Question about Fernandes FAB trimpots

[Picure]

Hello, I recently got my hands on a FAB 180 Omega that has original electronics. But I feel there is something off about the trimpots.

I have a Series one so I know how the trimpots work but on the FAB, they seem to work differently or mine has been wired incorrectly. The outer most trimpots do adjust the pickup gain, however, the inner two (for hum balance) it drastically changes the volume of the pickups also. And there seems to be a “jump” in volume once you go past a certain point. It does not really cancel the hum I feel.

Also, there seems to be way more directional buzz in the front pickup.

I would like to ask fellow FAB owners out there if this is the case with your bass.

[flavofive]

Hi - I own a FAB-170 bass and FAG-170 guitar.  Sorry to report this, but the circuit board in your photos looks completely different from any of the other ones I've seen.  Here is an example of what the original/stock circuit board looks like:








Notice that it's a manufactured circuit, labeled "AL-2".  The circuit board in your photos looks completely homemade - it's just made out of standard perfboard.  If someone told you those are the stock electronics, I'm afraid that's not correct.


It's POSSIBLE that they copied the circuit correctly, but the problems you're describing definitely should not be happening, so most likely something wasn't built correctly.
Actually, no, it's definitely incorrect - I can see that one of the trimpot values in the photo is 100k ohms, which is not correct for any of the trimpots.  In the stock circuit, the humcanceller trimpots should be 100 ohms (not 100k ohms), and the gain trimpots should be 47k ohms or 50k ohms.  So even the values of the trimpots don't match either the Fernandes FAB/FAG circuit (or the Alembic Series circuit, which is basically the same).


So the circuit in your bass might (sort of) work, but unfortunately it does not look like it's a correct copy of the original circuit - sorry to say.  That's very likely to be the reason why it's not working correctly.

[gtrguy]

Here is some info on my Ferbic FAG guitar board and some sort of schematic on the bass version. The Ferbic is designed to run on 9vdc as opposed to the Alembic 18v I believe .

[gtrguy]

I have a bigger more detailed version of this but it won't fit here.

[lbpesq]

The diagram shows two 9v batteries.  I haven't had to replace them for awhile, but IIRC, my Fernandes MH-110 Masterhand, a Series I clone with a Strat-like body, takes two batteries.

Also, any chance I could get a copy of the more detailed schematic?

Bill, tgo

[gtrguy]

Yes, I think the 2 9volts were just for redundancy. I'll try emailing you the schematic.

[Picure]

Thank you guys so much for this WEALTH of information! I’m pretty sure I can get an electronics repair guy to build me a new card from the info here (hopefully). Any more additional info is appreciated!

The bass does sound great! It’s just not really hum cancelling. The filters and the Q switches work as advertised.

Also, the 4 way rotary switch is bent (but working) on mine. What kind of rotary switch should I get to replace it?

Thanks!!

[Picure]

I would like to also ask. I do have 2 original Series 1 pickups left over from the electronics upgrade I got from before and I was wondering if I should replace the Fernandes ones with these. I don’t have the original dummy pickup because it was destroyed during the extraction process.

[flavofive]

Hey, awesome - that's the schematic I made!  Nice to see it's being passed around.  Here's a link to a full-size jpg.  I think you can right-click and "Save image as..." to download a copy.
https://i.ibb.co/bL3dSS3/2022-08-29-Fernandes-FAB-and-Masterhand-Schematic.jpg

Since everything else works, like the filters and Q switches, maybe you can just get someone to troubleshoot your board, and find out what the specific issue is.  It might literally just be the trimpots themselves.
It's lucky that nearly all of the parts on the FAB board are just regular, easy-to-find components.  There aren't any proprietary parts, specialized digital ICs, or anything like that.

Also to confirm:  The 2 batteries definitely generate 18V.  Or more specifically, it's +9V and -9V, but there's no practical difference - the total voltage range is still 18V.  Alembic Series are exactly the same.

The only voltage difference between Alembic Series and Fernandes FAB/FAG/Masterhand is that the IC chips (opamps) on Alembic Series boards are capable of operating at a slightly higher MAX voltage than the ones used on the Fernandes instruments.  This is only an issue if you use it with the power supply (Alembic DS-5 or Fernandes PS-5).  This may be way more detail than you need, but if you're interested, there's further information in this thread:
https://club.alembic.com/index.php?topic=27652.msg282255#msg282255

[Picure]

Thanks a lot flavofive! Initially when I searched the Fernandes FAB, I saw ur schematic and I downloaded it as soon as possible! 😆 I will definitely refer your info to my repair guy!

Thanks a million!

[flavofive]

Happy to help!  Good luck!