Zenith 25EC58 Restoration Thread

[old_tv_nut]

Originally Posted by TinCanAlley

I think I found the horizontal sync issue, maybe. The 755 horizontal driver transistor tests good as far as not being shorted, etc. However, the hFE is zero. According to the specs of the ECG157, it's replacement, it should have a minimum hFE of 30.

Ordered an OEM replacement.

You must not be measuring hFE correctly. A transistor cannot test good with a hFE of zero.

If the driver transistor were dead, you likely would not have horizontal scan at all. The driver transistor is very unlikely to be the cause of intermittent sync.

Again, if some function is intermittent, you should applied mechanical tapping, pressing and jiggling to find an intermittent connection.

[TinCanAlley]

Originally Posted by old_tv_nut

You must not be measuring hFE correctly. A transistor cannot test good with a hFE of zero.

If the driver transistor were dead, you likely would not have horizontal scan at all. The driver transistor is very unlikely to be the cause of intermittent sync.

Again, if some function is intermittent, you should applied mechanical tapping, pressing and jiggling to find an intermittent connection.

I double and triple checked with two different meters and both read zero. I also found my old horizontal board and pulled the 755 off and tested. it had an hFE of 82.

I've tapped the crap out of everything, but nothing changed. I pulled all boards and reseated, nothing. I took the horizontal, vertical and chroma boards out and checked all the resistors and transistors. I then remembered the video processor board. It gets the sync signals. I check the resistors and all but two are within the noted value. The others were so far out, I'm going to lift a leg for better testing. One is 4.8K out of 10K and the other is 6.8K out of 18K. But most concerning is the IC. The legs are black from some kind of corrosion and/or heat. I tried cleaning them, but it won't come off. It is has migrated down many of the legs. There a couple with bluing from heat.

[TinCanAlley]

I've uploaded a video of the problem. Maybe a visual will help. I really don't know where to look as there are multiple places the horizontal sync is processed (IF, video processor module and horizontal module). As suggested I have tapped on everything, wiggled all connections, removed and reseated modules and tested a number of components on the modules.

It no longer seems to be intermittent sync as I have not been able to get it back, even for a moment. Been working on it for about 7 hrs now.

I've ruled out the IF module. See attached pic. It is from test point C1, Detector Output. I see the section where blanking, color burst and horizontal sync take place, followed by the video information.

https://youtu.be/Ftj62sprzNY

[mrjukebox160]

The horizontal is running way off frequency. If the horizontal hold adjustment is a coil you might be able to bring it back in with that.

[BeamT]

+1 Horiz osc is way off frequency. This is almost entirely contained on 9-57 Horiz Board, with the exception of the horiz osc coil/hold control and the attached capacitor that forms the circuit that determines the osc frequency.

+1 on the Q803 121-755 Horiz Driver transistor not being the cause. ECG 190 is a closer substitute.

The question is, can the osc run on frequency or is the AFC driving it off frequency.

Do you have a spare 9-57 Board?
Is the plastic stop still in place on the horiz hold control? Has it already been forced past the stop?
Have you replaced any other components on the 9-57 Board?
Re-examine and tighten pins and connectors A11, U4, T13, that connect to the coil.

Common problems:

R808 330ohm 1/2w replace and upgrade to 1w
R821 2.2k 1w replace and upgrade to 2w

Lead sockets for the transistors are always problematic, most people will either remove the pins and solder the xsistor directly to the board, or solder the leads in the pins. Every rebuilt board I've ever seen, Zenith or PTS, soldered the leads into the sockets.

From there you need to troubleshoot the board. At least its not intermittant anymore?

On the Video Proc IC, I've had luck gently scraping with an Exacto knife, also taking a small piece of green scotchbright soaked in DeoxIT or CRC Cleaner, held with a needlenose and gently scrubing. Be care not to over manipulate the pins.

[TinCanAlley]

Originally Posted by BeamT

+1 Horiz osc is way off frequency. This is almost entirely contained on 9-57 Horiz Board, with the exception of the horiz osc coil/hold control and the attached capacitor that forms the circuit that determines the osc frequency.

+1 on the Q803 121-755 Horiz Driver transistor not being the cause. ECG 190 is a closer substitute.

The question is, can the osc run on frequency or is the AFC driving it off frequency.

Do you have a spare 9-57 Board?
Is the plastic stop still in place on the horiz hold control? Has it already been forced past the stop?
Have you replaced any other components on the 9-57 Board?
Re-examine and tighten pins and connectors A11, U4, T13, that connect to the coil.

Common problems:

R808 330ohm 1/2w replace and upgrade to 1w
R821 2.2k 1w replace and upgrade to 2w

Lead sockets for the transistors are always problematic, most people will either remove the pins and solder the xsistor directly to the board, or solder the leads in the pins. Every rebuilt board I've ever seen, Zenith or PTS, soldered the leads into the sockets.

From there you need to troubleshoot the board. At least its not intermittant anymore?

On the Video Proc IC, I've had luck gently scraping with an Exacto knife, also taking a small piece of green scotchbright soaked in DeoxIT or CRC Cleaner, held with a needlenose and gently scrubing. Be care not to over manipulate the pins.

The horizontal hold still has the stop on it and it hasn't been turned past it. The image does show slight change as I rotate the control.

I replaced the 755 on the current board yesterday, no change, so I have to agree, it's not the issue.

I have replaced all the transistors on the board in the past month, other than that, it is unchanged. Could a bad transistor in another part of the board cause it to loose synch? If so, I have all the original transistors. Maybe one of the new transistors is bad. I also have the original 9-57 board, but after 13 years I can't remember why I had to replace it. Guess I could swap it and see what happens.

The lead sockets on the current module have one crimped socket per transistor. I guess they did that to make sure they were secured.

I tested the resistors. R808 is 310 ohm and R821 is 2.4K ohm.

I was given a "new" video processor board years ago, but it never worked. Then I noticed the top corner of it had a red mark on it. I'm betting it was the bad one put back in the box and that mark was to let them know that. Somehow it got out. Never got around to finding out what was wrong with it, but I remember the board yesterday and took the IC out of it and put it in the existing board. Problems solved for that one.

Okay, I'm going to put the old horizontal in right now and I'll post my results. I'll also retest the transistors I put on the current board and see if any have gone bad.

[TinCanAlley]

Well now I know why I replaced the 57 module. When installed, there is no image or raster on the screen, but there is sound. I hear the buzz of HV, but the CRT is not active.

[TinCanAlley]

Okay, I think I know what happed to the old 57 module. See attached pic. Back when I first got the set, I was told to check all the transistors to make sure they are seated. I guess I pushed too hard on this one as the emitter lead left the side opening of the socket and joined up with the base socket. I'm no expert, but I think they call that a short.

I'm going to test that transistor, and possibly just replace it no matter what testing says. Then I'll reinstall the board and test again.

[TinCanAlley]

Okay, fixing the insertion issue with the transistor fixed it. I put the board in and it fire up. Slight adjustment of the horizontal hold and it's all good. I'm going to let it run for a few hours to make sure it has been fixed. Now to use the removed board to practice my troubleshooting and repair. This board is exactly what I need for that.

Appreciate all the help. I'm sure it won't be the last time I need it, that's for sure. When this is done, I have a B&W RCA 12" from '75 and a 13" Hitachi from '82. Not to mention a Canon Monochrome computer monitor that has a weak CRT. The socket adapter I have for my B&K had a couple wires pulled from the socket. I asked the previous owner and they said something about that socket had the wrong wiring for many of the monochrome monitors and he said someone switched a couple leads. Now I'm uncertain of using it, so I need to find the wiring diagram from B&K and compare it to the connector of the tester to make sure they all line up. Once that is done, I'm sure I'll have to go the rejuvenation route with it.

[TinCanAlley]

I've ruled out the tuners being the source of the jailbars. I have a modulator that outputs to UHF and tested a laserdisc and VCR on channel 82 and the jailbars are still there.

The next step is the IF module. I found out why the new IF is over saturated. I need to adjust the detector output. I'm going to just turn it down by eye at first to see if the jailbars are still there. If not, I'll do the SM adjustment procedure. If they are still there, I'll put the original back.

After that, I'm not sure I have the ability, currently, to go any further in trying to find the source. I won't stop trying, but all the while I'll enjoy watching what I've been able to accomplish so far in making the image as close to factory new as possible.

[TinCanAlley]

Can ringing like what is causing my jailbars come from the 240V and/or 750V boost circuits? The reason I ask is I realize the diodes have not been changed. The 750V was cut and tested, but the 240V wasn't. I have one new diode, but need to get the second.

Also, the capacitors across these are 2KV, but the diodes are only 1.4KV. Since their voltage ratings are not standard, and I haven't been able to find them, can a voltage of 1.5KV be used?

[old_tv_nut]

Diodes themselves cannot cause jailbars. Jailbars could be gnerated in a circuit that happens to contain the diodes, but changing the diodes would not fix them.

Are you asking about replacing the caps or the diodes?

1.5 kV diodes can be used in place of 1.4 kV diodes.

1.5 kV caps should not be used in place of 2 kV caps. The rating may or may not be important for steady state operation, plus, it could be specified to tolerate temporary overvoltage due to CRT arcs or some other condition.

[TinCanAlley]

Originally Posted by old_tv_nut

Diodes themselves cannot cause jailbars. Jailbars could be gnerated in a circuit that happens to contain the diodes, but changing the diodes would not fix them.

Are you asking about replacing the caps or the diodes?

1.5 kV diodes can be used in place of 1.4 kV diodes.

1.5 kV caps should not be used in place of 2 kV caps. The rating may or may not be important for steady state operation, plus, it could be specified to tolerate temporary overvoltage due to CRT arcs or some other condition.

Just thought that if the specs of the diode(s) were out, then it might introduce noise in the circuit that appears as bars. Since I have one diode, I thought I'd replace a current one, most likely on the 240V boost, then the other one when I get its replacement.

I remember years ago when I first started looking at the jailbar issue I was told to put a capacitor of the same voltage in parallel with the B+ filter caps to see if it helped. I didn't, but was wondering if the same would apply to the caps across the boost diodes and damper diode. I have some 560pf 2KV caps an was going to see if they worked in place of the 600pf across the damper and boost diodes. But now wondering if I could parallel them to see if it has any affect on the bars. If not, I have one real 600pf 2KV on an NOS flyback that's instructions say if replacing my current unit I'm supposed to remove it. So if I ever need the new flyback, the capacitor isn't needed. But since it's only one, and my set has three, I'd like to use it where it's actually needed.

[old_tv_nut]

The jail bars frequency is in the 100 kHz or so range. It takes significant inductance to resonate at that frequency with the caps that are across the diodes. Take your spare cap and connect it across an existing diode cap and see if there is any difference. You don't have to remove the existing cap to see if the capacitor across the diode is having any effect on the jailbars. If you do see an effect, you have a starting point for investigating from there. If you don't see any effect on the jailbars, you can stop wasting your time and look for some other cause.

This advice is based on the caps in question being across the diodes as you describe and not from one end of the diode to somewhere else.

[TinCanAlley]

Originally Posted by old_tv_nut

The jail bars frequency is in the 100 kHz or so range. It takes significant inductance to resonate at that frequency with the caps that are across the diodes. Take your spare cap and connect it across an existing diode cap and see if there is any difference. You don't have to remove the existing cap to see if the capacitor across the diode is having any effect on the jailbars. If you do see an effect, you have a starting point for investigating from there. If you don't see any effect on the jailbars, you can stop wasting your time and look for some other cause.

This advice is based on the caps in question being across the diodes as you describe and not from one end of the diode to somewhere else.

There are two that fit the criteria. The one on the damper diode and the one on the boost rectifier for the 240V.

The damper diode will have to be done from the bottom, but the boost can be done from the topside. The diode and cap for boost are across connection 13 of the terminal strip by the flyback. I can just connect the cap to those for testing.

Can I use clip-on leads to them? The damper diode is from collector to ground and the SM has the collector at 125V and the boost is 240V.

I find it interesting that the new flyback is one revision newer than current, has the same resistors, capacitors, etc..... Yet, it says when replacing my current version, I need to remove the 600pf 2KV capacitor. The capacitor on mine is on the FB side of the boost voltage to ground. Guess they figure since the supply line going to the diode has one of those capacitors across it, thing the supply side of it to ground isn't necessary?