[Collins] 75A4 sensitivity

[Jim Brannigan]

To expand on Jerry's comments:
Any boatanchor radio, Collins or not, is at least 30 years old and may have
seen hard service and have been subject to dubious modifications by several
previous owners.
It is worthwhile to carefully check the radio against the schematic and
determine if the modifications add to the functionality of the radio (in my
opinion, most do not) and if they were done to "good construction"
standards.  Does the work look like the factory wiring?  If not, fix or
remove it.

Every time I service a radio, I check and record the tube socket voltages
and resistance.  On an unmodified radio they should be within 20% of the
values in the manual.  I use an Excel spreadsheet to record the values.  The
spread sheet is set up like the tables in the manual, with empty cells for
the measured values.  Some simple formulas compare the measured values with
the factory (or last measured) values and let me know if I have a potential
problem.

A complete alignment is necessary and is also a good diagnostic tool.

Jim

> What does the 75A4 have for RF stage? IF stages? Are they the original
> types? Is the screen bypass on the RF stage paper or disc ceramic and
> are its leads short? Also the plate feed bypass? Is there a 100 ohm
> resistor in series with the grid?
>
> If the screen bypass has short leads and is a disc ceramic, and the
> plate supply bypass is the same, and it has the right RF tube (no 6EJ7
> or 6EH7), then I suspect a VHF parasitic oscillation. Adding a 100 ohm
> resistor in series with the control grid may help. It would help to be
> sure that the input tuning is actually to the signal frequency.
>
> 73, Jerry, K0CQ, Technical Advisor to the CRA