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

Tue Sep 3 13:15:21 EDT 2019

David:

Thank you for this info. I own ONE only ZM-11/U which I have never yet used. I also have a 
complete original manual for it. From your description below, I will immediately dig it out and 
put it to use.

Thank you again for your words of wisdom.

Ken W7EKB

On 3 Sep 2019 at 0:12, David Stinson wrote:

> 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
> 
> 
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