MA Scancommander motherboard repair
While doing repairments I came across lots of elder lighting consoles which have encounterd problems with booting, these faults where mostly caused by leaking backup batteries. When the battery starts to leak, the acid damages PCB traces and causes corrosion on components, it then can lead to all sorts of weird problems.
Crashed Scancommander
Leaking backup battery
In this case I have a MA Scancommander which isn't booting anymore. These desks were introduced in the early 90's and some desks are now at least a decade old. A leaking backup battery is common problem among Scancommanders and it also affects the Lightcommander range which used the same type of battery (The old blue/gray batteries where even more prone to leaking). It is wisely to replace the battery around every 5 years and check periodically for leakage. Don't wait until the battery doesn't work anymore! In this case it was overly clear I had to do with a leaking battery. A flat battery (normally) would not cause a lighting desk to crash, so more was going on.
Battery was already removed when I got the machine
Repair
First step was to clean the pcb, I use Kontakt PCC for this. Unfortunately not all of the solder-pads survived the leaking and they were completely gone. I had luck most traces run on the bottom of the pcb (it has a 4 layer pcb). You can scrape of some of the green coating of the top traces to get some bare copper to make a solder connection. I decided to replace all resistors, capacitors and diodes close around the battery. There were also evidence someone before me had a go at repairing it, doing a bad job tho.
It's a nice touch of MA to put the value's of the components on the silkscreen. Check with a ohm meter if you made a good solder connection, otherwise follow the trace to find a solpad close by and run a jumperwire. You can boot the desk without a battery to test it. If you still get the error message there could still be somewhere a faulty trace on the pcb.
Cleaned are repaired section
The new battery is now not on the pcb anymore, but placed under the large ribbon cable that goes to the frontpanel. The new battery has solder tabs and is connected to the motherboad by some thin wire and hold in place by zip ties. It has the benefit that if leakage will occur the pcb will not be damaged. If the battery has to be replaced, instead of the time consuming job of getting the motherboard out, just cut and strip the wire.
New battery under ribbon cable and new eeprom's with latest software
Troubeshoot
After cleaning the pcb and replacing parts, it still didn't boot. Somewhere was still a faulthy trace. Because it was clearly a issue started by the battery, I took a closer look at the battery circuitry. I don't have the schematic of the Scancommander, but I have one of the Lightcommander 12/2 and the whole MA commander range are internally quite related. D5 in the schematic is D4 on the Scancommander. When the desk is running on its own powersupply current flows trough the diode and powers the V_BATT rail. On this rail we find the real-time clock and sram memory chips. In these chips your present, chases, patch and memories are stored and this is archieved by keeping these chips always powered, even when the desk is off. When the desk is off, the battery is providing power to the V_BATT rail and the diode blocks the current to flow back into the rest of the mainboard, preventing the console running completely on the backup battery!
These ICs are connected to V_BATT
- IC17, IC31, IC18 SRAM (D431000)
- IC8 74AC00 (Even has "BAT" marked next to it)
- IC10 RTC72421 real time clock
Click image for high-res one (5,4 MB)
I measured the supply voltages of the ICs powered by the V_BATT rail. All of these ICs had a supply voltage around 2V when running on the main power supply. This was way lower then minimum conditions to operate. Further measuring proved that the connection between these ICs was good, only the trace between the battery and this group was at some point bad. Only thing now to do is solder a jumperwire between one of the sram chips and diode D4 to bypass the bad trace.
Jumper wire on the bottom to bypass the damaged V_BATT trace. The short jumpwire is for the software update
Succes
During the first boot after replacing the battery you will encounter some error messages. Because the battery has been removed the internal memory (sram) lost its data, clear them all by pressing the Clear button. To really clear the internal memory, keep all four buttons on top of the display pressed down when switching on the desk. The desk should work now fine again.
