75A-2 receiver dead on 20 meters and higher frequencies

[WB6VHE]

Greetings all!

I am now working on a Collins 75A-2, the later version with the various modifications.
After alignment (IF, etc.) I cannot get a signal through on 14 MHz and above.  I bypassed
the rf amplifier and injected a 14 MHz signal directly into the grid of the first mixer
stage: zilch!  Any ideas, or perhaps someone has had this same problem?

Regards,

[WA2TTP Steve]

I don't have an A2 but do have an A4. I would be looking at the first LO tube, maybe it's weak and cant osc at the higher frequencies. If you have a second general coverage receiver you can listed for the LO and see if it's running.

Good Luck,
Steve
WA2TTP

[w3jn]

That's the version with the 6BA7 first mixer, and 6DC6 first RF (instead of the 6AK5), correct?

Anyhow I concur with Steve.  Replace the first LO tube, sounds like it's crapping out at higher frequencies.  As I recall there are variable capacitors for each band crystal or perhaps a coil for each band; if misadjusted the thing won't oscillate.  Been a while since I've dug thru a A2 so I could be wrong here... check for a healthy oscillator with a scope.

[WQ9E]

The only thing I would add to the above is that one of the small modern PLL shortwave portables is very handy for checking whether an oscillator is running or not.  If it is, you will easily hear it on the "test" receiver.   Although it shouldn't be a problem with the crystal controlled LO in the 75A2, in some cases a scope probe can load the oscillator enough to stop it.  In any case, make sure to use a good 10X low capacity type probe.  A couple of times I have had to resort to my 1000X HV probe combined with a very high gain vertical plug-in to avoid circuit loading issues.

I really use my bench test receiver, a Grundig YB-400, quite a bit.  In addition to providing a quick check on oscillator health it provides a fast and sure method of adjusting the HFO on the old general coverage receivers in order to make sure you don't set it up for wrong side tracking.  I use my HP signal generator to provide a signal to align the RF and mixer stages but I set the Grundig receiver memories for the high and low set points on the band of interest and using this to listen for/set the oscillator is far faster than also having to tune the signal generator form point to point.  But the biggest advantage is you won't accidentally set the oscillator on the wrong side of the signal frequency.  Of course it makes a handy monitor receiver for a quick test on repaired transmitters also.

[KM1H]

A combination of a weak 12AT7 and sluggish crystals could do that.

Have you done a recap as well as checked for out of tolerance resistors?  My A2 had all of the above.

Carl

[WB6VHE]

Thanks Gentlemen!   The 1st LO level did seem lower on the higher bands when I peaked it up.
I did not check to see if it was operating on the proper frequencies, however.   I will get to work and let you all know how it goes.

73!

[N8ETQ]

  Love it,

  Sounds like the begining of another Thread, "Whats your Bench RX ?"

   I have found the R-392 is pretty handy for bench work with the RF/IF Stages readily available.. The Case makes a nice "Trash Can" for the shop also.


/Dan


from WQ9E :

"I really use my bench test receiver, a Grundig YB-400, quite a bit.  In addition to providing a quick check on oscillator health it provides a fast and sure method of adjusting the HFO on the old general coverage receivers in order to make sure you don't set it up for wrong side tracking.  I use my HP signal generator to provide a signal to align the RF and mixer stages but I set the Grundig receiver memories for the high and low set points on the band of interest and using this to listen for/set the oscillator is far faster than also having to tune the signal generator form point to point.  But the biggest advantage is you won't accidentally set the oscillator on the wrong side of the signal frequency.  Of course it makes a handy monitor receiver for a quick test on repaired transmitters also."

[KB2WIG]

I hope no one hates me; I use the S38.


klc

[KM1H]

Sounds like the begining of another Thread, "Whats your Bench RX ?"

That was the subject on another forum.

I move equipment around a lot but currently have a R-388 on one bench and a HP 3586B on the other as frequency spotters.  Neither helps with those high side LO's on the upper bands so then the spectrum analyzer gets cranked on.

Someday I'll find one of those DC to daylight receivers at a good price.

[ke7trp]

Eton XM1.  Has XM radio to listen to in the shack and it has wonderfull AM with a Sync detector. I use it as a test reciever all the time and use it for my main reciever when the boat anchors die.

C

[WB6VHE]

FYI, the update on 75A-2:  1st LO had all resistors out-of-tolerance, and the plate decoupling
resistor was WAY off (looked like it had gotten really hot once).  Replaced them all as well as
a couple of mica and tubular ceramic caps.  Also replace the electrolytics in the
audio/avc amplifier.  Now going great guns!  Thanks to all who posted
comments!


73, Ken