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Older CB Power Mics Similar to Cobra and RS

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Dawn

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Apr 5, 2003
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284
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Pinecrest,Fl
I recently did two repairs on old RS power mics that took the infamous 7V battery. The board inside is the same that's found in cobra,sears,jcp, and hygain mics I've been in so far with some difference in components. Both of these were dead. I haven't seen problems in other vendors mics and the circuit differs slightly form manufacturer especially towards the later 70's.

I've found leakage in the electrolytics in some, but still not enough to affect the operation. In these mics, they used 2 sc536 transistors. They ohm out ok, but apparently have high leakage. Replacing both in one shot with an equivalent is a good option althogh it's the first transistor that's usually the cultprit. Most all early power mics made japan used this same circuit and board, and I suspect that Merry corporatation made them all for the companies. This must have been a common earlier failure. Several of the mics have been in essentially rewired to straight dynamic output and bypass the board. Another thing I've seen is without exception, the lower post that holds the switch is sheared off from stress, but usually doesn't affect the mic due to a case screw holding it together. Superglue solves this.

Shame. These were great mics of their day and probably today using a very high quality dynamic cartridge then found in today's mics with a very heavy duty cartridge typically found in japanese recording mics of the day. The RS mics had a particularly very nice ergonomic factor then their original mics or their competitors squarish form factor. The odd mic used in RS's TRC-46 and TRC-48 is essentially the same mic but wired as passive and the pot controlling the volume control.

The schematics published never seem to match the actual components of the mics and they usually show 2sc458 tranistors and the 'lytic cap values never match the what's in the mics I've been into.

If these mics were used with later Alkaline equivelent batteries, it's best to clean them with some vinegar. Your're never going to get a reliable contact without regular cleaning and cocntact enahancemt. It would be easy to replace the contacts or possibly dremmel them down and remove the chrome and use a nickle plating pen kit. I haven't tried that. Batteries are available on the net for less then $3 in alaline form.The original TR175 Mercury has been banned for years, but alkaline replacements works perfectly.

I've never ran across a dead turner board, but these seem pretty common in Japanese/Taiwan mics
I've been into.
 

RC286

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Winnipeg MB
I run one of these in my mobile, wired 4pin on my DX-959. Picked it up NOS out of the classifieds. It is a fantastic mic.

I run mine off of 5x 1.5v button cells (type 364 IIRC) Lasts me months on a 5 pack of the dollar store cells
that run me a little over $1. Just stack 5 together wrap with tape to make a battery and pop in place.
Fits perfectly in place of the old TR175.

I could make some more comments about this mike, however you seem to have covered most of it.

Great mike.
 

Dawn

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Apr 5, 2003
Messages
284
Location
Pinecrest,Fl
Something else I forgot to mention is these mics particularly in the RS brand and also the mic that was used in the TRC 46/48 usually will have one of the posts that support the switch sheared off on the PTT side. It seems to be a structural design failure the way the switch is set at an angle. The only thing that's supporting the switch is the screw going through it. That doesn't really present any inital problems except a little play in the switch which may cause eventual failure or break the PTT plunger. Crazy glue seems to fix that nicely and seems to hold up. A thin piece of brass tubing from a hobby store can be slipped over the remaining piece and the hole in the switch can be widened with a drill bit and solve the problem once and for all.

The plastic on these mics is easily scuffed, you can take a tired, ugly old one and really spiff it up using some Nu-Vinyl from an auto store. A little tripoli, followed by rouge can buff out bad scuffs before using the treatment. Some plastics respond very well to the treatment. This mic does. It's a really handsome and great feeling mic, so it's well worth the effort. If your lady has one of those two sided jewelry cleaning cloths, you might be able to buff out the scuffs with one of those with a little effort. Use the white side first and then the red last.

Regarding batteries. This mic will run at 3V with less output. It's the same circuit used in the Cobra/Sears/and Radio Shack base power mics. (The black one with the tilting head and slide control on the support shaft). Those mics use two AA cells and it's doubtful that you need to run this mic all the way up. RS added a 15K resistor in series with the gain pot /output on the hand mics to further attenuate the output. None of the other brands I know of except Sears did that using the same circuit, so the Hitachi OEM'd radios probably have more gain in the mic stage. Removing it with with a lower voltage will probably bring the output up. Some other combination of button cells or other battery shimmed up with washers or 7/16" OD nut can be used like RC286 mentions and don't need to total 7V.
 
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