Focusrite Voicemaster Pro problem

Hi all,

I have a problem with a Focusrite Voicemaster Pro I have in for repair. This is what has happened and caused it to break:

A tubemic with it's pwn psu was connected to the channel. Than somebody accidently switched on the phantom power and this caused the channel to break.

Now, when you connect any kind of mic (dynamic or condenser) it suddenly drops volume and sometimes emits loud cracks and pops. Line-input behaves ok.

I don't have a schematic and it's all SMD. Any suggestions? I already contacted Focusrite but their answer was to send the unit to them..

Thanks

Hi Nele
I drew out the schematic of the 'rev 3' version mic amp a while ago...will try and find it when I get home this weekend and scan it for you
IIRC the differential to single ended conversion is after the mic/line/inst switching.
The mic amp is  made up of two (hot and cold paths) cascode circuits each sandwiched between a current source and a current sink and sits to the left of the relay on the mic amp pcb (component side).
The input protection was i think, back to back Zener 'fuses' between the base of the input transistor and ground

Cheers

tc

Here's the input board. It looks like rev 3. Is this the one you've drawn? That would be very helpful!
Cheers,
Corneel

Hi Corneel
Looks like the same beast..
IIRC I didn't bother with the insstrument input in the top right hand corner
The top left corner behind the XLR is the mic i/p and protection...mic pre underneath then Mic/line/inst switching
IC 3 the 5534 is the variable gain line stage and the circuit underneath the ribbon cable with the 5532 is the bal-unbal convertor

Cheers
tc

Great!
Do you have any idea what the parts beneath the bottom transistors are for (r6 - r14)? They look a bit fried. Are they used for the phantom circuit somehow?

Thanks!
Corneel

found it.. Needs redrawing will have a go this afternoon...

The resistors are part of the current source

tc

Hi Corneel,

the drawings are in the tech docs section.
Good Luck!

cheers

tc

Awesome!!!

Thanks
CC

Finally have this in for repair again.. and the mic-pre section is not working anymore. It's very noisy and 'clicks'. Instrument input is working.
Led 2 (Ld2) was not working anymore. I already replaced it but no change.

Attached is topcat's traced circuit.

Any ideas where to start?

As the LED between Q1 and Q6 was gone I'm thinking the problem lies with one of those transistors..?
Any suggestion on how to check? Is desoldering the only option? I already tried this but the board holds on to it's components!

Hi Nele,
Is the power supply stable.....  I've found dry joints on the power supply board
What's the voltage drop across the led?
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... on close inspection the the solder looked crystaline... a consequence of running hot?
If it's not that I'd look at the input transistor and check that the zeners haven't gone to valhala.
I was given a bunch (6 iirc) of these units a couple of years ago, fixed two and have decided to keep the others as 2u Boxes with a handy IEC and power transformer for prototyping.
The curse of pro-sumer devices...'good value for money'..lots of facilities for not too much cash but there is a price to pay in the long run. The Behringer mic processor was 'very similar' and much cheaper and an amazing piece of production engineering ( a truely disposable product) ...whilst it was available
There are some brands that buck the curve, we have for example a few low cost Drawmer units kicking around that are very robust
I work in the broadcasting industry and we tend to look for devices that don't have lot's of bells and whistles but just work for 10 years.  the rise of pro-sumer  has led to the adoption of devices like this, as may of our tradional sources quit the sector as the products are simply not profitable to produce...... Right now we are looking at repeating history and manufacturing, for our own consumption 3u Eurocard based mic amps and other periferals.
sorry for the rant.....

Topcat, thanks for the info! And I totally agree with you. This pro-sumer stuff is a pain in the ass to service. Had a similer experience with a isa unit.

Small update: I've checked the phantom power and it is unstable. It starts around 48v and then slowly rises till above 53v... Do you know where the phantom power curcuit is? Front pcb or back? Couldn't find it but I'll check again.
Also some other weirdness. The preamp kind of works if I squeeze/touch the frontpanel at both ends with both hands. Seems to be a grounding issue but ground measures fine. :

Hi Nele
IIRC the phantom power circuit is on the rear board at the top of the left hand (looking from the front) end.

Goodluck!