[Collins] 75A-3 Collins Recv'r

Fri Dec 30 11:35:11 EST 2011

Sounds like the PTO error is a half turn of the shaft. The span error is 
not unusual from that PTO at this age. Many versions had an adjustment, 
but its usually out of range and needs turns taken off that span 
adjustment coil. Be sure its PTO and not a replacement crystal that is 
on the wrong frequency. Being off on all bands would point to the PTO.

The high band crystals are overtone and the low band are fundamental and 
in the same oscillator overtone crystals tend to have lower output. Each 
crystal has its own resonant circuit to peak it up the output. As Al 
said section V.6 in the manual. There is a compromise adjustment to set 
all crystals so there's no change in dial setting to match the 
calibrator, at the cost of uniformity in mixer drive. A weak oscillator 
tube will contribute to the different mixer drive voltages, as will long 
leads on C36, the plate supply bypass to the oscillator. Adding a 1000 
pf ceramic in parallel with C36 may have an effect. At least in the 
manual I have it says that capacitor is ceramic. Many a radio of that 
vintage used a paper capacitor there and it was inductive at 33 MHz and 
so was a poor bypass. Crystals also age at different rates and have been 
known to quit from contamination or corrosion when the sealing wasn't 
perfect or the atmosphere left in the crystal case wasn't perfect. The 
since the quartz physically vibrates at the oscillation frequency, 
imperfect chunks of quartz can develop fatigue and so exhibit a lower Q.

In the tunable IF, remember the capacitor tunes its entire range in 1/2 
turn while the slug tuned coil tunes in 15 or 20 turn and probably has 
20% change for that many turns while the trimmer capacitor tunes 8 to 80 
or 5 to 25 pf in that 1/2 turn. So by turns the coil has a fine vernier 
and the capacitor does not. The best check of the effect of frequency on 
the IF bandpass would be with a sweep generator, detector and scope and 
ideally the bandwidth would remain constant from end to end, but in the 
practical world the bandwidth probably is a bit greater at the LF end of 
the tuning range simply because the loaded Q is lower from the circuit 
resistive load being constant, while the reactance of the tuning 
capacitor is higher.

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

On 12/30/2011 8:35 AM, Al Parker wrote:
> Hi  Joe,
> 	Your PTO can be slipped to center it's error.  To center it, set the
> readout in the center, then loosen the PTO coupling's shaft setscrew and
> move the PTO itself to zerobeat the proper calibrator signal.  But, it
> is still 11kc off in "span", and you'll be barely able to adjust the
> indicator enough at the ends.  The PTO should be removed and internally
> "tweaked" to bring the end to end span back to zero error.  The
> procedure isn't difficult, probably getting it removed and back in is
> the hardest part.  You can see some of the work I've done on them at
> <http://www.boatanchors.org/75A2.htm>   Similar work is shown in the
> R-388 pages.
> 	The higher band crystals will have lower output than the lower.  The
> output is adjustable by band, see sec. V-6 in the manual to optimize
> things.  Basically, as long as they're oscillating, you are in business,
> a more negative reading might improve things slightly.
> 	I think what you're seeing for the var. IF is normal.
> 	It sounds like you've been thru much of what needs to be done, and you
> have it working nicely.
>
> 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 12/29/2011 11:28 PM, Joe Walden wrote:
>> Hello,From Joe w5jdy in Okla...  The last time I posted here I was looking
>> for help with a 75A-3.  This time I have a couple of questions concerning
>> the same receiver...   Can anybody help me with these problems??   The PTO
>> unit is 42kc low on the low end and 53kc low in the high end.....  Is there
>> a fix??   Also,when measuring the crystal activity with a vtvm thru a 1 meg
>> resistor on pin 2 of V2,there is a significant change in the dc readings..
>> The 10 meter crystal shows less than -1/2 volt,the 11 meter crystal -1 volt
>> and the other crystals range -2.5  volts or more..  Is this normal or should
>> all the crystals show approximately the same negative volts??  (The manual
>> shows -4 volts..)    One last thing,the 2nd tunable IF,3.455mc to
>> 5.455mc,tunes but is is very broad tuning there is a nice peak on the
>> capacitor end but on the inductor end the tuning is very broad..   Is this
>> normal..  I hope these questions are not to dumb...   Joe W.
>> By The Way,the receiver work wells with good sensitivity,good volume and
>> good selectivity...
>> Just thought it might work better with higher crystal activity??
>>
>> ______________________________________________________________
>> Collins mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/collins
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:Collins at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
> ______________________________________________________________
> Collins mailing list
> Home: http://mailman.qth.net/mailman/listinfo/collins
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Collins at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>