Ibanez SST acoustic gutar preamp

[Jazz P Bass]

"I replaced the input assembly because of a bad 1/4" input jack"

My feeling is that you are missing a ground connection.

[ObsCuRitY]

Hi,

I'm currently facing the same Problem. Unfortunately Dough's workaround did not work for me.

Power supply seems to to be ok. I measure +2.7V +5.8V and -2.7V.

Has somebody here another idea where to look or some more detailed schematics?

Thanks in advance!

Andreas

[dmeek]

Make sure the pickup is OK. If you have a scope or an amplifier with a probe, you can check for
audio on either side of the FET. If the tuner is not working I would suspect the pickup.

Doug

[ObsCuRitY]

Thanks for helping!

The tuner is working. My problem seems to be the same like yours: When I plug in the cable to the amplifier, the audio quickly fades out.

Pickup looks ok for me. When i connect the pickup directly with the amplifier I get a very weak signal. Seems to be the same signal strengths as when I have the preamp in between. What's the reason for the faulty preamp? Are the capacitors aging aging, shall I replace them? OpAmp seems to be ok, power supply is ok, what other component should fail?

My colleague will give me an USB oscilloscope end of the week - maybe that helps to identify the problem.

Regards,
Andreas

[dmeek]

Its most likely the 2SK30 FET or switching control.
The FET is an audio switch that that turns off the guitar sound when the tuner is active.
Jumper the two outside leads of the FET which is at the left side edge of the PCB in the photo above.
If shorting the FET works then change R64 (47K) on the small board to 4.7K.
I don't recommend the 10M resistor mod I described, though it did work for me once.

[ObsCuRitY]

I got the USB oscilloscope from my colleague and traced the signal from the pickup to the FET. Looks pretty good. What I know so far is, that also the FET is working. On the output side of the FET I have a good Signal, when I put the tuner on it's muted.

According to your schematics the next part is the equalizer. After that the signal once again goes through the OpAmp (Pin 12 = +in). At this Point (Pin 12) I have no signal any more. Do you also have schematics of that part?

Thanks for your help!!!

Andreas

[Enzo]

It is usually difficult to see signal at op amp input pins. If you send a signal into pin 12, the place to look for it is the output pin of that IC, pin 14. Is the signal present at the volume control itself?

[MarkusBass]

If the configuration of the op amp is inverting, pin 12 (+in) may be connected to the ground (so there is no signal at all). Additionally, you won't be able to observe signal at pin 13 (-in) due to so called "virtual ground": https://en.wikipedia.org/wiki/Virtual_ground . Please check the signal before resistor that is connected to pin 13, or (better) on the output of the op amp (pin 14).

Mark

[Enzo]

His schematic in post #11 shows signal from volume control feeding into pin 12.

[MarkusBass]

Thanks, I didn't noticed the pin number there :-(. So this may be non-inverting configuration.

[dmeek]

This is all I have so far. The dashed "shape" box is the Shape slider and associated components that I didn't sort out.
You might have a bad Volume pot. See if you have signal at both the top and the wiper terminals.
If or when I see another of these preamps I'll fill in some of the gaps.

Doug


http://music-electronics-forum.com/a...1&d=1493905014

[ObsCuRitY]

Hi guys,

First of all: thank you very much for all your help!!

I had some time to look at this again: I found out that nearly all of the 50v electrolytic capacitors were defective. I changed them all and it works now. My preamp is working again! *yippieh*

Thanks again for helping!!!

Regards,
Andreas

[yato_ibanez]

I purchased an AEB10-E bass which turned out to have this problem ... and fixed it with this mod:
> If shorting the FET works then change R64 (47K) on the small board to 4.7K.

Just wanted to say thanks, Doug

Cheers,
Will

[dmeek]

Got an SPT preamp in for repair, it's almost identical to the SST so I was able
to work on the schematic.

The paralleled 100k resistors at the output is a little strange but that's how it is.

The input stage gives a treble boost above 2.5 kHz.

On the SPT the emitter of Q2 returns to ground not -3V. I don't know if that was
my mistake or the circuit was changed. R64 still goes to -3V.

After all that this SPT just had dirty battery contacts.



http://music-electronics-forum.com/a...1&d=1499352988

[Stogie D]

dmeek, I have a Ibanez SST that has failed also. I am not a solid state electronics guy. Been following this post to see if this unit can be repaired as a new one, although available, is just too much $ for this guitar. I have removed & disassembled the unit, tried the jumper on the FET & have volume loud & clear. My question is, can I solder a jumper on it and use it that way essentially scrapping the tuner. I run through a tuner on both of my rigs so I don't care if tuner on the guitar does not function. Please advise.
D.