[GreenKeys] (another) Dovetron Problem

Mon Mar 28 18:01:14 EDT 2022

A heatsink was an early thought. But the 2N3439 has a power dissipation 
of 1W at 50C and is only running 60ma * 0.5V = 0.03W. But it is near the 
loop current limiting resistor and the power transformer, so I think 
that is where the heat is coming from. Until I get a new transistor, 
I'll just leave the cover off and hope that lowers the ambient in the 
box enough.

Have fun,

Duncan
K2OEQ

On 28-Mar-22 17:49, Joe Duszyński wrote:
> "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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