1990 Musicman Stingray 3EQ schematic

[Randall]

I am working on a tortured old MM Stingray bass with the rectangular circuit board marked B-4 on the back, and Mid and Treble controls mounted to the board. When I got it, it didn't work at all, and the battery got real hot. Turns out someone had replaced the output jack and wired it wrong. I have the wiring diagram, and it shows the original output jack with four connections, output, ground and two for switching the battery hot. The jack that is installed is a Switchcraft 3 connection switching jack, so no way to switch the hot with that. I changed it to switching the battery negative, and sent the positive straight to the board.

Now it passes signal OK, but the EQ doesn't work. The TL062 is getting power at pin4, but I wasn't sure it was working correctly (it's hard to really tell just by tapping the pickup with a small metal screwdriver), so I swapped it with a TL072 that I had. No change. I can see signal on both output pins, so it looks like it's working up to that point. I can confirm it is wired correctly per the wiring diagram I have. I have run out of ideas here. Sure wish I had an actual schematic to look at.

Anyone have any insight?

[nosaj]

Will this help?
nosaj

[Jazz P Bass]

Nice1

[vintagekiki]

https://www.talkbass.com/threads/diy-stingray-3eq-preamp.883257/
DIY Stingray 3EQ Preamp

https://www.freestompboxes.org/viewtopic.php?t=8354
Music Man Stingray Preamp

https://www.freestompboxes.org/viewtopic.php?f=13&t=431
Baja Music Man guitar and bass onboard preamps

https://www.talkbass.com/threads/how-do-i-wire-up-this-3-band-stingray-eq.1177439/
How do I wire up this 3-band Stingray EQ

http://web.archive.org/web/20160306090725/http://myweb.ecomplanet.com/town8019/mycustompage0019.htm
Pre-EB and EB BASSES

http://www.musicmanbass.global/sr4-pups-and-preamps-pre-eb-eb-part-2/
EB 3EQ PREAMPS (1987 - Current)

[Randall]

I have seen a hand drawn version of the circuit nosaj posts, but wasn't sure if I could trust it. Question is in this circuit, how is it possible I get a clear signal through to the output, but that the tone controls have no effect? It also will pass a lesser signal with battery disconnected.

[Helmholtz]

Question is in this circuit, how is it possible I get a clear signal through to the output, but that the tone controls have no effect? It also will pass a lesser signal with battery disconnected.

That makes sense as the EQ filters are in the NFB path of the second opamp (active filters) and there is more than one path for the signal to get to the opamp input (and output when unpowered).
Check the tone pots.

Do you have a scope?

[Randall]

Yes, Have a scope. I tested the tone pots in circuit, and they appear to being doing their thing.

I disconnected the pickup and fed a sine wave into the input on the board. I get a nice strong signal at pin 7 and on one side of the 100pF cap, but nothing on the other side, and nothing on the pots. I replace the 100pF cap, but no change. According to this schematic, if I have signal on pin 7, I should have it on one side of the treble pot at the very least, but I don't. I am puzzled as to why this is?

After further testing, I find that I do have the smaller signal from pin 1 on all legs of all pots, but the controls have no effect. When my scope was dialed up to measure the higher pin 7 signal, the pin 1 signal did not register.

[Helmholtz]

As said, check the pots, especially the wiper contacts. Use your Ohmmeter to verify connections.

I assume you made sure that opamp outputs are sitting at 4.5V.

To check EQ circuits I prefer a 400Hz square wave input signal.

[vintagekiki]

Question is in this circuit, how is it possible I get a clear signal through to the output, but that the tone controls have no effect?

Remove IR and battery. Short circuit treble pot (IC pin1 and pin7)

[Randall]

I can verify that all pots seem fine. Pin1 output is at 4.1v, pin 7 output i at 4.6v.

[Randall]

Does anyone have any more suggestions? I'm at a loss here.

[The Dude]

The pots are part of filter circuits, so using a test tone won't tell you much. There isn't much 5kHz or 60Hz to cut on a 1kHZ test tone. Try testing with music or something full(er) bandwidth.

[Helmholtz]

. There isn't much 5kHz or 60Hz to cut on a 1kHZ test tone. Try testing with music or something full(er) bandwidth.

That's why I use and recommend squarewave testing with a scope (PUs need to be disconnected):
https://sound-au.com/articles/squarewave.htm

Test signals can be found online, e.g.
https://www.youtube.com/watch?v=gc1hHSWbg5o

The floating EQ filters are wired in parallel, summation point being the inverting input (pin 6) of the second opamp.
Scope especially the signals at the pot wipers.

[Randall]

I got it fellas. The strong signal from pin 7 was not making it to any of the pots. Turns out there is a trace that leads to the 100pF cap coming off pin 7. There are two smaller pads situated very close to that pad, but not touching. I solder blobbed the three together, and now it all works. That seems odd to me that they didn't connect them, but I'm in business now.

And BTW, I used music this time around for a test signal, but after it was fixed I ran a sine wave thru it at varying frequencies, and worked fine.

Thanks as always.

[Helmholtz]

There must have been at least one resistor or resistive path between the pin 7 output and the inverting input pin 6. Otherwise you couldn't have had 4.6V at pin 7. Rather the opamp output would have latched to either 0V or 9V.