Morley Sync Attack Box

[Eb7+9]

... and just in case it's not obvious, one can more confidently and easily confirm basic operation of the side-chain by starting out using a 3mm RED LED instead of the opto diode  ... the signal-path and side-chain are two independent circuits after all

[nbenning25]

Appreciate the responses and "dusting off" your efforts.  I'm not going to pretend I understand all the technical aspects of this thread conversation other then being able to test continuity and DC voltage readings

I happen to own one of these boxes.  Procured it back in the late eighties.  Recently pulled it out of my closet.  I've been trying to get it to work but it doesn't seem to create any tone changes whether it is off or on.  I remember back in the day when it was functional, it created volume swell tone.  Think of the beginning of the song "Bargain" by The Who.

I may play around with it this weekend to see if I can get it to function. I'd be happy to supply pics and contribute information where I can to your bread board efforts.

Cheers!

[pacealot]

Well, I proceeded to rebuild this on the breadboard, stage by stage, based on Eb7+9's thorough description of the action of each stage. Returning to 2N5087s for the first two transistors (and staying with positive ground/negative rail), I got the voltages they predicted it should have in each condition. Finishing the circuit otherwise as drawn (including the flipped MPSA12 Darlington), I'll be bleeped if it didn't actually work this time! Even with my less-than-optimal opto unit, it goes more or less fully "off" in pick-grounded position now. (I do intend to pick up an NSL-32SR3 for this, as it does appear to have more ideal light and dark resistance specs for this circuit.)

I will say that there is a slight level drop from unity, due presumably to the 33K in line with the signal (which is the other reason I presumed that to be a potential factor). But I think the effect's utility would be enhanced by a boost to juice up the swells above that anyway (for psychoacoustic reasons if nothing else), so something simple and P-channel FET-based should do the trick. I'm happy sticking with positive ground, as it will likely go along with several other circuits of like polarity.

Thank you Eb7+9 for pushing me to understand this more clearly in spite of my obvious EE-related shortcomings. Nbenning, if you need working voltages to help diagnose your own unit, I can post those soon-ish.

[pacealot]

As promised, voltages from the working circuit (2N5087s, MPSA12, positive ground):

Ungrounded
Power: -9.13V
Q1C/Q2B: -0.049V
Q1B: -0.634V
Q2C: -8.64V
Q3B: -8.62V
Q3E: -7.53V
LED cathode: -9.04V
{Q1E, Q2E, Q3C: all 0V}

Grounded
Power: -9.06V (interesting that it pulls a bit on the rail in grounded position)
Q1C/Q2B: -0.694V
Q1B: 0V
Q2C: -0.006V
Q3B: -0.022V
Q3E: -1.156V
LED cathode: -2.98V
{Q1E, Q2E, Q3C: all 0V}

nbenning25, I hope some of this can help you debug your vintage SAB! And don't forget — everyone here loves pics of vintage guts!