[GreenKeys] [External] Opinion on toasty ceramic resistors

[Harold Hallikainen]

On Tue, December 3, 2024 8:02 pm, Jones, Douglas W via GreenKeys wrote:
> From: Anthony Watson -- Tuesday, December 3, 2024 2:43 PM
>>  I have never had ceramics fail, and these aren’t failed - just look
>> well toasted.
>
> I remember back in 1974 when I was a student at the U of Illinois, we had
> an interesting ceramic resistor failure.
>
> Well, it didn't quite fail.  It did melt and drool molten glass on what
> was under it, but it continued to resist at its rated value.
>
> Full story.  We had a PDP-11 system, with punched-card reader and line
> printer, used to run student jobs.  It had a fixed head disk for fast
> access to commonly used programs and scratch files, and DECtape for the
> rarely used stuff.
>
> One day, the disk failed.  The DEC technician came to do his thing, and on
> opening the case, he found two problems:
>
> 1) The disk heads had worn neat grooves all the way through the oxide on
> the platter.
> Fix:  We only had one side worth of heads.  The other side of the disk was
> virgin, so he flipped the disk over, went to Kmart for a can of Turtle
> Was, carefully waxed and polished the unused side of the disk, and
> re-assembled that part of the problem.
>
> 2) The resistor melted because of a bad motor-start relay.
> Fix:  Replace the relay.  No need to replace the resistor!  Chip the glass
> drippings off from below the resistor.  Fortunately, what they fell on was
> structural, not electronic.
>
> When he left, everything worked, and continued to work until the end of
> life of that machine.

I'm trying to remember what ceramic resistor would be behind the keyboard.
I just looked inside my model 15, and there is no such resistor there. I
wonder if you perhaps have a governed motor instead of a synchronous motor
(which I have). If so, there is a resistor put in series with the motor
when the motor speed is too high. The resistor is then shorted when the
speed is too low. As the machine runs, the governor contacts cycle between
closed and open to regulate the speed of the motor (a bit like a "bang
bang" switching regulator.

But, speaking of stuff melting, I worked at a radio station that had a
Collins 20T transmitter (
https://bh.hallikainen.org/uploads/harold/Collins20T_1KWAM.pdf ). One
hight, the station went off the air. Upon arriving at the transmitter
site, I found that the oscillator had failed, which removed the grid leak
bias from the 833 final tubes. They had gotten real hot and sucked in the
envelope glass. I got the oscillator working again. (Murphy's Law has a
corollary that says "Amplifiers will oscillate, and oscillators will
not.") I also replaced the 833 finals, but as mentioned in this thread,
maybe they were still good...

Harold
https://w6iwi.org


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