Vintage Synth Repair

At 17:29 Uhr +0000 03.04.2013, alesisfusion8hd wrote:
> However, there is something weird in the filter section; the driving voltage
> (which should be negative according the service manual) is always close to 0
> (ADSR moves it a little bit). And the filter itself is always closed: to open
> it up, I have to set KBD CV, LFO and ADSR control voltages, as well as CUTOFF
> at max.
>
> I removed the filter and checked the output of the LM301; seems good but
>does > anyone knows what voltage range I should expect out of the LM301?

The LM301 is wired as an inverting amp which means the input will be
inverted; positive voltages on the input will become negative voltages at
the output.

I believe that if the slider for keyboard cv is in the top position there
should theoretically about the same negative voltage come out; so if the
keyboard CV is for example +2V the output of LM301 should be around -2V.
Same for the envelopes.

For the LFO it should be inverted and roughly triple.

You need to measure the original voltages directly at the switch or slider,
not at the LM301 input.

Please keep in mind that the 4023 has two control inputs which are both
used. The cutoff slider goes to input 4.

The test procedure is described in the service manual, section 4.3 - right
the first entry is for testing the inverter (which, after all, is the
LM301). My guess at the moment is that the LM301 is shot.