Neotek Elite -Mute LED conversion to 12v - Help needed
« on: April 23, 2018, 01:55:57 PM »
Hi guys,

I was trying to mod my Elite board or to be precise I wanted to mod a mod that a previous owner has done. The Elite is a 48 frame board and the first 24 channel were modified.

Originally there is a mute circuit on the individual channels of the board that is on a tiny seperate pcb. The channel mute is activated when pressing the mute button  (tactile switch). Along with the muting a little red LED is lit if the channel is muted.

The previous owner obviously wasn‘t happy with the visual feedback as he decided to rip out the extra pcb along with the switch and LED and to replace it with a latching press button switch lit by an old school 28v mini light bulb which are more or less directly powered by the 25v rail the power supply provides. In order to get this mechanically done he decided to drill holes to the front panels  (16mm diameter) so that was definitely quite a job. Of course it is easier to spot which channels are not on mute now but the board is lit like a console from the 1977 Death Star  .-)

The mod must have happened like 20 years or more ago when  switches with bulbs were en vogue. Anyway, they are aging and occasionally the bulbs fail. I am a bit tired to take care of those things so my plan was to replace the push button switches with new LED based ones of similar type. Dumb enough the only affordable and visually ideal push button switches you get today are  those 12v ones used in cars etc made in China.

Looking at the original schematics the circuit looks like a text book LED switching design. Aside from the switch  being in line with two 4001B NOR gates (group mute link to the maste section) the LED is switched by a NP2222 transistor running on 25v, with a 100k resistor to deliver some current for switching the red LED which is connected to a series resistor of 3.9k to drop the voltage down to around 1,8v.

Now looking at the actual board the previous owner did a clever od. He removed  the 100k in front of the base and bridged it with a wire, letting the transistor run idle and wired the switch to the 25v rail directly. That would power the 28v bulb in the switch so he could kick out the red LED. Prettty effective.

Anyway, in my wisdom I wanted to partially reverse the mod by reinstalling the 100k base resistor and replacing the series resistor of 3.9k with a 680 ohm resistor, actually wiring it the old way. In my mind at least tjat  drop the 25v down to 12v to be able to use those new LED based switches.

Dumb enough when pressing the switch one of the logic chips went up in smoke. So while I am able to understand schematics and principle looks like I amfailing on the practical side of things. So I would appreciate if someone could  tell me what I am missing here and would point me in the right direction.

Attached an image of the circuit....


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