1977, that covers a very close Sanyo. The set in the Sams has a click-stop UHF tuner, the controls are laid out differently, and the chassis only has 5 tubes where my chassis has 6 tubes; but, it looks close enough that I can use it to troubleshoot my HV problem. Since my set has the old style UHF tuner and is Korean, I'm placing it in the '74-'75 range. Any thoughts on that?

[ctc17]

Is there a chassis number on the tube location paper? I can't wait to hear what you find wrong, I have seen the boost filter cap do that. Also any of those green wax covered caps, they were in japan sets though. Korea is a trip, I can't remember seeing a Korean tube set, Taiwan yes.

[Jeffhs]

Federal law required in 1975 that all TVs be equipped with detent tuners for both UHF and VHF, so the set shown in your Sams schematic is likely, almost certainly, post-'75; however, the TV you have was made before the ruling took effect. The law was enacted because many people found it difficult to accurately tune in UHF channels on sets having continuous UHF tuners.

[radiotvnut]

Originally Posted by ctc17

Is there a chassis number on the tube location paper? I can't wait to hear what you find wrong, I have seen the boost filter cap do that. Also any of those green wax covered caps, they were in japan sets though. Korea is a trip, I can't remember seeing a Korean tube set, Taiwan yes.

Well, I didn't want to keep you waiting; so, I got on it today. The trouble turned out to be a bad horizontal output tube (33GY7). I checked the voltages against the similar schematic in an effort to see what voltages were changing as the condition worsened. The grid drive was right on at -27 volts, even after the TV started acting up. The screen grid voltage was supposed to be 124 volts and this voltage decreased as the set warmed up, settling at 78 volts just before the HV went away. Before I unsoldered any components for testing, I decided to sub the HOT. I had to borrow a tube out of another set; but, the HV stays up and the width remains normal with the new tube. This time, the grid drive measures -31 volts and the screen grid voltage is 118 volts. I tested the old tube in my B&K tube tester and it showed good emission with zero grid emission, even after the tube warmed up for several minutes. This is just another example where the best test for a tube is another tube.

The TV also needs the power supply electrolytic caps replaced because it has some hum in the picture. Also, the audio is somewhat weak and the plate voltage on the driver portion of the audio tube is about half of what it should be. This tube gets it's plate voltage from the 378 volt boost voltage thru a resistor.

First, I'm going to replace the bad power supply caps. Then, if necessary, I'll see what's dragging the voltage down.

Just for fun, I replaced two gray oil/paper caps in the vertical circuit and the boost filter cap. These caps checked out OK; but, the paper caps were starting to physically leak and the boost filter looked like it had been hot.

I looked for a chassis number on the tube chart and I couldn't find one.

[radiotvnut]

Here's the set: