Zenith 25EC58 Restoration Thread

[TinCanAlley]

Well it seems the replacement caps won't be here until tomorrow. No biggie as I was cleaning the G2 pots with D5 and F5 and decided to give the setup switch it's 5th or so cleaning when I decided to look at it closely with my video microscope. I couldn't see anything but the pads at the end for the setup mode and the contacts on the slider. They were pretty much black and it appeared the slider part wasn't making full contact even though the spring in it was providing pressure. Not sure this at fault, but it got me searching for a replacement and I managed to find an NOS switch and bought it. Should be in by end of week. So I'm going to replace the switch while the chassis is still out.

The G2s now rotate much smoother than before, so it can't hurt to have clean pots.

[vol.2]

Originally Posted by TinCanAlley

then if need be, I can use the 695 as a replacement for those other transistors?

If you show me a circuit (the schematic) with a transistor, and you give me both the original transistor and the one your are thinking about replacing it with (and their datasheets), then I can probably tell you with some confidence if it's going to work.

That's about the extent of my knowledge with this stuff; one at a time. I can't sign off on blanket replacements between transistors regardless of the use case. I'm not that good at it.

[TinCanAlley]

Originally Posted by vol.2

If you show me a circuit (the schematic) with a transistor, and you give me both the original transistor and the one your are thinking about replacing it with (and their datasheets), then I can probably tell you with some confidence if it's going to work.

That's about the extent of my knowledge with this stuff; one at a time. I can't sign off on blanket replacements between transistors regardless of the use case. I'm not that good at it.

Sounds good. I've tried to figure out which transistor package is from what year and if one is better than the other. For the 695s, I received 3 different package types, There are round white ones with black tops, black round ones that look like a water tower and the TO-92. I tried looking up the other 2, but I found no package numbers for them. I'm assuming the ones not TO-92 are earlier versions and might be of lesser quality.

The ones I'm looking at are the water tower looking one on the Horizontal module Q804 (121-447). Also, the Horizontal module is supposed to have a 121-931 (Q801), but there's a 447 in its place. Is this a suitable replacement, or is the 931 the best choice?

[vol.2]

Originally Posted by TinCanAlley

The ones I'm looking at are the water tower looking one on the Horizontal module Q804 (121-447). Also, the Horizontal module is supposed to have a 121-931 (Q801), but there's a 447 in its place. Is this a suitable replacement, or is the 931 the best choice?

Do you have the datasheets for the transistors? I looked around for them, but I can't find anything. I even went as far as doing a cross-reference on the an archived version of the NTE part cross-reference tool, but those numbers don't come at all.

I'm good enough to have an idea of which specs would be important to adhere to, but I can't tell you what the specs of the original transistor are without a datasheet.

In general though, both Q804 (waveshape) and Q801 (AFC) are not in super critical spots for either speed or gain. I would think that almost any decent modern transistor with the correct voltage requirements would do the trick. Something like a 2N4401 could work probably. Q801 would be more likely to have possible issues if the gain was too high because the AFC circuit might rely on the gain characteristics to lock in, but I can't tell you that because I don't know the gain of the original transistor.

[old_tv_nut]

It is ususal design process in transistor circuits to design for proper operation whenever the transistor current gain (beta) is above a certain minimum.

Looking at Q801, you see that the peak to peak waveform on the base is twice the size of the DC voltage, so this stage is being overdriven to clip the AC waveform. The DC bias uses feedback from the collector load. Given all this, it's likely that any general purpose transistor with a beta of say 100 or greater would work.

[vol.2]

Originally Posted by old_tv_nut

Looking at Q801, you see that the peak to peak waveform on the base is twice the size of the DC voltage, so this stage is being overdriven to clip the AC waveform. The DC bias uses feedback from the collector load. Given all this, it's likely that any general purpose transistor with a beta of say 100 or greater would work.

Ok. That's really cool of you to explain that. I think I get it.

I think what you're saying here is that because the voltage threshold is so low, the transistor is only going to turn on when the waveform crosses the conduction threshold. Which would be ~0.7V if the emitter was tied to ground, but it has a 270 ohm resistor there, so I guess it would actually be a little higher than .7 it starts, depending on what the voltage is on the emitter at any given time.

So that means the collector probably has a ~10Vpk squarish waveform with fairly short negative-going pulses when Q801 is in conduction, right?

[old_tv_nut]

Originally Posted by vol.2

Ok.
So that means the collector probably has a ~10Vpk squarish waveform with fairly short negative-going pulses when Q801 is in conduction, right?

I won't try to guess what the duty cycle is. I haven't seen this sort of frequency control before and I'm not sure how it works - maybe by varying the duty cycle?

[vol.2]

Originally Posted by old_tv_nut

I won't try to guess what the duty cycle is. I haven't seen this sort of frequency control before and I'm not sure how it works - maybe by varying the duty cycle?

If the collector is sitting at 8.72V, fed by the 23.5V, then I would assume that the resulting waveform is determined by the narrow window that the base waveform allows for Q801 to be turned on.

If the base is at .54V, and the emitter is at .27V, and the waveform on the base is 1.5Vpkpk, then that means the top of the waveform swing is like 0.90-1.29V, where Q801 is turned on I would think.

The collector is providing some weak feedback by being attached to the base through the resistor, so I suppose that would help with changes in current draw due to the reactance of the hor osc coil on the other side of the collector.

Maybe?

[TinCanAlley]

Well something happened that usually never does, but I was wanting replacement transistors for the horizontal and vertical boards, for now or the future. I kind of want OEM parts, but finding them all from once sources proved impossible. I could find them all, but buying individually with shipping on each was way more than I was willing to pay. Then it happened, something very rare. I searched for a 931 transistor and came across a listing I couldn't ignore. Not only did it have every single transistor I wanted, I also had a red G2 pot! It had a few other items that I might find a use for, but even if I don't, I got the entire lot for 16 bucks! I don't know if I need the red G2 pot, but now I can disconnect the current one for full testing and know I have a replacement if needed. I tried testing in place, but the readings were odd as it would go up, then down and back up again pretty consistently during testing. Not sure if being in circuit was the reason.

I got the new C202 and 204 capacitors yesterday 68pf and .0047pf. They test dead on, but still show a high ESR, so I now know my tester isn't up to the task for these values.


Tomorrow I carefully plan and execute the installation. I have my liquid flux, 200W iron and largest chisel tip I have and a couple clip on heat sinks for the leads of the resistors already in the solder buckets. Then by Friday I should have the new setup switch and have it installed same day. Once I go over all components to make sure I didn't disturb routing or created a short, back in it goes and I'll fire it up.

Been an interesting journey so far. Hopefully I've made things better, not worse.

So now from update to question. I want to test voltages to the CRT and will pull socket back just enough to clip leads on all three of the screen drivers. Since they are driven by the 750 boost, I should get no reading higher than that and the SM has the voltages noted for each gun. However, if the G2s control those voltages, what is the purpose of noting those on the CRT part of the schematic? Since they G2s are adjusted for greyscale, doesn't that mean those voltages will be different based on current adjustment position? What exactly should I be looking for? Also, if I do the same for the cathode and G1? Are any of the values given in the SM a for sure value or will any of them vary and I'm looking for values close to what is stated?

Thanks!

[old_tv_nut]

Voltages given for the G2's are just what was measured on the particular set that was used and will vary with adjustment, as you said.
Voltages on the cathodes also vary with video content, so may be quite different if you use a different test pattern from the one used in the service info.
Volatages on the G1's also vary with video content in sets where the color difference signals drive the G1's.

[TinCanAlley]

Originally Posted by old_tv_nut

Voltages given for the G2's are just what was measured on the particular set that was used and will vary with adjustment, as you said.
Voltages on the cathodes also vary with video content, so may be quite different if you use a different test pattern from the one used in the service info.
Volatages on the G1's also vary with video content in sets where the color difference signals drive the G1's.

So basically I'm just looking for something way out of the ordinary?

[TinCanAlley]

I reinstalled the chassis after inspecting all components. I connected all internal cables and verified proper locations based on pics taken before removal. I plugged it in and it immediately started a deep buzz from the speaker. I unplugged it and double checked everything and it is all correct. Plugged it back in and it did the same thing. I have it unplugged now and haven't tried turning it on until I find out why it is making this sound while off. Any ideas?

[TinCanAlley]

Never mind, ground loop. First time it's ever happen. Rotated plug and it's gone.

I haven't gotten around to doing any real adjustments as the room is too bright. I'll do them this evening. The new setup switch is late, so I left the old one in for now. Still won't bring up any lines from the G2 controls. Hopefully the new one will fix that issue. As for jailbars, here's hoping the new caps will fix it. If not, I'm going to look at removing the IF module and injecting my generator directly into the circuit on C1. Was told these bars can also come form bad RF, IF, etc.

We're getting there!

[TinCanAlley]

I've done all the adjustments. I got the convergence pretty much dead on, except for a little blue horizontal on the very right edge. Seems the 2 new pots and a very extensive D5 and F5 cleaning of the other pots helped greatly. It is the best I've been able to get it and I'm more than happy with it and will not touch it again.

I'm starting to think the jailbars are coming from some other source (IF?). They are still there with all new electrolytics using SM part numbers, new diodes and new ceramics in the horizontal circuits. I disconnected the tuner to IF and I got a bright white raster, so no way of telling if the jailbars were there or not. My next step is to remove the IF module and connect my generator directly to C1 and inject the signal. I'm not sure if I can bring up anything that will show if the jailbars are there or not. What about a direct composite feed? I'm sure it's not as simple as connecting the composite out signal to C1 and ground, right?

As for the peak pix control... Is that off fully counter clockwise, or does that over soften the image? I really can't tell, but it seems soft CCW and noisy fully CW. So maybe the center is where it should be?

Attaching a couple of pics. They're from a laserdisc.

[old_tv_nut]

Peak pix action was designed to be default at center, with CCW notably softer and CW notably crisper.