[Collins] 75S3 S-Meter Problem

[Dr. Gerald N. Johnson]

In my schematics the S-meter zero pot for the 75S-1 and 75S-3 is R13, 
250 ohms. Has a 68 ohm resistor R21 to the cathode pin of V5. The other 
end of the meter is connected to a voltage divider made of R16, 47K and 
R17 100 ohms. The circuit is different in the 75S-3B.

Carbon composition resistors are known to drift high in value with age, 
heat, and humidity so long as they are not IRC. IRC resistors were not 
allowed at Collins because they actually are metal or carbon film and 
don't burn open as fuses as reliably as pure carbon composition 
resistors and because over heating tends to char the insulation cast 
over the glass tube with the film they tend to go down with heat and age 
and to have different RF characteristics too. IRC resistors are 
detectable by the lengthwise mold line on the resistor and sometimes 
slightly rounded ends.

The pot has been known to go bad. Moving its range with a series 
resistor change can work, shunting with a 470 ohm wiper to ground will 
also change its position. It shouldn't be hard to find a suitable pot to 
replace it at Mouser or Digi-Key or Nebraska Surplus. It may be fixable 
with some magic fluids made for pots from Cramolin makers of DeOxit.

The S-meter uses the varying DC current of the AVC controlled stage as a 
DC voltmeter for AGC voltage. The RF gain in these receivers applies 
negative voltage to the AVC line to lower the gain manually. Many other 
receivers use a pot in the IF and RF stage cathodes to do that.

One other way the s-meter will kick backwards is when an AVC controlled 
tube develops grid emission from cathode material splattering onto the 
control grid. Then the reverse grid current makes the grid go positive 
and that is opposite to the normal AVC voltage. There are specialized 
tube testers for grid emission. When an IF tube is replaced, the 
receiver gain may need adjusting as well as the S-meter zero and the 
peaking of the adjacent IF transformers. Usually if its grid emission 
the receiver will power up with the S-meter zeroed and in a minute or 
less of operation the S-meter will drift negative as the grid heats 
slower than the cathode, purely by the heat radiated from the cathode 
glowing dull red.

I know my 75S-3B had the grid emission problem some 40 years ago and I 
think I've also replaced the S-meter zero pot. The S-line receivers get 
most of their gain in the two 455 kHz IF stages and so work those 6BA6 
fairly hard. These remote cutoff pentodes have more gain the greater the 
plate current.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 1/1/2017 5:42 PM, Al Parker wrote:
> Hi Bill,
> I don't have my skem close by, but I've just been thru this on 2
> 75S-1's. There is a zero adj. on the top of the chassis, with a series
> resistor to gnd. It's in the cathode of the S-meter tube ckt. In both my
> cases the pot was scratchy, and the series resistor had drifted high. I
> put in some lower res. resistors, and got the pot so at least it was in
> a better spot.
> I'm sure someone else here will come up with the res. nrs, but you
> should be able to find them from my description.
> 73, cu in the next CX,
>
> Al, W8UT
> www.boatanchors.org
> www.hammarlund.info
>
> "There is nothing -- absolutely nothing -- half so much
> worth doing as simply messing about in boats"
> Ratty, to Mole
>
> On 1/1/2017 5:39 PM, Bill Stewart wrote:
>> The s-meter on my 75S3 worked ok after I did some minor repairs
>> on the rcvr...that's been several years ago. But now, set up for CW
>> and RF gain at max.(full CW position), the meter pegs to the left pretty
>> hard and I stopped using the rcvr fearing damage to the meter. The RF
>> gain
>> has to be turned back CCW a good bit before the meter will come back,
>> off the LH peg, to the zero mark. Now I wanna get that rcvr back on
>> line. Before I go into it, I wonder if anyone can point me toward a
>> possible fix..could it be just an internal adjustment? Tnx for any
>> guidance.
>> 73 and HNY de Bill K4JYS
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