Zenith 25EC58 - won't turn on

[Adam]

So it's at it again -- 2 nights ago I decided I would hook up a complete set of video equipment for common formats (VHS, Beta, CED, LD, DVD) to this and make a nice place to watch all my video tapes and discs. So I was going to watch some CEDs, and it wouldn't turn on again.. It was late at night and I really was looking forward to watching those CEDs, so I tried something - My first thought was that it was the same problem, and I swapped triac and optocoupler from another set just to see if it would work - but it didn't. - now both of these Zeniths are out of commission.

Today after closer examination I noticed the circuit breaker was flipping off - Looking under the chassis I noticed the thermistor in the degaussing circuit broke apart - but that being open wouldn't cause this, something must be shorted. I disconnected electrolytic capacitors in the power supply and tested them with my old Solar CB-160 capacitor tester, some may be a bit leaky, but none are shorted (and there was no sign of 60 cycle hum in the picture the last time I used it) -- This TV gets a great picture (probably the best of any I have) - but what is it with the mysterious 'it won't turn on at all' problems...

[Zenith26kc20]

Is this a set with the large power transformer with the run cap (a oval can) next to it?
The oval capacitors fail and will give the symptoms you described.

[zeno]

Try with oil cap unhooked first.
Next check the hoz out collector to ground. Should be OL.
If a few ohms check hoz out, damper, & 4 lead cap it has one.
Thats the common stuff. Other things rare.

Zeno

[DavGoodlin]

Thats a rarely seen TV there! All of the flat-chassis consoles I saw were plain old standard video guard tuned 25DC56/57.

Does a 25EC58 have a 4-lead safety cap or several of the .0105uf (22-5001) like the '73 models?

[zeno]

Originally Posted by DavGoodlin

Thats a rarely seen TV there! All of the flat-chassis consoles I saw were plain old standard video guard tuned 25DC56/57.

Does a 25EC58 have a 4-lead safety cap or several of the .0105uf (22-5001) like the '73 models?

17, 19, 23 & 25EC45 uses the 22-5001 caps UNDER run 401. IIRC 5 of them.
Run 401 up uses 4 lead cap.
All flat chassii use 3 to 5 22-5001

73 Zeno

[Adam]

Yeah, that big oval oil cap is totally shorted - you can see it even with an ohm meter - nearly 0 ohms, dead short.

I just bought a NOS Quasar replacement at the exact value 3.5uF - 440VAC, but how critical is the capacitance for this - I see lots of 3uF or 4uF - 440VAC motor start capacitors around - would that sub for it if this old Quasar one doesn't check out?

And I replaced all the 22-5001 caps first thing back when I got this TV in 2016.

[DavGoodlin]

Originally Posted by Adam

Yeah, that big oval oil cap is totally shorted - you can see it even with an ohm meter - nearly 0 ohms, dead short.

I just bought a NOS Quasar replacement at the exact value 3.5uF - 440VAC, but how critical is the capacitance for this - I see lots of 3uF or 4uF - 440VAC motor start capacitors around - would that sub for it if this old Quasar one doesn't check out?

And I replaced all the 22-5001 caps first thing back when I got this TV in 2016.

370 volts AC (p-p) is the minimum rating, capacitances can vary a bit as the ratings seem to be given in ranges. 120 volts is AC rms, for P-P its 340 volts!

I often find similar caps inside appliances having a "permanent split capacitor" winding. A/C and Dehumidifier sealed compressors have a "start" winding with external cap. Any cap tester will read that value, regardless of it being used on AC. Many are open, but shorts in these caps (the smaller square plastic tub type) found in ceiling fans and as a high-power-factor* option in some ballasted light fixtures and exit signs. These short and cause smoky little "fires", one that recently emptied a movie theater.

* older oil-filled use PCB, beware if NOT labeled "non PCB"

The reason which Zenith used that heavy transformer, is superior voltage regulation with line volts low OR high (dangerous @ record-high 30+kV this Z makes) this cap is intended to work with the transformer's reactance to reduce current drawn yet maintain core saturation.

[vol.2]

Originally Posted by DavGoodlin

this cap is intended to work with the transformer's reactance to reduce current drawn yet maintain core saturation.

So if I read you correctly, you are basically saying that they chose the huge cap to help smooth out variations in household line power?

[Alex KL-1]

Or it is resonant with one trafo coil? Like with ferroresonant stabilizers? EDIT: I don't see the schematic, only mentioned what I remembered when I read about.

[Electronic M]

Originally Posted by Alex KL-1

Or it is resonant with one trafo coil? Like with ferroresonant stabilizers? EDIT: I don't see the schematic, only mentioned what I remembered when I read about.

It's a resonance thing. Those transformers could maintain rock solid output even if a full cycle of incoming AC was missing.

[Adam]

That new capacitor got this going again. So the idea is that that capacitor sits across the power transformer windings and its value is such that the circuit resonates at 60Hz effectively 'tuning out' any variations in the incoming power?

[Alex KL-1]

Pretty much like you said: LC resonant circuits have some unique properties.
Electronic_M remarked about the resonance working.

Besides, probably also the core (or some section, I dunno) of some type of reg trafos are made to saturate so the voltage wants to rise but core says "no" (saturated trafo). I believe some are a combination of the 2 (resonant/saturating). I dunno about the specific of this TV.