[GreenKeys] [External] Opinion on toasty ceramic resistors

[Anthony Watson]

Eek! Self-shrinking vacuum tubes!

Totally have the governed motor here - always is right on the dot looking through the tuning fork. This thing is a tank.  Today just a little smoky one. 


Anthony

Sent from my iPhone

> On Dec 3, 2024, at 8:53 PM, Harold Hallikainen via GreenKeys <greenkeys at mailman.qth.net> wrote:
> 
> 
> 
>> 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
> 
> 
> --
> Not sent from an iPhone.
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