Zenith 25EC58 Restoration Thread

[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!