There are always little maintenance jobs waiting to be done in a studio. One I had been meaning to get round to was replacing the VU meter in the mighty Focusrite ISA 430 Mk II Producer Pack mic pre.
What was wrong with it? The bulb had gone.
Cyril Jones, the founder of Raindirk and one of the great British audio designers, once told me that he used to tell his clients never to turn their desks off. He reckoned that the only time components were stressed were during power-on, when that initial surge of current hit cold components. Once powered up, he would say, the thing would go on working forever.
He had a point. I do try to leave my analogue equipment on, turning it off only when I know I won’t be using it for some time. But I got into a situation with the Focusrite; I was fooling with different grounding arrangements for the mains and measuring the effect on noise floor, so I was turning the thing on and off repeatedly. And inevitably, the bulb went. An incandescent bulb is an obvious case-in-point for Cyril’s advice. At the instant of turn-on, a high current momentarily surges through the bulb as it warms up and its resistance increases.
Other people to whom this has happened have moaned extensively that you can’t apparently replace the bulb in this type of meter. You have to replace the whole meter. Actually, it’s probably possible to replace the bulb (even with an LED). I’ll be experimenting on the old meter when I get a chance. I’ll tell you how I get on.
Obtaining The New Meter
Folks I’ve read about who found themselves in this position seem to tell stories of ‘finding’ a meter on eBay, or from such-and-such a supplier. Before you go down this route, and especially if you’re in the UK, you should reach out to Focusrite’s service department. After all, this is professional gear, not domestic or pro-sumer, and accordingly is intended to be maintained.
Focusrite supplied me with a brand new meter, with the all-important ff logo. The cost was £28.08 including VAT and shipping. At that price, why would you go anywhere else?
Fitting The New Meter
After removing the 10 screws to withdraw the lid, the meter is revealed, held in place only by two metal brackets which are bent into place:
(Take care lifting the lid; there is an earth strap running from the underside of the lid to main chassis.)
You can straighten the brackets easily with needle nose pliers, and the meter then simply drops out. The four connecting wires terminate in a connector at the extreme end of the main PCB, so you can remove the meter completely to do your soldering:
Then, it’s simply a case of transferring the four wires from the old meter to the new one. Make sure you don’t mix up any of the connections, then pop the new meter in. The metal brackets can be gently re-bent to hold the meter snugly. The whole exercise takes about 20 minutes.
Power Supply Connections
While I was in there, I looked at another problem that the 430s ( Mk I and Mk 2) can suffer. The main connection from the transformer to the PSU board is via a Molex connector. Now these, as anyone who’s had the privilege of maintaining an MCI multitrack machine knows, can be troublesome. People report burn marks on and around the connector where the molex is no longer making a good connection.
The oft-quoted ‘solution’ is to discard the connector and solder the wires directly to the board.
In my case, there was a barely-discernible discolouring of the plastic which you can see in this picture:
I decided this was acceptable. I cleaned the connector contacts and applied a little contact lubricant before putting it back together. I’ll make a note to look at it again in six months to see if it’s got any worse.
Finally, I noticed the mounting bolt for the toroidal transformer had worked a little loose, so I tightened it. This is well worth checking whenever you’ve got a piece of gear like this open for inspection.