So when it came to the frame of the Mark III tube mixer I decided to give the detail design and fabrication job to the professionals. I get all my sub-rack components from SRS here in the UK, not simply because I like to support local industry, but because they are a management buyout keen to build their business and they realise a successful business is built upon happy customers. So I sketched out what I wanted; a 30 degree sloping panel with a 3U space at the top (mostly for metering and ancillaries), a 6U space (for channel amplifiers) and a 3U space (for routing). Below this would be a horizontal 4U space for the faders. At the back would be a 9U space split into three 3U spaces. The top two are for the connections to the mixer and the bottom 3U space is for additional line amplifiers (Twin Line Amps).
The construction of the frame is really simple. It consists of a pair of side plates connected together by a bunch of standard length extrusions. The extrusions are placed in such a way as to provide the spaces required for the modules and to provide mountings for the back plane PCBs that the modules plug into.
From the users point of view the basic layout looks like this:
Because the new modules are 7HP wide (~35mm) you can fit exactly 12 of them into a standard 19 inch sub-rack that uses standard sized extrusions. So a basic mixer section is 12 modules wide. You can certainly make an 8 into 2 mixer in this format and possibly even a 10 into 2. If you want something bigger then you bolt a couple of sections together. This will give you a 24 modules wide mixer in which something like a 16 into 4 would be viable.
The two side plates that are connected together by the extrusions look like this:
The hard bit is designing the side plates. It would take me simply ages to work out exactly where all the holes need to go plus one plate needs to be a mirror image of the other so you can use countersunk screws to attach the extrusions (this is necessary so you can bolt sections together to make larger mixers). So this is the job I got SRS to do. I sent them these sketches and they responded with a fully dimensioned drawing and a very reasonable quotation. I requested a couple of minor modifications and then I ordered two pairs.
I forget how long it took SRS to make them but it was not long - maybe a couple of weeks. When they arrived they looked beautiful. Nice clean countersunk holes and very handy semi-shears on the inside to aid locating the extrusions and to prevent them rotating when you tighten up the screws. I already had all the necessary extrusions so I quickly built one section. I fitted the new back plane PCBs I had designed and started plugging in modules. All the 3U spaces worked perfectly. However, when I tried the first 6U high modules (Channel One of the previous post) it would not fit. The front panel seemed to be too tall. I tried slackening off the extrusions, pushing them apart and re-tightening but this made little difference. In the end I assumed my panel design was too tall and filed down the top and bottom of it until it fitted. To be certain, I found an old blank 6U panel make by Schroff - surely they must be the right size. To my surprise it did not fit either. It was beginning to look like it was not me after all but the SRS frame. A conversation with the supplier of the Channel One front panel and trying the Schroff 6U panel in a standard SRS sub-rack convinced me something was wrong with the frame.
So I contacted SRS and explained what I thought I had found. To their great credit they immediately suggested they send someone out to look at it. Considering I am a one man band working in a shed at the bottom of the garden I was impressed by their willingness to help. I was still unsure if I had done something silly in the assembly (knowing my reputation for building things wrongly) but when Martin from SRS visted me we went though it with a fine tooth comb and he confirmed there was definitely something wrong. He suggested he took the complete assembly back to base where it could be looked at in detail. A few days later I got a call to say they had managed to tweak it so a 6U panel fitted properly and they sent it back to me.
When it arrived I was eager to get stuck into building the first Mark III mixer but to my disappointment the 6U panels still did not fit. Whatever adjustments they had made had been lost during transportation so this was not a viable solution. After another phone call SRS agreed to subject the drawing to a thorough check. They soon called me back to say the drawings were fine so I expected to be told there was nothing they could do. But instead they said they were going to conduct a detailed examination of the complete manufacturing process to find out what had caused the problem. This took some time; about two weeks, during which time they discovered a number of small tolerances all of which could add up to the problem I had. I do not have a complete list of what they looked at but I know it included checking the length of extrusions because small variations in this parameter could cause one of an adjacent pair of extrusions to become warped when the end plates screws are tightened. I know they also checked the concentricity of the countersunk screws they were using and also the tolerances on their manufacturing machines.. In the end they made some slight changes to the design and added an extra semi shear to ensure all the extrusions are properly placed. They even checked their entire inventory of extrusions and discarded any that were not the correct length.
SRS then made me two new sets of end plates free of charge. One set they built up and thoroughly checked. The sent me this and the second pair of side plates. Again I eagerly tried a standard 6U panel in the 6U section and to my relief and joy it fits perfectly. Now I can get down to the interesting work of building a mixer.
I want to use this blog to publicly thank SRS for their tremendous support in solving this problem for me. After all I am probably their smallest customer.
Thank you SRS
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