[TMC] RF amp meters for a TAC-1

[John Vendely]

Hi Ed,

Unfortunately, I have no spare meters, so I can't help you out there.  
Finding original meters will be tough, but there are some good 
replacement options.  Are your meters missing, or simply defective?  I 
would also recommend checking the external RF thermocouples.  Blown 
thermocouples are very common in these tuners due to gross abuse, but 
they are sometimes repairable.

My TAC-1 had fallen into the hands of barbarians, and although it was in 
physically excellent condition in every other respect, both RF 
thermocouples were blown, and one meter had been  replaced with an AC 
ammeter, a completely ridiculous substitute.  I scrounged up a Triplett 
meter of the same style (commonly available with various movements), and 
made a new dial plate by duplicating the original meter's dial plate on 
an ordinary copy machine using heavy stock paper.  I trimmed the copied 
dial to slightly oversize, glued it in position on the substitute meter 
face, then carefully scraped around the edges to eliminate the excess 
paper. Believe it or not, this extremely crude expedient actually worked 
out beautifully, and you can't distinguish the replacement meter from 
the original.

Fortunately, the external thermocouples can sometimes be repaired with 
some tedious effort under a microscope.  Each end of the thermocouple 
wire is bonded to two terminal "bars" connected to the external screw 
terminals.  The thermocouple wires have some excess length, and if 
they're burnt out at a point near one of the terminal bars, they can be 
detached, repositioned, and silver-soldered to the terminal bars.  You 
gotta use a stereoscopic microscope, and work carefully with tweezers 
(the type used for surface mount component work).

I figured that after this operation, the thermocouples would never work 
worth a damn again, but I was pleasantly surprised to find this wasn't 
true.  The original meter could be easily calibrated against an accurate 
true RMS AC ammeter using a load resistor and variac at 60 cps, and 
showed good accuracy across the scale.  The replacement meter was then 
calibrated against the original.  One last check was done at 7 Mc with 
the tuner adjusted into a large 600 ohm load resistor, and driving it at 
1 kW from a SBT-1K transmitter.

I'd love to see a pic of your DDR-551.  I have one of those as well, 
probably not too many of them were made.  I'm looking for the IF, audio, 
and AFC units to complete a DDR-5, if you (or anyone else) has any for 
sale or trade...

73,

John K9WT


On 4/28/2014 7:03 PM, edmund weber wrote:
>   Greetings to the Forum,
>       I am posting this in hopes of finding the RF amp meters that are used in the TAC-1 antenna tuner...it's a long shot, but I know there's a pair
> out there somewhere! My DDR-551 is almost finished, needing only the AF and IF filter units, and the AC control panel.
>       I have several  TMC items that are surplus to my needs for sale or trade as well.
>   Thanks,
>   Ed, KA6ZMI
>   Near San Francisco
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