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Old 28th December 2015
  #1
Gear Maniac
 

JL Cooper MSB+ REv. II troubleshooting/fixing.

(pictures to come)

So last week I decided to really, truly, this time rewire my rack gear (again). So I pulled all the gear out, made sure all the cables were labeled, put it all back, including a couple new (to me) patch bays (the reason for the rewiring), and nothing happened. This rack is nearly all MIDI stuff, so nothing happening meant that no MIDI was going anywhere (audio is just fine).

Checking over my (so I thought) trusty (OK, secondhand) JL Cooper MSB+ REv. II, I noticed that I couldn't get it to change modes. Also, button #1 didn't appear to work (2-8 did). Now, that #1 button has always had a bit of trouble, as in you had to press on the lower part of it to get it to register sometimes, and the mode button sometimes acted like it wasn't debounced. But none of that should have kept it from sending or received MIDI data.

So let's open it up and see what we can see.

First off, the case is a touch odd. One piece of metal comprises the entire case except the faceplate. Off come the 4 screws, but the face remains on. A little messing with it reveals that it's still held by a pair of connectors. OK, I can pull it off. It's a pretty simple 2 sided board, with most of the traces towards the faceplate, which is still held to it by 3 nuts. The pushbottons are pretty simple SPST types with only the 2 solder points. There looks to be a few odd diodes in there, but no logic or anything else. I'll meter this later to see if the switches are working OK. The connectors are 15-pin.

The main board is held in the case by 5 standoffs. I'll have to remove the not and washer for the footswitch, too. It's apparent it's not the first time this unit has been opened. I can see a heat sink with a finger or two bent some, and there's some ceramic caps at the front of the board laying down flat against it. I doubt that they wre put in that way. I can also see that several of the chips are socketed. More than normal, even, So I pull the board.

On one side is a small power section. There's a 7805, and what is probably some associated circuitry. Seeing that the input is supposed to be 9VDC, I can see that this just brings the voltage down to what the chips need. The maibn processor is a Z-80, and there's a ROM socketed next to it (when I post photos I get the rev. info off it). Next is a series of larger socketed chips that I currently take to be RAM. There's a few various 7400-series logic chips, too, and some of them are socketed. There's Sharp 900 opto-isolators on each of the MIDI inputs, as you'd expect. And one large chip down at the other end of the board.

I can't see any reason why anyone would socket a garden-variety 7400 -series chip. And if it was manufactured that way, why not socket everything. That, and the somewhat crappy soldering job I see on some of the chips, convinces me that this has been opened before.

I check the seating of the socketed chips. I get the usual old slightly crunchy sounds you know by heart of you work on equipment like this. I'd bet even money that the thing will work now. And if it doesn't 2 to 1 that it's a bad solder joint on one of those chips.

There's also a battery. Well, it had been holding programs so it's probably good, right? I decide to check. It's a coin battery, and it's held in by a pair of metal pieces, one on top, the other on the bottom. OK, I'm not really familiar with this method, but it sure looks like I can remove the battery, right? Wrong. Those 2 metal bits have a couple of needle points sticking into the battery.

OK, I nearly always replace fixed batteries with removable, and I have a hold and batteries, so I just pry the metal bits off the battery. It's tests OK.

Since I'm getting a little tired tonight, this is as far as I'll take things. Mark which battery connector does to + and call it a night.

Tune in next time...