Focusrite Voicemaster Pro - overloading when turning on

[gpateluk]

Hi,
I've just got hold of a Focusrite Voicemaster Pro. The problem is when you turn it on the power and lights come on then the unit overloads.
So I'm here to see if anyone can help me work out how to fix the problem. Also where to start.

I've disconnected everything and still the problem persists. I've also turned on the 'process bypass' button to see if that makes any difference, but still the same.

I've attached some photos and a video to show what happens.

Any help is very much appreciated.

Thanks.

[J M Fahey]

What do you mean by "overloads"?

Thereīs no signal injected nor does the VU meter show anything so that word does not apply.

If anything, "the overload Led lights" which is not the same.

Overload circuits typically rectify audio, turn it into a DC voltage , compare it to some reference voltage and if surpassed turn a LED on, so unless proven wrong I suspect you have a problem in that area.

Does the unit pass normal audio and do its work otherwise?

Besides that bright LED, what problems do you find with that unit?

IF somebody has the schematic, it might help .

[gpateluk]

When I said 'overloads' I meant the O/L light comes on (which I thought was due to an overload in the signal). Sorry, I was wrong.

There is no audio passing through the unit at all. I did notice on the 'tube sound' section of the unit the big 'drive' LED quickly changes from green to orange/red. This LED shows the amount of overdrive applied to the signal (even though nothing is plugged in, in my case). The amount of distortion being applied shows blue (no distortion), through green, to red (high distortion). This LED shouldn't even be on I don't think as that part of the unit has not even been turned on. Also the peak level backlight goes off when the O/L red LED comes on.

I had a look again at the unit and thought about disconnecting some of the connectors to see if that did anything. Looking at photo 2 I disconnected the pre amp connector and the connector (seen bottom left in the photo). This connected the rear and front boards. I turned on the unit and smoke started to come from the rear pcb. I quickly turned it off! When I looked at the point of smoke I could see a 10 ohm resistor with black on the side. I took it out and it still reads 10 ohms. I checked around the area and all the parts look clean. The resistor looked the only burnt part. I'm going to order some new resistors and not put the resistor I pulled out back.

I got the unit second hand. The previous owner had it working for 5 years. He left it for a year, then when he turned it on this is what happened. No input or output. The unit has power as you can see in the video.

I did find another site that talked about a common problem Focusrite Voicemaster Pro problem "In a previous version of this preamp (Voicemaster) the symptoms you describe were quite common (c40% of the units we had) and the issue was solved by remaking the solder connections on the current source and sink transistors". I'll take a look at that too.

I've not found a schematic yet.

Thanks.

[J M Fahey]

Itīs a very complex preamp/processor so unless a schematic is found somewhere we canīt even begin imagining what the problem may be.

And you have a worse problem than just an indicator lighting improperly: the whole unit does not work, not even pass sound.

[The Dude]

If a resistor burnt, it didn't just go bad. Something in its path is shorted or drawing too much current. Putting a new one back in there without finding the problem won't solve anything. On the other hand, that is a good clue as to what is wrong with the unit. You need to find out WHY it burnt. Even without a schematic, you should be able to follow traces and see what it connects to. Is it maybe between a + or - 15v supply and an op amp? Collector of a transistor? Etc. I would leave the resistor out until you decipher what its function is. Use the burning resistor as information. It's telling you where the short is.

[MarkusBass]

There is no audio passing through the unit at all. I did notice on the 'tube sound' section of the unit the big 'drive' LED quickly changes from green to orange/red. This LED shows the amount of overdrive applied to the signal (even though nothing is plugged in, in my case). The amount of distortion being applied shows blue (no distortion), through green, to red (high distortion). This LED shouldn't even be on I don't think as that part of the unit has not even been turned on. Also the peak level backlight goes off when the O/L red LED comes on.

I'm confused with your posts. You haven't specified whether the unit works or not. When you turn it on does it pass audio signal?
You also shouldn't focus on the Overload LED. It seems that the unit has general problems and the O/L LED is only a result of the problems. The O/L LED circuit is just a basic comparator - it compares voltage from various parts of the preamp with fixed voltage. So, if it gets incorrect voltage from the preamp, it switches on. But clearly, all other indicators also show incorrect voltages so I wouldn't focus (rite) on the O/L indicator. Just for start I would check whether power supply rails (+15/-15V) are present.

I had a look again at the unit and thought about disconnecting some of the connectors to see if that did anything. Looking at photo 2 I disconnected the pre amp connector and the connector (seen bottom left in the photo). This connected the rear and front boards. I turned on the unit and smoke started to come from the rear pcb.

This is even more confusing. How disconnecting connectors can connect rear and front boards? Can you explain this?
I think that 10 Ohms resistors and in the power supply rails for some of opamps (additional filtering). So somehow you created a short circuit. Now you have to replace the resistor and find the short.
The preamp is very complex and you have to be very precise when fixing it and describing the problems you encounter. I fixed several Focusrite preamps (including Voice Master Pro) and I know that it requires a lot of thinking before you do anything.

Mark

[gpateluk]

I'm confused with your posts. You haven't specified whether the unit works or not. When you turn it on does it pass audio signal?
Mark

I wrote "There is no audio passing through the unit at all" which I think should answer your question. The unit turns on and I get no sound when anything is plugged in.

This is even more confusing. How disconnecting connectors can connect rear and front boards? Can you explain this?
Mark

Sorry, what I meant was the connector I disconnected did connect the two boards together. So when I disconnected the connector the two boards were no longer linked.

The preamp is very complex and you have to be very precise when fixing it and describing the problems you encounter. I fixed several Focusrite preamps (including Voice Master Pro) and I know that it requires a lot of thinking before you do anything.
Mark

Thanks for this information. I will try my best to describe exactly what is happening. One problem I encountered today was trying to get the front panel off. There are 4 silver screws (2 left and 2 right). Then 2 screws by the XLR input. But the silver panel seems to be held on in the middle. There is a small silver disc at that middle point which I think has the screw underneath it. But it won't come off. I've tried prying it off but all I've done is scratch it. Am I right that there is a screw under it? If so how do you get to it?

Tomorrow I'm going to draw out a simple diagram of what connects where to try and help me understand the flow. Looking at the unit, the connector I disconnected linked the rear PCB to the pre-amp section on the front PCB. The rear PCB has two other connectors that go to the output section (headphone level etc) of the front pcb.

I appreciate very much the comments I'm getting. Thank you.

[MarkusBass]

I wrote "There is no audio passing through the unit at all" which I think should answer your question. The unit turns on and I get no sound when anything is plugged in.

Yes, you wrote it in the second post. But in the first post you wrote that:

The problem is when you turn it on the power and lights come on then the unit overloads.

You focused on the OL LED but the truth is that the preamp does not work at all and the problem with LED is just one of many, many other problems. If you solve other problems, this one will go away. So the tittle of the topic is confusing and incorrect.

I encountered today was trying to get the front panel off. There are 4 silver screws (2 left and 2 right). Then 2 screws by the XLR input. But the silver panel seems to be held on in the middle. There is a small silver disc at that middle point which I think has the screw underneath it. But it won't come off. I've tried prying it off but all I've done is scratch it. Am I right that there is a screw under it? If so how do you get to it?

Are you trying to remove the panel, or remove the PC board? As far as I remember, I was removing the PC board. You don't need to remove the panel. Can you post a photo of the front panel PC board?
But before you remove the PC board, think what you can do before that. I suggested to measure at least the power supply rails for the opamps. Did you do this?

Mark

[gpateluk]

I managed to remove the front pcb from the front panel. There were 5mm nuts holding it in. So the little silver disc on the front I scratched up for nothing haha!
I am a bit confused about one thing. About the smoking resistor I took out, am I OK turning on the unit without it in? So i can measure power supply rails. As I don't want to cause more damage. So with that dilema I measured the voltages from the main source of power to the rear board. The connector has 6 wires and I measured each one:
Green/Yellow = 0 volts
Grey = 30 volts
Orange = 25 volts
Red = 45 volts
Black = 29 volts
White = 13 volts

As a quick test I did pop the smoking resistor back in to see what would happen if only the rear PCB was connected to the power supply. So none of the effects were connected (the front pcb and the mic input PCB). The resistor still began to smoke.

If it is OK to power up the unit without the resistor I'll get some more power supply readings. I'm still trying to track down a schematic for the unit too.

Thank you for the help.

[Jazz P Bass]

Can't say that it's safe to power up the unit 'without the burnt resistor' as there is no indication what it is for.

Obviously, something is demanding excess current to make a resistor smoke.

If you are dead set on getting power rail voltages, at least build a 'Lamp Limiter'.

Here is the contact info for the USA: https://us.focusrite.com/stores/dist...20of%20America

Maybe they can help.

[gpateluk]

Thanks, i'm building a 'Lamp Limiter' over the next few days. Got nearly all the bits, just need to get the right bulb. Finally got some more resistors too.

Will keep everyone updated.

Thanks again for all the help.

[gpateluk]

Lamp limiter built! Now for some testing

[J M Fahey]

I managed to remove the front pcb from the front panel. There were 5mm nuts holding it in. So the little silver disc on the front I scratched up for nothing haha!
I am a bit confused about one thing. About the smoking resistor I took out, am I OK turning on the unit without it in?

Who knows? We have no clue. We have not even seen the schematic. We donīt know what each part does.

So i can measure power supply rails. As I don't want to cause more damage. So with that dilema I measured the voltages from the main source of power to the rear board. The connector has 6 wires and I measured each one:
Green/Yellow = 0 volts
Grey = 30 volts
Orange = 25 volts
Red = 45 volts
Black = 29 volts
White = 13 volts

As a quick test I did pop the smoking resistor back in to see what would happen if only the rear PCB was connected to the power supply. So none of the effects were connected (the front pcb and the mic input PCB). The resistor still began to smoke.

If it is OK to power up the unit without the resistor I'll get some more power supply readings.

Who knows? We have no clue. We have not even seen the schematic. We donīt know what each part does.

I'm still trying to track down a schematic for the unit too.

Please do and then we might be able to help.

I mean, we want to, and if it were some popular widely known circuit, say a Twin or a Plexi or a Kustom or Peavet amp, we might find it on our own .... but being a modern complex somewhat mysterious processor ... canīt guess much if at all.
Thank you for the help.

[gpateluk]

The nice people at Focusrite are going to give me the schematic for the unit but first I need to sign a NDA (none disclosure agreement).

I'm guessing they like to keep their documents private and secure. That's why I couldn't find the schematics anywhere online. Doesn't help with this forum though.

[J M Fahey]

I suggest:
1) respect that agreement and donīt spread the schematic

2) use it as a guide for general troubleshooting , check signal flow path, power rails, connectors, check for the obvious.

3) IF you get stuck at some point, say signal gets to an IC but does not leave it, I guess cropping the schematic and showing *only* that IC and parts directly connected to it and voltage found at every pin is NOT an NDA breach in any way, same way as playing a few chords of some famous song to show, say, a pickup or guitar or amp sound is not a song copyright breach.

Good luck.