1973 SG Systems Standel/Gibson SG100G Amp w/ Maestro Phaser

[mort]

Ok so I bought this amp pretty cheap and it sounds horrible, takes up too much space, is extremely heavy and no one wants to buy it so I've decided to cannibalize it for the speaker, a few other parts and the phaser effect, if possible.


My music electronics experience has been mainly building known circuits from scratch and a bit of troubleshooting run of the mill issues on tube circuits, so this is a green area for me.


I'd like to extract the phaser circuit from this amp and build something to house it in.


1st question: Is it fairly straight forward to do this?


In general I'm thinking that it would need a power supply built for it, and locate the input and output of the effect, add some jacks and voila(?)


It even has the foot controller connector so if I ever found a foot controller that should just be plug and play.


Thoughts?


Thanks in advance.


http://www.prowessamplifiers.com/sch...ages/sg100.jpg

[nevetslab]

Well, as for anything being simple and straight forward, particularly without schematics, and presumably without a lot of experience in this sort of thing before, rarely is it simple and straight forward.

In your favor, though, it appears the layout is somewhat organized with the four circuit boards. Left front board appears to be the preamp section for the left input jacks, the middle board appears to be the preamp section for the other preamp, and the right front board probably the Masetro Phase shifter circuits. Follow the cables from the middle board over to the phase shifter board to see what you can figure out. The Phase Shifter circuit on it's own may require a gain stage so if stand-alone, plugging in a guitar you'd probably need some level boost. Though most effects pedals tend to appear to be unity gain, so you're still sending guitar-level signal to the amp. The rear PCB assembly appars to be both the power supply and the power amp to driver the speaker.

The middle board may also have the reverb send and return circuits to the tank thats probably mounted to the floor of the cabinet.

Looks like maybe there's a JBL D120F speaker? Seeing the aluminum center dome thru the grill cloth, though later products have added that dome to make them look like a JBL. This being very heavy may be from the heavy speaker.

Your stand-alone module still needs a power supply. You need to see what DC voltages are needed. I see a few IC's on the Phase Shifter circuit, so you do need to do some exploring with your DMM to determine basic voltages used to run the preamp sections, and whether its' running single supply or bipolar. You'll have to identify input and output on the Phase Shifter circuit. You'll also need to measure basic signal level into/out of the phase shifter.

Try and track down the schematics for this unit....THAT will be invaluable information to make this project a WHOLE LOT EASIER! Good luck.

[TomCarlos]

Check out this post on MEF - Photos, pictures of the Schematic

Gibson SG Systems G100 (Problem with built in 'Wave Form' Fuzz)

http://www.prowessamplifiers.com/sch...ages/sg100.jpg

http://img.photobucket.com/albums/v6...SG100G1of3.jpg

http://img.photobucket.com/albums/v6...SG100g2of3.jpg

http://img.photobucket.com/albums/v6...SG100G3of3.jpg

[mort]

1st post updated with schematic.



Ok so after looking more closely it looks like the lower right board is indeed the PS-1A Phase Shifter board (even labeled as such underneath it) and has two leads for power feeding 67vdc via the orange and red leads at the upper right of the board that drops to 22vdc after the r/c pair.

And 2 shielded cables which are the in & out.

There are 3 other leads that I'm not quite sure about. The yellow at the lower left goes to a center lug on the 3p3t main power switch, and the brown and red near the center of the board go to the same switch with one tied to lug 1 and the other to lug 3 on the same throw. Center position is "off", and the upper and lower positions of the switch are each labelled "on". So it appears that in either "on" position the yellow is being connected to the same spot on the board so I'm wondering why it is being switched at all. Should I just jumper the two spots on the board?

[mort]

Here are photos of the extracted phaser board.

Oh and yes the speaker is a 15" JBL M31-4 which according to some conversations found online appears to be a reissue of a D130F or something close.

[g1]

The yellow at the lower left goes to a center lug on the 3p3t main power switch, and the brown and red near the center of the board go to the same switch with one tied to lug 1 and the other to lug 3 on the same throw. Center position is "off", and the upper and lower positions of the switch are each labelled "on". So it appears that in either "on" position the yellow is being connected to the same spot on the board so I'm wondering why it is being switched at all. Should I just jumper the two spots on the board?

Look at the power switch on the schematic. It looks like the yellow gets grounded only when power switch is in either of it's 'on' positions. Maybe to deal with some turn on/off thump or something.
It seems the phase shift circuit is only shown as a 'black box' so we can't really know what it is doing.

[mort]

Look at the power switch on the schematic. It looks like the yellow gets grounded only when power switch is in either of it's 'on' positions. Maybe to deal with some turn on/off thump or something.
It seems the phase shift circuit is only shown as a 'black box' so we can't really know what it is doing.

Ah I didn't see that on the drawing. So just a ground. Got it.


This is looking simpler than I thought it would be. The effect worked fine so as long as I don't do something stupid it "should" turn out fine.

[icke2433]

Did you ever get this running?
I have the exact same board and have been working on getting it set up as a stand alone pedal. I'm replacing the caps and the pots, but the circuit seems slightly different from the PS-1A schematic. Mainly that op amp at the input and the transistor right by the output. 67 V seems like it's too large. I'm thinking I'll be using a +/- 24 vdc power supply

[icke2433]

Sorry to keep this dead thread going but I'm going to keep track of my thoughts in case someone else tries to harvest this board.
So far, I've been playing with this circuit and tracing out what I can. I've replaced the trimmer pots, the power supply electrolytic caps and plan on removing the first set of voltage dropping resistors as I'm using a stand alone +/- 15 Volt COTS power supply. This doesn't seem to match the PS-1A schematic you find when googling it online. I've attached the one schematic that seems to match, but it was quite the hunt.

The primary differences I'm seeing aside from the power supply from a PS-1A circuit is the line buffer op amp which I don't think will be a problem as it has a high impedance input and a low impedance out.
The board then traces to a TI559 transistor that is pulling a negative voltage from the power supply electrolytic capacitors prior to the voltage dropping 270 ohm resistors (so you get a larger negative voltage). Attached is a Q&D drawing of what I'm seeing.

I'm contemplating removing both sets of voltage dropping resistors so that I can get the most out of my power supply (I've read these circuits like more headroom) but have not quite figured out that second transistor at the output. I think if I needed to I could modify this circuit by removing the op amp and the second additional transistor and converting it match the attached schematic but will hold off until I get this thing in a box and see how it sounds.

Pictures are attached

[Helmholtz]

Pictures are attached

But unfortunately don't show .

[icke2433]

I somehow hit my upload limit without uploading anything. Here are my notes/schematics

[icke2433]

And here's the output transistor hand traced schematic

[g1]

The board then traces to a TI559 transistor that is pulling a negative voltage from the power supply electrolytic capacitors prior to the voltage dropping 270 ohm resistors (so you get a larger negative voltage).

That would be a TIS59 which is a FET transistor like shown on the PS1-A schematic. The rest of the statement I do not understand at all.

[Helmholtz]

Not sure what you mean with output transistor, but your drawing doesn't look correct/complete.

I'm contemplating removing both sets of voltage dropping resistors ....

If you're keeping the zeners remember that a zener always needs a current limiting resistor.

[mort]

Did you ever get this running?
I have the exact same board and have been working on getting it set up as a stand alone pedal. I'm replacing the caps and the pots, but the circuit seems slightly different from the PS-1A schematic. Mainly that op amp at the input and the transistor right by the output. 67 V seems like it's too large. I'm thinking I'll be using a +/- 24 vdc power supply

It's in a ziplock bag and I've don nothing with it as of yet.