Vintage Synth Repair

Hi group,

I'm currently restoring my whiteface Odyssey. I tuned it up (PSU, A and B boards), changed a few tantalums and changed the LM301 driving the filter. 

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? 

Any advice to test the 4023 filter module?

Thanks,
Francois

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.

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.
>

At 17:16 Uhr +0000 05.04.2013, alesisfusion8hd wrote:
> 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.

There probably wil be very little keyboard cv directly at the input of the
LM301 since it passes through a 100k resistor before.
If you look at the implementation of an inverting opamp you'll see that the
gain of the opamp is defined by the input resistor and the resistor in the
feedback loop.
The feedback loop has R9 and T5 as resistors; together they form
theoretically a resistance between 88.7k and 113.7k. The input resistor
(R4) for the keyboard CV is 100k. It's obvious that we're looking basically
at a gain of 1 here (so the signal amplitude stays the same, just the
polarity is reversed); and T5 is used to finetune the gain (because we want
proper tracking over the whole keyboard).
R4, R5 and R6 are not just there to decrease the control voltages; they
each define together with R9/T5 the gain for the control voltages.


I believe the keyboard CV should be 2V for the lowest key of the Odyssey
without any transposition and with the tuning knob centered.