Zenith 25MC33 Set

[bhegges]

I decided to go with four 1watt resistors in parallel and series to get the right ratings.

Moving along my next issue is the sound, I get a fair bit of hum and periodic static & crackling. It does seem to be affected by the screen content. I have replaced the film capacitors and checked all of the resistors in the sound circuit, tubes are used but test good, and the pots were cleaned with deoxit. Next will be to probe around to see if there is anything that jumps out. Hoping for a dirty tuner, pot, or maybe connection somewhere, hopefully not an IF alignment issue.

Might try to inject composite audio at the grid of the 6BN6, I would think the noise is ahead of the 6BN6. May need to build a small amp to supply the proper signal amplitude.

Any advice or troubleshooting ideas would be welcome.

[nasadowsk]

You might want to attack and solve the HV and sweep issues first. Leaky HV resistors might be getting picked up as noise in the AF circuits?

[trinescope]

Zenith TVs with the 6BN6 or 6Z10 quadrature detector tubes were prone to buzz, especially when there is a lot of fine, bright detail in the picture. Careful adjustment of the sound IF, detector, and buzz controls will minimize, but not completely eliminate the problem. The best way I have found is to display a test pattern that causes buzz in the sound, then adjust everything in the sound section for least buzz. The RCA style sound detector using a dual control pentode was a bit more immune to this buzz problem.

[Electronic M]

I suspect sometimes the buzz problem was audio IF tuning related. I've got a 71 hybrid set with the buzz tube. The thing regardless of the buzz control setting or modulator used has some buzz on all channels even when other Zenith's with the same tube won't buzz....A few years ago I got my hands on a vacuum tube video only RF modulator, and proceeded to add a tube FM modulator to it (generates a 4.5MHz FM carrier that I mix with the video signal at the AM video modulator) I found that modulator worked normally on all sets when set to 4.5MHz, and still buzzed on the Zenith.... Until, on a whim, I de-tuned the sound carrier in the modulator for best audio in that Zenith and the buzz stopped....I suspect if I did a proper sound alignment I could kill the buzz. I haven't gotten around to proving that hypothesis yet.

[trinescope]

In color sets the IF alignment places the sound carrier at minimum to avoid interference in the picture, then makes up for it with a sound IF amplifier stage. If the alignment were off, I could see potential problems with noisy audio. Color sets had to have video response to at least 4.1MHz for the color information, B&W sets could roll off response in the video amplifier and did away with some of the sound IF amplification used in color sets.

[damen]

Back in the old analog days, some Zenith customers complained about the buzz during weather maps or sports scores on the screen. I found I could slightly turn the 2nd video IF transformer core and eliminate most all the buzz without affecting the picture quality. Don't know what I did to the alignment curve, didn't care, customers were happy.

[bhegges]

Doing some further inspection and found one of the small ceramic capacitors way out of spec. SAMS lists C38 (Zenith #22-3990) as a 3.5pf, 5% part. With one lead pulled it measures 13pf. I am using an Almost All Digital Electronics L/C Meter which I believe can measure low value parts.

My question is how critical is that part and what about some of the other low value parts? Is there enough adjustment in the coils or would this cause havoc in the sound circuit?

[nasadowsk]

I’d say that guy would be pretty important.

[bhegges]

Quick update, once I pulled the part out completely and remeasured it tested at 3.3pf. I guess at that low value it is a sensitive part to measure correctly.

[old_tv_nut]

Originally Posted by bhegges

Quick update, once I pulled the part out completely and remeasured it tested at 3.3pf. I guess at that low value it is a sensitive part to measure correctly.

Correct.

Also, if you get an indication that a small capacitor has increased value so drastically (and it's not shorted), you should be very suspicious of the reading because there is no reasonable mechanism for capacitance to increase like that.
Failure modes should only be short or open.

[bhegges]

After more testing everything in the sound circuit measured ok so I proceed to make adjustments to the 2nd sound IF and quadrature (L18 and L19) per SAMS and after several turns of the quadrature coil the sound cleaned way up. I was surprised the coil was off so much as components ahead of this coil measured good.


With a full raster and sound ok my next task is getting the horizontal output tube running at a safe current. SAMS does not list a max current, elsewhere I have read under 200ma is good, 210-215 at max. My set is over this and don't know where to look next, please advise. I will also report back with other voltage & current measurements on the horizonal tube to see how they compare with what the SAMS schematics shows.

• Using an analog meter with a .1uf cap across the meter
• Line voltage of 122v yields a horizontal current of 240ma
• Dropping the line voltage down to 108v yields 210ma
• All of the horizontal circuit film capacitors have been replaced
• All other caps in the horizontal circuit measure ok
• All resistors in horizontal circuit measure good, only R119 out of spec and needed replacement
• Horizontal tube is a 6JS6C measuring very good
• Horizontal AFC/Osc/Discharge tube is a 6U10 and also measures very good
• High voltage set to 25kv
• Horizontal linearity adjusted to the dip and then raised off the dip ever so slightly

[zeno]

240 ma sounds a bit high. If you are riding it high you may be drawing
too much beam current. See what it measures at reduced brightness.
HINT ! You should be able to turn down the brite, contrast & color
level & extinguish , or almost so the raster, with no retrace lines.

CASE HISTORY. The Admiral M10 chassis was there first SS portables.
It used 2 beefy pass transistors in //. They ate them up often. Turns
out that if the brightness limiter was not set right you got too much
beam current but a normal pix. Over time it stressed things out then boom !
They short, & goes into HVSD ( pulsating on / off ).

BUZZ. You need to to balance the quad coil, buzz control AND the AGC control for least buzz. DO NOT use the AGC as a contrast control !!! In most cases the buzz can be eliminated or reduced to a slow & less annoying
tic-tic-tic on some scenes. I would also do this with a NOS 6BN6 / 6Z10 .

Time for another ale
73 Zeno
LFOD !

[bhegges]

A further update on what I have learned:

SAMS does not list horizontal output tube current for the 25MC30/33 chassis but on the 24MC32 schematic it lists the current at 220ma, this chassis uses the same 6JS6 horizontal output tube but a different flyback circuit. Unfortunately neither the Zenith CM-106 service manual nor the Goodman Zenith Volume One service manual list a current value. All three do describe the same method for adjusting the horizontal sweep. See below for the Zenith CM-106 service manual directions.

With all of the controls set per the directions I tested at multiple line voltages ranging from 105v up though 120v while setting the high voltage per the manual and adjusted the horizontal linearity coil to get the lowest horizontal output tube current which ranged 215ma up to 240ma respectively. The regulator current was just barely at .85ma. If I raised the high voltage up as high as it would go which was about 26kv at the highest line voltage I could get the current down ranging 205ma to 225ma.

I can run the set with a bucking transformer and also run the high voltage a bit "hotter" to bring the the current down to say 210ma but am I missing something, what is left to check? Could there be shorted turns in the flyback? Surely some customers had line voltages at 120v+ back when this set was new and that would have the 6JS6 running at 240ma+.

[trinescope]

Maybe it's just me, but I think there is a lot of angst and anxiety about horizontal output tube cathode current. I'd just set the linearity coil for minimum current, and if the width, linearity, and HV are correct I'd just leave it and run the set, unless the current is just too much, say 250mA or more. I personally can't believe there is much difference between running it at 210mA vs 205mA, for example, as far as tube or flyback transformer life.

Also, from what I see, the 6JS6 and 6JE6 tubes are just a repackaged 6DQ5 at least from what I see in the specs in the tube manuals, but later sets did run the tubes harder to get more HV and drive the 90° rectangular CRTs.

[Electronic M]

Depends on the set and flyback temperature. If the flyback is running hot to the point it's dripping wax and it's running beyond current specs it's probably not going to last long without current control and or forced air cooling...
On sets like the worst case above with some value and unobtainable flybacks this level of caution is warranted IMO.