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| Magnum neck pickup: Inside this mysterious beast|
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After 15 years of completely reliable service from my Magnum I, one day a few months ago I decided to tweak the volume trim pots on the neck pickup a little, and poof - no sound from the A string. After a little testing and contact cleaner yielded no improvement, I knew I was going to have to make an expedition inside this strange and wonderful pickup. (I wasn't thinking it was so wonderful in that moment, but it was before and after that.)
First task was to open up the frame the pickups are attached to. The following pic shows what the pickup looks like underneath. The pickup is a sealed case, with the back plate attached very firmly with some quite large globs of solder (with leftover flux still visible).
Removing this solder is fairly straightforward if you're decent with a soldering iron. I recommend using a broad tip and a solder-sucker - it makes melting and removing much faster than with a smaller tip and braided tape. But it still takes a little patience, so take your time.
Once the solder is removed, pickup case opens easily - only the solder around the edges holds it closed - and you are then able to access the full guts of the pickup.
The stock photo of the pickup that's in the Ovation manual is pretty accurate. Here's what mine looked like when opened up:
This gves you a good look at what I believed was the culprit: one of the little trim pots that are used to adjust the volume of each individual pole piece in the pickup. These pots are a great feature to have, because they allow you to balance string volume very precisely. But it also adds another part that can go bad (though one glitch in nearly 40 years of the instrument's life isn't bad!). Fortunately, as you'll see below, it's also very inexpensive to replace.
So how does this pickup really work, anyway? Before going any further, I referred to the patent documentation for the pickup to get a basic understanding of the principles involved in the various parts of the pickup.
Closer inspection revealed the source of the problem. These types of trim pots use a thin conductive carbon ring on a plastic substrate as opposed to a wire resistor. They are not sealed, which makes things a little more vulnerable to debris or other outside factors. As you can see in the following pic, the carbon ring on the A-string trim pot was missing. It had broken off, and I found it in pieces inside the pickup case.
At this point it was clear that replacing the trim pot was needed. 40-year-old unique pickup - how easy would this part be to find? Was it a custom part? I desoldered it and looked for any indication of its resistance or a model number. The part number turned up nothing, but some searching led me to the wiring schematic for the Magnum, which shows 100K resistors in line with the pole pieces.
Some online scouring finally led me to this part, which is the correct replacement trim pot for the Magnum pickup:
Removing the pot was a simple matter of desoldering the four contact points:
And then soldering in the new pot. As you can see, the original pots in the pickup had red adjustment dials. I've not seen red ones anywhere, even color versions of the original stock photo linked above (they're blue there, too). But no matter - a little change in color is worth it for everything to be working again:
And that was that! I tested out the new pot, and everything was back to working perfectly - the new pot worked just as well as the old, and it was again possible to balance the volume of the strings. I re-closed the pickup case (using a bit less solder than Ovation had, but still plenty to seal it firmly) and closed it all back up again. It was a little tricky getting the solder to start sticking to the casing - I highly recommend using a paste flux to help get this part of the job going. I bought some at my local Radio Shack that was cheap and did very well.
So if any of you have to take your own journey inside the mysterious Magnum neck pickup, I hope this little guide will prove helpful! If you have questions, feel free to let me know.
Edited by red_baron 2015-11-07 12:58 PM
|Photos should be working now. Let me know if anyone has trouble seeing them.|
Edited by red_baron 2015-11-07 12:59 PM
|Old Man Arthur|
Location: Keepin' It Weird in Portland, OR
I don't play Bass... but that was very educational.
|Cheers, Old Man Arthur! When that pot went out, I suddenly felt like giving up bass playing - for a moment, at least. But it turned out to be a rewarding little expedition.|
Location: NW Washington State
|Nice job and good documentation for the next guy! |
It looks like there's a resistor below each trimpot to keep you from turning any coil down too much and killing everything. But why did they add the extra resistor on the output?
edit: I see a section of the patent where they say that the output resistor is there to keep the impedance from falling too low if the trimpots are turned down.
The Breadwinner/Deacon preamps use similar trimpots for pickup balance and out-of-phase null. Easier to replace those since they're not buried inside a pickup.
Edited by numbfingers 2015-11-07 8:42 PM
|Thanks, Steve, I never knew that about the Breadwinner/Deacon - that does seem like a better design. More of a set-and-forget operation than something that often needs tweaking. The output of the Magnum pickup is so strong that perhaps they thought a ready way to tame its peaks was a good idea.|
Location: Corpus Christi, TX
|Thanks for posting this, very well written and indeed informational. Just curious as to why you didn't change all the trimpots while you had it apart.|
|Good question, Glen. I considered that, and bought enough of the new pots to do that. But the other original pots were working perfectly, were in perfect shape (their carbon rings were fully intact and not showing any wear), and I didn't see enough reason to mess with what was already working. The pots had been 100% reliable for the 15 years I've owned the bass, and just one replacement pot kept things closer to an "all-original" state. |
Using less solder to seal the pickup case means the next job, if it's ever needed, will be much quicker to do. The actual removal and replacement of the pots on the circuit board was a snap.
But yes, there is always a bit of "fingers crossed" in such a choice!
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