[GreenKeys] (another) Dovetron Problem

[Duncan Brown]

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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