Moog Opus3 Chorus fault

[lukeA]

Hi all,

I have a Moog Opus 3 which I am trying to repair. The main problem is I am getting nothing from the Chorus section at all.

I can verify that both the actual generators in the Chorus section are working- ie both the direct Organ and Saw string via the eq are functioning. Right after the 'Organ/string Mix' slider there are two resistors, one in series with the signal (22k) and one in parallel with signal and 9V (5.6k) which are there to "divide down to appropriate drive level" (from the service manual circuit descriptions).

I'm not sure what kind of signal the input to the chorus board is meant to be, but this divide down stage reduces a 1V P-P waveform down to a very small signal. I can still see this signal at the inputs to the BBD chips, however cannot see any signal on outputs of any of the three SAD512D chips. All of the other circuitry on the chorus board appears fine-clock generators and waveshapers.

I am not very familiar with these chips but it just seems hard to believe that all three are dead. That being said I don't know what may have happened with this unit in the past. Is there something else silly that I could be missing on this before I start looking for ridiculously overpriced BBD chips??

Thanks a lot,

Luke

[thealien666]

SAD512D chips were made by Reticon, and are notorious for being sensitive to power supply instability, and failing easily. But I admit that all three failing at the same time is a little surprising. Unless something went wrong with the PSU that busted them, maybe ?

BTW, there are 3 transistors (Q2, Q3, Q4), one on each output of the 3 BBD chips that are configured as low pass filters. You should check for a signal after these transistors, since they're there to filter out the clock signal from the audio at the output.

Also make sure that the +14.5 volts DC is present everywhere it's supposed to be on that board.

[lukeA]

IT is possible that someone went wrong with the PSU and fried them- I don't know the history of the unit.

Have checked the outputs of those transistors, no signal present there. Only millivolts of noise I would say.

I will double check the 14.5V around the board, though I'm pretty sure its present as all the rest of the clock circuit etc functions as it should.

Does anyone know what kind of signal level should be present going into this board and the BBD chips?? Mine gets divided down quite low right before it exits the main board and i'm not sure if this is correct, though the resistors and everything there looks to be ok. The signal I am seeing is biased around 9V, and is a very small level.

Thanks again

[thealien666]

From what I understand of the schematics, at pin 7 of U18 on board #2, you should have a 1V p.p signal there. It is then mixed, via VR99, with the organ section and then sent thru R98, and then biased with +9V with R97, and then sent to board #3 (chorus) where an additional 5.6K resistor reduces still the signal before being decoupled by a capacitor and being reduce yet again by a 47K resistor before entering each BBD input on pin 6.

I don't know exactly what the signal amplitude should be on pin 6 of each BBD chip, though.

Maybe Kevin Lightner, who is a real tech (not like me), could calculate the reduction in amplitude of a 1V p.p audio signal passed thru a series of resistors, who's total resistance comes up to about 75K ohms ?
(I'm not good at maths)

[lukeA]

Hey,

Yeah that all seems pretty spot on. The maths is where I fall short also! I may try to get an engineer friend of mine to check the schematic and see what he thinks, don't wanna take up too much of Kevin's time, he's also helping me with my Arp Quadra I recently acquried!!

I have the 1V p-p signal as you said, and after the Organ/string mix slider the mixed signal is still at a healthy amplitude.

It is around after R98 that things look suss, but I'm unsure if this is normal or not. The signal is biased up around the 9V area, but the amplitude of the original organ/string mix gets so low that it makes me think something could be wrong here... All the resistors you've mentioned test fine. Have replaced R98 but might just replace all those to be sure.

Hmmm I just wonder what else could be causing the issue.

Thanks again.

Luke

[thealien666]

From reading the datasheet on the SAD512D, it seems that the lower the input signal, the lower the harmonic distortion at the output. And it rises pretty fast at higher input levels. But then again, I don't know what Moog engineers decided it would be in the Opus 3.

Usually, in analog chorus pedals from Boss for example, the input is first low pass filtered (to limit bandwidth and avoid aliasing from clock signal interference), compressed to a signal level of about 0.2V p.p. (to limit output distortion), finally it is positively biased (to lower the noise floor of the BBD chip). At the output the bias is removed and the delayed signal expanded back to about 1V p.p. But these pedals use a different Panasonic BBD chip design. There is no compander in the Opus 3 chorus design.

I can only surmise that all three BBD chips could be busted (unlikely, but still possible), especially if something went wrong with the PSU in the past, or a momentary short was made on that board by a previous owner.

Alain.

[lukeA]

Hmmm yeah thats certainly what things are pointing too... I might just have to buy one and swap them out to see for sure.

In the meantime I will have another poke around to see if I come up with anything.

Thanks again.