Old Muntz TV for sale Locally to me, Should I pick it up?

[old_coot88]

Replacing the 500mf@5V cap won't fix your problem, guaranteed. It's already got a very low resistance across it, probably 10 - 20 ohms or so (wish we could see the actual schematic). So even if the cap were dead shorted, the increased load on the power supply will be negligible.

[Kevin Kuehn]

Agree the source of your excessive current draw is not the 500 mfd cap, but it needs to go none the less. Since you apparently have the Sams folder now, how about checking out the resistance readings chart on page 8. For starters check the resistance from the 5U4 filament pins(2,8) to chassis ground. Chart says there should be approximately 22,000 ohms there.

[Tom9589]

Looking at the schematic in msg #41, it looks like the voltage on the +370 line would vary according to the setting of the focus control. Also, it looks like the +360 line might vary some as well. Am I missing something here?

[Kevin Kuehn]

Originally Posted by Tom9589

Looking at the schematic in msg #41, it looks like the voltage on the +370 line would vary according to the setting of the focus control. Also, it looks like the +360 line might vary some as well. Am I missing something here?

It will change, but considering the focus control is in parallel with the 250 ohm focus coil it's overall effect is negligible. Sams will often say to the set the control to one extreme when taking voltage or resistance measurements.

[Tom9589]

Thanks. That makes sense. I'll be on the lookout for the SAMS note.

[vortalexfan]

OK so question, How do I go about measuring the B+ Voltage (the voltages on the 5U4G Tube) with the tube in place arcing while the TV is powered up? Wouldn't running the TV that long with the 5U4G tube arcing like that ruin that tube?

[jr_tech]

Don’t power it up again to check voltage! There is likely a short to ground that you must identify and remedy first. Do some ohm meter tests first.

Originally Posted by Kevin Kuehn

Agree the source of your excessive current draw is not the 500 mfd cap, but it needs to go none the less. Since you apparently have the Sams folder now, how about checking out the resistance readings chart on page 8. For starters check the resistance from the 5U4 filament pins(2,8) to chassis ground. Chart says there should be approximately 22,000 ohms there.

jr

[vortalexfan]

Originally Posted by jr_tech

Don’t power it up again to check voltage! There is likely a short to ground that you must identify and remedy first. Do some ohm meter tests first.



jr

OK So testing pins 2 and 8 of the 5U4G tube socket I'm only getting 25.5 Ohms on those pins, and not the 22,000 Ohms I should be getting.

I'm guessing that means that the issues I'm having with the excessive (4.5 Amps, 520 Watts) current draw lies there somewhere?

Or is that a sign that something else further down the line is causing that excessively low filament resistance reading on my 5U4G tube socket?

[vortalexfan]

This TV has me seriously stumped, there is NOTHING in this TV that looks like it could of shorted to ground.
The 26.0 Ohm Reading I'm getting on the 5U4G rectifier tube Socket rather than the 22,000 Ohm reading I should be getting is leading me to think that perhaps my 5v winding in my power transformer had failed in some way shape or form.

Everything else measures fine and there isnt' anything else in the TV that I can see that looks to be bad, all of the resistors test within tolerance (which is surprising) and leaky capacitors wouldn't cause excessive amounts of current draw to the point that the unit is drawing 3x the current its rated for.

It can't be the power supply caps because I had already replaced those with brand new ones and was careful to observe proper polarity and its still drawing over 4 amps of current and over 500 watts of power and the 5U4G tube is still arcing so like I said this TV has me stumped I cannot for the life of me figure out whats going on with this TV.

[Electronic M]

Originally Posted by vortalexfan

OK So testing pins 2 and 8 of the 5U4G tube socket I'm only getting 25.5 Ohms on those pins, and not the 22,000 Ohms I should be getting.

I'm guessing that means that the issues I'm having with the excessive (4.5 Amps, 520 Watts) current draw lies there somewhere?

Or is that a sign that something else further down the line is causing that excessively low filament resistance reading on my 5U4G tube socket?

Are you measuring 2 to ground and 8 to ground, or measuring 2 to 8 ( 2 to 8 ain't helpful)?

Either 2 or 8 should have a choke(in series with B+) or B+ line connected to it... unsolder the B+ line or choke and check 2 and or 8 to ground resistance. If the resistance is much higher than before then you have confirmed the rect and transformer are good. You'll then have to disconnect splits on the B+ line from the main line and check b+ to ground resistance till you find the point that has the short.

Most B+ shorts happen in ways that make visually identifying the issue impossible. You gotta rely on instruments, logic guided by the schematic.

It is not impossible for paper caps to dead short and for that to effect the B+... usually there is a power resistor in series that smokes out first, but not all sets are wired the same.

If I had it on my bench I could probably isolate the short in 30 min.

[old_coot88]

Originally Posted by Electronic M

...If I had it on my bench I could probably isolate the short in 30 min.

I could do in less than 1 min. Just kidding! Make it 2 minutes tops.

[vortalexfan]

Originally Posted by Electronic M

Are you measuring 2 to ground and 8 to ground, or measuring 2 to 8 ( 2 to 8 ain't helpful)?

Either 2 or 8 should have a choke(in series with B+) or B+ line connected to it... unsolder the B+ line or choke and check 2 and or 8 to ground resistance. If the resistance is much higher than before then you have confirmed the rect and transformer are good. You'll then have to disconnect splits on the B+ line from the main line and check b+ to ground resistance till you find the point that has the short.

Most B+ shorts happen in ways that make visually identifying the issue impossible. You gotta rely on instruments, logic guided by the schematic.

It is not impossible for paper caps to dead short and for that to effect the B+... usually there is a power resistor in series that smokes out first, but not all sets are wired the same.

If I had it on my bench I could probably isolate the short in 30 min.

I was measuring pin two to ground and pin 8 to ground, and those were the measurements I got, 26 ohms on pin 2 measuring to ground and pin 8 measuring to ground.

And as far as a paper cap being responsible for this goes, there are over 50 capacitors in there I'll have to check individually which will take me a while to test each and every capacitor with an ohm meter to see if the capacitor is still "good" or if it's open.

[Electronic M]

Originally Posted by vortalexfan

I was measuring pin two to ground and pin 8 to ground, and those were the measurements I got, 26 ohms on pin 2 measuring to ground and pin 8 measuring to ground.

And as far as a paper cap being responsible for this goes, there are over 50 capacitors in there I'll have to check individually which will take me a while to test each and every capacitor with an ohm meter to see if the capacitor is still "good" or if it's open.

If you are testing capacitance, don't bother. If the cap is open that is better. What you want to do do if you are hell bent on checking the paper caps is check the resistance across the cap and make sure it is too high for the meter to measure, and even that is a waste of time for two reasons. 1 some caps won't show leakage at the low 9V a DMM puts out (though that 25 ohms is registering on a meter so you might find it that way). 2 there is no guarantee it is a capacitor (could be almost anything on the B+ line) and it is far simpler (less, possibly MUCH less than half the work of checking all the caps) to start at the rectifier and split the B+circuit into segments by cutting it at nodes then measuring resistance to ground and splitting the lowest resistance section into smaller parts till you find the short.
Can you post a schematic of the power supply in your set?... I can write you a simple procedure to isolate the short if I can see the circuit.

[dieseljeep]

Originally Posted by vortalexfan

I was measuring pin two to ground and pin 8 to ground, and those were the measurements I got, 26 ohms on pin 2 measuring to ground and pin 8 measuring to ground.

And as far as a paper cap being responsible for this goes, there are over 50 capacitors in there I'll have to check individually which will take me a while to test each and every capacitor with an ohm meter to see if the capacitor is still "good" or if it's open.

Disconnect the lead going to the 5U4 socket from the B+ circuitry. It has to be in the B+ feeds, electrolytics, grounded filter choke, even the vertical output transformer.
As stated before, the smaller circuit capacitors throughout the circuitry of the set wouldn't cause a dead short in the B+ rail.
IIRC, the B- isn't going to the chassis but is forming a low bias source.

[init4fun]

With how many times now the poor 5U4 has been the purple guy here

it stands to reason that when the short circuit is finally found and fixed that the 5U4 may be too far gone to properly run the set .