6U Modules Part 3 and a hint of General Standardisation

It has been over a year since my last post here but a lot has happened. 6 months of work time was lost whilst I moved house and built and equipped a new workshop. Also,the separate Lunchbox project has really taken off and a version containing four tube mic pres and an integral power supply has proven especially popular. More on that later. The result is the Mark III has not received a lot of attention. Despite this, some progress has been made.

I finally got round to laying out one of the daughter board EQs, the REDD EQ. This turned out to be a lot easier than I expected. It essentially consisted of cutting and pasting the EQ from the main board to the new board.

It has four holes in the same positions as the main board so it can be attached using pillars. The only question now is how tall should the pillars be? At first I attached it to the main board using 20mm spacers as discussed in the original 6U modules post. Although there was a reasonable gap between the switches on the main PCB and the corresponding ones on the daughter board, the large EQ inductor was almost touching the underside of the daughter board. So I tried with 30mm spacers as shown below:

You can just see the large inductor behind the right hand switches. There is plenty of space between its top and the bottom of the daughter board. But the daughter board looks a little close to the module cover. WIll the switches and inductors fit on it?

As you can see, the switch fits comfortably but it is not clear if the inductor will fit. Also the switch looks a little closer to the to the top edge of the module than the bottom switch is to the bottom edge. We know that the bottom switches are:

 14.2 + 9.35 = 23.55mm from the left hand edge of the front panel.

Ideally, the switch on the daughter board should be the same distance from the right hand side of the front panel, and as the front panel is 70.9 mm wide, the daughter board switches need to be :

70.9 -23.55mm = 47.35mm from the left hand edge of the front panel.

Since the main board switches are 23.55mm from the left side of the front panel, the distance between them and the ones on the daughter board is just :

47.35 - 23.55mm = 23.8mm.

Since 1.6mm of this is the daughter board PCB itself, this means the pillars should be:

 23.8 -1.6mm = 22.2mm high.

So 30mm spacers are definitely too big but 20mm are definitely too small. Perhaps 25mm would be a good compromise.

Which brings us on to the other awkward mechanical problem we still have to solve. As you can see in the pictures above, the pan and AUX controls on the main board (red and blue knobs) are also soldered direct to the main PCB. But they are smaller than the Grayhill switches used in the EQ so their centres do not line up with the EQ switches. The original plan was for the EQ daughterboard to extend right across the module and also hold the second set of pan and AUX controls. There are a couple of problems with this. The smaller one is it would be much nicer if the pan/AUX controls lined up with the EQ switches. The bigger problem is that any time a customer wants a different pan/AUX combination we have to design a new main PCB and a new daughter PCB plus we still need to do seprate main board with EQ  for the mono mic channel versions of these modules.. I have already decided to go for a main board per EQ and a daughter board per EQ. Do I really want to redo these every time there is a minor change in routing requirements and do I really want to design another set of PCBs for the mic pre versions? The answer is very definitely NO which is why the daughter board only contains the EQ and does not extend into the pan/AUX area.

What we really need is some flexible means of adding any combination of mono/stereo pan/AUX mic/line controls in addition to the EQ. The obvious solution to this problem is to have a specific small PCB holding these controls that fits parallel to the front panel rather than at right angles to it as is the case for the main and daughter boards. This again provides the flexibility we had in the EZTubeMixer design but in a much neater fashion. The really big problem with this solution is finding a set of pots, push buttons, toggles and perhaps even rotary switches that can all be soldered directly to the PCB yet all interface neatly with the front panel. I have been working on this problem on and off for over a year but I think I now have a workable solution and this is what I meant when I included the phrase 'a hint of General Standardisation' in the title. This will be the topic of the next post.