[Collins] 75A-4 Low sensitivity on 11 - 10 - 10 Meters

[Dr. Gerald N. Johnson]

On Thu, 2006-07-06 at 08:19 -0400, Jim Walker wrote:
> Hello:

> I am in need of some trouble shooting advice. My 75A-4's sensitivity
> while in
> The 11/10/10 meter positions are very poor. Other
> band positions produce excellent signals in all modes along with strong
> calibrator marker signals. The calibrator on the three high bands is
> barely discernible.
> The 11/10/10 T-7 coil and C-16 variable capacitor are functional. I have
> gone through the manual's RF alignment
> procedure several times and have also changed out V2 to no avail. I put
> in the best 12AT7 into V4 that I have.  The antenna Peak trimmer
> produces ambient noise increase on each band but the signal are way low
> compared with the SP-600, R-390, 75A-2 and the ICOM 746.  Should I be
> suspicious of C-22, the 24 mmf (pf) padding capacitor?  I do have low
> plate (175 v 190)  and screen voltage (140 v 175) on the 6DC6 (which I
> also checked and substituted) but R11 reads 33.6K and R6 is 4.7k.  The
> grid resistor R11 measures 1.15 meg in circuit. Any other suggestions?

> Jim 
> 
> KZ5AW
> 
I don't have a schematic of the 'A4 to work from. Is that 12AT7 an
oscillator or a none Collins conversion first mixer?

The low plate and screen voltages on the RF stage can come from several
sources. If there are black beauty oiled paper capacitors in the radio
bypassing the B+ and screens, their leakage will lower voltages.

With a 1.15 MEG grid resistor that 6DC6 could easily be showing some
grid emission to move the grid positive and so increase its current.
There surely are some 100 ohm stage isolating resistors on the power
supply side of each plate coil (RF, mixer, or IF), and some of those
could have drifted value, sped up by leaky bypass capacitors.

Is this a new symptom? Has it worked better before?

In some vintage radios where paper capacitors were used for cathode and
screen bypasses on the RF and mixer stages, those capacitors were
inductive above 25 MHz and so dropped the stage gain. Replacing cathode
and screen bypass capacitors with ceramic disks has sometimes helped
performance above 20 MHz.

The padder could be suspect, but isn't highly likely. A slip of the
coupling link on the input coil for 10m is possible but not very likely
unless the coil has been fried by a lightning strike near or on the
antenna.

Switch contacts are quite suspect and could bear cleaning with a quarter
drop of DeoxIT per contact. A spray saturates the insulators which is
not good for the insulation. Very gentle probing with a sharpened
toothpick might show up a poor switch contact. It is possible there is a
bad solder connection in the 10m RF circuits.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer