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From: hr(bob) hofmann on 19 Jul 2010 23:06
On Jul 19, 8:49 pm, Mike WB2MEP <michael.w.appenzel...(a)lmco.com>
wrote:
> On Jul 19, 9:49 am, Chuck <c...(a)deja.net> wrote:
>
> > 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.
From: hr(bob) hofmann on 19 Jul 2010 23:08
On Jul 19, 3:42 am, "Arfa Daily" <arfa.da...(a)ntlworld.com> wrote:
> "Jim Adney" <jad...(a)vwtype3.org> wrote in message
>
> news:fefccde9-6445-418f-9b82-d2caf369870b(a)u26g2000yqu.googlegroups.com...
>
>
>
>
>
> > I'm trying to fix my '73 Zenith 25CC25. The problem is that the
> > horizontal sweep is not wide enough, so my picture is compressed
> > horizontally. I've lost about 10% of the width on each side. Other
> > than that, it's fine. I have both the Zenith service manual and the
> > Sam's for this set.
>
> > I've tried other hor osc tubes (6U10) and hor out tubes (20LF6) and
> > none of the ones I have (which should be good) worked any better (but
> > some were a bit worse. Not too surprising.)
>
> > This is a hybrid set, with 3 Zenith Duramodules. I have spares for
> > each module, but swapping in the appropriate spare made no difference.
>
> > I checked the B+ in the Horizontal stages, and it's fine, 274 V.
>
> > The Vertical also seems fine, but I just tried to check the 2nd anode
> > voltage, and got garbage readings from my HV probe. I'll take that
> > probe to work tomorrow and check it to see if I can nail down that
> > problem. It says I only have ~6800 V, which seems impossible. It's
> > supposed to be 26 kV, so if it was really that low I believe the
> > vertical sould be WAY overscanned, which it's not.
>
> > I've checked a few resistances that I can get to easily, and so far
> > they all seem good.
>
> > What I haven't done so far is try to get a scope in there to look at
> > the signal at each stage, so at this point I don't know where the
> > problem starts.
>
> > I'm hoping someone here will still be familiar with this old set and
> > can suggest some good places to look next.
>
> > thanks,
>
> 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- Hide quoted text -
>
> - Show quoted text -

If he really only had 7Kv, there would be a huge bloomed out barely
visible picture.
From: Jim Adney on 20 Jul 2010 20:28
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,
From: Jim Adney on 20 Jul 2010 20:43
On Jul 19, 8:49 pm, Mike WB2MEP <michael.w.appenzel...(a)lmco.com>
wrote:

> 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.

This set has a tripler.

Any other suggestions?
From: Chuck on 21 Jul 2010 09:33
On Tue, 20 Jul 2010 17:43:35 -0700 (PDT), Jim Adney
<jadney(a)vwtype3.org> wrote:

>On Jul 19, 8:49�pm, Mike WB2MEP <michael.w.appenzel...(a)lmco.com>
>wrote:
>
>> 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.
>
>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
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