The Maestro USS-1 Universal Synthesizer System was an early multi-effects unit from Tom Oberheim under the Maestro (Gibson) brand.
I've had the opportunity to service 3 of these units thus far.
These are some of my notes, in case others find them useful.
Disassembly begins with the back panel. Remove screws below the 1/4" jacks and on the bottom along the edge of the back panel.
#6 nuts hold the circuit board in place. Remove the molex connectors and any slider caps from the panel, then unscrew the nuts and the board should wiggle out for you. The ease of this probably depends on how bent any of the threaded rods and sliders are.
Quickly you realize that the connector scheme allows for ease of board replacement but not ease of board troubleshooting.
Before you even get to the board, I would remove the screws and nuts from the front panel (far left and right of the slider graphics) and the remaining bottom screws so that the whole case opens up, and you have good access to the rocker switches.
The rocker switches have a common wire and then each has an individual wire to activate the specific effect circuit. Use a low resistance check to confirm each switch is working, and clean contacts as necessary. I use a curved dental tool and some deoxit. Look for how people clean transistor organ contacts for more info on this. Note that the flexible wire is very similar to guitar string. When I've had to replace a broken one, I find a similar sized guitar string and solder that in.
After I fix the rockers, I don't put the board back. Instead, I will use jumper wire to activate the effects. I find the connectors are way too heavy duty for the kind of on/off action that troubleshooting demands. I only plug back in the power transformer secondaries so that the board can be powered up.
I solder in some solid core wire (I save clipped through hole resistor leads for tricks like this) to the connects that go to the 1/4" in and output jacks, then I use my alligator clip-to-1/4" jack adapters to connect to my test equipment.
Alternatively, you can just take the 1/4" jacks off the back. I can't remember why I ended up preferring the alligator clips.
The connectors seem to be prone to developing bad connections. If it looks like someone has serviced the unit before, definitely solder all of the pins on the connectors. Check all the wires carefully that they are not coming out of the connectors.
When you're ready to activate an effect, you just need to jump in a connection between the common switch connector and the desired effect connector. An alligator clip connection works, but I found it much easier to just use some solid core wire in the open connector slots.
If you use the schematic's numbering of the switch connector, this is the key:
#1 = Switch common rail. This is a transistor buffered +15V rail.
#2 = Connected to -V2 (-15V). Only used by the external connector on the side. Not involved in switching.
#3 = PHASE
#4 = The base of the previously mentioned transistor buffer. (#1 is the emitter). Only used by the external connector. Probably used to transfer “control” from the manual switches to the pedal switches (just my guess).
#5 = SAMPLE/HOLD
#6 = FILTER
#7 = SUB-HARMONIC
#8 = No Connection
#9 = ENVELOPE
#10 = AC (straight to power transformer secondary)
#11 = AC (other secondary) (#10 & #11 are probably for illuminating the pedal switches)
#12 = WAVEFORM
Here are my notes on some of the individual circuits:
TRIGGER CIRCUIT
The SENSITIVITY slider feeds a TRIGGER CIRCUIT comprised of Q32-35. This circuit isn’t very good. Try it in a SPICE program. The output at Q35’s collector will create multiple triggers. Ideally, this should be a Schmitt trigger that only triggers once and has a “dead band” to prevent multiple triggering. Maybe Oberheim liked the multiple triggers - it is sometimes musically interesting - but I would have preferred a Schmitt type trigger. Point of this note is that you shouldn’t distress (like I did) when you observe multiple triggers at Q35’s output. This is normal.
ENVELOPE CIRCUIT
A11a and A11b should have their outputs toggle. One should be high, while the other is low... never should they be the same.
Q36 resets the envelope voltage upon a new trigger.
The envelope output (emitter of Q39) should start low, rise up at a rate set by the ATTACK control, then A11 pin 1 will toggle A11a/b when the threshold voltage is reached, which brings pin 11 low, and the voltage at C37 drains through D11 at a rate set by the DECAY control.
My particular USS-1 had a bad Q40 with an open emitter terminal. The symptom was that the envelope didn’t work! More specifically, the envelope voltage at C37 would get stuck high if the ATTACK slider was set to more than about 5k or so (almost 0). Manually bringing the attack slider down would finally discharge the cap... this was when Q39 would finally get enough base voltage to push some “envelope” signal out its emitter, and allow A11a to toggle. Really tore my hair out chasing this one down, but it turned out to be Q40. Just some basic voltage tests for Q40 and Q39 made it pretty obvious. I think Q39’s collector was low, which didn’t make sense.
PHASER CIRCUIT
This is your classic “matched FET” phaser circuit. It’s probably pretty close to the other Maestro Phaser. Symptom I was getting was no phase, or sometimes a weird sounding phase. I’ve found these are common symptoms with “matched FET” phasers. The phase was only working with the bias and LFO depth trimmers fully ON (bias was about 0V). I pulled all the FETs, and found they all had a fairly high Vgs(on), and 1 was broken. The broken one was probably most responsible for the “weird” and lopsided sounding phase. I matched a set of 2N5458 that had a lower Vgs reading than the pulled FETs (the originals are 2N4303). When put in circuit, the replacements allowed me to adjust the bias voltage down a bit, and the phaser sounded like a normal phaser again.
I have encounted the “weird/lopsided” phase before with matched FETs. The solution is to pull the wonky set, then match a new set and install.
Here’s the link to RG Keen’s JFET matching circuit I used for this:
http://www.geofex.com/article_folders/fetmatch/fetmatch.htm
Note that RG uses a 9V single supply... I did away with his 4.5V rail, and I used +/-15V and put 0V where his 4.5V is. For the USS-1, I would do the +/-15 as it is what the USS uses. If doing a Phase 90 or other 9V powered thing, then the 9V test is more appropriate. If you have a bipolar supply, the +/-15V test is simpler than creating the 4.5V rail.
WEIRD PROBLEM:
Horrible popping and thumping when using the rocker switches, but works fine when using jumpers.... what is going on? Bad switches?? (The FILTER is popping...)
I was able to solve this by CUTTING THE TRACES that go from the AC power and the #2 and #4 connectors (all stuff that the external switch uses, not the main unit). This did it. Not a great “repair” but it worked. The cut traces are easily repaired if someone wants to fix this later.