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 Post subject: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 3:04 am 
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Wanted to point this unit out, as there may be some here who have no idea as to what it is. The listing is for a Clough-Brengle Model 712 Military ZM 11/U, and the chap want $75.

I have no interest in this sale, and do not know the seller. Too, I already own a ZM 11, so am not in the market for another.

I did want to point out, that if anyone is wanting a top-notch, vintage impedance bridge, this may be for you. Not only was the ZM 11 impressive when they came out in the 1950s, they are still pretty impressive today. These things did the normal measurement of capacitors, as far as capacitance, leakage, etc. But they also measure dissipation and power, can measure inductance, and also the turns/ratio for power transformers. The unite employs an eye-tube to set null, and has a meter for setting the correct voltage.

An all-tube measuring instrument, the ZM-11 is also very accurate when calibrated. Accuracy is only slightly less than lab grade impedance bridges such as the Gen Rad 650-A. Another plus is that these things consume very little space, as they were designed for military use (Navy and Marines), although there was a civilian version offered.

Another interesting feature of these units, is that they may be the first, or one of the first, capacitor analyzer/bridges to do a form of ESR testing. And this was in the 1950s.

Here is the bad: The very compact size makes for a VERY difficult instrument to work on. I would about guarantee that all the electrolytics have gone bad, and will need replacing. There are several of these 'lytics, and if I recall correctly, most are Sprague bath tub types. Since there is virtually no room under the hood, you have to stuff them.

The above is not the worst part however. Most of the capacitors can be pretty easily reached when the unit is out of the case. Unfortunately, there are at least two caps. buried deep within guts, and the only way to service them, is by "un-sandwiching" the top circuit from the bottom. This involves a lot of wire de-soldering, removing screws, and general dismantling. The manual for the ZM-11 even goes so far as to caution the intrepid tech. to go with such an operation with great circumspection. Alan Douglas had one of these units, and wrote a little about it in his book on vintage test gear. He held a pretty high regard for the design, but was also reluctant to completely open one up.

The ZM-11 I have worked when I got it some years ago, but eventually all the caps. finally died, and it would no longer null. I went through all the caps. I could reach, but did not have the fortitude to get to those last two deep inside. Unfortunately, those last two caps. are also bad, so the bridge sits unused. Perhaps someday I will develop the nerve to finally fix it completely.

So there it is. If you want a very good instrument, and are up to the challenge of doing the necessary repairs, you may want to look this one over. They don't come up too often.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 4:12 am 
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Steel yourself, take lots of digital photos, tag the disconnected wires with masking tape labels, and have at it! You can get it working again!

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 4:17 am 
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Thanks for the overview, J.D.

I almost PM'd you earlier to ask your opinion on these if you had experience with them, as I knew you were a fan of the brand.

I am always impressed by the C-B equipment I see. I'm sorely tempted by this one as I told the OP, but have too many projects waiting already.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 4:30 am 
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stevebyan wrote:
Steel yourself, take lots of digital photos, tag the disconnected wires with masking tape labels, and have at it! You can get it working again!


I know Steve, I really need to get that bridge working again. I really enjoyed the little bugger when I first got it.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 4:45 am 
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WoodchuckTN wrote:
Thanks for the overview, J.D.

I almost PM'd you earlier to ask your opinion on these if you had experience with them, as I knew you were a fan of the brand.

I am always impressed by the C-B equipment I see. I'm sorely tempted by this one as I told the OP, but have too many projects waiting already.


You may wish to reconsider. All things being equal, the price this feller wants isn't out of line. I think I paid near that when I got mine.

C-B gear was really pretty good. Their signal generators, at least that I own, use fundamentals for all frequencies, and were pretty well shielded. They a couple of pretty nifty 'scopes, and they had some very good audio oscillators. The last one produced looked very much like the HP version of the same era.

When it came to capacitor checkers/bridges, they got real good. I have several iterations from over a period of 25 years, and each successive new model was better built, and had more features. I have one from shortly after the war, and would love to get it working, but finding any paper or schematics has been impossible. Given the complexity of the circuit, number of tubes, and features, I would speculate that C-B used the experience gained from that bridge to create the ZM-11. Other than it has not been repaired yet, the thing is big and heavy. So given a choice, the ZM-11 will get repaired first.

Another thing I forgot to mention about the ZM-11, is that, unlike the vast majority of bridges, it has an actual 1 Khz oscillator for testing. Most checkers used the line freq. of 60 Hz.

Further, I just went to the bench and measured my bridge. The unit is approximately 8 1/2" square, and 6 1/2" tall. And this is with the cover on. The cover is perhaps 1 3/4" deep to allow room for the manual, test leads, and power cord. To give an idea of how cramped the innards are, there are 8 tubes stuffed into that little box, along with the power trans., and all those switches, dials, etc.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 5:03 am 
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I just hope somebody snatches it up before my resolve weakens any further (and you're not helping :lol:) or before he lowers the price.

As somebody said, we've already established what I am (an addict) we're just haggling price. :D

From what I've read, restoring my TS-303 B/G Chanalyst this winter is going to be a challenge about like the one you describe with the 712. Hopefully not quite that bad.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 10:01 pm 
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I've got one of these for each bench myself and agree they are
very nice and useful instruments. Well designed and built.
$75 is a decent price for a multi-use instrument like this that
will last a very long time. And they are getting hard to find.
Steve

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Tue 05, 2016 10:52 pm 
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If I didn't already have five capacitance bridges ...

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Wed 06, 2016 10:51 pm 
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I have one that I got at the last Hamvention for $20 . It needed only 1 cap and works well, but I agree $75 is not out of line . I've taken 2 of these apart and they are not the easiest, but do-able. as I remember, there were only a few wires that needed to be un-soldered to do it.
Good luck,

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Sat 16, 2016 10:08 pm 
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Received the Clough-Brengle 712 (aka: ZM-11/U) bridge today, and it came in fine shape. The seller, one teotwaki, packed the unit extremely well. Thanks teotwaki!

The outside of the case has the normal dings, scratches, sticker glue, etc., but is in overall good condition. With the lid off, the panel looks almost new, and is in much better condition than my old ZM-11. Somewhere along the line, the power cord has been replaced, and the plug is a thoroughly modern three-prong affair.

When I get the chance, possibly tonight, I will check the operation of the bridge. Since the 712 and ZM-11 are identical bridges, with the difference being in the case and knobs, my military manual should provide the correct procedures.

Did not intend on purchasing this thing, but I am glad I did. The ZM-11 made by Clough-Brengle rarely comes up for sale, and the civilian 712 version almost not at all. In point of fact, I do not recall another 712 bridge up for sale since I began collecting C-B gear over 30 years ago. I can now brag that I have BOTH versions.

An interesting side note: I am sure we have all noted differing "smells" or odors from radios and test gear. There is of course the rank stench of a unit that has had the magic smoke let out of it. And also the malodorous emanations from the occasional mouse house. But some times one runs across a piece of electronic equipment that has a unique "electrical" odor. Not a bad odor, just different, and sometimes sort of pleasant. I have noted this "smell" in some of the test gear I have. The qualities are not always the same, and can vary in pungency.

My ZM-11 has it's own unique "smell". I noted it the first time I opened the case years ago. It still has that odor, and it is one that, for me, reminds me of some tech decades ago doing testing on a long forgotten piece of communications equipment. I do not recall anything else that is exactly like it.

This 712 bridge has that exact same smell, and it hit me pretty forcefully when I opened the lid. Again, not a bad odor, just different. It must be coming from a certain component deep within the circuitry, and one shared by both units, and from the same manufacturer. Either way, I am glad it is there. Kind of like an old friend.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Sun 17, 2016 4:15 am 
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Greetings to J.D. and the Forum:

The smell associated with older electronics gear, especially military stuff is often the fungicide treatment. Most vintage equipment made under military contract was fungicide treated for use in unforgiving climates. I'm not sure if Clough-Brengle treated their civilian market gear in the same way, but it is not beyond the realm of possibility.

Regards,
Jim T.
KB6GM

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Sun 17, 2016 1:28 pm 
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Jthorusen wrote:
Greetings to J.D. and the Forum:

The smell associated with older electronics gear, especially military stuff is often the fungicide treatment. Most vintage equipment made under military contract was fungicide treated for use in unforgiving climates. I'm not sure if Clough-Brengle treated their civilian market gear in the same way, but it is not beyond the realm of possibility.

Regards,
Jim T.
KB6GM


I had forgotten about the fungicide Jim, and it certainly may have something to do with the odor. That said, the only other true military test equipment I have from the period in which the 712 and ZM-11 were made, is a TS-505 multimeter.

The 505 also has a unique odor, but is not quite the same as the 2 bridges. Who knows what causes that odor. Thankfully, at least it isn't repugnant.

One of my Clough-Brengle 'scopes, a 127 Graphoscope, I firmly believe was pulled from a chicken coop. Came complete with straw and feathers. Thankfully I found no mites or other vermin. Didn't smell too well when I first warmed it up, but is OK now.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Jan Wed 20, 2016 6:00 pm 
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Operated the bridge a couple days ago, and it seems to work pretty well. Seems to get somewhat low readings on a few of the capacitors I checked with it, but it balances OK. The one function that does not work, is the capacitor quality test. In this test, you must first zero the meter out using a specified procedure. Unfortunately, the meter cannot be zeroed. Checking the manual, it can be something as simple as a faulty tube in the oscillator circuit, the voltmeter tube, or something amiss with the associated components. Since the meter functions satisfactorily in other test modes, it appears to be something in the oscillator circuitry.

An examination of the instrument reveals pretty significant differences between it, and it's military brethren.

For one thing, the case is not "militarized", in that there are only two snaps to hold the lid on versus the four on the ZM-11. The 712 also lacks the two screws in the bottom of the case to better hold the chassis that the ZM-11 has. There are no strap "D" rings on the 712, and the inside of the lid doesn't have provision for lead stowage, or manual placement.

Panel faces are exactly the same, except that the 712 uses phenolic knobs for the "D" and "Q" controls versus the nickel plated ones on the ZM-11. And while both units employ aluminum as the case and chassis material, the ZM-11 is virtually non-magnetic due to the almost total lack of ferrous material. The handle, many (most) screws, posts, stand-offs, etc. on the ZM-11 are brass/aluminum, or SS.

Internally, the circuitry and placement of components appears to be the same. However, the 712 use several tubular electrolytic capacitors in place of the bath-tub types in the 11. The few bath-tubs in the 712 are also riveted in, whereas the 11 uses no riveting in the construction anywhere I have seen.

Lastly, there is shielding within the 11 that the 712 lacks. This is not surprising since both units employ two oscillators, one producing 10.75 MHz., the other 1 kHz. The shielding in the ZM-11 is no doubt installed to try to suppress radio freq. emanations that could be detected by an enemy. The manual (military version) I have a copy of, cautions against operation of the oscillator functions during periods of radio silence.

When posting about this instrument in the Classified section of ARF, a discrepancy was noted between the weight of the 712 vs. the ZM-11. The seller stated the 712 was approximately 10 pounds, whereas the manual for the 11 lists that unit as being around 15. It is readily apparent why the 712 is much lighter (by almost 5 lb.). The extra screws, shielding, brass, and heavier components no doubt account for this.

Am quite pleased to be able to add this interesting instrument to my collection. Not only do I own another Clough-Brengle piece of gear, but there is a practical aspect to having it. I can now more easily "un-sandwich" the ZM-11 (to re-pot the faulty 'lytics buried deep inside) without much concern as to where all the disconnected wires need to be soldered back on. All I need to do, is lay both units side-by-side when I put the 11 back together.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Feb Wed 17, 2016 8:10 pm 
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JD,

In case you need another ZM 11/U:

http://swap.qth.com/view_ad.php?counter=1248760

:mrgreen: :mrgreen:

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Feb Wed 17, 2016 8:47 pm 
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Thanks for the detailed description of this unit!

I will be keeping my eyes open for one in the future.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Feb Thu 18, 2016 1:27 am 
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The fancy fungicide smells just like old-school liquid Lysol.

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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Sep Sun 26, 2021 4:39 am 
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This thread is more than 5 years old but I am stirring it up because today I picked up one of these at a swap meet. Mine, however, is a ZM-11A/U and I don't know what the differences might be. And Google doesn't recognize the A model.

After fooling with it for a while, I don't find it so user friendly. One thing, the capacitor leakage test is supposed to go from 1 - 500 Volts but mine goes from about 10 to over 700 Volts. Since I already have a couple of impedance bridges, I will probably end up swapping this one for something else.

Anyone familiar with this, and maybe could point me toward the proper manual?


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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Sep Sun 26, 2021 5:03 am 
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goto:
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 Post subject: Re: ZM 11/U Bridge in the Ads
PostPosted: Sep Sun 26, 2021 9:00 pm 
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Count yourself lucky you have an A version ZM-11. The original one has carbon (drifted) resistors instead of film, and the capacitors are likely still good and not leaking like the wax covered ones in the earlier model. You can get over 500V no load; however it will damage the circuitry doing it. Usually getting to 500V is a problem with the older one, as the bathtub cap on top tied to the voltage control is leaking.

The A model is essentially the same as the original one, with better quality components. The manual for it is Navships 91704A (original manual is Navships 91704).

https://www.navy-radio.com/manuals-test.htm


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