Focusrite Voicemaster Pro - overloading when turning on

[gpateluk]

I got the info this morning. The guy sent me some other documents too about calibration etc which was nice of him. There is a lot to look at on screen so I'm going to get them printed out on A3. Then the fun begins!

[J M Fahey]

Ok, start with the obvious,
1) check that you have all the supply voltages
2) that said voltages reach the boards
3) IF said boards use Op Amps, that each of them has +/- 15V where it belongs, and 0 Vdc relative to ground.
4) if everything checks well, which I doubt, start injecting signal at the input and tracing it until it stops.

That alone means MANY fun filled evenings

[gpateluk]

I'm back after a few months! I have found time here and there to work on the Voicemaster pro. The first thing I wanted to do is work out why the resistor kept burning up. So using the schematic of the board I traced what was connected to what etc and found that a 2200uf 16v capacitor was faulty. So I swapped that out. Then just to double check that the voltage regulators were working fine I tested the 7815a regulator. The output I got was 14.4v. I read the output range should be between 14.4v and 15.6v. So mine is right at the bottom of the scale. So the question is, should I replace it? or will it be fine?
Thanks.

[The Dude]

14.4 is fine.

[gpateluk]

Thank you.

I just tested the 7915 voltage regulator and I got -13.5v. I can't find a range for this one but -13.5v looks pretty low as its supposed to be -15v. Also if I swap it out, do I need to swap the 7815 out too? As pairs?

Thanks.

[The Dude]

No, you don't need to swap both of them out. It certainly could be a regulator issue on the 7915, but I have doubts. They rarely fail "low"- usually shorted if there is a failure. I suspect something else like either a lower than expected load on the regulator output, poor filtering on the supply to the regulator, or low supply to the regulator. Before changing the regulator, check for high ripple content on the input pin of the regulator and check for lower than expected DC voltage there. The regulator can't regulate if the input voltage is low. For starters, you can compare the negative regulator input to the positive regulator input. I would think they should be very similar. Maybe you have another defective filter cap?

[gpateluk]

I tested both regulators 7815 & 7915 the same way. I took them out of the circuit and applied 19 volts (dc) to them using a laptop charger (I watched a Youtube video on how to test voltage regulators. I know, just because its on YouTube it doesn't mean its the right way). I plugged the charger into a dc jack socket component I have. Then double checked the connector pins for voltage. The jack pins i attached wires. Those wires i soldered to the voltage regulator input and ground pins. Then with a multimeter i measured the output voltage. 7815 I applied 19v and the output was 14.4v. 7915 I applied -19v and the output was -13.5v.

[The Dude]

Well, that pretty much confirms that the regulator output is indeed a bit low. I don't think it's low enough to hurt anything, and it should be fine to use it as is. On the other hand, you already have it removed from the board, so you may want to install a new one.

[gpateluk]

I tested a LM2940CT -5.0 P+ and that had a reading of 15.4 volts instead of 5 volts. So i'm swapping that out also. Parts have been ordered

[gpateluk]

Parts came so I replaced all the voltage regulators and decided to give it a shot of turning it on. I connected the bits back together and took a deep breath and pressed the on button. The unit powered up! No smoke from the resistor. The peak meter light came on blue and the dial that shot to the right didn't do that! I left it for a little longer then shut it off expecting the thing to blow up haha!!

I'll have a real test when I've got more time and update here to let you know if it's fully working or not.

[gpateluk]

Yesterday I put it all back together and switched it on. It all seemed to start up fine. I plugged in my guitar via the line in and lights went up and down etc. Today I plugged in my mic via the xlr with phantom power. I used headphones too. I got a clean output but now and again I would get some crackling. I thought it could have been my mic cable as when I unplugged it the crackle stopped. So I need to confirm why the unit crackles. Does anyone know what usually causes crackles?
So after replacing parts the unit is working! The unit has lots to it so I'll be testing it more and more over the next few days if I get time. I need to route it into my soundcard and then see if it crackles there too. So many little tests to do. But I have to say I've learnt so much from this repair.

[Enzo]

This is serious, not just being clever. Asking what causes crackles in an amp is about like asking what causes rattles in a car. If unplugging a mic from the input causes your crackles to stop, then I doubt it is the amp.

[g1]

If the mic is using phantom power, the connections or cable could be responsible.
(same would also be true for non-powered mic)

[gpateluk]

I've had more of a play and the unit making some weird noises. I've disconnected everything and there is a slight hiss through the headphones. But what I notice if I press some of the buttons turning sections on and off I can hear a 'peeeeeeeeeeoooooooooowww' noise in the headphones. As though something (a component?) in the unit winding down. With my guitar in the unit is fine but when the mic is in there is a lot of noise. I have tried the same mic and cable plugged directly into my soundcard and that is very clear.
I'm thinking the unit needs to be calibrated as with all the schematics Focusrite sent me was a sheet for calibration. Where it says stuff like 'Now set oscillator output to +1.30dBu @1kHz sine wave'. I also think the unit needs to be warmed up as the unit needs to be on for 20 minutes before calibration starts.
Oh the fun!

[Enzo]

I think you will find the calibration is about distortion and headroom. Probably not causing noise. Of course never hurts to go through the procedure.

You should always calibrate a circuit after it has warmed up, so that all the parts are up to full temperature and stable. If you fire up a unit and quickly adjust it, you may find that 20 minutes later, your settings have drifted. This applies to most anything, not just your unit.