Hi, I use a MX2424 and a tascam M3700 (automated version of the M3500). First up lets just clarify that there are line level inputs and tape inputs. The returns form the MX should be connected to the Tape inputs.
Here are the problems, the MX runs at the +4dB line level standard. The M3500 runs in -10dB line level standard. Some M3500 consoles had the optional balancing kit that had tape returns at the +4dB standard with balanced inputs. However this is not common, you will know if you have them as there will be D-sub connectors on the back of the M3500.
However all is not lost.
M3500 Tape inputs: What you need to do is to make up a
simple pad on the returns form the MX, the pad can also at the same time convert balanced to un-balanced. This can also be built into the back of a patch bay if thats easier.
M3500 Direct outs: The direct outs are at -10dB and the MX needs +4dB, what I did was to get a tech to make up a line level matcher (24 channels) using small amp chips and a power supply. Tascam also made a unit that converted -10dB to +4db 8 channels at a time, but these are hard to come by. However another studio in town also had a M3700 and a MX2424 and they said there was no problem driving the line outs hard to achieve +4 as the Console has plenty of headroom before clipping. You just have to realise that the console vu's will show a hot signal.
With the connections, as a rule (channel 1 d-sub) you have pin 24 hot, pins 25 and 12 bridged as the shield (cold). The connection going to the MX is the same. However what I would do is to wire the D-subs as a fully balanced connection and do all the bridging/unbalancing at the console end.
What connection are the tape ins and direct outs on your console? The US had RCA connectors the rest of the world had unbalanced jacks as standard.
Cheers
Alan