Zenith 25CC25 horizontal sweep problem

Not familiar with this set specifically, but still recall (fondly !) my days working on UK hybrid TVs. How about the horizontal output tube's screen feed resistor, or decoupling capacitor ? Also, the tuning cap for the HOPTx, or even a leaky S-correction cap ? Bad width control ? I seem to recall that we had lots of trouble with these going high in value to cause low width. Also, any other high value resistors in the area. As to your garbage readings on your high voltage probe, what sort of meter are you using it with ? If it's old like the TV, it will have been made to work in conjunction with an analogue meter with the 'standard' 20k ohms per volt characteristic. Use with a different characteristic or digital meter, will result in wrong readings.

Arfa

I started out working on tube stuff seeing that's all there was way back then. You'll need to find a print of the HO stage with nominal voltages illustrated. I'd assume without much doubt you've got a resistor or two gone high value and some weak capacitors in that stage. You're going to stumble around in the dark unless you know what to expect volts-wise on the 20LF6

Did you try replacing the damper diode or tube? Chuck

Also, if your set has a HV Pulse Regulator tube like a 6JH5 or 6HV5 (same size/shape as Hor Out, but no top cap) it can cause this problem. The Pulse regulators work on the primary side of the Flyback, and can load down the horiz. sweep. Gassy? I have a 25CC50 from '72 and have seen all three of these tubes cause this problem. I'm not familiar with the 25CC25. If it uses a HV tripler like the 4-tube hybrids, there won't be a regulator tube.

Mike WB2MEP

I would be suspicious of the screen grid. Either the feed resistor or the bypass capacitor or both.

ays

eed

we

so,

's

If he really only had 7Kv, there would be a huge bloomed out barely visible picture.

Thanks to everyone who took the time to respond. I'll try to respond to each of your comments.

I've measured the 2nd anode voltage twice, with 2 different meters, one analog and one digital, both with 10 MOhm input impedance. In each case I got low answers, 7 kV and 13 kV. I really do know what I'm doing here, so I'm puzzled by the low readings and I can't explain them. I took the probe and one meter to work to check them and they checked out perfectly at 5 kV. My picture isn't as bright as I'd like, but it really doesn't seem like it could actually be down to half voltage or less. There's no blooming, either. Right now I'm going with operator error....

I have tried different hor out (20LF6) hor osc (6U10) and dampers (19DQ6.) None improved the problem. I didn't swap in vert output tubes (10JA5) since the vertical seems to be fine. Those are the 4 tubes in this set.

This is a hybrid set. The hor signal coming out of the 9-48 Duramodule measures 21 V p-p. Sam's says it should be 16 V but the Zenith manual says it should be 24 V. Swapping in another 9-48 doesn't help the problem. There are differences in the schematics shown in the 2 manuals, specifically in the 24 V supply for the solid state modules. My modules are getting about 22.5 V, but that's one of the options that Sam's shows. That bus is dead quiet: less than 0.1 V p-p of ripple, exactly as it's supposed to be.

It appears that some of these sets fed 22.5 V to the Duramodules and some supplied 24 V. I'm tempted to rewire this set to supply 24 V, but the fact is that it worked this way for 40 years, so that would be skipping over the real problem. Nevertheless, I'd be interested in hearing from those of you with real world experience about which version is actually better.

So far, I've just been working from the top of the chassis, picking up test points that correspond to tube pins that I can get a DVM or scope probe on. Tomorrow I'll clear off the junk that's sitting on top of the set, flip it over, and dive into the belly. Yes, the first thing I plan to do is check for carbon resistors that have gone high. Unless I find the smoking gun that way, I'll then look at voltages at each of the test points shown in both the Sam's and Zenith manuals.

One of the signal voltages I found late in the hor circuit was low, so I'm pretty sure the problem is somewhere in the 6U10 or 20LF6 circuit. With all the connections to the convergence and the HV, there's so much going on there that I'm really not sure about what parts can affect which other parts.

I very much appreciate everyone who made suggestions. I'll try each one of them. I'm skilled at electronics, but I have little practical experience with TVs. It's good to get input from real experience.

thanks,

This set has a tripler.

Any other suggestions?

I seem to remember Zenith sets of that time period having a width coil which would mechanically fail so the core would move or fall out. Since I don't have a schematic for your set, I don't know if this chassis was one of the ones affected by this problem. You might check the coil to see if the core is still there or , if it is, try turning the core in both directions to see if there is any improvement. Chuck

Are you sure about that 10 meg load? That would way overload the EHT supply. The Fluke EHT probe is around a GIG ohm load.

Why do you care about TV experience? Are you expecting a resurgence in analog CRT standard def sets? It's good to know stuff but why this?

G=B2

The meter has 10 MOhm input impedance. The probe adds 990 MOhm to that, for a total of 1 GOhm. That makes it a x100 probe. IIRC, the Fluke probe is actually a x1000 probe.

I was just trying to express gratitude for those of you who have been willing to donate your experience to help me out.

;-)

Okay, the problem has been solved. As usual, I'm embarrassed at how long it took me to find it.

The key was in the FACT that the HV was low. It really was. There is a HV adjust pot, but I had not tried it because it was quite hard to access and I knew I had adjusted it carefully, years ago. Once I tried it, to see what it would do, I found that I got no response for most of the rotation, but at about 90% rotation the HV suddenly jumped from ~10 kV to ~23 kV. That pot was open near the end of its travel.

This was a 4 MOhm pot which applies a bit of pos input to the HOT grid bias. Without this input, grid current tends to bias the HOT off. So I had low HV and low hor sweep. Once I fixed that, things got a LOT better.

It turns out that as the HV goes up, the hor sweep goes up slightly faster, so that by the time you get to 20 kV, the picture fills the screen. The vert deflection shrank as the HV came up, but I was able to adjust it back up to fill the screen.

Unfortunately, I still can't get the HV to the 25-26 kV that this set is supposed to have. I replaced a 27 kOhm resistor in the hor osc grid circuit that had gone to 35 kOhm, but that didn't help. I tried several 20LF6s and they all worked the same except for one which was clearly worn out. I finally installed a 26LX6, which is also correct for this set, and that gave me the same 22-24 kV that the others gave. And the HV pot is still cranked all the way up.

Finally, I checked the DC bias on the HOT. The Zenith manual says it should be -80 V, so I was expecting it to be lower. It's actually -65 V. So there must be something else wrong in that circuit.

At this point, I've got a working set that looks great, but I'm troubled by the HV dilemma. I'd like to know what's wrong, so I can fix it and turn the HV pot down a bit.

BTW, in this process I came across a component I've never seen before. Zenith call it a "voltage dependent resistor" and doesn't give ANY specs, just a Zenith part number. Sam's doesn't even try to offer any subs. Any clue what this might be? The particular VDR in this circuit is Zenith #63-5440. Anyone got one of this in stock? It just looks like a big resistor with 2 wide color bands on it. A Google search comes up with some places that seem to have these in stock. They seem to be calling these varistors, which makes sense, but this would be the first time I've seen a cylindrical, axial lead, varistor.

Thanks, everyone, for all your help.

Yeah, Zenith used those VDRs in various places in the horiz. sweep circuits in their late 60's - early 70's tube sets. I had one that caused a width problem similar to yours. I discovered the open VDR when the width improved while I measured voltage at a certain point with my DMM. From the schematic, I found my DMM was in parallel with a VDR. If the 10 meg DMM input improved things, it must be open. That one I was able to use a plain resistor for, I started with a few meg ohms and went lower until the width was satisfactory. Another Zenith had one in the HV regulator circuit that went open and lost regulation. The HV was running around 30 kV on a 19" set. A regular resistor did't work on that set and I had to get the correct part.

It looks like

Mike WB2MEP

Good grief ! Did you not have x-ray protection circuits on the sets your side of the pond ?

Arfa

Not on tube sets, and early solid-state ones. X-ray protection came into widespread use when they started running the horiz. sweep from a regulated B+ supply and did away with regulation of the HV itself.

Mike

That surprises me. It's of course been a very long time ago, but I'm sure that the early tube sets that I worked on, which employed a type PD500 shunt stabilizer for the HV. had at least some crude form of x-ray protection, in the form of sensing the beam current and using the result to back-bias the H-out tube into cutoff. At 30kV on a 19" CRT, the average beam current must have been well over the top, and fairly easily sensed as a fault condition ? Perhaps they were just a bit more 'picky' about such things this side of the pond ? :-)

Arfa

On 7/24/2010 5:50 AM Arfa Daily spake thus:

So at what HV voltage do (did) TV sets start emitting X-rays?

Early Zenith solid state sets had a horizontal tuning cap that would open, then the high voltage would skyrocket until the neck would fall off of the crt. RCA sets with SCR sweep had a failure mode where the high voltage would go over 60 KV. The vertical deflection was designed to collapse, when the hv rose, which was supposed to signal the customer that they should shut off the set. However, many people would leave the set on and just listen to the sound. I always wondered why RCA didn't just kill the horizontal drive. Chuck

In comparing my Sams & Zenith manuals, I find that Zenith says the HOT control grid bias should be -80 V but Sams says -48 V. I now have -65 V. I'm going to guess that Sams is right and I need something closer to -50 V, which should turn things up a bit more and get me to the 26 kV that I should have for the HV. I also checked the tube data sheets for the 20LF6 and 26LF6 and found that typical control grid bias values are -10 and -20 V. So I can afford to boost that voltage quite a bit higher (more positive) than the -65 V where it's at now.

The set is put back together now, but I'll pull it apart again once our houseguests (arriving tomorrow) are gone. In the meantime, it looks like I should try to buy one of those odd 63-5440s. AFAIK that's the one remaining part in there that might be the problem, but I'll check around the circuit, too, to make sure I haven't missed something.

As an aside: Was it common for the Sams to be more accurate than the Zenith manual? Or, to put it differently: Did the Zenith manuals have a poor reputation?

thanks,