[Heathkit] Completed rebuilding the IT-12 Signal tracer, a couple glitches

[Phil]

Hi All,
First, when I received this unit, I fired it up with the dim-bulb-tester 
just to be on the safe side.  It fired up, first with a few "crackles" 
and some hum.  Turning the volume control up past about halfway, give or 
take and it would break into oscillation.  I figured that was probably 
due to flaky filter caps and didn't worry about it at the time as I 
figured I'd re-cap it anyway.  Also, the eye tube was quite dim, could 
see it ONLY if the room lights were off.  I hoped that was due to low B+ 
or related.

Further examination showed that most of the resistors were out of 
tolerance on the high side, most of the higher values ones by as much as 
50%.  NOT unusual for carbon resistors.

So I decided that a complete rebuild was in order, not just a basic 
re-cap.  So I tore it completely down to a bare chassis, gathered all 
the resistors and capacitors together and started the rebuild just like 
it was an unopened kit.  Even the tube sockets were replaced with 
ceramic ones (with tube shield for the 12AX7) (except the 1629 eye tube 
socket).

The parts that were reused was the speaker, output Xformer, power 
Xformer, all controls & switches and of course any hardware/binding 
posts etc.  ALL small electronic parts (caps, resistors,rect etc) were 
replaced.

I was VERY careful to make sure that the wiring is exactly per the 
manual, doing it step by step and check-marking the manual. The original 
can type filter capacitor was replaced by a couple terminal strips and 
discrete capacitors.  The 50uF input cap was replaced by a 40uF one and 
the 50uF output filter cap (after the 10K resistor) was replaced by a 
100uF unit (both rated 260V). This because they're what I had on hand in 
my new stock.

The "Wired-in" permanent probe cord was replaced by a 1/4" jack in the 
same hole, this because I don't like "wired-in" probes and other 
dangling things.  The power cord was replaced by a polarized one, and a 
.01uF cap placed from the neutral lead to the chassis. I made sure the 
"hot lead" went to the power switch.  I chose this route rather than a 3 
wire cord (original was 2 wire non-polarized).   NO other changes were 
made to the unit.

OK, here's the "glitches" I mentioned.
1.  The eye tube is still dim and will need replacement (whew, 18.00 at 
Antique Electronics).  Voltages on it are all correct.

2.  It STILL oscillates when the volume is turned up about half way, 
give or take.  Doesn't sound any different in that regard than it did 
when it arrived.  Putting the 12AX7 tube shield on/off doesn't change it 
any.  It's high pitched, wife thought I was playing with crickets in 
here ;-)  I tried subbing a 12AT7 thinking the lower mu device might not 
oscillate, but it made little/no difference.

3.  Hum is now negligible until the volume is turned up moderately, 
indicating any hum pick-up is 'probably' due to the close proximity of 
the power Xformer to the front end circuits. The unit is rather tightly 
packed and there isn't as much spacing between things as I'd do it if I 
designed it (I've done a lot of home and pro audio work).

I wonder if any other list members have experience with the IT-12 signal 
tracer?  This is the one in the VTVM sized cabinet, I'm NOT sure if the 
larger cabinet ones would have similar oscillation problems, even IF 
they used the same circuit. . .

-- 
73 de Phil,  KO6BB
http://www.qsl.net/ko6bb/  (Web Page)

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