Zenith 25EC58 Restoration Thread

[TinCanAlley]

About to install the new setup switch. I got the old one out and opened it after testing. Testing on the new switch showed no resistance. On the old switch, it showed resistance, mainly in the "setup" position. Upon inspection of the contact pads, I can see where the resistance is coming from.

Took plenty of pics to make sure I get the wiring on the new switch correct. Sure wish there was more play in the wires for easier install and soldering.

I've attached a pic. Can you guess which pads are for "setup"?

[TinCanAlley]

The tripler has a 500ohm 7W resistor that is one of the cement (sandbox) type. Since I'm replacing the tripler I figured I'd go ahead and replace the resistor as well. Finding the cement ones is not easy, so I just ordered a WW in the proper resistance and wattage that is a 1%. It also has a temp of up to 250c.

[old_tv_nut]

You do not need or want a capacitor in the external power supply lead. That would defeat the insertion of 7.2 volts. The 7.2 volts is to force a known AGC voltage in the system when making the adjustment of the output.

[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.

[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?

[old_tv_nut]

"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?"
A cap across a diode and a cap from one end to somewhere else serve different purposes. One place doesn't sub for the other. There must be something different about the flyback transformer itself to require removing the cap.

Re clip leads: keep leads short and be very careful about accidental shorts, as the diodes would not like that.

[TinCanAlley]

Originally Posted by old_tv_nut

"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?"
A cap across a diode and a cap from one end to somewhere else serve different purposes. One place doesn't sub for the other. There must be something different about the flyback transformer itself to require removing the cap.

Re clip leads: keep leads short and be very careful about accidental shorts, as the diodes would not like that.

If there is something different with the flyback, I can't find it (visually). The capacitor only stays in if you're replacing the -02 with this -02. The original -00, -01 and -03 deletes the capacitor. Not going to put anymore thought into it. Just glad I now have a 2KV capacitor. If I do find that one of the current ones is not up to spec, I have a replacement.

The leads I have are 10" and rated for 600V. Clips are insulated except for the bite section.

[old_tv_nut]

Voltages in some parts of the horizontal section can exceed 600V, so i would play it safe.