[Milsurplus] ZM-11/U Repair: Two for the Books

[David Stinson]

My ZM-11/U Cap-Induct-Resist-Transformer Bridge test set is like my 
scope or multimeter; if it ain't working, ain't *nothing* getting 
fixed.  Absolutley essential on my work bench.  Actually have two.  Had 
three, but repaired one and gave it to an old friend for Christmas.  My 
"main" one rolled-over on me a few days ago.   The second one I got at a 
hamfest years ago.  The bridge function didn't work, but it still tested 
cap leakage fine and figured I'd get it fixed sooner or later.  Well, 
with the "main" in cardiac arrest, figured this was a good time to fix 
them both. The "main" wasn't tough- bad solder joint at a tube socket.  
The backup-however, has been "interesting."

The ZM-11 uses the triode section of a 6AV6 as a 1000 cycle RC 
oscillator, which feeds a 6J6 dual triode as a push-pull amplifier into 
a transformer (looks like a little TV flyback) to provide a signal to 
the bridge circuit:
https://photos.app.goo.gl/vz5gy5zsoaRVL8Xz9

   Scope showed the oscillator was barely making any power out to the 
cap under test and the bridge indicator was weak and erratic. Started 
with voltage measurements at the tube bases and right off the bat- the 
plate of the 6AV6 Oscillator was at 99 volts when it should be about 
155.  This is voltage buss B2, derived from buss B1 (300+V) by using a 
10K dropping resistor and a filter:
https://photos.app.goo.gl/RM77iN1ez6fdRAbSA

C112 tested good.  Measured resistance along the B2 buss from the 6AV6 
plate to the cathode of the 6X4 rectifier.  It read 27K. Should read 
10K... that's odd.  Lots of other stuff gets buss B2 and if there were a 
bad load in one of those stages, we'd see lower than 10K, not higher.   
Then a dim bulb lit-up the dark and dusty hollow between my ears-  
27K...  I looked at the schematic again, especially resistors R125 and 
R126 in the voltage divider, then measured from the B1 buss to the top 
of the cap leakage test pot which sets buss B3: 10K.

Looked at the board where these two resistors are mounted:
https://photos.app.goo.gl/4VFzAKAUrxdcEJVg6

The two resistors had been swapped at the factory.  Right out of the 
box, this test set never worked.  Not the first time I've seen something 
like this.   The unit is taken from the box, it doesn't work, the tech 
puts it on a shelf to "take a look at it later." It gets kicked-arond 
and moved and stuffed in a locker and taken out and plugged in and 
cussed-at and back in a box under the bench etc. etc. until one fine 
day, two guys with a clip board stack it on a palette with a bunch of 
other junk, it gets sprayed with paint and marked "surplus,' beginning 
the long, circuitous journey to multiple hamfests until it finally found 
my bench.    Swapped the wires and we now have a lively oscillator, but 
it's intermittant- "jumpy."   And that brings us to the second "one for 
the books."

The intermittent was mechanical, because moving the 6AV6 in its socket 
set it off.  Worked with all the leads and parts around and the only 
thing that had an effect on the intermittant was wiggling the tube.   
THe tube tested excellent on the TV-7.  It seemed it must be a bad tube 
socket.  So I changed the tube socket- big pain, that.   It didn't fix 
it.  Wiggling the tube still fiddled the intermittent.  More trying to 
localize yielded no results.  As Sherlock said:  "When you have 
eliminated all the other possibilities, whatever remains, no matter how 
improbable, must be the answer."  Put in a new 6AV6 and that cured 
it.    The glass at the tube's base is not broken.  The pins are clean.  
Never the less, when the tube was flexed in the socket, the oscillator 
displayed the erratic behaviour.  I assume the mechanical stress on one 
or more of the pins is being transferred into the tube envelope and 
opening one of the elements, unlikely as that seems. The bad-boy tube is 
now resting quietly at the bottom of my dust bin, awaiting it's eternal 
slumber in the land fill.

There are still some issues with this test set- on the lower cap value 
ranges, something is attenuating the oscillator output to the bridge 
circuit, but it's getting enough to work.  I think we got this one "on 
the run."

Good night all,
73 OM DE Dave AB5S


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus