McMurdo Silver 903(A)

[WA3VJB]

Picked up what I thought would be a good project box at the hamfest last weekend.
Then I researched "Silver" and found out it was a brand of McMurdo Silver.

The piece I've got is the 903A, the rectifier module that connects by cable to a larger chassis in the 903 model.

I've found plenty on the 903 itself, and too bad this got separated from the main unit.

Is it useful for anything on its own?

If not, it shall become the project box I intended.

[k4kyv]

Interesting.

I can't help you with any info on that box, but here is my story.

After studying the Micromatch article in the April, 1947 QST, I decided to build one.  The Micromatch is a reflected power meter that works with balanced line.  The one in the a article is designed for 3-30 mc/s at 70-300 ohms, but I figured I could make one that worked at least through 1.8-7 mc/s, up to 450 ohms. I was looking for a box to build it in, and picked through a pile of small metal boxes in my junk collection.  I found what appeared to be perfect for the job, and lo and behold, it was a McMurdo-Silver Micromatch!  No idea when I acquired it or where I found it. The meter was missing, but I had a suitable replacement. Then I found that the 1N34A diode was bad.  I replaced it with an ECG-109, which is supposed to be better than than the original 1N34A.  Turns out the bank of carbon comp resistors that make up the 1-ohm resistor in the circuit had drifted in value, up to about 2 ohms.  Using the formula given in the Micromatch article in QST, I replaced the fixed mica capacitor with a suitable value to go with the drifted resistor, to allow it to work up to 450 ohms.  I also replaced the original trimmer cap with a better quality one with wider spacing.

The thing works very well on 160-40, and nulls down to near zero with a suitable non-inductive resistor.  Working with my 440 ohm feed line feeding a balanced tuner, I can null it down absolutely to zero.

Sorry about the fuzzy picture, but that was the best close-up focus I could achieve with my cheap camera in the ambient lighting available when I took the shot.

Notice the similarity in style of the two boxes.

[WA3VJB]

That is interesting, thanks for sharing the story. Did you remember how the box got in your junk pile ?

Must be the shape of the boxes that catches our eyes, plus the black wrinkle (nod to Derb).

This poor thing was $1.00, and I thought it was a piece of early '60s Japanese tin because of the cheesy graphics "SILVER."

Didn't even look at it for days, it just sat in the garage with the usual hamfest box marked "educational materials" that I use as a passport into the house here.

Well, if it's not doing anything for anyone I'll rip the front sheet metal off of there and see what's inside, and cut a new piece for the front for the project I've got in mind. 

Do you think the meter might be helpful in setting yours right ?

[k4kyv]

I think the original meter had a large square face, based on the markings on the panel.  I replaced it with a standard size round meter, about 3 1/2" diameter. Mine has an aluminium front panel, and the box is steel, painted grey (smooth, not wrinkle I believe - it's out in the shack and I am in the house).  I don't think your meter would fit mine any better than what I already have, which works very nicely for the job.

I would guess its vintage to be late 40s to about 1950, given that the QST article was in 1947.  The Micromatch was the first ham reflected power meter, and it's what introduced hams to the concept of SWR. To measure SWR you have to reverse the input and output connections, set the sensitivity to full scale, then swap back the input/output terminals to read reflected power without changing the sensitivity.  There is a chart in the article to correlate reading on the 0-1 MA meter to SWR.  A lot of trouble; I think most hams just tuned for minimum reflected power.  Not long after that article, all the reflected power and SWR meters in the ham publications seemed to be designed for 50 ohms unbalanced. Assuming the commercial manufacturers followed suit, that would most likely put it in the late 40s.  

Because of the similarity in the design of the front panels of both boxes, I'd put yours about the same era.  McMurdo-Silver started building broadcast receivers in 1933. http://www.antiqueradios.org/gazette/silver1.htm  Not sure if they are still in business, but I haven't heard of anything they produced post WW2 other than our two boxes.

I'd open the thing up and see what is inside, and if it would be worth keeping as is. I don't have any idea where I got mine or how long ago, or if I paid anything for it.  It may have been included in one of the piles of junk that people have given me over the years instead of throwing out. I just found it on a shelf with other metal boxes of various shapes and descriptions. If I paid money for it, it wasn't likely that I would have given much more than a quarter. But it turned out to be unexpectedly valuable when I had a use for it.

[The Slab Bacon]

they also made test equipment. I had a really kool McMurdo Silver VTVM years ago, It was a big box with a built-in /rf probe. (Actually on it's own umbilical cord with a 6AL5 inside of the probe.) It is one of the many things I once had and wish I still had.

I dont know much else about their test equipment, but their radio's were considered "high end" stuff.

[Don, W2DL]

McMurdo-Silver also made a whole series of amateur radio receivers and transmitters just after WW II, and although none of them were in any way exceptional performers (to say the least) they did allow a 15 yr old fledgling ham such as me to afford a "commercial" rig while still in high school. Some of their products post WW-II until maybe 1952 were the #701  HF xmtr, used a 6AQ5 tri-tet oscillator driving an 807 final. Modulator was a pair of P-P 6AQ5's, directly driven by a carbon mike via a P-P mike/grid xfmr. If you shouted it would do almost 100% mod !  The same modulator was also used in their #700 2 and 1 1/4 m xmtr, which had a 6AQ5 tri-tet osc, 2 6C4 doublers and an 832 P-P final. Produced around 20 w out on a good day. They also made 4 receivers: #801 was a 6BA6 RF amp, a 6AU6 regenerative detector, a 6C4 audio amp and a 6AQ5 audio out. Much better was the #802 "single signal superhet" which used a 6BE6 converter/osc, a 6BA6 regenerative IF stage, a 6J6 det., another 6J6 BFO,ANL and a 6AK6 audio output. The #801B battery operated rcvr was very similar to the AC powered #801.The #800 2 + 1 1/4 m rcvr used a 6AK5 RF, 6C4 super-regen detector, a 6AU6 first audio amp and a 6AK6 audio output.  It sold for $36.95 . Before you laugh too hard at this unit, remember the National 1-10 unit of the day had  the same functional stage lineup (RF-super regen-audio-audio out) and cost $ 67.50.  All receivers and xmtrs  used plug-in coils (sold separately, $ 1.00 each!) And, they made a #907 keying monitor that took any RF input from a transmitter at any freq and made a keyed tone from it. The previously mentioned VTVM with the 6AL5 active RF probe was very good up to around 150 Mhz, it was actually a great unit for around $24 !. I owned (and sort of used) the #700 2 m xmtr and the 701 HF xmtr, both worked once one got used to their tuning method, etc. More on that if anyone needs to know. Bottom line is, if you ever see any of these at a hamfest pick one up - they were really amazing for what they did, each in the exact same cabinet 10" x 5" x 5 1/4". Oh, also, each needed an external power supply, Where else could a 15 year old get a xmtr and/or receiver that actually worked and looked good for $36, or in kit form for $25 in 1947? Harvey-Wells in their first TBS-50 used a very similar P-P tubes driven directly by a carbon mic., McMurdo-Silver was ahead of the times in some respects.

What a deal!

Don, W2DL