Ibanez MC924 EQ Repair

[milosman]

I am fixing up a 1983 Ibanez MC924. The problem I'm trying to solve is this--one of the knobs in the bass's 3-band EQ is non-functional.

Suspecting that the knob's potentiometer might be shot, I took it out and tested it with my multimeter...and it seems to be operating just fine.

If it's not the pot that's the culprit, I'm at a bit of a loss. Anyone have an idea of a next step I might take in terms of troubleshooting this? Would it be worth ordering a new pot, just to see if it might help? i.e. Is there a chance that testing the pot's resistance with a multimeter didn't completely answer the question of whether it needs to be replaced?

I'm hoping there might be another fix available before I get into replacing the entire EQ.

Thanks!

 

[Abner]

Do you have the schematics? There are diagrams in the Musician Club thread and wiki.

If the pot is fine, next easiest thing to check is continuity of the inductor used for that band.

 

[milosman]

Do you have the schematics? There are diagrams in the Musician Club thread and wiki.

If the pot is fine, next easiest thing to check is continuity of the inductor used for that band.

Here is what the circuit looks like. The failing band is the "bass".

I believe this is the schematic:

 

[Abner]

That's the wiring diagram. The schematic will show you the components on the board itself.

Both schematic and wiring diagrams in this post.

All three bands are actually in parallel, so if two of them are working correctly, it should be fairly easy to track down the faulty component. On the wiring diagram, the bass pot is labled "a", "b", and "c". And actually, all the outer terminals of the tone pots are connected together, so first, check for continuity between the outer terminals of each pot (for example, points "a", "d" and "g" should all read 0 ohms between them, likewise for points "c", "f" and "i").

If all is well there, the only thing left is the inductor coil, the capacitor, or the resistor on the board coming from point "b"..

 

[milosman]

thanks very much @Abner. I'll give this a shot and will report back.

 

[Abner]

Oh... the schematic in the post I linked is probably closer (if not identical) to the 1983 version. The one you posted above is the earlier version with the gain control (1979-81). Not much difference, but, you know...

 

[Vene]

Hi there,
would the tone and volume pots be log or linear?
Vene

That's the wiring diagram. The schematic will show you the components on the board itself.

Both schematic and wiring diagrams in this post.

All three bands are actually in parallel, so if two of them are working correctly, it should be fairly easy to track down the faulty component. On the wiring diagram, the bass pot is labled "a", "b", and "c". And actually, all the outer terminals of the tone pots are connected together, so first, check for continuity between the outer terminals of each pot (for example, points "a", "d" and "g" should all read 0 ohms between them, likewise for points "c", "f" and "i").

If all is well there, the only thing left is the inductor coil, the capacitor, or the resistor on the board coming from point "b"..

 

[Abner]

Hi there,
would the tone and volume pots be log or linear?
Vene

Different manufacturers use different codes for their pots, there does not seem to be a global industry standard.

That being said, in the the above diagram, the balancer pot has two different tapers, so we know it's not linear (as there is no "reverse" linear), and it's not the typical M/N taper you see on balance or pan pots, so that leads us to "A" being log (or audio) taper, "C" being reverse log.

The "D" taper of the tone pot seems to be a modified log taper. You could get away with a regular log taper for this, I would imagine, with very little difference.

Now the "G" taper pots might be a little more difficult to source — they have a log taper from one end to the centre, and then reverse log from the centre to the other end. Ideal for eq controls, but I see the main analogue volume control is also labeled "G" taper... not sure about that one, could be a typo on the diagram.

Hope that helps.