Zenith 25MC33 Set

[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.