Ibanez preamp

[Mark Black]

As has been said, the preamp should likely still work to some extent even if there is indeed a missing cap.

I think there are bigger problems going on: the corrosion visible in the photo immediately reminds me of other surface mount automotive PCB's I have worked on which had numerous discontinuities everywhere. I think if the solder mask is scraped off the darkened areas of the traces the true extent of the horror will be revealed, and it probably exists underneath components as well as at the interfaces between the traces and solder pads. Indeed some of the components on the automotive boards were separated by corrosion from the solder blobs at the ends and the parts only held in place by the assembly adhesive dots underneath.

Could be a very deep rabbit hole indeed...

[J M Fahey]

Build this :


You can replace the inductor with a single transistor gyrator.

Or design your own: basically the blend pot and gain stage, and replace Bartolini 3 band EQ by a simplified Boss or MXR 6 or 7 band Grapic EQ chopped down to 3 bands.
Pick the 3 bands most useful to you.

EDIT: found this which makes it very doable.
With real inductors, of course:


didnīt check it wire by wire, might match this wiring diagram:

[MarkusBass]

The remark regarding the corrosion is very important. You can clearly see places on the board that look very suspected. Like this one:

If you fail to make the preamp working, I could make such a preamp for you (I mean the exact copy of the original preamp). But then you would need to send the preamp to Poland . I can see the schematic from your photo but some traces are under opamps and other components.
Is the MID frequency pot in good condition? It is very special potentiometer and I'm not sure whether it can be purchased. Everything else can be purchased without any problems.

Mark

[MarkusBass]

The Vari-mid part of the preamp looks like this:

I redrawn the Hungarian schematic posted previously. Once it is redrawn, you can clearly recognize typical topology of a parametric equalizer.
On your board just few components are different: you have 2k resistors instead of 2k2, 430R instead of 1k, 5k6 instead of 4k7. Nothing very complex.
And the BASS/TREBLE part looks like this:


One of the caps near the BASS pot is missing. I think it is exactly as the other capacitor - you can just measure it and solder exactly the same value in the place of the missing component.

Mark

[MarkusBass]

I wonder whether this preamp was ever fixed? It seemed to be an easy task .

Mark

[Axtman]

The preamp/controls/wiring had a lot of issues. Instead of spending the rest of my adult life repairing it, I installed a new preamp (I forget the exact model) that I ordered from Best Bass Gear. The new preamp had the exact control layout and configuration. I sold the Ibanez bass to a friend of mine for a very reasonable price (family discount). He LOVES the bass and it has become his main gigging instrument. So it was a Win-Win!

[MarkusBass]

Thanks for the info. But it's also a pity that you haven't try to fix the old one. I would say that it required just 1 hour of work (including missing components, wiring problems and corrosion problems).
Anyway, it's good that it's now fully operational and it's used on a daily basis.

Mark

[Axtman]

I'm the type of guy that will spend hours and many dollars trying to repair a part that I could order on line for less money.

This preamp was toast. It had a lot of problems. The wires kept breaking on it the more I handled it. I was not set up to do surface mount component repairs. The controls were rusted and one was frozen. I just decided it would be better to let it go and buy a new part. When my friend walked away with the bass, I wanted to feel confident that he would not call me up later to say that it was working.