Magnavox T931

[LukeSimon]

1967 Magnavox T931



I need help: this TV has AFT, which can be disabled via a switch on the back, but I can’t find the fine tuning dial. How is fine tuning done on this TV? The outer dial changes the VHF channel number and the inner dial changes UHF channels number. The Sams schematic is cryptic. It indicates a manual fine tuning dial exists, but it is not clear what they are indicating.



The picture tube tests good.



I powered up the TV gradually using a variac, dim bulb, kill-a-watt, and then removed the dim bulb. At full voltage, the TV uses 290 watts. The Sams lists 310 watts, so not too far off. The problems: audio volume is weak even at max volume, brightness must be turned all the way up to display a picture, color is not working, reception is weak, focus is bad, and I can hear a clicking or ticking sound every few seconds from the HV cage. After 15 minutes of warm up, reception got better, which suggests some of the tubes need to be replaced.


I will measure B+, boosted B+, and ultor voltages tomorrow to track down the brightness and focus issues. Again, the picture tube tests good, so the chassis must be applying the wrong voltages to picture tube electrodes.

Please help me answer the following questions:

1. How is manual VHF fine tuning performed? Is the UHF dial used for fine tuning of VHF?
2. Can I use a 40kv rated probe for measuring focus voltage?
3. How should I fix the clicking / ticking sound coming from the HV cage?

[John Adams]

Long shot, but will the inner knob push in?
I assume that it has a strip type vhf tuner and each strip has a fine tuning slug at the front of the strip.

[ARC Tech-109]

Looks like my T940 Astro Sonic chassis the only difference being the focus rect, mine uses the tube. Nevertheless what you're dealing with is low B+. Powering up the set and trying to "reform" a 50+ year old capacitor is a gamble at best. The best advice I can give before replacing any tubes or making any assumptions is to replace the power supply caps and check the dropping resistors on the side board. C105 is located near the power transformer, C107 and C130 are behind the HV cage, see the attached pics. My Astro Sonic has been in the family since early 1970 and in my care since 1984 when dad replaced it with a Zenith S-3... the Magnavox survives to this day. I'm using my T940 as an academic example here.

The only tube that has been a problem with these is V204 the 6MU8, the other issues you describe point to low B+ caused by bad/leaky capacitors behind R312, it's a resistor divider chain that supplies most everything in the set. Poor focus and low HV, limited brightness and what I can see in your pic is a lack of color caused by reduced voltage to the horiz section. This also provides the burst gate keying signal and when low will not open the color killer to allow the colorburst into the chroma bandpass amp. Low B+ will also cause weak audio and washed out contrast, the convergance & pincushion on the left reinforces my conclusion. Measuring the focus with a 40KV probe might make the meter wiggle some but the numbers are meaningless. HV ticking is leakage in the cage caused by dust that needs to be cleaned with some Windex and small towels after connecting a cleap lead to ground then the center contact of the 6BK4 plate cap, learned this the hard way at 14! Let things dry for an hour before power up.
Looking at your pix tube readings they are cathode currents and still strong. The voltages going to the pix are arbitrary, its the differentials between K&G1 that are important. A weak power supply is going to throw them all off. My T940 uses a 6MD8 triple triode for the R-Y G-Y B-Y drive and G-1 bias, I don't know what your T931 uses but it should have roughly 195 volts difference between cathode and grid-1. The picture tube section of the RCA Receiving Tube Manual details this better.


The T940 SAMS is Set 1109 Folder-1 and you mentioned having a Sams as well. I have over 40 years of experience with mine and it's posted here, look toward the bottom of the page http://videokarma.org/showthread.php?t=273555 and if you look at Image34 it details how I addressed mounting the replacement capacitors.

What you have there is an EXCELLENT Magnavox console, a frosted front CRT without a cataract? WOW! Really complements your PortaColor.

[AlanInSitges]

John is right, push in the middle knob to fine tune.

[LukeSimon]

Thanks for the help.

I will measure B+, boosted B+, and ultor (aka high voltage cap) today and report back to this thread. I ordered newly made in 2022 Mallory capacitors so that I can keep the chassis looking as close to original as possible.



I have a theory about the no cataract. The TV has sliding doors that cover the screen, and this TV was a bedroom TV, not the main room TV. So it got light usage and was closed up and off for most of its existence.



Inside looks to be in good condition too. Definitely a secondary, back bedroom TV, and wasn’t the main TV of the house.

[zeno]

There was a cap, IIRC in the burst that killed the color
Boost usually feeds G-2 controls, audio & vert size. Looks
bright enuf so probably OK.
OK on HV probe, focus 5-6 KV apx & control near center of
range for best focus
Push & turn for VHF FT.
There is an auto color switch on the back. Best to leave it off.
It causes overall brownish color & was known as the "brownalater". Yuk !
Ticking, look inside the cage for arcing with room lights OFF.
Also do a complete clean up from HV rect to the CRT. It may also
be your focus problem.
BTW the pix looks about normal brightness. It will NOT be as bright as a newer Chromacolor jug or newer in line sets.

73 Zeno
LFOD !

[LukeSimon]

Originally Posted by zeno

Ticking, look inside the cage for arcing with room lights OFF.
Also do a complete clean up from HV rect to the CRT. It may also
be your focus problem.

Opening the HV cage unplugs the HV rectifier (red circled cover). How am I supposed to open the HV cage without unplugging the HV rectifier? Am I supposed to observe the arcing with the rectifier unplugged?

[Electronic M]

It looks like there's a top lid that is held by a lone 1/4" bolt above the red red circled area.

Some sets (cough RCA CTC-16 and up) weren't designed to allow the door to be open and the rect to be plugged in.

[zeno]

IIRC the lid swings out. Arcing can happen to both the HV & focus including
both fillament windings.
You can also pry the cover to the HV cup ( red circle ) & look in there too.

Zeno

[LukeSimon]

My wall outlet measures at 125VAC, so I used a variac to step down the voltage to 117VAC to match the Sams. According to my Kill-A-Watt, at 117VAC, the TV uses 302 watts and 2.92 amps, which is within 5% of the Sams spec. Stepping down the wall outlet from 125VAC to 117VAC made the arcing clicking and ticking sound from the HV cage completely stop!


The most readily accessible areas to probe B+ and Boosted B+ are the blue circle and red circle in the schematic, that is, the two leads of the spark gap, which are right above the blue screen pot. The B+ measurement should be accurate at this point, but the boosted B+ should be lowered by the resistor network in the screen grids to 1012V.




B+ measures 383.12V, which is within 5% of the Sams spec of 370V.


Boosted B+ measures at 960.92V, which is within 5% of the Sams spec of 1012V (the theoretical voltage if each pot and resistor in the screen resistor network measures exactly to spec).


So B+ and Boosted B+ appear to be good. I will measure ultor voltage and focus voltage tomorrow. I bet focus voltage is low, which would explain the blurry picture.

[LukeSimon]

I exhaustively measured all B+ sources and summarized them in the table below. The B+ 220V source is the farthest from the schematic at 270V, which is 23% higher than specification. Since color is not working, this may be a symptom of the root cause of the picture being black and white.

[damen]

If you haven't already tried it, see if switching the AC input to the 128 volt tap of the transformer helps with the high voltage issue without the need of the variac. ARC Tech-109s diagram of his set shows a switch, your set should have the black-white primary wire connected to a terminal on the PC board. They use a slip on connector with a blue wire to select the 120 or 128 volt tap.

[ARC Tech-109]

simple things first, check the color killer control? Turn it to the left on a blank station and the screen should look like it has colored confetti all over. No change from the control? Check the phase det diodes CR701 and chroma sync diodes CR702 (T940 schematic) the nominal voltage should be about -2.0V on the grid of the triode section of the killer/burst amp 6MQ8/6KE8. The diodes are germanium 1N34's and are known to die without warning. I pulled mine and wired in a pair of 6AL5 twin diodes on wafer sockets. Same with the horiz AFC diode pack. If the burst amp isn't getting a signal it won't feed the color killer and that will keep the bandpass amp grid-1 cutoff so make sure it's opening up the bandpass.

Check for chroma subcarrier coming into the chroma take-off coil, it's fed from the 1st video amp plate via 18pf ceramic cap (C742). Look at the the output of the bandpass amp pin-6 of the 6HG8 V707a. Signal present then check the output of the 3.58 osc, you can pick up the output on the X-Z demod 6MK8 at pin-9 and it should have greater than 15V P-P. Signal present? Check to see if the auto color circuit or the switch is cutting off the chroma subcarrier. In my T940 its broken away to feed the TAC just before the color control and the total auto color circuit is switched in & out by a slide switch on the back, regardless of the setting the subcarrier passes thru the board then down to the main color control R402 before feeding G-1 pin-7 of the 6MK8 demod. I completely bypassed my TAC auto color board as it never completely shut off. That board was too far advanced for the time and proved to be a big pain in the back side, germanium diodes and transistors are not heat tolerant.

The best advice I can give here is to follow the signals on the schematic, the big challenge I had with my T940 was drifty carbon comp resistors. Cold they were fine but would drift up with heat causing all kinds of weird problems later in life. At some point I will be replacing them all on the chroma/luma and IF boards, the sweep board got all new 5% films in 2005 to address horiz & vert stability problems.

There are a number of those mylar type capacitors that I've had trouble with over the years. They are as follows in my T940:
C715 0.1uf 400V DC filter on leg of 6DT6 audio det from boost B+ (low distorted audio)
C721 0.033uf 100V AGC line to tuner
C722 0.1uf 100V IF AGC drive (washed out video)
C732 0.047uf 200V killer output bypass cap.
C749 0.1uf 100V 3.58MHz osc control R/C in series with R764
C773 0.047uf 200V. XZ demod screen bypass
Normally Mylar is like a lifetime capacitor but being baked over half a century they will die. I replaced them all with orange drops from AES.

Seeing the glow in your set makes me want to pull mine out of storage, unfortunaly it would take up a large chunk of my apartment being a lowboy console. Someday I will get my sanity in check and dump my plastic fantastic sets for the AstroSonic.

[LukeSimon]

Thanks for the advice. Turning off the color killer has brought in color, but it is lots of purple and green colors.


When the color saturation knob on the front of the TV is turned 50% away from fully counter clockwise, I get lots of purple colors.


If I turn the “color” saturation knob on the front of the TV fully counterclockwise, as expected, the picture becomes black and white (aside from dynamic convergence issues).


If I turn the “color” saturation knob on the front of the TV to about 25% away from fully counterclockwise, I get green colors and a little purple.


One successful thing I accomplished is I got focus working! The slug in the ajustable focus buck boost transformer was not unscrewing as far as it should. I didn’t realize this a few days ago, but after playing with it for an embarrassingly long time, I was able to get it to screw in all the way and unscrew almost all the way. Maybe it was jammed with dust or gunk and constantly screwing it back snd forth unjammed it. Regardless, the cathode rays are focused sharp enough to show gaps between the scanlines when displaying 240p pictures.





Tomorrow I will see if I can get dynamic convergence calibrated, and if I get that working, then I will take an oscilloscope to the color circuit.

[ARC Tech-109]

Excellent progress on the set, you will prevail and when you're done I have a nice Panasonic CT34WX54 widescreen CRT just for your collection