los angles area repair for 1970 Magnavox color tube console?

[jimmydonc]

Hi all!
I just joined this group because Im hoping to find someone in the Los Angeles area to help. I have a 1970 Magnavox console with a color TV in very good condition, but i think it needs a bit of basic maintenance. The vertical hold jumps a lot between scenes and the horizontal position could be tweaked. Im guessing there are some caps or other components that need to be replaced but i dont know how or trust myself to do those kinds of repairs myself.

in general the console works great. Just hoping to do some basic maintenance now so that i can keep it working for years to come.

I know this is complicated work and am willing to pay a good rate for the job if anyone has leads. Not looking for cheap work.

[jimmydonc]

heres some photos. the test pattern is so you can see the horizontal position i mentioned.

[jimmydonc]

Heres a short video of the TV playing so you can see what i mean by the vertical jumping between scenes: https://youtu.be/nTQzEOjErC4

[Electronic M]

There's usually a horizontal centering knob near the HV cage in most sets. A small tweak of the AGC control may improve sync behavior on scene transitions.

[old_tv_nut]

The 1967(?) Maggie I had years back had no centering control. IIRC, there wasn't even a convenient place to kludge one in. It just depended on sweep being wide enough to prevent black borders. I wonder if they put one in this chassis.

[jimmydonc]

there are a lot of controls on this one. the horizontal centering control bustedwhen i tried turning it .It was one of those pots that you have to turn with a screwdriver but its made of plastic and was a bit sticky. My fault on that one Im afraid. im guessing replacing that pot is one of the repairs it needs.) as far as other controls yeah there is a vertical hold that i constantly adjust but never quite stabilizes the picture completely. Maybe I’ll fiddle with the AGC control again and see if that helps..

heres some more photos. hard to get a picture of the broken horizontal centering pot because its in a weird location and i dont want to disassemble anything. but i tried:

[jimmydonc]

i even bought a repair manual off ebay thinking maybe i could figure it out myself. (i can do basic soldering if i know the exact parts but i dont really inderstand what im doing as far as electronic components).
pretty sure this is the correct schematic but I dont understand it at all. The images below are probably too low res to read but I could lend the originals or scan them high res if i could find a repair shop.

[Popester]

Maybe leave Shango066 on YouTube a message. He is in the LA area.

[jimmydonc]

Originally Posted by Electronic M

There's usually a horizontal centering knob near the HV cage in most sets. A small tweak of the AGC control may improve sync behavior on scene transitions.

i made some adjustments and its not bouncing like before. maybe that AGC control was the problem. the horizontal is still off. wonder if that pot is something i could swap out myself. might be a bit of a project..

[zeno]

To see if you have the right manual compare the chassis numbers.
It will be on a small tag on the chassis. Format is
T-###-##-??

Centering control is a wire wound that gets quite hot. Often the
plastic breaks up.

Vert roll looks like its one of the big filter cans in the way it seems
to be at a regular time interval.

This is NOT a good beginners set ! Usually you do a few AM radios then
a few B&W's to learn the basics.

The good news is the CRT still looks strong !

73 Zeno
LFOD !

[jimmydonc]

Originally Posted by zeno

To see if you have the right manual compare the chassis numbers.
It will be on a small tag on the chassis. Format is
T-###-##-??
73 Zeno
LFOD !

yeah thats what the first image in my first reply is up above. took me a while to find that number because that little tag was totally coated in dust. ha. But yeah thats the manual/schematic I got.

Thanks everyone. i played with some of the controls and did get a better more steady image, but Im sure one day, before theres no one left with the knowledge to work on these, Ill want to get it properly serviced. I guess if it aint broke dont fix it!? ha.

[zeno]

Good you got the right manual. Maggies were a PIA, they
would use different chassis in the same model #. Model #'s
are ONLY good for cabinet parts. And those damn little tags often
fall off & get lost !

Its probably best to leave well enuf alone unless you can find someone
that knows there $#!T. Just moving this can stir up trouble at its age.
Just never leave it running alone.

The centering pot & cans are not hard to change. Basically you pull out the main chassis far enuf to flip it on its side. I would use a box to put it
on just behind the cabinet & do the change leaving all the other stuff
together as much as possible.
Bottom line is if you want a new & fun hobby start small then do this
last. If not leave it be, its in nice shape as is.

73 Zeno
LFOD !

[dieseljeep]

Originally Posted by zeno

Good you got the right manual. Maggies were a PIA, they
would use different chassis in the same model #. Model #'s
are ONLY good for cabinet parts. And those damn little tags often
fall off & get lost !

Its probably best to leave well enuf alone unless you can find someone
that knows there $#!T. Just moving this can stir up trouble at its age.
Just never leave it running alone.

The centering pot & cans are not hard to change. Basically you pull out the main chassis far enuf to flip it on its side. I would use a box to put it
on just behind the cabinet & do the change leaving all the other stuff
together as much as possible.
Bottom line is if you want a new & fun hobby start small then do this
last. If not leave it be, its in nice shape as is.

73 Zeno
LFOD !

I see the CRT is the original one. Still looks pretty good.
I never was too crazy about Magnavox.

[ARC Tech-109]

Bet it even smells like one! Sweet early Mag T940 chassis, unfortunately I'm not in the L.A. area anymore... you could box up the chassis and send it my way for a complete recap & resistor replacement but it won't be cheap. I reworked my 1970 Mag T940 Astro and added an external video input years ago and while the world went digital it can hold its own with a composite source, see attached images.

To address the horizontal tearing & stability issues this is going to be more than just tweaking the adjustments, 50 year-old carbon comp resistors are anything but predictable and just the heating from the nearby tubes is enough to drive them crazy.

I didn't see what the schematic was that you have but the T940 is a SAMS set 1144/folder-1

The issue is most likely out of tolerance resistors in the horizontal AFC circuit on the deflection & osc board specifically R-531, R-534 and R-530 as any shift in these will also shift the error voltage going to the Horiz osc grid. I could not get your attached video clip to play on my end but with 40+ years of experience I have a good idea of whats going on.

The biggest problems Ive encountered with vintage electronics is bad/leaky/dry or shorted electrolytic supply caps, a small amount of ripple can cause all kinds of stability problems. Image34 details how I mounted a number of 450v electrolytic caps to the chassis.

During the rework I swapped out the dead solid state audio amp for a Heathkit AA-100 tube amp running 7591s, it also received a complete recap and resistor replacement and with 25 watts/ch it rocks. Turntable still works as does the tuner but with iHeart radio dominating our local market it doesn't get used much.

[ARC Tech-109]

UPDATE...

Was able to view the video clip you posted and it looks like a stability issue from the video amp section, the picture washes out during the rapid transition between the light & dark scenes and this would affect the vert sync and AGC, follow the line from pin-2 of V703 which is the AGC keying, as this shifts it also affects the cathodes of 1st & 2nd video IF stages causing them to shift and that causes the output of the video amp (V204) to shift, as the 1st video amp drops off the voltage to the AGC and this pushes the 1st & 2nd IF amp cathodes more positive and 'round it goes.

That 6MU8 video amp was a problem in these sets, as they age the picture washes out from lost gain, also make sure someone didn't swap the IF tubes around at some point in the past. V201 is 6JH6 and V202 is 6GM6, Ive seen them swapped before.

Finally don't overlook the RF modulator you're using to supply the signal with, many are poorly designed and the older tube sets were never very forgiving to begin with.