[1000mp] Details for MP Key Click Mod with 3rd keying stage included

[Juha Kajava]

There has been some interest on this modification. Therefore I will =
describe it. Sorry, there are no pictures.=20
This is not a very easy to do modification. INRAD mod is easier.=20

It is possible to do this modification with adjustable rise and fall =
times in a range of about 3ms to 7ms with one trimmer resistor.=20

 There are three keying stages in MP. Two are in RF-board and one in =
IF-board. ALL of those cause key clicks. FET Q1034 on RF-board is the =
worst one.=20

First I explain the modified circuit and then later give some hints how =
to do the modifications. It is possible to modify the RF-board without =
removing it.=20


1. Q1024 on RF-board  (This is not included in other modifications)
   This transistor is keyed with Q1019. Add a  4.7uF capacitor between =
Q1019 base to emitter. It will be parallel with the original 0.22uF =
C1129.


2. Q1034 on RF-board
   This FET is keyed by CMOS gate Q1026-2 via resistor R1147 (47k). =
There are two options now. A is simple and B allows rise time adjusment. =


  A:
 Add a 10uF capacitor between Q1026-2 output and ground. Rise and fall =
times will be about 5ms.

  B:=20
 Cut the line between Q1026-2 output and resistor R1147. Add a resistor =
R1 =3D 2.2k (or adjustable 4.7k trimmer) to Q1026-2 output. Then add =
another resistor R2=3D10k between R1 unconnected end and R1147 =
unconnected end. Now there are two new resistors in series between =
Q1026-2 output and resistor R1147. Add a capacitor 4.7uF from R1 and R2 =
junction to ground. This creates  RC low pass with R1 and 4.7uF. R2 is =
there only to keep weighting constant with high speeds. If you want you =
can make R2 adjustable too.
=20

  3. Q2033 and Q2038 on IF-board

 The SEND-signal (=3D keying) will be modified. Cut the line from =
connector J2014 pin1 to resistor R2200 (100k). Solder a small silicon =
diode (like 1N4148) anode to J2014 pin1. Solder a 1k resistor from =
cathode to  resistor R2200 unconnected end. Add 100nF capacitor  from 1k =
and R2200 junction to ground. 1k resistor is there to limit the current =
at rising edge from IC Q5014 on control-board.=20


As I have no pictures it would be a good idea to check the other well =
documented modifications. Thanks W8JI and others for putting so much =
usefull information on your pages.

You will need a rather small solder iron. 100W is too big. If this is =
your first soldering excercise please let some one else do it. All =
normal rules for repair work apply here.

                        Modifying RF-board:

1.  C1129 0.22uF
 This electrolytic cap is rather close to 13.5 DC input connector and =
LIN switch. Solder a 4.7uF cap parallel to C1129. Use same polarity as =
the original cap has.

2. =20
 Locate electrolytic cap C1127. It is near BAND DATA connector and the =
big fuse. There are 4 almost parallel oriented folios between C1127 and =
board edge. The folio we need is the one that has its end closest to =
C1127. The folio is also bent near C1127.=20
Do A or B, not both.
A: Remove some solder resist from the folio. Solder 10uF capacitor =
between folio and ground. Do not bend the capacitor at all after it is =
soldered! The folio could be damaged. It may be easier to solder a thin =
flexible wire to the folio and route that to the cap. Then there is more =
freedom to place the cap and the folio stays better on the board.

B:
Cut the folio with a sharp tool. Remove some isolation (solder resist) =
from folio on both sides of the cut and solder thin flexible wires to =
both sides. Solder a 4.7uF caps gnd leg to ground. You can remove some =
solder resist from ground area or use C1127 ground leg. Solder a 10k =
resistor to the wire that is soldered to folio ending near C1127. Solder =
2.2k or 4.7k trimmer to the other wire. Solder both resistors free ends =
to 4.7uF capacitors free end.=20
 =20

                            Modifying IF board

3.=20
Cut the folio between connector J2014 pin 1 and resistor R2200. That =
folio is on chip component side of the board. Pin 1 is marked with a =
white dot on circuit board. R2200 is 100k with text 104 on top of it. =
Solder diode anode side to connector pin 1 and cathode to 1k resistor. =
Solder the 1k resistor other end to R2200 unconnected end. Solder a =
100nF cap from 1k resistor and R2200 junction ground. Do not make a =
short circuit to pin1. That may damage IC Q5014 on control-board.



A note:

A cap of 10uF at CMOS gate output is not a problem because that 4011B =
gate is operating on 5V supply. I measured 8mA current to the cap when =
loading or discharging it. Typical sink or source current value found =
from Philips data is 5mA. One gate can tolerate 100mW dissipation. So we =
are far from that.


I hope that I remembered everything essential.=20


73  Juha  OH1NOR

e-mail:  oh1norATsral.fi   (replace AT with@)