Sirius 24 output voltage low

[dniggema]

Hi,

 

I have a slightly odd problem with my nearly-resurrected Sirius 24. After replacing some damaged hardware (faders and keycaps), I found that the battery wasn't holding charge and needed replacing. I replaced the battery and then found that this would discharge again within hours. After some speculative component swapping I finally found a defective diode (1N4148) on the CPU board that was causing the battery to discharge quickly. I replaced this,and now the battery holds charge and the board boots reliably. However, when it first starts up the output voltage at 100% fader position is low. In fact, it's about 8.5V, and the 0% output voltage is also high, maybe just over 1V. The voltages are proportional to fader position, though. Over a period of about 30min-1h it settles down to the correct voltage. I can't seem to stop it from doing this, even after replacing the resistors that are connected to the jumper block that controls the output voltage and some nearby capacitors that I thought might be questionable.

This seems to have improved the situation slightly, but so can sticking your finger on the leads to the resistors- they're between 300K and 470K so skin resistance will make a significant difference.

 

I have no idea what causes this. Has anybody had the same problem?

The other Sirius we use in our regular venue seems to have the same problem, but less seriously. It takes about 5-10 minutes to 'warm up' to the correct voltage level, so that all the dimmers are slightly 'low' on start-up.

 

Thanks,

 

Dirk

[Mars]

From the service manual

 

Vbat:

from plus16V: D42 1N4148 in series with R69 2k2

from plus5V: D41 1N4148 in series with R68 220R

parallel to battery: C67 100nF

Note: "with PSU off: +4.3V to +3.6V"

Check D42 and R69 and ground of the battery (coppercorrosion)

 

Faders are supplied with the buffered Vrefs from IC39

Vref2 on outputpin 1 of 39a from Vref1 on inputpin 3 and Vref 1 on outputpin 7 of 39b from Vref on pin 5, Vref = VR of IC 34 on pin 10 (the internal ref of the ADC); all at 2.55V. This ADCVR is supplied by R67 1k5 from the Plus5V supply and has a bypasscap C64 4u7F.

Check R67 and ground of C64.

 

Request a service doc (email Keith).