Zenith 25EC58 Restoration Thread

[TinCanAlley]

Since I'm starting from scratch, I'm creating my final thread on this chassis/TV. I know multiple threads are frowned upon and that's why this one. It will serve as an update and help request in a single thread.

Okay, so I am tired of chasing issues, so I'm going to tackle the entire set, piece by piece. I'll start with the chassis and replace all electrolytics. I did that 13 years ago, but I know a little more about replacements than I did back then. I have ordered, where available, the exact replacement noted in the SM (Vishay and Cornell). I will also be testing, as best as can be while in circuit, all resistors and diodes.

My first task is to measure B+ and scope it. I will start at the test point and from there, test and scope at all the in and out points on each module. From the schematics I see that from the regulator it goes in this order: Chroma-->Subcarrier-->Video Processor-->Vertical-->Horizontal-->125V source for a complete loop. I want to do this before and after recapping and other diags to see if there is any noticeable change. I figure if it starts out clean, then gets noisier, I can pinpoint the issue better.

After the main stuff, I'll be replacing pots on the convergence board with NOS, replace IC on sound board in hopes of getting tone control back and fixing the channel motor as it gets stuck pretty often and requires a change up and down to get it to fully change channel. Once that is done, I'll do all adjustments per SM.

So that's my plan. Parts will be here Thursday, so I have plenty of time to measure and scope. I know I'll be here often to ask questions and request help, that's for sure. I've learned quite a bit from everyone here, so I think this will go well.

Now for my first questions. If the B+ is 125V, does that need to be dead on, or is there a +/- that's acceptable? The filtering comes from the multi-cap canister that I bypassed already, wanting the keep the canister for aesthetics. I wasn't able to find 400uf 150V, so I used 390uf 160V. I'm pretty sure that falls within the 20%. See attached pic.

Do the RGB output transistors have affect on image and such? The reason I ask is it originally came with 121-743 TO-5 package, but to replace one cost more than buying all three 121-868 TO-220 package. While this works, I miss the original look of the heatsinks with the plastic cage. As it stands now, they are just plugged in with a small clip-on heatsink on each. Looks crappy. If they do the exact same job, I'm going back as I got my hands on an NOS TO-5.

[TinCanAlley]

Well what do you know. I have another question.

I am searching for the 3 pots I don't have for the convergence board, but Zenith parts aren't out there. I started looking under the other part numbers. I managed to find all I need, but I'm not sure if they'll really do the job. The resistance numbers are off. For instance, the 300 ohm factory's replacement is 500 ohm, 60 ohm replacement is 50 ohm, 120 ohm replacement is 100 ohm and so on.

I can see how the 500 ohm works as it goes beyond the 300, but the others don't even reach the original. Zenith did put them down as replacements, so they must work, right?

[vol.2]

Originally Posted by TinCanAlley

My first task is to measure B+ and scope it. I will start at the test point and from there, test and scope at all the in and out points on each module. From the schematics I see that from the regulator it goes in this order: Chroma-->Subcarrier-->Video Processor-->Vertical-->Horizontal-->125V source for a complete loop. I want to do this before and after recapping and other diags to see if there is any noticeable change. I figure if it starts out clean, then gets noisier, I can pinpoint the issue better.

I just want to make sure you know not to scope the B+ unless your set is plugged into an isolation transformer, or you're using an entirely battery powered scope. You cannot scope anything in the set without isolating it or using a battery powered scope because the neutral line will short out to the ground lead on your scope probe because the neutral and the earth are connected at the circuit breaker.

I would, in general, recommend that an isolation transformer always be used for doing any live testing on a set, no matter how safe you think you're being, but I know not all people will do that.

Sorry if this is unneeded advice, but I just wanted to make sure that at least someone said this to you.

[Electronic M]

Originally Posted by vol.2

I just want to make sure you know not to scope the B+ unless your set is plugged into an isolation transformer, or you're using an entirely battery powered scope. You cannot scope anything in the set without isolating it or using a battery powered scope because the neutral line will short out to the ground lead on your scope probe because the neutral and the earth are connected at the circuit breaker.

I would, in general, recommend that an isolation transformer always be used for doing any live testing on a set, no matter how safe you think you're being, but I know not all people will do that.

Sorry if this is unneeded advice, but I just wanted to make sure that at least someone said this to you.

IIRC all Zenith CCIIs, besides the one flat chassis variant that had a switch mode module, had a big honking power transformer with full isolation so you don't need an outboard isolation transformer on these.
Although transformers can short between windings and keep working as though nothing is wrong (I had my outboard bench isolation transformer do this to me and ironically cause the very problem it existed to fix... I learned the hands on way) and it's wise to check chassis to scooe ground voltage with a DMM before hooking up a scope.

[vol.2]

Originally Posted by Electronic M

IIRC all Zenith CCIIs, besides the one flat chassis variant that had a switch mode module, had a big honking power transformer with full isolation so you don't need an outboard isolation transformer on these.
Although transformers can short between windings and keep working as though nothing is wrong (I had my outboard bench isolation transformer do this to me and ironically cause the very problem it existed to fix... I learned the hands on way) and it's wise to check chassis to scooe ground voltage with a DMM before hooking up a scope.

Yeah, I just automatically use my isolation transformer for any DUT. Even with an internal transformer, you still have mains on the hot side of the power supply, and accidents can happen

[TinCanAlley]

I decided I would give my battery operated scope a shot at it. I do know that back in 2012, I scoped just about everything major on it with my Tektronix before anyone mention "hot" chassis. I did inquire about it after reading that and had mixed responses about using an isolation transformer. Some said it was ok not to since it had a transformer for all sources of power, while others said it was better safe than sorry. I am in the camp of better safe than sorry, so I'll be using battery operated and only if that doesn't work will I bring out the big gun.

Maybe someone can tell me if I have this correct.... I want to scope the B+ DC coupled. If I then do AC, won't that show me ripple? If so, how much ripple is too much ripple? The filter caps for the B+ came off the canister cap, but other than the values in it, I can't find anything about ripple. I do know about what I replaced it with and it has a ripple of 1.5A. I don't know if that's too much, but all the caps I could find were between 1.4 and 1.6A for ripple. I've learned quite a bit over this project, but somethings still mystify me.

[vol.2]

Originally Posted by TinCanAlley

Some said it was ok not to since it had a transformer for all sources of power, while others said it was better safe than sorry.

Yes, it's a better safe than sorry thing. The issue could arise if you accidentally touched something that is on the line side of the transformer. If that happens, it will short out your probe ground to line voltage.

However, if you were to accidentally touch the line side of the transformer with you hand, you could then be connected to the breaker yourself, and that could hurt or kill you. This is generally why some people recommend an isolation transformer no matter what. I do enough electronic repairs that I bought one and just use it.

Maybe someone can tell me if I have this correct.... I want to scope the B+ DC coupled. If I then do AC, won't that show me ripple? If so, how much ripple is too much ripple?

I feel like I would start getting suspicious if it was more than 100mV, as that might indicate weak filter caps. If it's much over 150-200mV that would make me concerned, and any much higher than that I would turn it off and start hunting for the bad component. Certainly if it was over 1V or more.

In terms of AC Vs DC probing: The DC probing doesn't show the ripple because you have to crank up the volts/div so high that you lose the visual resolution to see the ripple. Looking at AC only means the DC component is gone, and you are then just looking at the waveform centered in the middle of the scope (vertically). So that's why you see the ripple in AC coupled.

I don't know what that particular set looks like, but my 1975 Sony 9" is about the older thing I have and it's B+ is rock solid at 130V with less than 20mV of ripple, so I know it's possible.

[TinCanAlley]

Originally Posted by vol.2

Yes, it's a better safe than sorry thing. The issue could arise if you accidentally touched something that is on the line side of the transformer. If that happens, it will short out your probe ground to line voltage.

However, if you were to accidentally touch the line side of the transformer with you hand, you could then be connected to the breaker yourself, and that could hurt or kill you. This is generally why some people recommend an isolation transformer no matter what. I do enough electronic repairs that I bought one and just use it.



I feel like I would start getting suspicious if it was more than 100mV, as that might indicate weak filter caps. If it's much over 150-200mV that would make me concerned, and any much higher than that I would turn it off and start hunting for the bad component. Certainly if it was over 1V or more.

In terms of AC Vs DC probing: The DC probing doesn't show the ripple because you have to crank up the volts/div so high that you lose the visual resolution to see the ripple. Looking at AC only means the DC component is gone, and you are then just looking at the waveform centered in the middle of the scope (vertically). So that's why you see the ripple in AC coupled.

I don't know what that particular set looks like, but my 1975 Sony 9" is about the older thing I have and it's B+ is rock solid at 130V with less than 20mV of ripple, so I know it's possible.

I don't know what the ripple is, it was more a question about the ripple noted for the capacitor. I'm assuming that's the maximum ripple current the capacitor can/will pass. While my jailbars have reduced in intensity, they are still there. All the Zenith jailbar fixes I can find aren't for this chassis, but they all point to filtering capacitors near the power transformer. So I'm hoping it will be one of the caps in that area.

I'm going to start testing voltages tomorrow. Right now I'm installing an IC socket and new IC on the sound board in hopes of restoring tone control. I've check the wiring and pot and it all appears good. That leave the IC as the tone control goes directly into pin 1 of the IC. Seeing as how the IC on the Chroma board caused tint and color saturation issues, I'm willing to take an 8$ gamble and install an NOS IC.

I don't need a fancy isolation transformer, do I? There's a unit on Amazon for 136 and it seems decent for the price. It handles 500W. Is that good enough, or it is wiser to get the 2000W version? I'm not sure if this set exceeds the 500W unit's 4A.

[TinCanAlley]

Not that this is the cause of my loss of tone control, but it shows how things fall through the cracks. It's been working this way for 52 years, but they could have done better. "Quality goes in before the name goes on...."

[BeamT]

Obviously bad connections, easily reflowed and cleaned up. Could very well be your problem. You will know soon enough.

I usually find the 125v B+ in the flat pan chassis such as your 25EC to be 120-130 depending on the line voltage. IIRC this supply is regulated by the resonant circuit formed by the power transfomer and the 3.3uf oil capacitor. There is no regulator that you can adjust. The 24v supply does have a regulator, set by the zener diode in the base circuit.

If your jail bars are vertical, as they typically are, the "ripple" is at a horiz rate leaking into the video, more likely the boost source.

Are you looking at Sams? Which one? or the Zenith CM?

[Electronic M]

Ripple current rating of a cap usually isn't tied so much to B+ ripple voltage as it is to series current. Without seeing the schematic I can hazard a guess that the cap with that rating is either the resonant cap in the transformer or it's a series AC pass cap in a B+ doubler circuit. Ripple current in a cap usually means that that caps primary duty is not so much to filter DC as it is to pass large amounts of AC power along to somewhere else. Some B+ doubler circuits the main B+ cap does double duty and is there to handle large AC current and filter DC.

[TinCanAlley]

Originally Posted by BeamT

Obviously bad connections, easily reflowed and cleaned up. Could very well be your problem. You will know soon enough.

I usually find the 125v B+ in the flat pan chassis such as your 25EC to be 120-130 depending on the line voltage. IIRC this supply is regulated by the resonant circuit formed by the power transfomer and the 3.3uf oil capacitor. There is no regulator that you can adjust. The 24v supply does have a regulator, set by the zener diode in the base circuit.

If your jail bars are vertical, as they typically are, the "ripple" is at a horiz rate leaking into the video, more likely the boost source.

Are you looking at Sams? Which one? or the Zenith CM?

I'm using Sams. Been unable to find the Zenith.

I'm attaching a couple pics of the main transformer and circuits as well as the horizontal and boost circuits. I'm getting better at using schematics, but get a bit lost seeing multiple voltages coming from the transformer to different points and combining and turning into completely different voltages. For instance, the voltage going to the Chroma module is 125V, but in the schematics, it is derived from two 100VAC lines, one 235VAC line. Don't get me started on the return of it going to a separate line derived from 26VAC line to a voltage regulator creating 23.5V.

[TinCanAlley]

Originally Posted by Electronic M

Ripple current rating of a cap usually isn't tied so much to B+ ripple voltage as it is to series current. Without seeing the schematic I can hazard a guess that the cap with that rating is either the resonant cap in the transformer or it's a series AC pass cap in a B+ doubler circuit. Ripple current in a cap usually means that that caps primary duty is not so much to filter DC as it is to pass large amounts of AC power along to somewhere else. Some B+ doubler circuits the main B+ cap does double duty and is there to handle large AC current and filter DC.

Do wish they canister cap had it's main function noted and more about it's specs.

[TinCanAlley]

Also, in the horizontal section, I replaced the damper diode (CR218 on previous post pic) on the HOT and lifted the leg of the boost diode (CR219) and it tested good, but I'm not sure if that excludes it from being an issue. The only things left in that area would be all the ceramic caps around them and the resistors. I remember someone, some time ago, saying that ringing is reduce/removed by using diodes and caps and if one part of the circuit is bad our out of spec, ringing can return.

[TinCanAlley]

Just checked the B+ 125V and got 129.2V from the test point and 129.1 from the Chroma module through the last module to the return point. So I'm assuming that is good. Also, the scope showed around 1V AC on top of the 129V DC. Not sure if that's good or bad.

Oh, and I reflowed the solder on the sound IC, but it didn't help. I took it out, put in a socket and installed the new IC. I now have tone control again. One more misc issue down.