FYI, we measured the voltage coming out of the relevant pins from the actual power supply output cable, and those read ~16v... so it seems like it's losing the voltage just immediately after the connection to the power input on the desk or some such?

Andy Halliday, post: 465399, member: 52035 wrote: ... we've taken the bottom off and measured the voltage between the earth and red on the board both at the point the power enters the desk itself, and the MPU board - both read 1.6v... so it seems like there is not enough power actually reaching the board.

Andy Halliday, post: 465400, member: 52035 wrote: ..we measured the voltage coming out of the relevant pins from the actual power supply output cable, and those read ~16v... so it seems like it's losing the voltage just immediately after the connection to the power input on the desk or some such?

I don't follow where you read the 16V figure - was it with the power cable disconnected from the mixer?

If you read 16V with it disconnected and 1.6V with it connected, then the fault is likely to be in the PSU itself rather than the mixer.

Does the -ve analogue rail behave in the same way? (NB it doesn't go to the MPU board.)

Yes, so we measured the voltage coming from the PSU with the cable disconnected from the mixing desk itself. It read ~16 but when we reconnected the cable and measured via the connectors on the back of the desk (where they connect to the desk) and also on the MPU board, both are reading only 1.6v...
Yes we reckon it's the PSU as well. Should it be simple to fix the PSU?

Maybe just the +ve rail regulator. It's an old-school design of PSU, but I've never seen any schematics for it.

Seems i get a few years out of each repair thus far. My mixer broke down again for the forth time and seemingly for the same general set of reasons we all experience.. This time might be a bit different for me as, i think its the diode feeding the 5v regulator input that has gone out this time. Last time was the regulator itself. I get 15v on the line side of the diode and just a couple volts on the load side and of course zero volts on the regulator output..

It does raise a question for me about how the pair of BC549s play a role in the circuit. The Arrow box with PWR written inside confuses me in the PSU schematic below. I know it feeds the P1 break feature on the 80c32 chip but im wondering does the arrow box show direction, like, are the BC549s triggered by the 80C32 or do the BC549s trigger the 80c32?? Trigger is probably no the the right word. I know its a logic symbol of some sort.

Just a screen shot of the PSU part of the MPU below. In red i listed the volts i was getting at various spots which lead me to the diode on the positive line being out. Any other reason i would get these voltages?? Like could the BC549s or the C5 cap be sucking the volts down??. 

The symbol PWR on the schematic is simply a way of showing this point connects to another that is identically labelled, but maybe on a different sheet. If that point is a pin of the MPU (80C32), then the transistor output is an input to logic on the MPU chip. On power up, the first BC549 does not initially conduct, but when the power rail voltage reaches around a volt or so, the second BC549 will turn on, forcing the open-collector PWR signal low. PWR will stay low until the first BC549 turns on and pulls the base of the second transistor low, turning it off. A quick calculation shows that this point in the sequence will be when the input rail voltage reaches about 15.5V.

I expect that PWR acts as a reset line, holding the MPU and other logic in an initialised state until the input power rail has achieved its design voltage. By then, the 5V regulator would normally be powering the MPU and it can be started up by releasing the PWR signal. Note that an external 5V pull-up resistor must be present on the PWR line; this is probably shown on the MPU schematic.

What I have described is the 5V power-up sequence on a working board. In the case of your faulty board, it's clear from your voltage measurements that the 5V regulator is not giving any voltage output, so the power-up sequence fails. There are a few possible reasons for this: (1) the voltage across the IN4002 diode in the forward direction should be around 0.7V and not 13V, so this diode is not working correctly, (2) the output of the 5V regulator is at ground potential, meaning that either it is being held to ground by a shorted component or connection on the 5V rail, or that it is not functioning as a regulator, (3) C5 has a conductive fault.

If C5 is faulty, it is probably holding the regulator input voltage at the 2V level, causing excess current through the 1N4002 diode, causing that to fail. Alternatively, there could be a short on the 5V rail, and this has caused the regulator (it's an old-fashioned type) to draw excess current, again burning out the 1N4002.

A few more checks would be needed to determine which of these possibilities is your problem. Firstly, I would perform voltage measurements (using a battery-powered multimeter) directly across the 1N4002 diode and across the regulator. Also, if you can, switch to mV range and measure the 5V output rail with reference to the 0VD ground point. Note whether this is (a) a constant zero to within a millivolt, (b) several millivolts above ground but steady, (c) some millivolts above ground but waving about.

Then disconnect the PSU from the mixer and use the multimeter on Ohms range to measure the resistance of the 5V power input pin on the mixer to ground (chassis). Depending on the voltage that your multimeter uses to measure resistance, this reading will be either be below about 20 Ohms or more than a few hundred Ohms. The first indicates a fault and the second means the mixer is probably OK. Come back with the results, also saying whether you are handy with a soldering iron to lift pins and replace components in the PSU.

 

Thank You Boswell. Yes, The PWR does connect to the 80C32 on pin 12 (Int0 external interrupt) ,, and there is a 4.7K pull up resistor between the 5v rail and pin 12.Thanks for the details on how the circuit works. I have gathered all the parts i might need to attempt a repair, so, im going to replace the 4002 diode and the C5 330/25 cap. I have a replacement voltage regulator as well if i need it. I dont have a 330/25 cap so im going to use a 470/50v. I think that should be fine but do let me know if you think its a problem. I am going to pull the PCB out now and get it over to the bench for surgery. I will update soon. Thanks again.