Modifying the Kenwood TM-531E Transceiver for 9600 Baud Packet Service
  ====================================================================== 

Trunk backbone links are increasingly being upgraded to 9600 baud working.
To achieve this, bolt-on modems such as the G3RUH 9600 baud unit are used
with TNC2 clones or DRSI PC*PA Type 1 cards.  Most of the 23cms trunk links
in the U.K. use Kenwood TM-531E radios which work perfectly at 1200 baud,
unfortunately they do not work at 9600 baud without some modification.

The problem is basically a lack of low-frequency response at 9600 baud data
rates because of the response of the VCO.  A few components can be changed
inside the VCO box to rectify this situation.  I would add that this is not a
job for the faint-hearted since VCO removal is a difficult job which needs a
steady hand!

Kenwood (UK) commissioned Chris Lorek, G4HCL (who also happens to be the
SysOp of the GB7SMC PacketCluster) to look at improving the VCO response of
the TM-531E radio to work at 9600 baud.  Reproduced below is a summary of
G4HCL's modification based on first-hand experience:

Modifications for TM-531E to work with G3RUH Modem at 9600 baud
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1) Audio output is available without de-emphasis at Pin 6 of the microphone
   socket.  This is an unsquelched output and is suitable for the G3RUH 9600
   baud modem

   n.b.  This high impedance audio output point is not suitable for driving
   normal 1200 baud TNCs unless they have been fited with a state-machine DCD
   modification.  The Kantronics Data Engine internal 1200 baud modems are a
   good example of a suitable unit.

2) Audio input to the TX PLL is applied to connector W301 on the TX/RX unit
   (X57-3340-xx) by the following method:-

   Cut all wires which are connected to pin designation TO (coloured
   off-white) and the earth connection (coloured black).  Connect audio coax
   to these two wires, and take this out of the set via the slot in the side
   of the rear heatsink.

3) Remove the VCO box from the main PCB.  On the PLL unit (X58-3490-11
   inside the VCO unit).  To do this, remove the screen on the underside of
   the TM-531E PCB.  There are four soldered lugs holding the VCO box to the
   PCB and a number of pins which also need desoldering.  Use a small
   soldering iron and a desoldering tool.  Once the VCO box is removed from
   the main PCB, the next step is to get at the board which needs modifying.
   There are two PCBs stacked one on top of the other inside the VCO box.  In
   addition, both these PCBs are soldered to the outer casing of the box.
   You have to remove both boards since the board which needs modifying is
   stacked above the first board.  The components which have to be changed
   are really tiny, so take great care when replacing them.

   Change the following SMD components:

   R56 - change to 3.3K
   C56 - change to 10uF 10v
   C57 - change to 10uF 10v

Reassemble the radio CARFEULLY and test.  It should be possible to see a
good data "eye" pattern with G3RUH modems at each end of the link by
carefully perming the link settings for the default EPROMs provided with the
G3RUH modem.  G3RUH can supply a specially optimised EPROM for his modem to
match the audio characteristics of the TM-531E.  With these EPROMs fitted at
either end of the link, the recovered 'eye' patterns are optimized.

The TNC TXD time must be carefully adjusted for maximum data throughput on
the link.

73 Steve Wilson G3VMW @ GB7LDS

(Downloaded from GB7BBS)

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