[TMC] That pesky PAL 350/500

[Chris Bolkan]

Thanks Gary!

I have been looking at the bottom of the chassis. Never thought to look at
the top! I will remove the ceramic ring and look at the top.

Yes I absolutely have the tubes removed because I did not want
interelectrode capacitance affecting my measurements. This helps a lot!

I will figure out how to properly measure the cap with what you have told
me. This makes a lot more sense as to why my measurements were identical.

Maybe the sockets aren't bad afterall.

More to come.

Chris

On Thu, Oct 21, 2021 at 9:34 AM Gary Schafer <garyschafer at largeriver.net>
wrote:

> Hi Chris,
>
> The screen bypass capacitor should be a large ring that mounts on top of
> the chassis. It will have finger stock around the inside where the screen
> ring makes contact. The screen ring is just beloW the plate.
> If you had the tube removed when you tried tried to check the cap via pin
> 1 it would be disconnected. Look at the bottom of the tube and you will see
> a small strap from pin 1 to the screen ring.
> Check capacitance from the finger stock to ground.
>
> 73
> Gary. K4FMX
>
>
> Sent from my iPhone
>
> > On Oct 21, 2021, at 10:54 AM, Chris Bolkan <bolkyboats at gmail.com> wrote:
> >
> > Hi TMC lovers!
> >
> > As you know I'm struggling with my PAL 350. Power output isn't near what
> it
> > should be. The help and comments from the group so far have been
> wonderful.
> > A lot to think about and digest, but so far no ah-ha moment.
> >
> > Right now the amp is apart in order to focus on what looks like it could
> be
> > the problem or at least a contributor.  Screen grid voltage is fed
> through
> > inductors to pin 1 on the final sockets (screen grid) and inside the
> final
> > sockets is supposed to be a 1000pf capacitor from pin 1 to ground.
> >
> > I'm struggling with this 1000pf capacitor built into the tube socket.
> I'm
> > not seeing any evidence of it visually or by measuring it. Both sockets
> > read exactly 4pf from pin 1 to the case of the socket which is the part
> of
> > the socket that mounts it to the chassis.
> >
> > A lot of the socket is translucent plastic and the pins all look exactly
> > alike in the plastic. Where is the capacitor hiding? I know these are the
> > original sockets. All the screws have the TMC marking dope on them. There
> > are no external 1000pf caps on the pins but 1000pf caps are shown on the
> > schematic. This amp must have worked as shipped from the factory.
> >
> > Has anyone ever taken one of these sockets apart to see how they
> > implemented the capacitor?
> >
> > I'm so stumped. How did they both fail EXACTLY the same way? Im my world
> > that doesn't happen very often.
> >
> > Thanks as always!
> >
> > Chris
> > KI7NXY
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