LED's not lighting up but check out OK? [Solved]

Thread Starter

Alan3666

Joined Jul 22, 2021
15
Hi,

I'm still looking for circuit diagrams but this is just a general query I thought I'd ask the forum.

I'm working on an Alesis MMT-8 sequencer. It is fully functional but I have three LEDs that appear not to be working. When you press one of the buttons marked 'Song' the LED is supposed to light up. Same for 'Edit' and 'Echo' buttons. I thought at first the MMT-8 was faulty, but when you press each of these buttons the unit carries out that action. It echos the midi input, it goes into edit mode and it selects a song. So I thought maybe the LEDs had simply blown, so I opened it all up and checked the leds with a meter on continuity with a buzz. All of the LEDs lit up and buzzed with the meter connected , including these three. Nothing wrong with the LEDs.

I'm baffled that, although pressing the buttons makes the circuit and carries out the action it doesn't light the LED, but the LED is good?

Any initial thoughts anyone?

cheers

Alan
 

Audioguru again

Joined Oct 21, 2019
3,675
If the LEDs are simple indicators then they have their own driving circuit that could be defective but the circuit that is being indicated works fine. Maybe the LEDs are missing their series current-limiting resistors?
 

Thread Starter

Alan3666

Joined Jul 22, 2021
15
Fault fixed. Replaced IC U6 on logic board, a D-type flip flop that fed the three LEDs that were not working. There are three of those chips on the board but the schematic wasn't very clear which chip fed which leds. The middle chip fed the channel leds U7. I could see the signal change on pins 19 to 12 for channels 8 to 1. So it had to be U6 or U8. The pins on U8 had some kind of clock signal on its output pins so I opted for U6 and All leds now working. Result.
Also put new battery in Alesis HR16 and it now retains its memory. New battery £10. I had tried soldering in a Blackberry battery but that didn't work.
 
Top