[Collins] 75A-4 dead on 40 mtrs only

[Joe Eide]

Hi Dave,

It is possible you have found a problem with C10 in the RF section. 
Sometimes L1 also gets damaged from lightning or transmitting accidently 
into the receiver input.

However I would suggest a few other things to try in the oscillator section:

1.    Rotate L13 (the 40 meter xtal inductor) 1/4 turn cw.  If that 
doesn't work, turn it 1/2 turn ccw.  Continue this procedure a bit 
further if necessary while remembering your start point.  This may 
restart the oscillator on 40 mtrs.

2.    If that didn't help replace C41 (130 pf) capacitor.

3.    Repeat step 1 if necessary.

4.    If this doesn't work, you may have a bad xtal.  I think the xtal 
for 40 is 9.3 mc.  You can sniff loosely in the area of V4 or V3 for a 
9.3 mc signal.

5.    I have also seem a bad 12AT7 that tested good kill the oscillator, 
but that's not so common.

Hope this helps,

Joe - KB9R

On 4/23/2018 7:20 AM, David Propper wrote:
> HI all,
>
> Just finished re-capping my SN1135 75A-4 and it was working beautifully for 2 days. Last night, 40 mtrs just went absolutely dead. Receive sensitivity is fine on all other bands. I pulled all the crystals just to be sure it wasn’t an intermittent contact. Looking at the schematic, I suspect that possibly C10- a 300mmf mica padding capacitor across the first stage coil might have gone south.  This cap (postage stamp red mica) does have a blister on it at one end.
>
> Any other thoughts / recommendations as to the culprit ? I probably won’t have a chance to work on the receiver until later this week.
>
> Any help would be appreciated.
>
> Thanks and 73,
> Dave, K2DP
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