Zenith 25CC25 horizontal sweep problem

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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,


Re: Zenith 25CC25 horizontal sweep problem




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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


Re: Zenith 25CC25 horizontal sweep problem


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

Re: Zenith 25CC25 horizontal sweep problem



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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

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Re: Zenith 25CC25 horizontal sweep problem


On Mon, 19 Jul 2010 13:02:54 +0000 (UTC), Meat Plow

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Did you try replacing the damper diode or tube?   Chuck

Re: Zenith 25CC25 horizontal sweep problem


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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

Re: Zenith 25CC25 horizontal sweep problem


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

Re: Zenith 25CC25 horizontal sweep problem


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,

Re: Zenith 25CC25 horizontal sweep problem


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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?

GB2%

Re: Zenith 25CC25 horizontal sweep problem



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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.

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

;-)


Re: Zenith 25CC25 horizontal sweep problem


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.

Re: Zenith 25CC25 horizontal sweep problem


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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 www.moyerelectronics.com has a 63-5440 sub. in stock.

Mike WB2MEP

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

Arfa


Re: Zenith 25CC25 horizontal sweep problem


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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

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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


Re: Zenith 25CC25 horizontal sweep problem


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

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


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Re: Zenith 25CC25 horizontal sweep problem


On Sat, 24 Jul 2010 13:50:38 +0100, "Arfa Daily"

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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

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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,

Re: Zenith 25CC25 horizontal sweep problem


On Mon, 26 Jul 2010 08:19:04 -0700 (PDT), Jim Adney

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If I remember correctly, there were usually more mistakes in the Sams
manuals.  Chuck

Re: Zenith 25CC25 horizontal sweep problem



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This set has a tripler.

Any other suggestions?

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