Ibanez SST acoustic gutar preamp

[dmeek]

This is a long shot. Does anyone have the schematic?
Sounds OK for a fraction of a second when the cable is
first plugged into the guitar, then volume fades out quickly.
Tuner seems to work though.

Doug

[Jazz P Bass]

I don't recall ever seeing a schematic for any preamp.

Check any & all opamps for Vdc on an output pin. (that would indicate a failed IC)

If not that, you may have a coupling cap going south.

[doctor]

Try to address to Ibanez Collectors World forum:
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[dmeek]

The problem seems to be in the power supply section, It runs on 2 penlite cells and must have a voltage converter.
The main board TL064 pin 4 is +6V, pin 11 is -3V. A little odd but could be OK.
On the tuner board there is a 358 dual op-amp with -3V on pin 4, -3V on pin 8. Definitely not OK.
I see on another Board this was a common problem and was fixed by replacing with a new version preamp
I'll check the Ibanez Collectors Forum.

Thanks
Doug

[dmeek]

I fixed the power supply problem. There was a bad connection on a capacitor in the charge-pump circuit which is an SOT23-6 chip marked "SO3A" (LM2664). However that didn't fix the audio. The sound is clean and loud for the first half second, then it fades out. DC voltages are centered at the op-amp outputs. The mystery continues...

Doug

[Jazz P Bass]

It is really tough repairing these little things!

No schematic or board layout.

Punt: check any & all of the opamp output pins.

In all likelyhood, one of them will show as dropping out.

[dmeek]

After spending way too much time on this, tracing parts of the schematic and general messing around, I found the solution.
There is a 2SK30 FET for muting the output when the tuner is on. I had to add a 10M resistor from the gate to +3v to make it switch on and off the way it should.

Doug

[Jazz P Bass]

Yeah, time consuming, to say the least.

I like when the traces run under a bunch of parts & Then go into a through hole.

Great fix, but it is does beg the question: 'what is wrong that you had to do that '?

[dmeek]

I'm guessing it's a marginal design. A little leakage current would keep it biased the right way, but not reliably over time.
There is a discussion about it here: AEQ-SST preamp stopped working. Does any other preamp fit... - Technical Support - Ibanez Forum


Here is my Mod:

http://music-electronics-forum.com/a...1&d=1395267829

[tchewtch]

My AEL2012E has recently developed this same issue. I figured that I may as well give dmeek's fix a go before looking for a replacement. Dis-assembly is relatively straightforward, you will need to de-string in order to put a hand inside, through the sound hole. You will need a very fine bit for your soldering iron, but the mod is very simple to do, any electronics hobbyist could do it. The fix has successfully restored the audio output from the preamp. I would like to thank dmeeks working this one out.

Dave.

[dmeek]

I have an update on this.
I had another SST to repair with the same symptoms but my fix described above
didn't work. I traced out more of the circuit and found the transistor (Q2) which
provides the switching voltage to the gate of the FET, wasn't switching reliably.
I changed the smd emitter-base resistor R64 on the smaller board.
Was 47K (473), now 4.7K (472).

Doug


Here is a schematic for those interested. My modifications in red

http://music-electronics-forum.com/a...1&d=1413833651

[dobrota]

Please have You complete schematic of Ibanez SST preamp ? Thanks.

[dmeek]

I wish I did. I spent a few hours tracing most of it. the result is a very messy
drawing full of errors I'm sure. I didn't trace the power supply fully. It is an
smd LM2664 voltage converter which outputs +6V, +3V, and -3V.
If I have time I'll clean up the rest of the schematic which will at least
be a rough guide for signal tracing and repair.

Doug

[DrewV]

My AEL2012E has recently developed this same issue. I figured that I may as well give dmeek's fix a go before looking for a replacement. Dis-assembly is relatively straightforward, you will need to de-string in order to put a hand inside, through the sound hole. You will need a very fine bit for your soldering iron, but the mod is very simple to do, any electronics hobbyist could do it. The fix has successfully restored the audio output from the preamp. I would like to thank dmeeks working this one out.

Dave.

So two days ago my 2005 AEL20 had the same problem. The 10M fix worked with no issues. Radio Shack has a five pack of of 10 Mohm 1/4 watt resistors for $1.50. To aid in install put a tiny hook on one end of the resistor and feed it around the gate leg. (You will have to trim the lead length first) then bend the other lead so it will land right on top of the target resistor, but don't trim until after it is soldered using it as a handle. Put a tiny bit of solder on the hook and the spot that will solder to the board resistor. Put the 10M in position and touch the gate joint with the soldering iron. do the same on the other end and trim excess lead. Thanks for the info!! DrewV

[DrewV]

Hum

So two days ago my 2005 AEL20 had the same problem. The 10M fix worked with no issues. Radio Shack has a five pack of of 10 Mohm 1/4 watt resistors for $1.50. To aid in install put a tiny hook on one end of the resistor and feed it around the gate leg. (You will have to trim the lead length first) then bend the other lead so it will land right on top of the target resistor, but don't trim until after it is soldered using it as a handle. Put a tiny bit of solder on the hook and the spot that will solder to the board resistor. Put the 10M in position and touch the gate joint with the soldering iron. do the same on the other end and trim excess lead. Thanks for the info!! DrewV

Since doing the mod I am getting a low level 60hz hum on both the 1/4" and XLR outputs. I can eliminate it by touching the input jack. I replaced the input assembly because of a bad 1/4" input jack. Is anyone else gotten this with the mod? thanks DrewV