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

[Dr. Gerald N. Johnson]

Could be any of many causes. The band switch contacts as suggested by 
Bob AD3K certainly are a possibility. Complete loss hints at the crystal 
oscillator, broken signal path tuning might pass strong signals yet, but 
not necessarily. If you have another general coverage receiver you can 
try to tune it to the crystal frequency and see if it can be heard. Its 
at 9.3 Mhz according to my late 75A4 manual.

If the crystal is oscillating and the signal is getting to the mixer 
tube, you can check for signal loss with a signal generator applying 
signal to the grid of the mixer, then working through the circuit 
towards the antenna. Its easiest to do it sequencialy but most 
effincient to do a binary chop. Inject signal in the middle of the path. 
If it works next inject signal half way from the middle to the front. If 
it doesn't next inject signal half way to the mixer from the middle of 
the path. Split path segments in half until the fault is located.

Without a signal generator, the calibrator can work as that, use a small 
coupling capacitor from the plate of the calibrator and a short clip 
lead to connect it to various places in the signal path to find where 
the signal is being lost.

A tube shouldn't have a failure mode to take out one band. Switch, 
crystal, or tuned circuits are more likely when the part is only used on 
40 meters.

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

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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