Vintage Synth Repair

Hi Malte

thanks for your advice. Output of the slider seems ok (1.36 V w/ KBD CV way up, LFO & EG down), but input of LM301 still 0. I removed the LM301 from its socket -- I already changed it several years ago, and then put a socket -- and try to get a chance to test it this week-end.


--- In vintagesynthrepair@yahoogroups.com, Malte Rogacki <gacki@...> wrote:
>
> 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.
>