Magnavox C-36-20-11 TV chassis troubleshooting - Videokarma.org TV - Video

uncleputz · #1 12-22-2023, 11:29 AM

This chassis is somewhat covered in Sams 591, which I have. The exact schematic may exist in factory documentation, but I don't have access to that. I have the Sams, and a 1077B Analyzer, the usual meters, but no scope. I am beyond a novice at TV repair and am TERRIFIED of high voltage. I collect Magnavox console stereos, models with P-P audio and I have six. From a Concert Grand, down to the cheapest model. The Stereo Theater is third from the top in the lineup both in livery and in price. My Stereo Theater has a TV chassis that I am trying to get sorted.

I have a good raster with full deflection. The 27ZP4 CRT does not test the best, but it may be 30 years since it was turned on. Cutoff is iffy, it flunks the life test. I think it is fair to say the CRT is a bit soft, but, hopefully, it will produce a picture. The goal is to be able to watch the Lawrence Welk show on PBS, over the air, one time, then after that I don't really care if it works or not. And I will have learned something and will have done the work myself. But I need a bit of guidance if someone would be good enough to lend a hand.

I have a good raster, good brightness, but no video and no audio, no "hash" on the screen. I suspect the tuner, but removing it is a horror show due to the remote control setup. Sams suggests checking V1, V2, V3 and V4, which are tubes in the IF strip and audio IF. Plus X1, which is the video detector, a 1N60. I think the video detector diode is in a can that looks like an IF can. How do I check that part? Plus, I have read that the horizontal phase detector diode is a problematic part, so I want to check and replace it (them?) while I have the chassis out. I have test sockets so I can check voltages and will check voltages in the IF strip. Removing the tuner requires removing the safety glass and mask to access a screw on the remote channel indicator. The tuner itself is buried behind the radio tuner. I want to cover all my bases without having to discharge high voltage. I am not sure how to do that, and would be too chicken to try. I don't have a high voltage probe. Sams does not say which part I need to replace the horizontal phase detector diodes. Any suggestions?

So here we are, a novice scared of high voltage working with a TV chassis. It may take a while, but I'll get it eventually. If someone would be good enough to fill in the blanks, I'd really appreciate it.

zeno · #2 12-22-2023, 03:20 PM

First dont be afraid of the HV. I got zapped at least once a week for 40 yrs.
DO respect the AC input, main B+, hoz out & damper. Those can harm
you !! Also use an isolation transformer.
So some hints.
Run the set & change channels. See if you hear or see the effect of
the tuner contacts making & breaking.
If so look to the tuner.
If not look at the IF's.
If not look for where the sound is taken off. The problem is before
the take off point. Good old time "no equipment" quick bench tests !
good luck & Merry Christmas !
73 Zeno

LFOD !

uncleputz · #3 12-22-2023, 04:59 PM

Now I don't have a raster, no video, no sound. Next step is to check B+. It draws 250 watts with the HOT out, just the filaments in the TV and the rest of the stereo figured in that. When I put the HOT back in, the screen remained dark, and the wattage didn't increase. I changed channels, no noise. I have an iso/Variac. My thinking at this point is that I had no B+ to the IF strip before, now I have no B+ anywhere. Thank you for your reply and Merry Christmas to you as well.

uncleputz · #4 12-22-2023, 06:02 PM

I have B+, in fact, it's right on the money. I can't induce noise in the audio anyplace. I'm wondering if the cable from the chassis to the tuner is open. I should be able to inject noise into that cable and hear it in the speakers. The emissions on the CRT have come up considerably since I ran it for a couple hours with just the filaments hot. Cutoff is reasonable and the emissions have gone up to 400 with 300 being minimum acceptable on my tester. I seen to have lost high voltage. I tried the Shango "spark jump" test and got nothing. This thing has more than one problem.

damen · #5 12-22-2023, 08:18 PM

A common problem back in the day with these sets was burnout of the filament of the half of the 6GH8 used as the AGC keyer. Magnavox put full B+ on the cathode and any slight leakage to the grounded filament blew it out. Also, make sure all 3 IF tubes are lighted, sometimes hard to tell with their shields on. Since you have the combo model, the 6GC5 audio output tube is only being used as a voltage divider, supplies B+ to the tuner and the IFs.

uncleputz · #6 12-23-2023, 05:40 PM

Now I have no high voltage. I went to the shop today to run some tests then discovered a black screen. I tried the spark jump test and it for sure is dead. The damper, HOT, and high voltage rectifier all check good on my tube tester. Nothing red plates, nothing appears like it is getting hot. There is no wax puddles under the flyback. I'm not sure where to go next. I will probably need to pull the chassis, but I want to make necessary tests, if any, before I do that. I tested all the tubes and they all tested good. I can't see if the IF tubes are lit or not. I can barely see them light on the tube tester. They all have B+ on plate and screens. I have not checked filament voltage on the IF strip, but that's a good idea.

damen · #7 12-23-2023, 09:00 PM

If the shields feel warm after a few minutes, the filaments are probably okay.

uncleputz · #8 12-24-2023, 05:46 PM

I have filament voltage in the IF strip. I don't have a high voltage probe. I suspect that I may have high voltage, and that something is driving the tube to cutoff. I need to measure voltage on "the yellow wire" then run some more voltage checks, verifying that the horizontal oscillator is running. Like many who work on old TV;s for the first time, I struggle to be patient. I'm trying all the suggestions I have read, starting with certain voltage checks. Every voltage I have measured so far has been right on right on. I need to get the raster back before I can do much else in the way of troubleshooting. If a control, such as the brightness control, were to open, would that cause the CRT to be dark?

zeno · #9 12-24-2023, 06:34 PM

Tape a little NE-2 lamb on apx a 1 foot plastic stick. Place it near the flyback.
If it lights you have HV there. If not no HV.
Can also check with your hand. Put back of hand near screen ( front).
If you have HV you will feel it tugging on your tiny hand hairs.
As Damon says the set has stacked B+ so look around the audio out, voltages.

Zeno

Electronic M · #10 12-24-2023, 11:55 PM

If the gun bias is good and it passes Zeno's neon bulb test (a CFL will also work instead of neon if it's what you got) then it's probably a bad rectifier tube... you can check by connecting the blade of a well insulated screwdriver to ground and trying to slip it under the CRT HV suction cup...If you hear it arc there's HV if you can't there's no HV. If the neon says the flyback is making HV but there's none at the suction cup then the HV rectifier tube is bad, the heater winding/resistor feeding it is bad or the heater winding or HV lead has shorted to ground (insulation can fail).

uncleputz · #11 12-25-2023, 12:29 PM

Thanks for all the response. My next step is to measure voltages in and around the HV circuit, for learning purposes if nothing else. Then measure voltages at the CRT. This stereo pulls 250 watts with the TV on. There are 36 tubes hot when the TV is on. Sams says the TV chassis pulls 160 watts, the stereo pulls about 160 watts without the TV chassis on. So I am about 70 watts or so low from where I should be. This may be not very scientific, but this is the best I can do with available equipment. The wattage is the same with the HOT in or not. This is what leads me to believe there is no high voltage. I'll try the CFL trick and see if the hair stands up on my hand in front of the CRT. I never thought of that. I can also darken my shop nearly totally dark. I can do that to see if something is cutting off the CRT. Pardon my fumbling, but I'm learning a lot. I'm sorry, but I am just not comfortable placing a screwdriver under the HV suction cup on the CRT. If I have to check that, I'll wait and find a reasonably priced high voltage probe and do it that way. The damper, horizontal output, and high voltage rectifier tubes are all NOS RCA and test good. I realize that doesn't always guarantee they will work in the circuit....

How long will it take for the high voltage to bleed off naturally? Overnight? Then, could it be possible for the fine tuning to be so out of adjustment that it would totally cut off video and audio? The fine tuning on this tuner is hard to manipulate with the channel selector knob on. This set is alleged to have an automatic fine tuning feature, but I suspect the fine tuning needs to be set, then it is automatically adjusted as you change channels. It seems like I can turn the fine tuning without feeling any stops. I know there is a plastic gear in the tuner that can break, but it feels like I am turning "something" but I can't seem to make a difference. I have a B&K 1077B available now as well. As soon as I play with it a bit to familiarize myself with how it works, that may help answer a few of the questions I still have.

Electronic M · #12 12-25-2023, 09:09 PM

The screwdriver method can be used to disch the CRT. (A series resistor around 1M helps prevent dielectric bounce back).

The HV isn't dangerous unless you have a heart condition. The voltage and charge stored on a color CRT is slightly less than the worst that of a bad static jolt from shuffling on carpet and touching a doorknob. Voltage doesn't kill current kills...30mA to be precise. A color CRT only draws 1mA max and a monochrome 1/3 of that... the HV winding of a flyback is not able to source lethal current. 400V B+ in your stereo amp designed to source 150mA all day on the other hand can absolutely kill you. I've been bit by live and stored HV countless times and haven't been harmed, B+ is more dangerous than the CRT HV lead.

uncleputz · #13 12-26-2023, 06:37 PM

I have a short wooden handle out of a windshield scraper I can rig up to a screwdriver and insulate myself from HV. It's something I need to do and to learn not to be afraid of it. I'll work something out so I can check HV. I'm doing a lot of fumbling, but am trying not to make it harder than it needs to be.

Electronic M · #14 12-27-2023, 02:01 PM

Quote:

Originally Posted by uncleputz

I have a short wooden handle out of a windshield scraper I can rig up to a screwdriver and insulate myself from HV. It's something I need to do and to learn not to be afraid of it. I'll work something out so I can check HV. I'm doing a lot of fumbling, but am trying not to make it harder than it needs to be.

If the screwdriver has a plastic handle and you keep your fingers at least an inch away from the metal shaft you'll be fine. When I say well insulated I mean not grandpa Larry's 1910 wood handle driver that's been outside and has moisture in the wood. Adding a chicken stick to a plastic handle driver is like sticking pool toys to the outside of your car Incase of an accident...

uncleputz · #15 12-27-2023, 06:08 PM

I got brave and did it your way. I have a long screwdriver that I used when I was working on cars and I knew the handle to be well-insulated. I THINK I have high voltage. I get a good spark at the HOT, the hair stands up on a pissed-off cat's back when she is held in front of the CRT. (She may not hang around the shop as much for a while, bit I digress). I then measured voltages on the CRT. No voltage on pin 6, an orange wire. The Sams I have is for a 23MP4 CRT, mine is a 27ZP4 and I suspect the connections may be different. The "yellow wire" has 75 volts on it. I believe I remember seeing on this forum a color code for CRT connections. The yellow wire is for the cathode, right? I don't have any references that give me the connections on the 27ZP4. If someone has that information plus what the color codes are for CRT connections, that will help. Before I got this set, someone had been playing with it because it was populated with all Radio Shack tubes. The CRT plug had come apart so I repaired it. I need to verify it is wired right. I cleaned and checked all the brightness, contrast, vertical and horizontal controls and they checked good. I had raster, turned the set off, then had a dark screen when I started it again.