Re: {Collins} 75A3 15 M



Hi Dan,

You have a signal generator so you have the diagnostic tool you need.

First, listen with a second receiver that is gimmick coupled to the 12AT7
xtal injection oscillator and make sure the oscillator is running.

Then, using a 0.01 ufd disk cap, capacitive couple your generator to the
grid of the 6BA7 first HF mixer on a frequency in the middle of the 15 meter
band. You should be able to hear this. Use about 300 microvolts rms at 21.3
mc let's say.  If you cannot hear it, the difficulty is in the oscillator or
mixer and we will go  from there.

Now, move your injection point forward in front of C29. Check there. That
should be fine since the other bands work.
Then. Go to the plate of the 6CB6 RF Amp and go on either side of R69
Thence forward to pin 1 grid and then to the high center pin on S1-G and
then to center pin on S1-H (You should have antenna terminal 2 at ground and
then go to the antenna terminal 1 with the generator.

You are looking for:
1. A injection oscillator not running on 15 for some reason
2. Bad band switch contacts on 15 at either the RF/Mixer inter-stage or the
RF amp input
3. Bad fixed shunt cap on one of the 15 meter tuning coils
4. Bad coil winding

The other stuff in the path has been verified by the operation on other
bands.
Whatever the issue is, the trace back process from the mixer back to the
antenna input will at least tell you where the failure is within two or
three components.

My experience is that you are probably looking at an oscillator problem or a
bad cap in one of the tuning circuits.

If you have a grid dip meter, you could also lightly cap couple to L-3 and
L-9 to verify you have a dip at 21.3 while you are tuned for the generator.

It seems unlikely, but the coils could have been mechanically damaged or
even done in by lightning. While at first blush, it may seem that this is
not probable because obviously T-2 is good at the front end, I have seen
very strange things happen in secondary lighting hits inside radios where
the front end and the back end are fine and the EMP pulse just flew around
and chose something in the middle at seemingly random.

Good luck Dan and let us know how it goes.


Bill Carns, N7OTQ
Trustee K0CXX
President, Collins Collectors Association
Editor, Signal Magazine
Wimberley, TX
512 618 2762  (Cell)
512 847 7010  (Home)



-----Original Message-----
From: Collins [mailto:collins-bounces+wcarns=austin.rr.com@xxxxxxxxxxxxx] On
Behalf Of Dan Presley
Sent: Friday, October 31, 2014 10:38 PM
To: Collins@xxxxxxxxxxxxx
Subject: {Collins} 75A3 15 M

Hi guys-a while back I posted here asking for any ideas on my  75A3 which
persists in being deaf on only 15 M!  I suspected the15 M crystal (23.3MHz)
in the oscillator section and that still may be the problem. What I've done
so far is build a simple crystal checker  and the 15 tests fine. Short of
having a spare to plug in I think I may look elsewhere. To reiterate-when
hooking up my sig gen there's no signal at all on 15, but all other bands
are fine. Plenty of audio so that's not the issue-just no detection. I
suppose next step is testing the various L/C circuits. I don't have a lot of
fancy test gear or much experience testing these, so any ideas are welcome.
I have an old signal tracer that I haven't had a chance to use so maybe this
is an excuse to try it out-never used one before. Is there a better way to
test these out, or an easier way to troubleshoot? Thanks for any help.
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