Zenith 25EC58 Restoration Thread
 

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.

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?

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.

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

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.

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.

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...."

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?

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.

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. :lmao::confused:

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

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.

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.

I realized now that I forgot about what I had done years ago to the boost circuit. I replaced the boost rectifier, tested the resistor, but never changed the ceramic capacitor as I couldn't find the correct replacement. Zenith uses a 600pf 2kV and I couldn't find any NOS and for new all I could find were 560pf or 680pf. The same for the HOT. I replaced the damper diode, but not the capacitor for the same reason.

I know Zenith chose that value for a reason, but I don't know how important that exact value is to the circuit. It's the last thing I can think of that might be causing the jailbars.

Do you have a picture you can post of a blank raster?

Assuming this is Sams 1370, take a look at C270 10uf 300v on the 240v boost supply from the Horiz Out Transformer. That boost source feeds the RGB Video outputs.

Did I understand correctly the "jail bars" are intermittant?

No, they aren't intermittent. They are always there, but can be reduced by bring down brightness and contrast to an unusable level. :thumbsdn:

Posting a pic from the DVD player screensaver.

I have a new 10uf 300V capacitor on hand as I want to replace them all again to ensure I have the correct replacements installed. I replaced them 13 years ago, but I didn't have the SM, so I ordered solely based on value. This time I used the part numbers from the SM and got all but one based on that info. The other one is supposed to be 25uf 15V non-polar, but the SM has no part number and all I could find was a 22uf in non-polar.

I'm attaching a pic of the section with C270. Out of this section, my concern is C258 in the pic (small gold radial). I'm trying to go back in memory as to why I used a Nichicon radial of the type I used for restoring my Pioneer SX-1080. Not sure what that cap does, exactly, but I would assume it is up to the task. Still, I ordered the correct replacement for it as well.

The C270 I left the leads long so I could position it better after the safety capacitor upgrade. Don't know if the longer leads affect anything.

This might not look good, but it is possible to parallel four 150pf 2kV capacitors to make the 600pf I need, right? I've found 3 I'd like to replace (flyback, HOT and boost sections). It's impossible to find 600pf and anything close is either higher or lower.

You could, if that's what you have.

Very low value film caps don't tend to come in high voltage variants, but ceramic capacitors do. Some cap types are chosen for a specific purpose, and can't be changed, but that's not always the case. Ceramics, especially very old ceramics, tend to drift in value with temperature a lot, so they aren't used when the value has to be really specific, but newer ceramics of high stability types will do very well at this point, and can be subbed for films in many places. Maybe not a good idea if it's a tuning cap.

There's a very stable Y5R 680pF cap: https://www.digikey.com/en/products/...P6UK5R/2831053

But I can't comment on the specific place in the circuit that your cap is in. Someone else will have to confirm that 680 is okay, and that the small amount of drift isn't a problem.

Since they're caps in the flyback and horizontal circuits, I thought they might affect the signal and cause ringing. If they don't serve such a purpose, then I'll leave them be.

Found issue with one cap. It's in the wave shape section of the Vertical Output section. The cap in there I tested and it's was at the low end of tolerance (81uf of 100uf), so I decided the new cap would go in. Upon looking up the schematic, I noticed that the cap is in backwards. The schematic shows the positive side connected to the diode and the negative to the resistor. It occupies the same connect points as R292 and one leg of R293. I've attached a pic of how it was and the part of the schematic. In the pic you can see the negative side of the cap connected to the diode.

Am I interpreting the correctly? Even in the SM, the crappy pic of the underside it appears that the positive side is down on the diode side.

I traced the path exactly as the schematic shows. The green/red lead connects to the diode, the other end of the diode connects to both the positive side of the capacitor and 470ohm resistor. The other ends of the capacitor and resistor join up with the lead of the 68ohm resistor and the other end of that resistor meets up with an orange wire that goes to pin 6 of J201. So if the last cap was put in backwards, why didn't it fail or cause issues with the circuit?

The picture you posted is not clear enough and is too close-up, so I can't orient what I am seeing. I can't see any of the cap polarity markings, or the diode clearly at all. Test the diode and the resistors.

Assuming the rib on the cap is positive. I believe the cap was in backwards. Should be 100 uf @ 25v. I would go to 35v or 50v for a little more margin. Attached is more of that circuit, which is dynamic convergence and associated wiring, from Zenith CM-122, confirming that SAMS in that area is correct. In the SAMS picture, Cap positive, was oriented to the back of the chassis.

I don't pretend to understand dynamic convergence circuits but you could have impacted several coils and controls on the convergence board/clover. From your prior threads you have been working on that board. Did/do all of the controls work?

Do you have any pictures of the chassis from your prior recap?

Agree with BeamT about the cap.

However, 80% wouldn't generally cause a circuit to fail. It's an indication that it might be on the way out or that it was stressed (possibly from being in backwards), but it wouldn't generally be the actual cause of anything not working.

On the other hand, 80% and high ESR could cause something to fail if the ESR is high enough. You can imagine ESR as adding a resistor in the circuit across the capacitor, which will throw things off by creating voltage dividers and stuff.

Long story short, check ESR if you're troubleshooting. Capacitance is only telling you part of the story.

Okay, I managed to find an under chassis pic from when I first got the set, before any work. The cap is definitely installed positive side to the diode. In the second pic, it is after the recap. The recap was done for me by a family member as my soldering skills were pretty non-existent at that time. The cap was put in backwards as the negative stripe side is to the diode. The replacement cap was also quite a bit smaller. The replacement I have, that I just installed per schematics (positive to the diode) is quite a bit larger and near the size of the original. The new cap is the Sprauge part number in the SM.

I have been having issues with doing proper convergence, but the pot that was bad wasn't the one connected to this lead and the other one that I want to replace causes the screen to shimmy during adjustment and sometimes continue to shimmy afterwards. I'll just replace that one and give another cleaning to the rest and see if this reversed cap might be the issue with convergence.

Your SM shows what mine does. I've posted two more pics of before and work and after the first recap. I am doing the second recap with part numbers from SM.

I've tested the resistors and they are at correct values. The diode passes 572 one way and infinite the other. So it seems the diode is good.

I have been checking ESR as well as value. The one I removed had an ESR of 540 milliohms with a 100K frequency.

Would it be possible for you to post the adjustment section of the CM-122? I want to see if it compares to the Sams, or if it has better/different procedures.

Horiz ringing and jail bars in my experience can be very difficult to troubleshoot. Sometimes I've had good luck with a scope, if you want to try that route. The source can be a number of places and can be very subtle. Otherwise I have had some success examining one circuit at a time. I try not to just throw parts at it.

To finish where you left off, I would look at:

C258 10uf 50v B+ Horiz Module (you mentioned above)
C264 47uf 10v Secondary Horiz Drive Transformer

and then work your way through the horiz blanking and then into the video path. This could easily be caused by horiz blanking.

I find it helpful to work with a blank raster, from a known good generator, tuner mid channel, or IF disconnected. Chromatic switch off, color off. This eliminates any outside source issues and gives you a good baseline.

Has this set always had the jail bars? Has the tripler been replaced? Zeno (RIP) always commented on non-Zenith HV triplers causing Jail Bars. I never encountered this but after replacing a separate tripler and focus divider in a 25DC56 with a Zenith combined unit 977-36, I ended up with jail bars. Putting the separate tripler and divider back in, eliminated the jail bars.

What part of the CM-122 Set-up procedures are you looking for? or all of it.

Safety first, the boost circuits is these sets are not only at a higher voltage, they can deliver some current, lots of RF energy.

This set has always had jail bars to some extent. It was initially recapped because of other odd video issues associated with bad caps. Once the recap was done, the bars were more noticeable once brightness and contrast were set.

I have replaced C258 with the SM replacement part from Vishay. C264 tested good with and ESR around 430 milliohms and a value of 45. I have a replacement, just in case.

I've been trying to figure out where to inject the signal from my generator, but lack the knowledge of where in the circuit to inject it and what I need to disconnect when doing so.

Years ago, you can search for them, I posted quite a bit during the initial repair and search for ringing source. I used the scope of many areas and found ringing in a couple, but was told that it wasn't the cause of an issue. Found it in the brightness limiter circuit, HOT base and B+, but I was pretty new with the scope and I went with it not being the jail bars. The waveforms in the Sams aren't the highest quality, but enough to note difference is what I got.

The set has the original tripler. Only thing it need was the focus divider and I got an NOS and it's been fine ever since.

Once the set is back up and running, it's going to need some adjustments, so I thought I'd run through all the adjustments for AGC, ACC, APC, brightness limiter, color killer, convergence (static and dynamic), etc. Not going to mess with the IF stuff, just the basic stuff. Just wondering if the factory SM has the same procedures.

Thanks for all you time and help!

Well trying to test all the pots on the convergence board isn't going to work. I was able to test 3 correctly. Those 3 had correct values and smooth readings. The other 6 didn't show anything useful. I'm assuming it has to do with those 6 being tied into a bunch of resistors, diodes and many connected to each other. One started around 1ohm and gradually moved up to almost 30ohms, but instead of continuing upwards, it started to go back down and was almost 2ohms at the other end. I know this isn't supposed to happen, so it must be the other connections on the board interfering?

So I just gave them another dose of D5, followed by multiple turns and finished up with F5. If that doesn't help, I have several new pots I can install, but will need to get some Mallory pots noted in the SM as I can't find the ones I don't have. What I find odd is there is one pot that has specs of 120ohm 2W, but it has a different part number than the other three 120ohm 2W pots on the board. They all feel wire wound, so I wonder what the difference is.

Topside is done. It has all new transistors (2nd and 3rd video amps, brightness limiter, vertical blanker, blanker and video outputs). Might be a little overboard, but I had access to them, so why not. Also used contact cleaner on all the modules and IF contacts as well as all contacts in the plugs. With the replacement video outputs I was able to reinstall the original heatsinks and support cage.

In the horizontal blanking, all the resistors were replaced and the diode tested good. Is it possible for a diode to test good and be weak?

While troubleshooting by substitution can be a strategy, replacing large swathes of parts can have a real risk. At some point in time you can find yourself with a set that doesn't want to work right, and unraveling layers of parts and changes can be extremely difficult.

In the end this is your set and your journey!

Always wait for the DeoxIT to evaporate and dry before you energize.

Diodes can be leaky, the horiz blanking diode CR201 would be a good part to substitute. This needs to be a fast switching diode like a ECG177.

I would take some time to look at the voltages on the Blanking Amp Q202, Vertical Blanking Amp Q217, and 3rd Video Amp Q203. The line between the collector of Q217 to the base of the Blanking Amp Q202 gives you a perfect place to scope the blanker at both the vert and horiz rates. Good waveform to look for in the SAMS. C204 and C202 are critical to horiz blanking.

Were any of the resistors in the Blanking circuit out of tolarance?

Out of curiosity does the Peak Pix control work?

Does this set have a separate focus divider?

Do you have the shield for the Subcarrier module?

Each of the subs were tested earlier in the set. I swapped them in and out until I was certain they all worked as expected. Just never go around to doing the permanent swaps. Now I have and it won't be hard to figure out which one if any fail. I still have all the previous parts.

I can scope them again. I did this back in 2013 on all those outputs and they measured at rated voltages. I also replace CR201 back then and still have the one I removed as the new one didn't help issues, so the old one is still good. Last go around I replaced the 750V Boost diode, Damper diode and Horizontal Blanking. The only other diode I was supposed to change, but didn't, was CR202 Zener Diode 24V across the emitter and collector of Q204. I have that diode and it would be an easy swap.

I'm going to order those two capacitors you noted as I was eyeing them myself, but is wondering what other pieces I might want as I'd like to maximize the order to justify the crazy shipping rates now.

The peak Pix works, the resistors in the horizontal blanking circuit were replaced and it does have a separate focus divider. I replaced that shortly after getting the set as it would sizzle on humid days and upon inspection it was drying and cracking.

This is the alternate module (9-69) for this chassis and it doesn't require the shield. If you look closely at the board, there is a shield around the sensitive section only. I think the cover is the reason the pots on the original were messed up. They were poking through the holes for make adjustments and messed the up. They were pretty mangled.

Do know if this set will run with the video IF module removed? If so, I can connect my generator's signal via it's injector connection directly to the C1 test point into the video amp section. I am unsure if connecting my generator to this point while the IF is working would be a bad thing as it seems there's roughly 6V on it. This would let me see if the jail bars are being injected from the IF stage.

I believe you can run this set without the IF module in place. The 125v B+ supply interlock does not run through the IF module. You wouldn't need to feed a signal to the IF output, you should still have a raster. You might need to turn the brightness up a bit to see it.

If you are going to replace C202 in the horiz blanker, it needs to a temperature compensated N750 Capacitor.