Magnavox C-36-20-11 TV chassis troubleshooting
 

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.

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:smoke:
LFOD !

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.

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.

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.

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.

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

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?

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

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).

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.

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.

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...

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.

27ZP4:
https://frank.pocnet.net/sheets/168/2/27ZP4.pdf

27MP4:
https://frank.pocnet.net/sheets/127/2/27MP4.pdf

jr

Thank you so much for the drawings. That looks a lot how mine is wired. I see I need negative voltage on Pins 2&6. I'll check it again.

The drawing says I should have from 16.5 K volts to 19.5K volts on the anode. I don't think I have that much. That is almost color TV voltage. This thing has a vertical chassis and a test jig would come in handy to check some of the voltages I need to check. I'd say I have a sizzle of high voltage, more than a spark. The way the wires are run in this stereo, it will be nearly impossible to run this chassis in the cabinet to check voltages on the back side of the chassis. Pending a few more tests, my guess at this point is I don't have enough high voltage to light the CRT. I tested the CRT yet again and it has really decent emissions and cutoff since I have been running it so much. It did take awhile for it to wake up. If I didn't know better, I'd say I have lost B+. This thing does nothing except light the tube filaments. So far, I have B+. I'm going to concentrate on the deflection board. I'd think there has got to be a missing voltage somewhere, but what do I know?

Bad capacitors in the B+ divider tree holding down the various voltages on the other side of the resistors? Insufficient drive from the deflection is going to get insufficient output from the horizontal tube, a lack of B+ from the divider will do just that. I don't think there's a free lunch with this one.
The only correct way to verify the HV is with a good probe, the big B&W sets had almost 20KV on the anodes but even with 10KV you should get some fuzzy light on the screen.

Personally I have a Mag T940 Astro Sonic combo, pics are posted here... somewhere.

The problem, or part of it, is in the B+ divider. The chassis has been recapped, but I'll still check the associated caps. One half of the horizontal oscillator has no B+ on the plate. That same half has 300 volts on the cathode where 5 volts is spec. The 6GC5 cathode follower has one leg that is supposed to have 235 volts, but only has 150. I have no boost voltage, which supplies B+ to the audio detector, but I have no high voltage, hence no boost voltage. The reason the voltage on the CRT was off was due to no power to the brightness control. Yeah, you're right. This ain't gonna be an easy ride. But I will have learned a bit, which is part of it, right? Thanks for your input.

We all start somewhere.
I would need to see the schematic to get the picture of how the horizontal oscillator is layed out

A really dumb question is the 6GC5 actually in the correct socket? I can't believe the number of times I've stuck the wrong tube in the wrong socket then chased my tail for hours sometimes days looking for the problem... 6U8 vs 6GH8 vs 6BL8 and so on.

Yes, the 6GC5 is in the correct socket. It is located on the sound board. It is a nine-pin tube, the other two on that board are seven-pin. Some versions of the 36-series chassis use this tube as an audio output tube, the version I have, in a Stereo Theater, uses it as a cathode follower. I have fumbled more on this project than I have in a long time. I got too worried about what I didn't know then forgot what I did know.....

At that point it's best to take a step back

I found a wire from the TV tuner that goes to the IF strip. It has a shield that has been cut. Will that shield wire being open kill the output of the tuner?

It can. Many sets use the IF shield as the return for the B+ and heaters, so if it goes open the tuner looses all power.

Also with the shield open you can get weird interference and signal issues.

Thank you. The drawing does not show that shield at all. I'll repair that just to be on the safe side. Next rookie question. The AGC resistor had drifted low. Spec was 9.1 Meg, the one in there measured a little over 1 Meg. I put in a new resistor that measured 9.113 Meg. It measures a little over 5 Meg in the chassis. That resistor is not paralleled with anything else that I see. Should I look for something possibly loading the AGC line? If so, where do I start? Rookie question number two....there is a choke of some kind in the plate circuit to the 6DA4 damper tube. One lead on the choke opened, killing B+ to the plate of the damper. What is the purpose of that choke and what happens if it is not there. I fixed the break. The choke measures an ohm or maybe less.

I have made some progress. I have a good raster. I may even have video, since there is snow on the screen. No sound at all. The audio detector gets B+ from the boost line. I need to verify voltage on the sound board. Then I'll connect the TV Analyzer to the antenna terminals and try to adjust the fine tuning. I got the raster going late this afternoon and didn't have time to go any further. I can change channels and get a flash on the screen. Is this an indication that the tuner is working, and my problem is in the IF/audio sections? I appreciate everyone's help on this.

I have a raster, I have an image when I inject a signal into the antenna terminals with the 1077B Analyzer. The vertical height is off, probably linearity as well and the horizontal may be too fat. I think I can say the tuner is okay. I still have no sound. I have run continuity tests on the sound board and can't find anything open. I have B+ on the audio detector tube and the other voltages are okay. I have correct voltage on the 6GC5 cathode follower tube. I have correct voltages on the audio IF tube as well. I'm not having any success feeding an audio signal from the Analyzer to the sound IF. I'm not sure how to troubleshoot the audio section. There is a quaduature coil. I'm not sure what it does or how to test it. I haven't connected a DVD player to it yet, but the image from the Analyzer looks good as far as focus and contrast is concerned. I had to play with the fine tuning to get it close enough to get an image. If I could figure out why I have no sound, this project might be successful yet. Thanks for any advice you can give...

Finding all the voltages on the sound board to be correct, I decided to examine the drawing and check component by component, every component in the audio stream. I noticed a notation on the drawing that said "Mute connection (where used)". Hmmmm. This stereo has remote control. It turns power off and on, changes volume in three steps, and changes the TV channels in one direction only. Apparently, it has a muting feature so the movement of the tuner is not heard in the audio (good old Magnavox!). Apparently, that switch had shorted, killing the audio. I disconnected the RCA plug on the end of that wire and the audio came to life. I have adjustments to make, I have no idea what high voltage I have, no idea of cathode current, vertical is off, horizontal may be off a bit, too. This thing allegedly has "automatic fine tuning" so changing the fine tuning with the knob is made purposely difficult. I'll remove the channel knob to get to the fine tuning knob better to adjust it. I've made progress and am satisfied with the outcome so far. And, I've learned a lot.

The muting feature is a leaf switch on the back of the channel motor. When the motor is activated, the motor shaft goes into the gear box and the switch closes, when the motor stops, the spring loaded shaft pops back out and opens the switch. The automatic fine tuning was what everyone else called pre-set fine tuning. Each channel tuned individually, adjusting any one channel didn't affect the others.

I have found that switch is stuck. Or just shorted. I haven't figured out exactly how it works, but I can take it apart then figure it out from there. Channel 3 on the Analyzer actually comes in better on Channel 4 on the TV, so fine tuning is pretty far off. Like I said, I have lots of tinkering yet to do, but i'm taking my time and experimenting and learning as I go.

First, let me thank everyone here who helped with this project.

I haven't taken pictures yet, but I will get some and get them posted if there is interest. This TV works really well. After the no sound situation got squared away, I connected a DVD player to make my finally adjustments. I had the fine tuning on channel off. You had to turn to Channel 4 to get Channel 3 to come in. I got that sorted. Originally the CRT checked fairly weak. After running for five or six hours, retesting shows the CRT tests almost like new. Who knows how long it sat unused. This old Maggotbox doesn't perform like a Zenith, but certainly better than I expected. The sound is awesome, although not stereo, since it passes thru the stereo amp and speakers. It could use about a quarter of an inch more vertical height yet. Very clear and sharp picture. It doesn't bloom at higher brightness. Vertical and horizontal are rock solid. I still need to sort the record changer and I want to build a new back for the cabinet. This will not only let the electronics show better, but it will provide better ventilation. Every tube in this stereo are on when the TV is on, even the Multiplex adapter. You think they would have shut off the components not necessary for the TV to work, but nobody worried about pulling 320 watts in a black and white TV back then, I guess. I am thrilled with how this project turned out and I thank you all for all you did.

That's great to hear.

One thing you can do on these if you would like stereo sound is identify the TV sound input on the stereo chassis and figure out where the 2 stereo channels get shorted to mono then un short and bring a line level audio stereo input out the back for your VCR/DVD player/HDMI to composite AV adapter/ whatever you are using as a signal source to connect to.

Some machines have a stereo input connector and the shorting to mono is done in the cable from the TV, others have a mono jack and the stereo to mono short is somewhere between the mode switch terminals and the input jack.

I'd be tempted to add a switch on the back that would allow you to select between external stereo audio and internal TV sound.

Some piccys please. :yes::banana:

I tried posting pictures. The software says my pictures are too large. So...

The model number is 1MR418M. It was the top line 400 series of Stereo Theater Magnavox built in 1962 using a 27" 27ZP4 picture tube. It uses the 93-series stereo audio amp, 70-series Multiplex adapter, 79-series AM-FM tuner and a solid state remote control chassis. Magnavox built two other versions of the Stereo Theater, the 300 series with a 24" CRT and the 200 series with a 23" CRT. Magnavox also built a color Stereo Theater, but these are exceedingly rare and I've never seen one in person.

My Stereo Theater was found by a member of our console stereo forum in an abandoned house that was being torn down in Detroit. The house had a hole in the roof. This old stereo was found under a staircase with debris all around it that had protected it somewhat from the elements. He rescued it and put it for sale on the forum. Since it was one of the models on my bucket list, I made arrangements to make it mine.

The cabinet is 60" long, compact for a three-in-one unit of that day. IT had been exposed to so much cigarette smoke, the top of the cabinet was black with nicotine and dirt. Everything inside the cabinet, all the electronics was coated in sticky cigarette residue. There was a mouse nest under the amplifier. The picture tube was ready to fall thru the front of the cabinet. It couldn't fall backwards so the shadow mask and safety glass kept it from falling to the front. But, typical for even neglected old Magnavox stereos, the radio worked, the changer did not, the TV did not. The TV chassis would not even let the tubes light.

I removed all the electronics and wiring from the cabinet, including the CRT. It took me two years to get the electronics cleaned and repaired. A proper schematic or service literature for the entire unit did not exist. I never found anything for the tuner. I got a "close" schematic for the TV chassis, which served me well. The amp I was familiar with and I finally found a schematic for the remote control, although I have never done anything with it and don't plan to. We took the cabinet as far apart as we felt was safe. We cleaned each piece with mineral spirits to get rid of the smoke scum, then sprayed the inside of the cabinet and all its parts with three coats of shellac, followed by three coats of satin polyurethane to seal the cigarette smoke and to keep it from smelling like cigarettes. The outside of the cabinet received 15 coats of gloss polyurethane, sanded from 400 grit to 2000-grit to knock down the gloss a tad. We used gloss material to heighten the grain effect in the cabinet. It turned out nice. Magnavox advertised that they used "10 coats of hand-rubbed oil finish" probably tung or teak oil. We used black grille cloth to give it a "mid century modern" look. It originally had two different kinds of grille cloth, for some odd reason. I got this Stereo Theater in 2017 and just got the TV working. My next task is the changer, but that should be a piece of cake compared to the TV.

I realize this is just a Magnavox, not a Zenith or anything important and worth messing with. I couldn't be happier that I finally got it going and I'm as proud of this thing as I would be of a CT-100 or whatever. I put a lot of myself into this project and it fills a slot in my Magnavox collection between the big Imperial and the Symphony.

One thing. The CRT Cabinet Mask, and CRT Shadow Mask are 2 different things. Only color CRTs have a shadow mask and it's inside the CRT between the phosphor and electron gun....It's basically extremely high-precision window screen designed to keep the red gun from hitting green and blue phosphor dots and vice versa for the other guns.

All TVs have a cabinet mask but only color sets have a shadow mask, and if you ever see the shadow mask with the naked eye there's either internal coating missing or your color CRT has been de-constructed in some permanent manner.

IMO average (PP) Magnavox audio consoles beat average (single ended ) Zenith consoles, though high-end PP Zenith's give most Maggie's a run for their money.

I redid the phantom remote on my Maggie roundy, and IIRC I did a YouTube video on it. If you have a scope and a little bit of patience those remote units aren't difficult (the bellows hand units can be a bit of a headache if not functional, so using a tuning fork or electronic remote hand unit from another set may be easier).

I told you I was a TV rookie. I meant the plastic thing, shaped like the picture tube face, located between the picture tube and the safety glass. It has a Magnavox emblem on it and a channel indicator for the remote control. I've never owned a Zenith audio console, so can't compare those to Magnavox. I probably won't since I avoid most non-TV things made by Zenith. I respect their TV sets, but their radios are infested with rubber wiring and SMD. And the famous green crud.....