View Full Version : A Lemon set?


radio nut
03-21-2013, 12:00 PM
My Muntz from a previous thread burned up the flyback. I have since bought one and will buy a back-up if the videokarma member can be patient with me(money is tight).
Maybe I should have known what was going on but here is what happened....

New caps and out of tolerance resistors replaced. The horizontal frequency did not have a scope picture to adjust it. the diagram said to adjust horizontal drive clockwise until the picture distorts and then turn counter clockwise a little and stop.

When I tried that The pic would not distort... the fly would squeal.

If I ran the set every day everything seemed fine. If I left it alone for a week then the set would come on with no pic and I had to turn the brightness up to see it. the crt still tests the same though, really strong.

Does this sound like a flyback or other issue? i have checked/ rechecked ALL resistors and they are still ok.
All tubes were also checked and still tested fine. Also I put in a new doorknob cap and did try subbing the horizontal circuit tubes. All of this was before the fly went POOF!

Zenith26kc20
03-21-2013, 01:13 PM
If the picture seems to get too big to fit the screen and is very dim I would suspect high voltage (blooming).
I would think more along video or screen grid voltages on the picture tube are off value. I would check the schematic voltages and compare them.
Also, is the horizontal output or damper running hot (red plate)?

holmesuser01
03-21-2013, 03:04 PM
How about the drive signal from the horizontal oscillator tube to the output tube?

Does the horizontal output tube run abnormally hot, but not red plate?

Did the old flyback seem to run hot? Most of my set flybacks get warm, but never uncomfortably warm to the touch.

kramden66
03-21-2013, 03:12 PM
Yes good point the fly should not get too hot , if it does it is either overdriven , too much voltage going to it or the fly itself has a shorted winding.

mike

radio nut
03-24-2013, 06:19 PM
If the picture seems to get too big to fit the screen and is very dim I would suspect high voltage (blooming).
I would think more along video or screen grid voltages on the picture tube are off value. I would check the schematic voltages and compare them.
Also, is the horizontal output or damper running hot (red plate)?

no on the blooming. when the set had sat for a week or sp it would turn on with zero picture until the brightness was turned up. I did in the middle of this issue recheck all resistors actually a couple of times before the flyback went poof. I will check all voltages with new flyback in it.
no red plate on the hot tube.

radio nut
03-24-2013, 06:24 PM
How about the drive signal from the horizontal oscillator tube to the output tube?

Does the horizontal output tube run abnormally hot, but not red plate?

Did the old flyback seem to run hot? Most of my set flybacks get warm, but never uncomfortably warm to the touch.

Not sure if hot tube was too hot. What would that indicate? I do not have a scope to check horiz. drive signal. never checked fly by hand for temp. thought about it after I turned the set off but got shocked by something so I gave up.

Each time I ran the set day after day it seemed fine. With a week or two break it would come up with no pic until brightness turned up .

radio nut
03-24-2013, 06:42 PM
Well, Two different problems I guess.....

I have not used this set for about two or three weeks since the fly went.
Out of curiousity I Rechecked the crt and turned the lights off after I set up heater voltage and turned it to emissions... Purple...... plus it took 20 minutes to get the needle into the good section....
thinking crt leaking and going to air.
So all thats left is the fly.

radio nut
04-01-2013, 09:25 AM
Well I put in a used fly I purchased from a Member and a different crt and so far the picture is good.
One side question.... I can't seem to get all of the buzz out of the sound. Hooked to a dvd player the sound has a slight buzz that starts as soon as the dvd player is turned on.
The same player does not cause any buzz with a modern tv. No matter where the audio If or other adjustments are put the buzz remains.
Can I add a cap to filter This?

If so what part of the sound circuit?

Zenith26kc20
04-01-2013, 10:13 AM
Usually a leaker will come up to emission test fast as the air ionizes. When you turn the set on after sitting is it purple for a while. Any popping noises from the CRT?
My Magnavox was dark for a good while and slowly got bright as it ran. The CRT finally went PURPLE and that was the end of that one.
A new CRT cured that.

radio nut
04-01-2013, 10:54 AM
No on the popping noise.

The old fly was probably bad to begin with on this set. When I set up the horiz. sweep the fly would squeal when I turned the drive adjustment clockwise as the diagram states. The replacement fly will eventually distort the picture like the diagram says. so once I get the sound buzz figured out I will be happy. Oh it does quit buzzing at full or really high volume!

You are correct that it would eventually get brighter after it ran. Someone told me that a gassy tube could pull more power. If they are correct I am lucky that I do have a spare crt!

kramden66
04-01-2013, 12:58 PM
try unhooking one terminal on the antenna connection causing a weaker signal and see if it buzzes , is it doing the buzz durring everything or just text ?

it is a muntz so there are missing things inside that may be causing this

mike

Don Lindsly
04-01-2013, 01:15 PM
Make sure the 1B3 is good. If the set uses porcelain insulators on the 1B3 socket, they go bad and cause blooming.

High voltage rectifier should always be replaced when changing flyback.

For horiz alignment, short the coil section with the cap and adjust for proper frequency. Then remove short and adjust the other end of the can for proper freq, then touch up a half turn to prevent "motor-boating".

Make sure the 180uuf horiz osc coupling cap is good. Replace with silver mica only.

Check HO tube screen resistor for proper value.

For sound buzz, check 4uf cap in sound detector and make sure the aquadag CRT coating is well grounded.

miniman82
04-01-2013, 05:23 PM
Audio buzz can be cured with an alignment or a slight tweak to a coil or two in the IF strip, but you really have to be familiar with the set to go poking around without a scope. If slightly adjusting fine tuning cures the buzz but jacks up the picture, you need an alignment.

radio nut
04-02-2013, 11:41 AM
try unhooking one terminal on the antenna connection causing a weaker signal and see if it buzzes , is it doing the buzz durring everything or just text ?

it is a muntz so there are missing things inside that may be causing this

mike

The buzz is worse on text but the set starts out pretty good. The buzz gets worse as time goes on.
Well, in the sound circuit I am using new caps and New ceramics.

Is the new ceramics a mistake meaning I should have used Mica?

radio nut
04-02-2013, 11:45 AM
Audio buzz can be cured with an alignment or a slight tweak to a coil or two in the IF strip, but you really have to be familiar with the set to go poking around without a scope. If slightly adjusting fine tuning cures the buzz but jacks up the picture, you need an alignment.

Ok, will check. Just curious.....Does the yoke have to be pushed up against the bell of the crt or is a small gap normal?

Don Lindsly
04-02-2013, 12:58 PM
The yoke should be snug against the CRT in a B&W set.

Einar72
04-02-2013, 11:16 PM
I don't wanna go too far out on a limb having not seen the schematic, but there may not be enough performance engineered into the poor old thing to not have some buzz. I think it was Zenith that had a pot somewhere in their design called a "buzz control". The usual cause of buzz on white-intensive video (like text) is the strong white-level signal "spilling over" into the sound circuit. I wouldn't expect the poor little Muntz's AGC circuit to handle the overmodulated video signal from a modern VCR/DVD/Cable Box/NTSC Converter modulator, try an attenuator on the antenna input, since there's no more need to have weak-signal capability...

radio nut
04-02-2013, 11:42 PM
Audio buzz can be cured with an alignment or a slight tweak to a coil or two in the IF strip, but you really have to be familiar with the set to go poking around without a scope. If slightly adjusting fine tuning cures the buzz but jacks up the picture, you need an alignment.

This set does not have a fine tuning adjustment........high end ! Huh?

AdamAnt316
04-03-2013, 12:14 AM
This set does not have a fine tuning adjustment........high end ! Huh?

Oh, it was "high end", all right. Madman Muntz was legendary for getting rid of 'superfluous' features such as fine tuning and AFC in the sets he sold, stripping them down to their absolute bare essentials. It is said that he carried a pair of insulated wire cutters with him, and would start snipping components out of a running TV until it stopped working, then have his engineers put the last snipped part back in. They don't call it "Muntzing" (http://electronicdesign.com/boards/whats-all-muntzing-stuff-anyhow) for nothing. ;)
-Adam

dieseljeep
04-03-2013, 09:19 AM
I don't wanna go too far out on a limb having not seen the schematic, but there may not be enough performance engineered into the poor old thing to not have some buzz. I think it was Zenith that had a pot somewhere in their design called a "buzz control". The usual cause of buzz on white-intensive video (like text) is the strong white-level signal "spilling over" into the sound circuit. I wouldn't expect the poor little Muntz's AGC circuit to handle the overmodulated video signal from a modern VCR/DVD/Cable Box/NTSC Converter modulator, try an attenuator on the antenna input, since there's no more need to have weak-signal capability...

Zenith sets had a buzz control because of the type of audio detector circuit they used.
You're right regarding, attenuating the signal. Muntz didn't use much of a AGC circuit. They seemed to run the IF and RF "wide open".
You also had to try several different 12AT7's and 6BL7's to get those circuits to work properly. :scratch2:

radio nut
04-07-2013, 03:13 PM
What does having the volume all the way up have to do with the sound buzz disappearing?

radio nut
04-07-2013, 05:28 PM
Well, got curious and checked voltages on the 6t8 audio detector tube...
pins 1-2 -3 should be minus.5 minus 1.8 and minus .5 respectively.....they check 20 volts pos. at least on those pins with a digital volt meter. with analog the needle moves off the scale to the left even with the neg lead or pos lead to ground......
now I am confused...

Don Lindsly
04-08-2013, 08:11 PM
The reference voltages were likely taken without an antenna. The voltages you are seeing are normal.

radio nut
04-14-2013, 02:23 PM
Well I took the diagram to an expert because something just did not look right but I could not put my finger on it.

He looked at it and said the ratio detector tube is isolated from voltage and should be signal only. He stated that the ratio detector transformer was probably shorted.
I called Moyer's.,....got a new one.....put it in and sure enough it works and there is only Macrovision buzz that goes in and out in conjunction with the lines on the screen that likewise come and go.
I can't wait to try a tape/disc that is not encoded with Macrovision!
I tore the old transformer apart and there was a dark brown spot on the side where it looked to be arcing.