MRT-90 is a rugged old-scool semi-pro mixer. Why rugged? Because the case is made from real steel sheets. Why semipro? Because it is intended for smaller clubs or discoteques. Auto-start interface is present – you move the slider, CD player starts playing. There is only one single XLR connector – for the disk jokey microphone. Most audio connections are to be made via RCA jacks.
I like the kind of attitude that engineers were able to put on/in their products … due to worldwide political correctness this is not possible anymore:
Here is the inside view of the mixer:
Looking at the element base:
The actual issue with the mixer was the malfunctioning PSU. PSU is very simple and based on LM78xx and LM79xx series chips. Unsuccessfully, the safety limits for 12V stabilizer were too low. After accidentally shorting the lamp connector, the capacitor in front of LM7812 has died and taken LM7812 with it. LM78/79 chips on the PCB are getting too hot even after the repair, especially the chip for 12V.
Throughout the repair, the particular PCB was populated with new capacitors and chips:
Yet another problem – due to the crash of the 12V source, the backlight lamps within the VU indicators were not working any more. A crazy idea arised – to change lamps for white LEDs. Here you see how the lamps before the repair:
A lamp was substituted for one resistor and two LEDs:
Here you see how the LEDs look like when switched on: