75S-1/2/3 Service information

75S-1/2/3 Service information ... 75S-1/2/3 Service ...
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The following items are intended to be used as aids in trouble shooting

6BF5 tube selection. To eliminate the necessity for tube selection to meet distortion and gain specifications, the fixed bias on audio output tube V8 has been reduced by changing resistor R54 from 27k ohms to 22k ohms 1/2 watt.

To eliminate the necessity for tube selection of the 6U8 in the high frequency crystal oscillator to meet output voltage specifications, the feed back capacitor C79 has been changed from 100pf 500V to 47pf 10%, 500V.

Capacitor C120, 15pf (CPN 912-2760-000), has been added in parallel with C75. This will eliminate the possibility of misalignment of the crystal oscillator on 21 Mhz.

If the BFO crystal oscillator fails, check for a shorted mica capacitor.(C93 or C94).

Any lack of sensitivity between upper and lower sideband will most likley be the BFO crystal being off frequency.

75S3C AGC

Just a note on my experiments with the RCVR AGC. I got a note in from Topi, OH7RN, about changing out the value of R88 which is a 680K resistor in the AGC circuit. He said moving the value down to about 68K would slow things down. Boy did it.
It made the slow position about as slow as my Drake R4C in the slow position or about the same as my Icom 751A in slow. But the fast position is way too slow. So I experimented with several values of caps and resistors and I came up with a very nice combo that makes the slow almost as slow as the R4C and the fast about the same as the medium position in the R4C.
I use the R4C as my reference because it is one of the best overall RCVRS I own. I use the medium AGC postion for most CW work up to about 30 WPM and it is very nice.
The R4C fast AGC is for me too fast and it pumps a little. The medium is very smooth on the decay time and pumps ever so slightly on attack. Slow is very slow and smooth, without pumping at all.
I changed C50 in the 75S3C which was a .1uf over to a .47uf. Then I added a 220K in parallel with R88, which is the 680K resistor.
This gives the value of about 190K. It was easier to tack in various values in parallel then to clip and add. So this is the reason for the odd value at net.
What I now have is a slow position almost as slow as the R4C in slow, and a fast speed about the same as the R4C in medium. However I will say the action is MUCH better in the Collins than the Drake. There is no indication at all of pumping and the decay is so smooth you have to look at the meter to see the difference between the fast and slow.
There is a difference of about 3 to 4 times in speed on decay between the two positions, much nicer than the stock setup where I could not see the difference between the two speeds. You cannot hear a difference, it is that nice.

All of the above is just what I like to work with. You may prefer different values. Just remember, the lower the value of R88, the slower it all gets. If you like it all a little faster, find a value a little higher than 200K and less than 680K. C50 seems to work at its best around .47uf. It is smoother with the maximum difference between speeds with this value. I tried from .1 up to 1 uf. The middle was best.
The whole operation should not take more than 30 mins. to install. Just clip out C50 an replace. Then add another 220k over R88.

73, Bruce K1XR

Topi Partanen wrote:

Bruce,

If you study the circuit diagram more carefully you find that C50 (0.1uF) is always connected from to agc line to ground (wiper of RF gain pot) so it is used when the switch is at 'agc fast' position. If the switch is at 'agc slow' position C137 (0.47 uF) is connected in parallel with C50 and the time constant is increased six times. C153 (0.01 uF) does nothing for the agc time constant, it is merely a minimum loading capacitance that is always connected from agc line to ground even it is in series with C50.

C50 or C50+C137 are always charged via R88 (680 k) that prevents short peaks to hang the agc voltage to block the receiver. R88 is however too high in value so that it prevents C50 and C137 to charge even at ssb or cw signals so that agc time constant seems always to be fast. Just replace R88 with 68 k to 'enable' slow agc function. 68 k is high enough to prevent agc to hang too badly during noise peaks.

I have done the same modification for my 75A-4, KWM-2A and 51S-1 and listening ssb or cw signals changes much more pleasant. KWM-2A uses same 680 kohm resistor that should be changed to 68k.

75S-3 S Meter Zero Problem:

Interesting thread about the S Meter Zero wandering. This is an aggravating situation because it certainly would be nice if the S Meter would stay zeroed!

My receiver is a 75S-3C and its S Meter wanders. I have to continually adjust the zero potentiometer.

John K7JB

Reversing the leads to the outer connections of the zero adjust pot moves the zero seting to another area of the pot. Thus old pots that are somewhat erratic with heating and adjustment are now being operated on a "virgin" region.

Ian, K6SDE

Service Bulletins

75S-1 Receiver PUB NO. NOTE DATE DESCRIPTION SIL 2-75 10/15/75 Extended Operating Range SIL 0-60 2/3/60 VFO Oscillator Drift 75S-2 Receiver PUB NO. NOTE DATE DESCRIPTION SIL 2-75 10/15/75 Extended Operating Range 75S-3 Receiver PUB NO. NOTE DATE DESCRIPTION SB 1 1/11/62 Revised 3/9/62 To Eliminate BFO Parasitic Oscillations SB 2 5/10/62 To Minimize Background Hum Level SIL 2-75 10/15/75 Extended Operating Range 75S-3A Receiver PUB NO. NOTE DATE DESCRIPTION SB 1 Not Issued SB 2 5/10/62 To Minimize Background Hum Level SIL 2-75 10/15/75 Extended Operating Range 75S-3B Receiver PUB NO. NOTE DATE DESCRIPTION SB 1 2/27/67 Revised 5/11/67 9/29/67 Improved Performance SB 2 3/1/68 Revised 8/15/68 Second Mixer Plate Circuit Improvement SB 3 6/15/76 Revised 4/15/73 8/1/75 10/15/75 Convert 75S-3B to 75S-3C SIL 2-75 10/15/75 Extended Operating Range 75S-3C Receiver PUB NO. NOTE DATE DESCRIPTION SB 1 2/27/67 Revised 5/11/67 9/29/67 Improved Performance SB 2 3/1/68 Revised 8/15/68 Second Mixer Plate Circuit Improvement SIL 2-75 10/15/75 Extended Operating Range

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