Zenith 25MC33 Set
 

Working on a Zenith 25MC33 color roundie chassis and would like some troubleshooting advice. I have gone through and replaced any resistors over 20% which was only 4, weak tubes were swapped with good tested tubes, replaced the electrolytic capacitors, and like others had to replace the horizontal linearity coil as the plastic form had fallen apart.

The set powered up but I can not get full vertical deflection, it appears to be folding over short from the top. I can fill the screen by adjusting the vertical size and vertical linearity but then can not get the vertical hold to lock.

• 117v line voltage
• adjusted high voltage to 25kv
• 230ma HOT current (a bit high, 110v line voltage brings it down to 210ma)
• 4:3 ratio DVD player feeding composite video/audio into RF converter outputting to ch3
• quick and dirty first pass on color & convergence

Troubleshooting so far:

• I have tried substituting both vertical tubes (V7 & V10) but get the same results.
• Knowing that the vertical output transformers are subject to fail I went in and replaced the film capacitors and resistors in the vertical circuit.
• I also tried replacing K1 & K2 with a homemade circuit, thought they might be the problem and swapped the originals back in (same performance)
• I took pictures and now have rechecked and retested each component to ensure I didn't make a mistake

Here is what I get:
https://i.imgur.com/ft7DLIO.jpg

Highlighted parts replaced:
https://i.imgur.com/xcv2xa5.png

Have you tried adjusting the centering rings (fishing line loops protruding from the yoke are how you adjust them) to move the image up then trying to set the height and linearity?
That may help.
Subbing the vertical tubes also may help.
If you can get it high enough you could adjust it to the edge of lock and try paralleling capacitors across the the existing capacitors to try shifting the operating frequency such that it can stay in sync with full screen deflection.
Voltage checks may also be worth doing...If a socket has carbonized and created a leakage current path or some similar nonsense that changing parts won't fix then voltage checks should catch it... Maybe do this before the parallel cap decade box engineering hack.

Foldover at the top is going to be a problem with the start of the scan waveform, check the waveforms with a scope at the grid of the vertical output tube and the yoke winding of the output transformer for nonlinearity at the start of the ramp.

Did some more troubleshooting and ultimately found the issue was the vertical output transformer. Installed a replacement transformer and I now get a full raster.

Next up continued testing & monitoring.

Where did you manage to find a replacement horz lin coil?

To my surprise Talon Electronics had a Miller replacement coil in stock. I must have gotten the last one as they now show out of stock: https://talonelectronics.com/shop/item.aspx?itemid=6636. The failed coil was wound on a light blue plastic, the new Miller coil was on white. Part of me still worries that this one will fail in time too; to that point I installed a 250ma fuse to protect the HOT and flyback. The fuse holder also provided an easy access point to connect an amp meter to.

https://i.imgur.com/vCWEd7z.jpg

I would think a person could search old parts catalog and look for a generic coil with a matching inductance range. The coil should come with the metal clip that locks into the chassis via drilling a small hole.

Another viable option is if you come up on a scrap without that coil, but with others that use that form is to wind the wire from the original onto a good from from a different Zenith coil.

FWIW, here is an exerpt from a post of several years ago, although it doesn't mention the light blue form:

It's easy to verify that the coil form is mechanically sound by 'wiggling' it briskly. If it's compromized, it'll be obvious and the coil will act like a Slinky.

There were three types coil forms that were not prone to this problem:

1. Brown phenolic
2. Yellow plastic
3. Solid white (opaque) plastic

The one that's always suspect is a whiteish semi-translucent plastic.

Next item:
I am checking over the flyback cage and found the 18M resistors way over tolerance. I was looking to use high voltage rated resistors which are very large or could use a combination of parallel and series resistors to get close on resistance and power.

My question: was the 2 watt power rating for withstanding voltage vs power dissipation? If so than a single 39M 1 watt HV resistor would make for a simple replacement: https://www.digikey.com/en/products/...5JAC00/9693129 they even make a 36M value but are not in stock nor sold per each.

https://i.imgur.com/G7UhIhO.png

this probably has something to do with High Voltage Creepage and the fact that resistors back when the set was made had a less potential to handle high voltages like the ones made today. IE one can do today where 2 or 3 was needed 50 years ago.

2x 18M 2W in series is 36M 4W. I would be concerned that a 1W part wouldn't be able to handle that.
There's no harm in placing 2-4 1W or higher resistors in series and or parallel to spread out the power. If I'm spending the money to special order unusual values of parts for a set I rather over build then under build.

I just used the same type of resistors in my Hoffman Colorcaster in the 6BK4C area, it did not specifically call for HV type ones, but I chose to use them, as it did say 1W, but these type do not come any larger than 1W(10kv), however. :(

I'm thinking that in a series circuit, the current is always the same, not double?

https://www.digikey.com/en/products/...2W618/11651620

These will most likely work for you.
They are not HV rated, but I doubt that's what was in there in the first place anyway, and 2 in series will be what was in there to start with.

You're not wrong, but power is not current. Power is voltage times current, or current squared times resistance. Replacing 2 resistors of equal value with a single part you're doubling the resistance in the second equation and doubling the voltage drop across the resistor in the first equation so 1 part needs to be 2x the wattage of 2.


I like Yamamayas solution of 2 18M 2W parts... Can't get the math wrong if you are not doing any math.

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.

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?

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.

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.

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.

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.

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?

https://i.imgur.com/KX4RnvF.jpg

https://i.imgur.com/EMFgJ1o.png

I’d say that guy would be pretty important.

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.

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

https://i.imgur.com/RlxhiUZ.png

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:beer:
73 Zeno:smoke:
LFOD !

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+.

https://i.imgur.com/a1N8LyZ.png

https://i.imgur.com/Q5pF9BS.png

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.

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.

Quick update: I purchased a replacement 6JS6C horizontal output tube from eBay and was surprised that a different tube brought the current down 20ma. The tube was listed as tested good and I lucked out, NOS tubes are priced crazy no thanks to transmitters like the Yaesu FT-101 transmitter :(

On my B&K 747 the tube I was using measures 111 from a scale of 0 - 120 with good starting at 65, I thought was actually very good. The new tube measures above the meter range (120+) so maybe it was NOS. I have a bucking transformer I plan to use which drops the line voltage down to 106vac and has the horizontal output current running about at 190-195ma. I would suspect as the horizontal tube ages the current will rise.

Next is to run through a full setup and put this set into use.

I tackled the cataract tonight. The tube is a rebuilt Sylvania 21FJP22A/21GVP22 type which tests strong, not sure how translatable one CR70 is to the next but all 3 guns measure the same at 12.1v on the scale. To remove the safety glass I pulled a guitar wire through, it pulled very hard but slow and steady got the job done, also wore heavy overalls & coat, helmet, and thick gloves. Acetone and lots of paper towels to remove the sealant. Tomorrow I will work on sealing it all back up.

https://i.imgur.com/Mk7IN6a.jpg
https://i.imgur.com/pHLnUSN.jpg
https://i.imgur.com/ucGLLuy.jpg
https://i.imgur.com/yJDhzCR.jpg
https://i.imgur.com/XOMq2pp.jpg

I can smell that from here! :D

Hopefully that 12.1V is the emission measurement, and not the heater setting.

Yes, that would be bad, when I had the tester out and made notes I guess without thinking used the heater voltage scale as the reference. But hey at 12.1 volts I should have excellent emissions, longevity might suffer a bit :)

I am trying to minimize new threads but if needed I can start a new thread, also note this post is a duplicate to a thread on Antiqueradios that I started in regards to injecting a composite signal. Finally note this is the same set that I just finished a full alignment on so I believe everything is working correctly.

To summarize the picture will not sync when injecting at test point C1 and am looking for help.

• I built an op-amp circuit to amplify the signal to match the voltage at C1 which is approx. -2.5 volts and 4 volts P-P. Note I added batteries to drive the signal down negative to -2.5 volts.
• Also used a Sencore VA62 to feed signal into the injection point. The VA62 allows you to adjust the drive voltage positive or negative. A positive signal is inverted and the color bars are wrong, a negative signal is correct but again I can not get the picture to sync. I set the VA62 to 4 volts P-P, note measuring at C1 shows no DC voltage, and again no picture sync.
• Finally tried moving back the injection point by lifting the anode of the video diode and then keeping L5, C18 in circuit, this made no difference.

Relevant parts of the schematic with Injection point and switch
https://i.imgur.com/aa8Gxk7.png

Op-Amp
https://i.imgur.com/vQh105A.jpeg

Composite jack and switch
https://i.imgur.com/nsxnHWY.jpg

Did some more testing and while I initially thought the polarity was correct it might not be. From the bits and pieces of out of sync image I see inverted colors.

Assuming the negative drive on the VA62 implies an inverted output this aligns with what I am seeing but I would expect the signal to then be in sync. Also tried to feed the VA62 inverted signal through the op-amp to add the DC offset but that did not help.

Pulling the 3rd IF tube seemed to help and could almost get the picture to sync. Is there something related to AGC or other signals from the IF that are needed for the sync circuit?

I want to make sure of your terminology of a positive signal or negative signal from the VA62.

It looks to me like the polarity of the video detector diode means the signal at C1 should have negative-going sync (casually called a "positive signal"). So, a signal from the Sencore VA62 with negative-going sync should be the proper polarity, it seems.

I wonder if you are having a problem due to the AC coupling in your amplifier (even though you are inserting an average DC with the battery).

Can you check with a scope set to DC coupling or maybe just with a voltmeter whether the VA62 output runs negative from ground (like the regular C1 voltage) or floats around zero?

In short there is no DC voltage offset from the VA62. As I look over the schematic it looks like everything that gets signal from test point C1 is capacitively coupled so does the DC offset even matter? Now for more details I Took some measurements from both the VA62 feeding the RF generator into the tuner, and then also the VA62 injecting composite video (currently at L5 & C18 with the diode lifted).

Using the tuner:

• C1: -2.97 Volts DC
• C1: 0.25 Volts AC
• https://i.imgur.com/T7urrBD.jpeg

Injection:

• C1: 0.043 Volts DC
• C1: 0.25 Volts AC
• Note I did adjust the VA62 signal amplitude up to match the 0.25 Volts AC
• Note the scope pictures are with DC coupling and no changes were made other than adjusting the trigger to get a clear waveform.
• Note with the tuner still tuned to the RF generator which is sending a blank signal I can kind of get the picture to sync, when I turn the tuner to an empty channel the sync from injecting is very scrambled.
• Note due to the camera shutter speed the below image looks way better
• https://i.imgur.com/beGKQia.jpeg

Interesting, see below for the signal diagram from Zenith for this set. Still studying the schematic to see how the sound detector signal feeds the sync circuit, but this would explain the sync issue from just feeding signal at the video detector alone. Maybe lift a lead on L14 and additionally inject video at test point C2.

https://i.imgur.com/HWzSoPs.jpg