Customer brought this nice unit in to look at as two meter lamps were out, wanted them sorting and also needed investigation/service as had some concerns over the calibration and signal routing (Master fader not working).
Took out the meter panels and the master module (which turned out to be ok in the end, just cleaned up fader and pushbutton switches on it)
I also noticed during initial testing that a couple of the meters got stuck at full scale deflection too (probably just be dirt/sticky stuff [nicotine from years past?!]).
As the incandescent lamps (10mm wide) seemed to be made of unobtainium the decision was made to upgrade them to LED which posed a challenge due to limited space – the first attempts using 20mm 12v dual LED modules at the base didn’t work as the bezel couldn’t be refitted, however this photo shows a clear brightness difference and slightly yellower appearance which wasn’t wholly unattractive:
Due to the fitment issues, chose to abandon the 12V LED module idea and work with some 3mm ‘top hat’ white LEDs which needed a bit more surgery to remove the old lamps and replace with LEDs, but they fit nicely and didn’t cast as much shadow as first feared:
Blu-tac to the rescue! – holding component in place after lead forming ready for soldering (quickly, or those black plastic pegs would melt), and testing at low voltage:
Comparison check to see if customer happy. Had a little trick up sleeve too which makes this mixer unique in the world, following below!
Sticky needles (seen sticking to the white ceramic ‘stops’) – cleaning with a good solvent seemed to do the trick!
All ten meters received the same upgrade. Testing all to work out the current limiting required to run off the +/- 17VDC supplies:
It was decided to make the brightness level adjustable! So a discrete hole similar to the meter zero adjustments was created between the RIGHT and 1 meters, and a bracket was fabricated to secure a brightness level potentiometer that can then be adjusted with a small screwdriver through the new hole:
Meter panels fitted back in during testing:
The sheet metal base of the mixer was removed to check cabling, connectors and a good amount of dust cleaned up!
A number of the gain pots had dried up grease and were very very stiff (close to knob breaking stiffness!) so a solvent was used to free up and relubricated accordingly.
A couple of the input module fader sliders (very high quality) were a little scratchy and not as ‘buttery smooth’ as they should be, so were removed and disassembled for cleaning and relubrication.
All the rear jacks were then cleaned/deoxidised prior to testing every single connection, button and knob for the correct behaviour and signal routing, and meters recalibrated.
Customer very happy with their serviced 400B mixing desk now – and probably now the only one in the world with LED-lit meters + brightness adjustment modification!