Strange issue with M100B

[doob67]

I recently acquired a M100B in pretty good condition. I was able to get it playing fine. But, a few days ago, while scanning, there was a pop (near the bottom of the mech) and a puff of smoke & it stopped scanning. Nothing.



I checked the fuses on the selection receiver (recently repaired by Bill Bickers) & the one circled in the attached pic had blown. This was one I had just changed. I changed it again & powered the machine up. Still nothing. Checked the fuses again & the same one had blown. Being unfamiliar with the machine, I thought I'd try another fuse. Six fuses later...it still happens. Just that one fuse.



Any idea?

[Ron Rich]

Yep--all kinds of ideas-- first--you need to READ "Hi Newbies", in the FAQ's section above. Second, you need to realize that fuses need only be changed, ONCE. If they blow again, something's wrong, that NEEDS "fixin'", BEFORE, you replace that fuse (with the correct, or smaller, type/size !) again.
My GUESS, judging from what you have written, is that either C-2, or SW-6 on the mechanism is shorted/broken/welded together. Ron Rich

[doob67]

Hi Ron-

Thanks for your follow up. I do realize the fuses need to be replaced only once but was hoping it would allow me to see where the issue was be it from smoke or a spark, etc. I am using the fuses posted on the sign above the ports. 3 amp 3ag fuses.

i will check the areas you suggested & hope to get this thing moving again!

[Ron Rich]

That's a VERY dangerous, method of locating any electrical problem- You may wind up with "more smoke", then you are looking for !!
Ron Rich

[doob67]

yeah. I know.

[robnyc]

Aug 27, 2018 13:23:18 GMT doob67 said:

I recently acquired a M100B in pretty good condition. I was able to get it playing fine. But, a few days ago, while scanning, there was a pop (near the bottom of the mech) and a puff of smoke & it stopped scanning. Nothing.



I checked the fuses on the selection receiver (recently repaired by Bill Bickers) & the one circled in the attached pic had blown. This was one I had just changed. I changed it again & powered the machine up. Still nothing. Checked the fuses again & the same one had blown. Being unfamiliar with the machine, I thought I'd try another fuse. Six fuses later...it still happens. Just that one fuse.



Any idea?

Yep, and it happened at the point where it reversed. It is the reversing switch and as Ron pointed out, one set of contact points have welded together preventing them from opening before the others make contact.  Typically, if these have not otherwise been tampered with (bent) the cause will simply be pitting which reduces the effective contact point area. This overloads that point causing the metal to become plasma and weld together. Use a small point file (such as used for spark plug points) and clean with alcohol. Check operation with power off and make sure one set opens before the other closes.

RobNYC

[doob67]

absolutely right! That is EXACTLY when it happened. So happy I found this board!!

[Ron Rich]

That's where I thought this would go, and happens ONLY onaccounta acuse some "yahoo" "messed with" the contact adjustments--You may also have a broken blade here--however I strongly urge you to read the info on contacts posted above ! I also urge you to remove C-2, if it has not already been done ! You will also probably need to re-form the reversing switch limit bracers. These are on both ends of that switch, and should form an "upside down V". They MUST limit "over-travel" on the center blades to a MAX., of 15 ths of an inch. See your Service Manual !! Ron Rich

[doob67]

Patiently awaiting my copy of Ron's book before I dive in. It's better to have more info/resources when beginning such a project.