[GreenKeys] (another) Dovetron Problem

[Joe Duszyński]

"Heatsink" if it doesn't have one! Small computer fans also work wonders
too on vintage stuff that you can roast weenies on!!.


On Mon, Mar 28, 2022 at 3:59 PM Duncan Brown <duncanancy at earthlink.net>
wrote:

> Tom,
>
> Thanks for detailing your Dovetron problem.
>
> I just had a strange problem with a Dovetron MPC-1000CR at the AWA
> Museum.  It had been on all day (normal occurrence) and late in the day,
> the TTY stopped printing. When I looked at the loop current it was only
> varying from about 50 to 60 ma with traffic.  I took the Dovetron home
> to look at it more closely in my shop, but it printed OK and loop
> current looked OK. I looked it over & poked at it some, but couldn't
> make it fail.  Took it back to the Museum and it wouldn't print! Loop
> current not going below 40-50 ma with traffic.
>
> Took it home again and looked at the loop keying circuit more closely
> and recorded voltages.  Let it run all day today and it finally failed
> here at home with a scope connected into the keying circuit.  I could
> see that the base of the keying transistor, Q7 (2N3439) was not going
> down all the way to 0V, unless I put it in standby, which shorts out Q7
> (removing 120V from Q7-C).  Pulling the Regen board connector opens the
> input to Q7, with only a 2K resistor on the base to ground, but the Q7
> base still reads about 0.35V (with 120V on Q7-C)
>
> Q7 definitely leaky, especially when hot!!
>
> 2N3439 available at Digikey, so I've ordered some.  Should fix the
> problem.  Meanwhile, I'll run the Dovetron with its cover off.
>
> Another problem solved!
>
> Have fun,
>
> Duncan
> K2OEQ
>
>
>
>
> On 24-Mar-22 19:30, Tom Hunter wrote:
> > My civilian Dovetron just lost its loop supply.
> >  I wanted to document my progress.
> >
> > This Dovetron has been copying news for testing
> > Teletypes for a couple years. First ITTY and more
> > recently my news stream.
> >
> > Today I noticed I had no loop current. First I checked
> > the patch panels in my radio building and basement.
> > There was nothing plugged into the loop in either place.
> >
> > So I went back to the den and pulled the Dovetron out
> > of the rack, plugged a meter right into the loop jack -
> > no current. I checked the loop fuse - not blown.
> > I pulled the top and bottom off and brought up the
> > manual - no schematic, just a block diagram.
> >
> > Oh well, that will have to do. Next I checked the shorting
> > contacts on both loop jacks - good continuity. Next I
> > tried to trace the jack and current pot wiring back to
> > the loop supply. Not easy when I could not pinpoint the
> > loop supply. I was sitting there staring at, as one does
> > when stumped. I noticed a spot of gray ash on the
> > bottom of the board. I checked the top and it was
> > the brown wire from the current adjust pot on eyelet
> > 71 and the top of the board was slightly blackened.
> > Checked the continuity from the pot to the other end
> > of the trace - nothing. Checked it within a quarter inch
> > of the eyelet - nothing. checked it to the bottom of the
> > eyelet - nothing. I think I found my problem. I am still
> > trying to decide what  to do about it.
> >
> > That should not have happened with 60 mils. my best
> > guess is that is was a high resistance solder joint from
> > the factory
> >
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