1990 zenith no HV

[john f]

I was given, or rather, paid to take away this zenith. It had an awful screaming when plugged in which got worse if it was turned on. Regardless, it passes audio but had no picture. I found and soldered a few cold joints, and replaced a 35v 100uf electrolytic that appeared to be leaking. The screaming is gone, but no crt filament and no static when I touch the face. The only voltage I can find at the crt board is 188v. At this point I'm past my knowledge. I'd like to give this TV to my sister, and I'm willing to spend up to the $20 I was paid to take it away. Ideas?

[davet753]

I remember replacing a lot of horizontal output transistors in those sets. When they short, the fuse usually blows but not all the time. If the transistor's shorted, replace it and start looking for an open resistor.

[zeno]

The 200V for the CRT comes off the HV usually a tap on the
primary so you must have sweep. The fil comes from the secondary along with the low level B+ & vert output voltage. Shouldnt have
audio without HV unless the triplers open. But should have fil in that
case. Something not adding up.........
Post the model & the main board number 9-****-** I will
see if I got the Sams.

73 Zeno

[john f]

Thanks Zeno. I appreciate any advice offered. Model number is SF2017W. Board number is 9-592-04.
When I first visited the person who gave me this TV, it had a full picture, but it was very VERY dark, like someone had turned the contrast and brightness all the way down. In the few days since I got it home, it lost picture completely, even before I touched anything inside.

[john f]

If it helps any, I get a quick flicker on screen if I power it up and turn it on, static if I bring my hand close to the face of the tube for a few seconds, but if I try again a few moments later there is no more static. No visible filament glow.

[radiotvnut]

Usually, there is a low ohms resistor in the filament line to the CRT and these resistors can open. It would be best if you had an HV probe to see exactly what's getting to the CRT anode.

If I'm not mistaken, these Zenith's with the switching power supply can lose horizontal sweep and still have audio because the voltages for the signal and audio circuits are taken off the switching power supply transformer, not the flyback. I think the only voltages provided by the flyback are for the vertical output circuit, HV, focus voltage, filament voltage, and B+ voltage for the video output stage. If the horizontal circuit is not operating, you won't have any of these voltages.

Since you have 188V at the CRT socket, I'm betting the sweep circuit is working and you have an issue in the filament circuit.

[rca2000]

You are correct---this is a cold chassis. The SMPS powers most stuff. Not sure if they only "boost" the 200 volt line with the flyback or if it is totally fly derived. This chassis was introduced in 1987. It lasted a few years before replaced with something likely cheaper to produce.


It sounds as if the 'screaming noise" was a runaway SMPS---I have seen this many times on this chassis and many others in all things. I do wonder if you have HV. If the G voltage is at the CRT--you DO have HV. Maybe that resistor for the Filamant line got damaged or such, during repair. Maybe even something has come off on the CRT board to the fil. line. One way to PROVE this--connect a small bulb, such as a # 47, to the fil line on the CRT board. If it lights decent--the fil line is ok. If not at all--you have no fil voltage. If there is no G voltage, likely no HV or fil voltage either--as those come totally from the fly.


At this age--look for a number of bad caps in the vertical and flyback areas. I recall a LOT of cap problems in the vertical ckt. on these sets, would cause retrace lines and/or linearity problems.


I used to work on these chassis qiute a lot. not a bad set, totally cold and fairly built (for the time period). Have one or two, and a 13" model for sure. KInda like them, and Flybacks did NOT fail often. Sometimes would lose H drive, from a bad driver transformer or a resistor feeding it , IIRC. a 1990 set SHOULD have a decent tube if not ran to death, that was right before the "switchover" to the Goldstar tubes. I now the 19" sets GS tubes did not die as often as the 25 and 27 inch sets--but they DID go bad some.

[radiotvnut]

I recently had the 19" "cube" version of this TV that had bad capacitors in the vertical circuit. I also replaced the ones in the power supply, since they can fail and take the STR53041 regulator with it. That set had a great picture; but, despite that, it took me two months to get $20 for it. Back in the mid '90's, I could have moved it for $75 real quick.

[zeno]

Finally looked & I dont have the manual. Would have done
it sooner but went to Hampton beach to thaw out.

Depending on your level about the only thing left is
being sure the SMPS is putting out. The common Zenith SMPS
had 2 small caps in the primary that caused problems.
Beyond that I would need a manual or the set in front of me.

73 Zeno

[john f]

I stayed up late last night and changed 5 more caps near the vertical IC (radiotvnut's youtube video regarding this same tv was a huge help, thanks!). Also found one cold joint on the CRT board, which got my filament back. Adjusted the G2 control, focus, and color controls and it's looking pretty good. Only thing I notice is the little whistle from the fly is pretty noticeable.

[radiotvnut]

Glad you got it working. Sometimes, you can stop a noisy flyback by unsoldering it from the board, reseating it, and resoldering it. Sometimes, you can get by with just resoldering it. Other times, you can silicon the core in place to reduce the noise. If this doesn't work, you may just have to live with the noise.

[zeno]

Nice work ! For squeal narrow it down. I cant hear them unless I
use a paper towel tube. Can also come from SMPS & coils in
the hoz. Poking usually helps find it too.

73 Zeno

[john f]

Found the source of the squeal. Bad 22uf 63v cap next to the voltage regulator. It was right near to the flyback. At least the noise is gone and my previously decent but a little dark picture is perfect now. Had to readjust brightness and vert size to get it all right again.

[rca2000]

I expected that you had some bad caps. That 22/63 is the FEEDBACK cap--and if it goes bad, the regulator does NOT get the proper reference voltage from the Switch Mode transformer to know if it is producing enough voltage. SO, as a result it "floors it", pushing the SMT transformer to produce as much as it can, which is often OVER 250 volts (on the 135 or so volt line).!! This can and WILL damage many things, if the set is allowed to run this way at ALL.

That "squeal" in ANY SMPS is a dead giveaway of an off-frequency supply, and usually an over-voltage condition, due to lack of feedback. Sometimes it is due to "overcurrent " too, from an overload on some output line. IN EITHER case--it is a condition that CANNOT be allowed to persist--as it WILL damage both the supply and its loads.