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From: Jim Adney on 18 Jul 2010 21:14
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,

From: Arfa Daily on 19 Jul 2010 04:42


"Jim Adney" <jadney(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

From: Meat Plow on 19 Jul 2010 09:02
On Sun, 18 Jul 2010 18:14:46 -0700, Jim Adney wrote:

> 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,

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

--
Altopia.com $12/m 10 unlimited connections
From: Chuck on 19 Jul 2010 09:49
On Mon, 19 Jul 2010 13:02:54 +0000 (UTC), Meat Plow
<mhywatt(a)yahoo.com> wrote:

>On Sun, 18 Jul 2010 18:14:46 -0700, Jim Adney wrote:
>
>> 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,
>
>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
From: Mike WB2MEP on 19 Jul 2010 21:49
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
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