YAY!
I had the input channels off the mixer again today to try and cure:
1. The overload led on channel 5. It is fully lit
2. The overload led on channel 8. It slowly creeps on, though not as much as channel 5
3. The hum that has appeared on channels 4,7,8 and 10
4. The noise on the CUE MONITOR BUSS when then unit is in any mode other than 'MIX'.
I started by having another good visual look at the BUSS PCB with my super goggles. I couldn't see anything obvious, so I elected to run solder over all the joints. After inspecting the input boards, it was more likely that if there was a dry joint between the input boards and the buss board, it would be on the buss board, as the ones on the input board are doubled up (they are like the female socket which the male buss pcb inserts into). I then set about removing a couple of transistors from around the led to check them. I compared the ones on channel five with the ones on channel four, and couldn't see any difference.....so these are not the culprits. To make sure I swopped them over with one another. It was only at looking at channel 4, that I noticed the ground pin on the XLR was loose. They are soldered straight onto the board. I couldn't imagine when it came loose, but then wondered if the same thing had happened to the other boards on removal and re-fitting. Sure enough, channels 7,8 and 10 all had loose ground pins. I then elected to remove all the input boards and run new solder over all four joints.
I put the 688 back together and was disappointed that the re-soldering of the buss board hasn't made a difference to the CUE MONITOR problem. Also, I hadn't cured the LED on channel 5 either.....though now led 8 illuminates straight away....I wonder if in time, they're all going to light up?
BUT....and it is a big but......the unit sounds tons better. The hum has all disappeared on all channels. I really wanted this sorting, because I want to use the pres on a drumkit and I also want to insert a Presonus ACP8 that I have on up to 8 channels. The CUE MONITOR buss fumctions as it should if you keep the 'MIX' button enabled......which really isn't a problem. I just did a test recording and the matrix switching is silent and the unit sounds tons better.
So, for now, I'm going to start recording with the unit and see how far I get. I'm pretty sure the 'noise' problem is component related....because it gets less and less the longer the unit is on.
Onwards and upwards....