Hi All,

Just a note. As happens to all of us, one of those
little ceramic trimmers in an FG540 was broken. The
screw turned and turned, but the ceramic disk on top
didn't. It was frozen solid.

Bother...

I didn't have a spare, so I removed the trimmer, and
found the ceramic disk frozen solid to the ceramic
base... no trimming going on here!

First, I put the capacitor into some IPA to soak for
a bit. There was more than enough stinky flux on
the cap, so I think the flux was gluing the cap stuck.

After it soaked for a while, I was able to turn the
disk with my fingers... don't use tools, it will break!

I put it in for some more soaking, and twisting, until
it was good and clean.

Next, I took a small dot of cyanoacrylate glue, and
spread it around the screw. A small dot, not a flood.
[If you can't control your glue bottle, put some CA glue
on a piece of plastic, and use a toothpick to bring a dot
to your trimmer.]

Then finally, I gave a rag a squirt of "Kicker", a CA
glue accelerator, and put the rag near the trimmer... The
kicker's fumes are enough to harden the glue almost
instantly. If you don't have any kicker, just let it set
a while, it will harden from the moisture in the air.

I gave the screw a twist, and all was back to normal.

-Chuck Harris

Wow, Chuck, it was worse than I thought, but it sounds like you have prevailed.

john

You mean these trimmers?
https://www.mouser.com/ProductDetail/?qs=HHuYZtQ5iDWVDisM2QQAHw%3D%3D

I had them fail in 2 ways. The caps became loose so the wipers had no contact, or they get dead spots which don't seem to be reversible at all. I want to replace these with sealed Bourns trimmers as possible. My FG504 came very clean and new looking and with a calibration sticker, like it had been used in a climate controlled building. In my 465B scope, one of them had the cap break off in the middle where the shaft is. I didn't have a replacement so I wedged something between the trimmer and some wires in that area to keep it in contact. And there it remains to this day.
The brown cap is plastic, not ceramic. They seem to get brittle over time, perhaps accelerated when exposed to chemicals. I wonder how long yours will last after this treatment.


On Tuesday, August 18, 2020, 08:51:04 PM CDT, Chuck Harris <cfharris@erols.com> wrote:

Hi All,

Just a note.  As happens to all of us, one of those
little ceramic trimmers in an FG540 was broken. The
screw turned and turned, but the ceramic disk on top
didn't.  It was frozen solid.

Bother...

I didn't have a spare, so I removed the trimmer, and
found the ceramic disk frozen solid to the ceramic
base... no trimming going on here!

First, I put the capacitor into some IPA to soak for
a bit.  There was more than enough stinky flux on
the cap, so I think the flux was gluing the cap stuck.

After it soaked for a while, I was able to turn the
disk with my fingers... don't use tools, it will break!

I put it in for some more soaking, and twisting, until
it was good and clean.

Next, I took a small dot of cyanoacrylate glue, and
spread it around the screw.  A small dot, not a flood.
[If you can't control your glue bottle, put some CA glue
on a piece of plastic, and use a toothpick to bring a dot
to your trimmer.]

Then finally, I gave a rag a squirt of "Kicker", a CA
glue accelerator, and put the rag near the trimmer... The
kicker's fumes are enough to harden the glue almost
instantly.  If you don't have any kicker, just let it set
a while, it will harden from the moisture in the air.

I gave the screw a twist, and all was back to normal.

-Chuck Harris

Hi All,

Just a note. As happens to all of us, one of those
little ceramic trimmers in an FG540 was broken. The
screw turned and turned, but the ceramic disk on top
didn't. It was frozen solid.

Bother...

I didn't have a spare, so I removed the trimmer, and
found the ceramic disk frozen solid to the ceramic
base... no trimming going on here!

First, I put the capacitor into some IPA to soak for
a bit. There was more than enough stinky flux on
the cap, so I think the flux was gluing the cap stuck.

After it soaked for a while, I was able to turn the
disk with my fingers... don't use tools, it will break!

I put it in for some more soaking, and twisting, until
it was good and clean.

Next, I took a small dot of cyanoacrylate glue, and
spread it around the screw. A small dot, not a flood.
[If you can't control your glue bottle, put some CA glue
on a piece of plastic, and use a toothpick to bring a dot
to your trimmer.]

Then finally, I gave a rag a squirt of "Kicker", a CA
glue accelerator, and put the rag near the trimmer... The
kicker's fumes are enough to harden the glue almost
instantly. If you don't have any kicker, just let it set
a while, it will harden from the moisture in the air.

I gave the screw a twist, and all was back to normal.

-Chuck Harris

On Aug 18, 2020, at 12:37, Martin Whybrow <martin@thewhybrows.co.uk> wrote:

My 7854 has issue with the readout, the characters are lines (no Y deflection) and are in the wrong place (all at the central grid line); I want to go through the display setup process, but that requires the keyboard in order to enter the program to generate the calibration pattern; unfortunately I never got the keyboard when I bought the scope, is there any way around that little problem?
Martin

One of the nice features on the 485 is that you can switch the calibrator from 1kHz for the standard compensation adjustments to 1MHz for the rise time checks when otherwise the trace would be very faint. The PG506 has intermediate rep rates as well.

Regards,

Roger