[Collins] 75A4 sensitivity
jfuhrman
[email protected]
Tue, 8 Jul 2003 14:55:02 -0500
I had a similar problem with my 75A4.
But, first back to the beginning. I found several caps that should be on
the list. The following describes my initial replacements:
12/11/2002
75A-4 Replacements / Mods / Changes
During the previous decade many 75A-4 receivers were brought back to life by
CCA/CRA members. During this time various capacitors were noted and
recognized for their likelihood of being defective or a source of problems.
Some of the various capacitors were also recognized for their danger to life
due to the PCBs used in their manufacture. These caps are often referred to
as the “deadly capacitors.” They are typically called “Black Beauties” for
their color but many manufacturers have used a variety of different colors
of plastic. Light green, pink and light tan come to mind.
PCB is the acronym for Polychlorinated Biphenyls. PCBs have been
demonstrated to cause a variety of adverse health effects. PCBs have been
shown to cause cancer in animals. PCBs have also been shown to cause a
number of serious non-cancer health effects in animals, including effects on
the immune system, reproductive system, nervous system, endocrine system and
other health effects. Studies in humans provide supportive evidence for
potential carcinogenic and non-carcinogenic effects of PCBs. The different
health effects of PCBs may be interrelated, as alterations in one system may
have significant implications for the other systems of the body. Detailed
information is available on the EPA website.
Since the PCB danger is real and proven, all capacitors suspected of
containing PCBs should be carefully removed and replaced, regardless of the
brand of equipment.
“Orange Drop” Mylar & foil capacitors have received a great deal of
“coverage.” While they are a quality capacitor, there are several
metallized polyester (mylar) film caps that have non-inductive
characteristics with smaller size and lower costs.
The following list of parts was gleaned from multiple sources. QST, CQ
Magazine, HR Magazine, Electric Radio, CCA reflector, and CRA reflector are
the primary sources. Rather than put all identifiers in ascending order
they are arranged by diagram location for easier locating. The list starts
in the upper left RF amp area and works across to the right and then returns
to the lower left for another pass to the right.
I did not have another 75A-4 to use as a pattern for returning everything to
original. Not having a standard of comparison gave me license to repair /
replace / modify as I saw fit. If I did not like something, I could change
it without encountering guilt feelings. For those who consider this a
sacrilege, you had better stop reading here.
Identifier description Code* Notes
C20 0.1 ufd R use polyester / mylar type
C34 100 pf R use DSM type**
C37 100 pf R -do-
R15 8K2-2w C Chg of value to reduce screen voltage to safe voltage
C52 5 C use DSM type. 4 pf not readily available.
R25 8K2-2W C Chg to 2 watt
L32 2 mH M Mod to remove B+ from mechanical filter input
C145 62 pf M -do-
C144 1000 pf M -do-
C62 0.1 ufd R use polyester / mylar type
C54 0.1 ufd R -do-
C68 470 R use DSM type
C70 0.1 ufd R use polyester / mylar type.
C71 1000 pf R use DSM type
C75 470 pf R -do-
C92 100 pf R -do-
C93 .01 ufd R use polyester / mylar type
C94A 47 ufd/350 V R relocate / place from pin 6, V11, 12AU7, to ground
point
C94B -do- R Mount two terminal strips across bottom of old can cap-
C94C -do- R and mount 2 new caps between them.
C95 .01 ufd R use polyester / mylar type
C96 .01 ufd R -do-
C97 .0.1 ufd R -do-
C98 .01 ufd R -do-
C146 0.1 ufd R -do-
R71 100 K-1/2w M Eliminates bass “bump” at 100 Hz
C100 .01 ufd R use polyester / mylar type
C101 .047 ufd M Smoothes out audio response
C102 1 ufd R use polyester/ mylar type
C124 .5 ufd R -do-
C104 470 pf R use DSM type
C108 0.1 ufd R use polyester / mylar type
C111 0.1 ufd R -do-
C112 0.1 ufd R -do-
C113 50 ufd/300 V R
C137 50 ufd/300 V R
C115 0.1 ufd R use polyester / mylar type
C116 0.1 ufd R -do-
C204 .01 ufd R use 500 V disc
C205 51 pf R use DSM type
C206 .01 ufd R use 500 V disc
*CODE:
R= Replaced
M=Mod/chg installed per note
C=chg of value
-do- means same as line above
**
DSM = Dipped Silver Mica
Power Supply change to Solid State Epoxy Rectifiers
The 5Y3 rectifier tube failed early in the rebuild effort. Due to a spares
shortage and the desire to improve power supply reliability, the high
voltage supply was modified to include 4 each 1N4007, 1 A / 1000 PIV diodes.
The 1N4007 is readily available from multiple sources, even Radio Shack, for
a few cents per diode. This change will remove a 10 watt filament load from
the power transformer which promotes a longer life.
The glass envelope of the 5Y3 was carefully broken inside a paper bag and
the Bakelite socket salvaged. A series connected pair of diodes (four
total) were connected from pin 6 to pin 8 and pin 4 to pin 8. Be certain of
correct polarity. The banded ends should “face” pin 8. A cap plug can be
placed over the old base after installing the diodes.
The new epoxy silicon diodes have less than five volts drop compared to 60
volts or more for the 5Y3 tube rectifier. To reduce the B+ to a safe level,
a series resistor must be installed between pin 8 of V17and L29. Remove
the lead from L29 to pin 8 of V17 socket and install a series resistance of
800 to 850 ohms with at least a 20 watt dissipation capability. The 75A-4
current demand is 110 ma, nominal. The voltage drop across the 800 ohm
resistor is 90 VDC. Therefore, the power to dissipated is about ten watts.
I used four 200 ohm / 5 watt resistors in series placed on two solder
terminal strips mounted to the side of the inner vertical chassis wall of
the center sub-chassis behind the PTO.
The 5 volt filament leads from the power transformer can be removed from the
tube socket, folded back, insulated and placed out of the way where they can
not short out
AVC / AGC improvements for SSB usage
The issue of receiver poor AVC performance has been visited by several
others over the decades since the 75A-4 was in popular usage. Multiple AVC
circuit changes have been tried. Each has had both improvements in
performance but usually with other depreciating effects on performance. All
the various AVC articles invariably have a follow up to fix the “new”
problems caused by the modified circuit.
The biggest improvement was to stop the RF amp from oscillating. On 10
mtrs, by rotating the ant trimmer you could hear the oscillation swish past
the receive frequency. It was also present on 11 & 15 mtrs. Chging tubes
made no dif.
I chgd screen bypasses, grid bypasses, grid blocking cap, et all. No chgs.
Still unstable.
The RF amp grid which has the trimmer C18 and the tuned circuit for each
band evidently has some ground loops. I changed the grid circuit slightly.
I removed the 18 ga bus bar from the band switch to the terminal strip and
the 18 ga from the trimmer to the terminal strip. I installed a single 18
ga from the band switch to the trimmer stator. I installed a 33 ohm from
the stator to the terminal strip.
The ground paths were chgd and the 33 ohm damped out the spurious
oscillations. AVC action improved, sensitivity improved, and the AVC "pop"
stopped.
Good hunting.
John
K0LFA
----- Original Message -----
From: "Bill Russell" <[email protected]>
To: <[email protected]>
Sent: Monday, July 07, 2003 1:11 PM
Subject: [Collins] 75A4 sensitivity
> This might be a long question.....
> I have a 75A4 that exhibits a lot of background noise. I will try and
explain.
> The 7 deadly caps have been replaced by the previous owner although the
> value is larger than the originals. When I got the radio, it would
overload
> very easily. I found that the IF transformer for the AGC amplifier was way
> out of adjustment and I had very little AGC voltage. After alignment of
> this IF can, I now have plenty of AGC voltage and no more overload but the
> gain of the radio seems very high. With the IF gain control set at minimum
> and the s meter sensitivity also set at minimum, I always have a noise
> level of at least S9. When I remove the antenna, the S meter will fall to
0
> but as soon as I connect the antenna its back up S9 or more. When I set
the
> AF level for a comfortable listening level on a fairly strong station, the
> background hiss is very annoying. Its not noisy band conditions because
> when I switch to my SP600 or NC303, the band is quiet. As a matter of
fact,
> my SP600 S meter will read about S1 but the A4 will read S9 to 10 over!!!
> Remove the antenna and back to 0 on the S meter and the background hiss
> will drop but I think its still noisy.
> Any ideas will be greatly appreciated.
> Thanks much,
>
> Bill Russell
> KC2IFR
>
> Sponsored by the Collins Radio Association
> http://www.collinsra.com
>