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From: stratus46 on 21 Jul 2010 22:26
On Jul 20, 5:28 pm, Jim Adney <jad...(a)vwtype3.org> wrote:
> 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,

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?

G²
From: Jim Adney on 21 Jul 2010 23:43
On Jul 21, 9:26 pm, stratu...(a)yahoo.com wrote:

> 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.

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.

> 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?
>
> G²

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

;-)

From: Jim Adney on 22 Jul 2010 00:22
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.
From: Mike WB2MEP on 23 Jul 2010 03:58
On Jul 22, 12:22 am, Jim Adney <jad...(a)vwtype3.org> wrote:
> 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 www.moyerelectronics.com has a 63-5440 sub. in stock.

Mike WB2MEP
From: Arfa Daily on 23 Jul 2010 12:59


> 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

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

Arfa

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