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I recently acquired a PG502 from Tek surplus (aka Country Store) and found several setting to be intermittent. Expecting that a good cleaning would be in order, I was astonished to find that cam switch drum appears to be decomposing. It looks dull, powdery and parched like mud flats after a dessert gully washer!
In fact, there was a lot of fine debris on the contact board. Cleaning the board and the contacts made no difference because the cam lobes are deteriorating. I have not touched the cam drum with any cleaner or other chemicals.
This unit was used in the production test area; perhaps its entire life. It was last cal’d June 2022. Perhaps it is the result of 45 years of continuous up time in a rack.

I have never seen a cam drum look like this.

Has anyone seen such a thing?
(See photo gallery for “Decomposing Cam Switch Drum.”)

Hey Clark,

I've never seen such a thing (/g/TekScopes/album?id=301415),
how strange. I wonder if anyone here knows what sort of plastic Tek used
for these?

I've never looked closely at one of those drums, but it looks to me that it
would be relatively straightforward to 3D print a replacement nowadays.
I've recently dusted off my 2nd hand Prusa, so now everything looks like a
nail to me :). The commercial sintering services should be able to produce
a better part than the original. Even a lowly FDM printer should be able to
produce a usable part, though perhaps a little bit of beveling would be
necessary to avoid sharp overhangs.
The problem would be to produce the CAD design, maybe I'll go see if I have
something to play with in my box of mystery modules...

Siggi

On Sat, Mar 22, 2025 at 8:29?PM Clark Foley via groups.io <clarkfoley=
[email protected]> wrote:

I recently acquired a PG502 from Tek surplus (aka Country Store) and found
several setting to be intermittent. Expecting that a good cleaning would be
in order, I was astonished to find that cam switch drum appears to be
decomposing. It looks dull, powdery and parched like mud flats after a
dessert gully washer!
In fact, there was a lot of fine debris on the contact board. Cleaning the
board and the contacts made no difference because the cam lobes are
deteriorating. I have not touched the cam drum with any cleaner or other
chemicals.
This unit was used in the production test area; perhaps its entire life.
It was last cal’d June 2022. Perhaps it is the result of 45 years of
continuous up time in a rack.

I have never seen a cam drum look like this.

Has anyone seen such a thing?
(See photo gallery for “Decomposing Cam Switch Drum.”)

I was fortunate to find an NOS replacement for one drum , 105-0509-00. That allows me to experiment with the bad unit; however, I am not sure where to start. One thought is to find a compatible resin, cyanoacrylate or epoxy to “rehydrate” and seal the parched surface. Hopefully I can find something to fill the cracks and create a smooth surface and restore internal strength. If successful, I can apply it to the other drum, 105-0510-00. If it doesn’t work, then I need to find or fabricate a replacement.

On Mon, Mar 24, 2025 at 04:17 AM, Sigur?ur ?sgeirsson wrote:

I've never seen such a thing (/g/TekScopes/album?id=301415),
how strange. I wonder if anyone here knows what sort of plastic Tek used
for these?

I have some (PG506 for one, IIRC) cam switches that are cracked on the ends, and I expect they're on borrowed time. Nothing as bad as in Clark's photos, though.

I've never looked closely at one of those drums, but it looks to me that it
would be relatively straightforward to 3D print a replacement nowadays.

I've had the same thought, more recently with 7A29 cam lobes. Those are a slightly different beast, but suffer severely from cracking (and I have one that's missing entirely).

Thinking about this for the more typical long cam drums, I suspect one could convert the dotted alignment diagrams in the service manual to a table, and produce cam rings for an individual contact using a given column of on/off sequences for a given contact to generate the lobes (assuming equal intervals?) on a given drum diameter. I think this would be straightforward in OpenSCAD, but have never looked closely at the ramp geometry. Stacking those rings would give you the full cam drum, and then there'd be a mildly annoying detent gear and/or bearing section at the ends, plus grooves for circlips. I suspect that would be tedious, unless it could be parameterized somehow.

All purely academic speculation, as my free time is about to go towards outdoor chores instead of playing with the 3D printer.

Adam

To me, that appears like someone has used an incompatible contact cleaner or such on it.

Russ

Look under "Surface Delamination."

Greg

I had problems with broken individual cams and bushings in the attenuator of a 7A29, which I think are the same gray plastic. I had to do some tricky repairs to put them all back together. The story starts here and there are some nice pictures associated:

/g/TekScopes/message/206747

In my case, the cams did not wear down or anything like that - they just broke open from around the shafts. The plastic was still shiny and had good surface, but simply cracked apart I think due to being too flimsy to take the stresses - those should have been stouter or of a stronger material. I also think the too-radical lift in the cam and follower design and lack of lubrication aggravated the situation, as described in the discussion and picture captions. OTOH, I also noted that the bushings are low stress items yet cracked anyway, so there's an aging/chemical deterioration issue too.

Ed

The cam switch drum is one of the many plastic parts in the 7000 and 500 series with this issue. The material becomes brittle and develops chalky white residue on the surface. From what I gather, the problematic plastic is delrin or acetal resin.

One of the reasons I divested myself from this series. The intricate plastic parts break left and right, and when they do, you have quite a problem on your hands.

It seems that the color rules are:
1. Avoid blue plastics used by Tek
2. Avoid red M&Ms

Neither of them last 40 years!