Boss KM-60 mixer meters problem

[Nele]

I have a Boss KM60 mixer in for repair with dead meters. The meters say "VU Level" but they are not standard VU-meters.
If I measure them with my multimeter on continuity test the needle jumps to the end but with audio nothing happens. They also have built-in peak leds but no separate power to drive them.

Attached is the driver circuit. Does anybody know where to start looking? All the components test ok but maybe the transistors are bad? Both channels are not working. Mixer is otherwise working ok. Someone had done some weird stuff with the psu but that's fixed.

 

[ruffrecords]

Check for signal across R709 and then for a half wave rectified version between the junction of D1 and R710 and ground.

Cheers

Ian

 

[Nele]

Thanks Ian, Ill scope it tomorrow. I guess the meters are probably some kind of volt-meter?

 

[zamproject]

Hello

Usually Vu are (micro)amper-meter
That's why it jump when injecting the small current of your multimeter in resistor mode.
This leading to conclusion that if your meter "jump" as you say the issue is elsewhere.

Best
Zam

 

[JohnRoberts]

Yup meter is ammeter (milli or micro probably micro)...  Driven by DC current rectified by D1, C709 smooths the current, attack through 3.9k release through the meter discharge. The fact that meter responds to your VOM (it injects current to measure resistance from the voltage drop)is a good sign meter probably works. Look for changing DC voltage on C709. If it charges up but doesn't discharge may be bad wiring to meter. If no DC voltage on 709 could be open diode (unlikely), open C708 (more likely), or lack of AC on transistor collector.  That simple transistor gain circuit is self biased up to a DC voltage mid supply (not middle), should measure roughly 0.5v on base.

They don't get much simpler than that, check solder connections, good transistor, etc.

JR

 

[Nele]

It turned out capacitor C708 was worn out. Temporarily replaced it with 4.7uF I had lying around and the meter works! Looks like the rest of the board might also be up for a recap given that all the capacitors are from the same series and age.

Thanks for all your help!

 

[JohnRoberts]

good job... caps lose electrolyte over time and become high impedance.

JR