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

Tue Sep 3 15:25:10 EDT 2019

You're very kind, Ken.  I know there's other ways to check capacitors, but I don't know how I ever fixed anything without it, LOL. If I ever see one at a hamfest, if it has a reasonable price tag on it, it will be snapped up. I recommend them. But if you have to work on it, you're going to need some patience. They packed the stuff in there pretty tight.  ;-)73 Dave S.------ Original message------From: Kenneth G. GordonDate: Tue, Sep 3, 2019 12:15 PMTo: milsurplus at mailman;David Stinson;Cc: Subject:Re: [Milsurplus] ZM-11/U Repair: Two for the BooksDavid:

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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