Are these things I should consider bad - Page 9 - Videokarma.org TV - Video

Findm-Keepm · #**121** 10-02-2013, 09:28 PM

Quote:

Originally Posted by jr_tech

I have been looking at the schematic and scratching my head over that cap... what does it do? Is the transformer somehow set up to be a constant voltage device?

jr

That transformer supply is ferroresonant - an L-C tank, and causes the transformer to provide some level of regulation itself. If the 440V cap goes bad, you'd know it, with a dead set, or really low voltages. If you get B+ and other transformer secondary voltages, move on, as the ferroresonant circuit is working. Zenith stressed this after seeing a spike in folks ordering the cap. Reliability testing showed the cap had a 40+ year life, and shops that didn't understand the circuit would order and replace the cap to eliminate that as the cause of whatever problem they had. I've only seen one go bad- and a dead set was the result, with the blown input fuse as well.

We always kept the caps from the sets we junked, and I used one two summers ago as a makeshift run capacitor in my HVAC Gaspack to keep the AC going in July 2011. Date code on the cap is 1979....

Cheers,

TinCanAlley · #**122** 10-02-2013, 09:28 PM

Quote:

Originally Posted by andy

It doesn't look that bad to me. I think your scope is showing more of those sharp negative going pulses than what ever scope Sams used. If you ignore that really fine negative pulse, it ends up being around 8-10 volts which is well within reason. I doubt Sams would have used a 100MHz oscilloscope because no technician in the 70's would have had anything even close to it.

I can just about guarantee nothing in the HOT base drive circuit is causing your jail bars. Generally, if there's anything wrong with the base drive, the HOT will overheat and fail. The HOT acts as a switch. It's either completely on, or off.

I think you need to be looking around the flyback, and yoke. Something is causing the flyback, or yoke to resonate at about 10x the horizontal scanning frequency. It's not going to be caused by a slightly wrong drive signal, or slightly off voltage.

Have you looked at L218 and C274?

Okay, just to make sure, is this attached pic of the L218 you're referring to (the brown cylinder with the coil around it)? It took me a moment to find, but it is connected to tie points 3 and 4. I should check its resistance and see if it matches what Sams has across tie points 3 and 4? Also, I can't find the capacitor shown (the .047).

You have any tips on checking the yoke? I've been trying, but I can't see to get any meaningful results. I've tried checking pin pairs, but according to the schematics, each has multiple connection points. So how am I supposed to check the resistance? All I can really do is check continuity. The first pair I tried should have a resistance of 220ohm, but my meter shows me 8ohms.

TinCanAlley · #**123** 10-02-2013, 09:39 PM

Quote:

Originally Posted by andy

Have you looked at L218 and C274?

Okay, I checked L218 and I got .7. It's supposed to be .12, but I checked and my leads are .6. So subtract the leads and I get .1 (my meter doesn't show anymore places to the right).

I think I found the capacitor. It's red and axial and there are actually two of them. Is this them in the pic?

TinCanAlley · #**124** 10-02-2013, 10:18 PM

Okay I finished the testing of the FB. I don't know how accurate it is as my leads seem to have .6 resistance of their own and I don't know if that should be disregarded or subtracted. I usually disregard it, but the numbers I'm looking at for the FB are pretty low and if .6 is added by the leads, it could easily triple some of the results. So if anyone knows what should be done in this situation, let me know.

I'm attaching the FB section of the schematic so you can see where/what I tested.

Below are the connections, the results w/out subtracting me leads and the values on the schematic. The first three are at the tie points on the board next to the FB. The rest were done at the chassis connections of the FB leads.

Connections Results Sams
3 - 9 / .8 / .1
4 - 8 / 2.2 / 1.6
9 - yoke plug / .12 / .5

G - R/W / .10 / .5
R/W - Blk / .8 / .7
Y - Blk / .7 / .1
Y - Vio / .8 / .2

I don't know if anything is thrown off by all the leads being connected to the chassis still. Nothing in the Sams says anything about tolerance, so I don't know how close or dead on they have to be.

Username1 · #**125** 10-03-2013, 08:38 AM

I'm happy your check of the low voltage power supply went well. As long as the 125v is not as high as 140v and your measurement of the 24v regulated showed the same readings on both test instruments, lets go back to tp5. I know there are not many who believe something can be wrong here, but there has to be a reason the drive voltages are so far off. I'm posting 2 more pictures. In the horiz osc, driver, I would like you to measure the voltages of the driver transistor and post them. I believe its q803. I would also like to check the caps attached to the coupling transformer between the driver and HOT.

Also check that 2200 ohm resistor between 124 and 123 is it ok....?
If the emitter voltage of the driver is too high, you'll have to check that 40 ohm resistor in that circuit.

In that picture you posted of the flyback, is it melted or is that some repair made to it ? its not running hot is it?

Sams gives an emitter current for the HOT on that schematic, I think it say 1.8A, if you think its running hot, it may be worth measuring that.... You have to use the 10A DC current scale on your DMM, - to ground + to the emitter. Only if you think its running hot..... or the FB is melted....

TinCanAlley · #**126** 10-03-2013, 11:13 AM

Quote:

Originally Posted by Username1

I'm happy your check of the low voltage power supply went well. As long as the 125v is not as high as 140v and your measurement of the 24v regulated showed the same readings on both test instruments, lets go back to tp5. I know there are not many who believe something can be wrong here, but there has to be a reason the drive voltages are so far off. I'm posting 2 more pictures. In the horiz osc, driver, I would like you to measure the voltages of the driver transistor and post them. I believe its q803. I would also like to check the caps attached to the coupling transformer between the driver and HOT.

Also check that 2200 ohm resistor between 124 and 123 is it ok....?
If the emitter voltage of the driver is too high, you'll have to check that 40 ohm resistor in that circuit.

In that picture you posted of the flyback, is it melted or is that some repair made to it ? its not running hot is it?

Sams gives an emitter current for the HOT on that schematic, I think it say 1.8A, if you think its running hot, it may be worth measuring that.... You have to use the 10A DC current scale on your DMM, - to ground + to the emitter. Only if you think its running hot..... or the FB is melted....

The two caps you noted between the coupling transformer are new and were tested before installation (jail bars were there before the replacement).

I think you're looking at the blob of RTV covering the lead so it doesn't arc from the top of the flyback to any metal in its surroundings.

The HOT runs around 81 degrees and cool to the touch (well not cool, but barely registers to the touch as it's no hotter than my fingertips).

I'll pull the horizontal board today and check the values of the cap and resistors noted. If they are okay, I'll put it back and scope the H-OSC transistor.

Gotta catch up on some other duties this morning, but will be back inside the set in a couple of hours.

TinCanAlley · #**127** 10-03-2013, 01:51 PM

[QUOTE=Username1;3084085]I'm posting 2 more pictures. In the horiz osc, driver, I would like you to measure the voltages of the driver transistor and post them. I believe its q803. I would also like to check the caps attached to the coupling transformer between the driver and HOT.

Also check that 2200 ohm resistor between 124 and 123 is it ok....?
If the emitter voltage of the driver is too high, you'll have to check that 40 ohm resistor in that circuit.QUOTE]

Okay, finished testing what you noted. The 20pf (C814) is 19.8, the 2Kohm resistor (R821) is 2.5Kohm (this one has no percentage band so I don't know if that's too far off), the .22uf (C811) is .23uf and the 40ohm resistor (R819) is 47ohm.

Do you want me to test the voltage of all three legs of (Q803)? Do you just want DMM readings, scope or both? This little bugger's a pain as it's right behind the large capacitor and it's a pain to get the hooks on the legs.

Username1 · #**128** 10-03-2013, 01:57 PM

Yah, I would like to know what the voltages are on that driver transistor....
No need for the scope, the shape on the output looks ok, so no need.
q803. Follow the trace to the next component if its more convenient to take a voltage reading. You can measure off the resistor, that 40 ohm, just get the side that goes to the transistor... Same with the base, get it off the r814, one side is ground.... 180 ohms another 5% resistor...

That 40 ohm resistor says it should be 5%, but its off by more than 10%..... close to 20% off.....
Any discoloring on that resistor....

Was that HV Transformer arcing? did you add that silicone to the HVT?
Do you have any idea if the HV to the picture tube is on target, high or low...?

If you were to take out the HOT and check the voltage at tp5, does it come back to normal....? Not sure if you have run the set without it in there or not....? Not entirely sure if its safe, I would think it is....

TinCanAlley · #**129** 10-03-2013, 02:18 PM

Quote:

Originally Posted by Username1

Yah, I would like to know what the voltages are on that driver transistor....
No need for the scope, the shape on the output looks ok, so no need.
q803. Follow the trace to the next component if its more convenient to take a voltage reading. You can measure off the resistor, that 40 ohm, just get the side that goes to the transistor... Same with the base, get it off the r814, one side is ground.... 180 ohms another 5% resistor...

That 40 ohm resistor says it should be 5%, but its off by more than 10%..... close to 20% off.....
Any discoloring on that resistor....

Okay, got in there with a small j-hook. The Collector is 68V (Sams 63.7V), Base is 2.28V (Sams 2.42V) and Emitter is 2.68V (Sams 2.07V)

Username1 · #**130** 10-03-2013, 02:35 PM

Sorry, I have added stuff above....

That diode in the HOT collector, is that been tested, and do you know if its the right one? It would need to be a high speed diode, not a power supply 60hz diode, but a fast one...

And I see there is a 1.5 ohm resistor on the secondary of the coupling transformer going to the HOT base, is that 1.5 ohm resistor been checked...?
goes to ground, and one leg of that transformer, secondary.

TinCanAlley · #**131** 10-03-2013, 02:46 PM

Quote:

Originally Posted by Username1

Sorry, I have added stuff above....

That diode in the HOT collector, is that been tested, and do you know if its the right one? It would need to be a high speed diode, not a power supply 60hz diode, but a fast one...

I got 442 in one direction and 1232 in the other direction yesterday, but that's with the HOT installed. If I remove the HOT, will I be able to get a better reading or do I need to lift a leg? I'm pretty sure the 1232 should be infinity and the HOT might be interfering with the readings.

I couldn't tell you if it's the correct one, and I do have an NOS replacement if needed. The replacement isn't a cylinder like the current one. It's shaped kind of like a barrel.

TinCanAlley · #**132** 10-03-2013, 02:53 PM

Quote:

Originally Posted by Username1

Yah, I would like to know what the voltages are on that driver transistor....
No need for the scope, the shape on the output looks ok, so no need.
q803. Follow the trace to the next component if its more convenient to take a voltage reading. You can measure off the resistor, that 40 ohm, just get the side that goes to the transistor... Same with the base, get it off the r814, one side is ground.... 180 ohms another 5% resistor...

That 40 ohm resistor says it should be 5%, but its off by more than 10%..... close to 20% off.....
Any discoloring on that resistor....

Was that HV Transformer arcing? did you add that silicone to the HVT?
Do you have any idea if the HV to the picture tube is on target, high or low...?

If you were to take out the HOT and check the voltage at tp5, does it come back to normal....? Not sure if you have run the set without it in there or not....? Not entirely sure if its safe, I would think it is....

The RTV, I believe, is factory. Doesn't look like it's ever been disturbed. It's also on the tripler end.

I don't have an HV probe, so I couldn't tell you. This makes me think of something I've been meaning to ask. Whenever they talk about safety they get on the fact you need to discharge the CRT at the anode. They say to use a screwdriver connected to ground. So I take a long screwdriver, clip an alligator clip lead to it and clip the other end to chassis ground. I then slip the tip under the cap and touch the connection. They say you should hear a "snap" and should do it a couple of time. I've done this several times while working on this set and not once this it make a noise or give any indication that it discharged. Last time I took out the chassis I just turned off the set, pulled the plug and in less than 3 minutes later I tried to discharge and nothing.

When I killed the HOT the other week, I ran the set without it (was the only way to keep the circuit breaker from going. I don't see why I couldn't remove it and measure T5 again. Do you want just a DMM voltage or a scope waveform?

Username1 · #**133** 10-03-2013, 03:21 PM

Well here is my thinking...
The horiz driver transistor, 67v 2.x, and 2.x tells me that it looks to be within reason given something is wrong elsewhere in the immediate area.

The large signal on the base of the HOT leads me to think possibly open 1.5 ohm resistor in the secondary of the coupling transformer... You should check that.

Then there is that diode in the collector of the HOT. That diode is to arrest, or stop a sharp negative pulse coming from the HVT when the transistor turns off. If this diode was not turning on fast enough would there be a sharp negative pulse there, and would it be traveling back to the coupling transformer???? It could also be allowing the HVT to be producing those jail bars.... Its all reason, What's going on, and testing. There are some clues around that area, so its just trying to see what part could reasonably be bad, still let most of it work properly, but still be causing some minor problems.... That doide, If you have a new one, might be smart to put it in since it will be spec. when you do....

And on that picture tube discharge, on all tube sets there is more likely a long term charge held, but on a trippler set there are little bleeder resistors, and leakage through the caps in there, more so than a tube rectifier alone...
While its best practice to discharge always, if there is no change, don't loose sleep over it.

TinCanAlley · #**134** 10-03-2013, 03:52 PM

Quote:

Originally Posted by Username1

Well here is my thinking...
The horiz driver transistor, 67v 2.x, and 2.x tells me that it looks to be within reason given something is wrong elsewhere in the immediate area.

The large signal on the base of the HOT leads me to think possibly open 1.5 ohm resistor in the secondary of the coupling transformer... You should check that.

Then there is that diode in the collector of the HOT. That diode is to arrest, or stop a sharp negative pulse coming from the HVT when the transistor turns off. If this diode was not turning on fast enough would there be a sharp negative pulse there, and would it be traveling back to the coupling transformer???? It could also be allowing the HVT to be producing those jail bars.... Its all reason, What's going on, and testing. There are some clues around that area, so its just trying to see what part could reasonably be bad, still let most of it work properly, but still be causing some minor problems.... That doide, If you have a new one, might be smart to put it in since it will be spec. when you do....

And on that picture tube discharge, on all tube sets there is more likely a long term charge held, but on a trippler set there are little bleeder resistors, and leakage through the caps in there, more so than a tube rectifier alone...
While its best practice to discharge always, if there is no change, don't loose sleep over it.

Okay, I'm going to pull the chassis and change out the diode and the 1.5ohm resistor. I have purchased all new resistors for that area in case of finding any bad ones.

If you can think of anything else while the chassis is out, that would be great. It's much easier to test/replace when it's on the bench.

Username1 · #**135** 10-03-2013, 04:37 PM

I know its trying to be taken in different directions by different people with ideas about what can be wrong. I'm quite surprised you have not called us all a bunch of nuts by now and thrown that tv off the roof and went and got another off CL for free or $10. and just said the hell with it...... : )

If its not right in that area of the coupling transformer, that diode, open resistor, then it's got to be the pink fairy inside the flyback, she probably can't get any air cause of that extra silicone on it.....

I know it's a Letterman thing, but you may be able to plead your case to Lenno, he's on your coast, maybe he'll make an exception and do a Letterman and toss it off a roof for you and put it on national tv..... Could be the best solution to the problem...