APX-10A - System 29 Preamp Issues

APX series guitars are designed with a focus on playability. Their thin bodies and cutaways that provide greater access to the upper frets, offer smoother switching between electric and acoustic instruments.

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Island Mojo United States of America
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APX-10A - System 29 Preamp Issues

Unread post by Island Mojo »

I'm on the global search for info on locating an original replacement preamp (used of course) for an APX-10A. It came with the System 29 preamp. This same preamp was also installed on these APX guitar models: 8A, 8CS, 10C, 10CS, 10CT, & 10NA.

The problem I'm having is that the output on the 3 lower strings is considerably and noticeably lower than the output on the 3 higher strings. Sadly, and to no one's fault but my own, the guitar was stored in a non-humidity controlled environment for a few years, and I suspect some circuitry corrosion has taken place. So ultimately I'm looking for a replacement, but any ideas and/or suggestions will be appreciated.
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TheSupposedStringMeister New Zealand
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Re: APX-10A - System 29 Preamp Issues

Unread post by TheSupposedStringMeister »

Island Mojo wrote: Tue Jul 09, 2024 3:15 pm I'm on the global search for info on locating an original replacement preamp (used of course) for an APX-10A. It came with the System 29 preamp. This same preamp was also installed on these APX guitar models: 8A, 8CS, 10C, 10CS, 10CT, & 10NA.

The problem I'm having is that the output on the 3 lower strings is considerably and noticeably lower than the output on the 3 higher strings. Sadly, and to no one's fault but my own, the guitar was stored in a non-humidity controlled environment for a few years, and I suspect some circuitry corrosion has taken place. So ultimately I'm looking for a replacement, but any ideas and/or suggestions will be appreciated.
I have the APX-10 which comes with a totally different pre-amp. However, I don't believe that the the guitar living in a non humidity controlled environment will affect only part of the pre-amp circuit. All my guitars live in a room without humidity control and no issues.

I note your photo has the EQ bass set to nearly -12dB so that would remove most of the lows. Anyway, I would put a new battery in, set all control to their middle position, equalisers at 0 and try it again. If it doesn't change then perhaps your LOW volume slider might need some contact cleaner spray applied.

Attached the document which has some good info on the system 29, including the circuit diagram.
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Saul Ukraine
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Re: APX-10A - System 29 Preamp Issues

Unread post by Saul »

It is always possible of course, that you have some sort of corrosion/oxidisation of the electronics. Have you checked the capacitors? They can leak and/or dry out over time.

The first thing I would do though, and you may have already done this, is remove the saddle and check the pickup for any damage. Also, make sure the saddle is sitting perfectly flat against the pickup.

Check the connector from the pickup to the preamp. Unplug it and look for corrosion, maybe spray some switch cleaner in there and plug it in and out a few times.
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Re: APX-10A - System 29 Preamp Issues

Unread post by Island Mojo »

Thanks for the helpful info guys. Issue in my original post still there, no change. I took the steps you recommended, except for checking the capacitors for leaks or drying out. I did check the saddle for a good seat, adjustments were made and the saddle fits nice on the pickup. Also replaced the pickup connector where it goes into the preamp. Apologies for the photo I attached, the settings were in their wrong places, but no matter, the issue remains no matter where the settings are. Mainly, it's that area right around the D string that's kind of a dead area. The E, B, and G strings area is strong, but then just drops off as I migrate toward the lower strings.

Honing in on the control board perhaps, I did not access it and spray it, looks pretty difficult to get to, but I guess I have to find a way. Perhaps spray it good with some DeoxIT, then do a visual on the capacitors. This is where it gets beyond my scope, I don't really know what I'm looking for, and certainly don't have any testing equipment....if I did probably wouldn't know how to use it. Any more ideas, suggestions, or helpful advice will be greatly appreciated. Thank you. ~IslandMojo
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Re: APX-10A - System 29 Preamp Issues

Unread post by Saul »

Ok when looking at the top of the capacitors they should be flat. If they are bulging even slightly it means they are drying out. Visible leaks will be obvious due to staining around the side or base of the capacitor.

Apart from that it could be the pickup itself is faulty. Hard to really know without replacing it. I had a similar issue with a Fishman a couple of years ago and replacing the pickup did the job with that but I have never needed to replace a Yamaha pickup.

It could also be a fault within the preamp.

It's all a process of elimination. Sometimes it is just easier to replace the whole thing, preamp and pickup and be done with it. Although in the case of the APX 10A, you would probably be looking at a third-party one.
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Re: APX-10A - System 29 Preamp Issues

Unread post by Island Mojo »

Good info once again, thank you. Will do exactly that, will let you know....fingers crossed. If everything looks good on the board I'll replace the pickup. If that does not good, unless I can locate an original replacement preamp, I'll replace with a new one. Personally, that's my last resort. Don't know how, don't know why, but those old Yamaha preamps are fantastic.
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