Detective stategy on coaxial pickup in thin hollowbody

[Emory]

I have a Vox HDC77 with what they call coax pickups, each pu with switch to go between "lead, crunch and clean" (their description). Hadn't played in couple of months, and when I did noticed appreciable difference between neck and bridge (bridge about 25% volume of neck). Tried height adjustment (nope). Cleaned jack, gingerly cleaned 3 way pu selector. unscrewed pu, omg not your typical wiring (sorry I didn't take photo)... I'm antsy about trying to pull the harness out of this, and don't know any techs here in Thailand I would trust to work on it.

So question or two: is there an agreed upon order for looking for problems (easy to hard, I would assume) some might suggest for this issue? Anyone know a resource where I might get schematic and values for these unique pickups? Hopefully will post a couple photos. Looks a tad tricky to pull guts out (and moreso to put back in). Any help would be appreciated.

[WRGKMC]

The pickups on guitars typically have less winds on the neck pickup because the strings over that pickup vibrate much greater then they do over the bridge pickup so manufacturers intentionally make that pickup to get a weaker output.

 

High gain pickups may not have as much variance because they distort easily and the imbalance simply isn't as noticeable as a vintage wind like that Vox has.

 

My first electric was a Vox Apollo and the pickup in it wasn't very powerful. They installed active electronics to give it all the various tones. Matched up with a Vox amp they sounded wonderful, but with other amps that lacked the midrange and top boost they could sound pretty whimpy.

 

It makes no sense at all to pull the harness. Wire doesn't go bad. Copper lasts for millions of years and its unlikely the harness is the issue. You'd more then likely cause more problems then you would solve them.

 

If this was a vintage guitar I'd suggest checking to see if the magnet in the pickup came loose. Magnets were typically held in place with some lacquer and when the lacquer got old and turned to dust the magnet could dislodge making the pickup weak.

 

What I'd suggest is getting an ohm meter and connect it to the end of the guitar jack so you could read the resistance of the pickups.

Turn the volumes and tone up full the select one pickup and the other and compare the resistances. Then I'd check and see if you can find the Dc pickup resistances on line. The guitar isn't that old so you could even email Vox to find out.

 

Your values may be off a couple of hundred ohms or more because you are also reading the pots in the guitar but the main thing you want to look at is the ratio of differences between the two pickups. With typical PAF humbuckers you may have a bridge that's 7K and neck might be 1000 ohms less making it weaker in output. They could also use different magnet strengths so resistance isn't the only method of knowing so its a place to start.

 

The other thing is that guitar has some unusual switching configs. I believe it has a Humbucking and split coils mode to give you a P90 tone.

When you switch the switches you should see a doubling of resistance between the single and humbucker modes.

 

If the switches aren't working right you may have the neck stuck in the P90 single coil mode and the other set for Humbucking. This would account for your volume differences. The fix is to simply give the switch a squirt of lubricating contact cleaner and work it a bit. Do the switch that seems to be at fault only. Don't go squirting everything in there. Cleaning switches and pots when its not needed can actually reduce they're lifespan.

 

if the switch still doesn't function right then maybe you need to take it to a pro who knows how to do those kinds of repairs. This guitar has fairly complex wiring. Unless you feel confident about finding the correct replacement switch, soldering a new one in properly and snaking a harness in and out of a guitar its not one of the easier jobs to do on a semi hollow body guitar.

 

Do the ohm meter tests and then try the switch cleaning if you discover the Humbucking mode isn't working.

Your chances of finding your problem there is well over 90%.. That guitar isn't very old so I don't see the switches actually going bad yet. Especially if you don't play it allot.

[onelife]

How are the pickups connected to the harness? Are the wires soldered to the pickup or do they use connectors?

[Emory]

Hmmm just checked ohms via guitar cable. Neck is somewhere around 8.6, bridge no movement of meter at all. None. Yet I still get some output like I said. The 3 way lead-crunch-single switch still functions to change sound.... should I fish out the volume knob ya think? Looking at serial number, I am guessing guitar was made in 2012, so not all that old, although Thailand's climate does age things quickly. Oh, all solder joints to high quality hardware.

[onelife]

Based on the pictures you posted, it looks like the switching arrangement is the most likely source of the problem. With only one volume control, any issuses with the control would affect both pickups.

 

What happens when the pickup selector switch is in the middle?

 

I would still like to know if the pickups use a connector or if the wires are soldered (at the pickup)

[Emory]

Soldered at the pickup. Middle position of pickup selector switch has a bit of sparkle from bridge pickup. Is it possible to have poor solder joint that sends some but not all of what should be going to volume? Thanks for taking some interest in helping me

[Emory]

My crapola Chinese solder gun blew up other day, and I'm in Bangkok for a coupe of weeks away from guitar. But I did have idea of sorts.... I could get the switches and pots near F hole one at a time (pickup also, but just take out) and perhaps use alligator clips and jumper to go around possible source of problem.... nuts or worth the time?

[Emory]

Problem fixed, hopefully will last update. Read interview with pup designer, and he said neck and bridge identical. So got my ohm meter out and took readings. There are 5 or 6 spots on bottom of each pickup that could be measured (had little idea which one went to which). Anyway, all numbers similar so (thank goodness, somewhat) reckoned it wasn't the pickup.

Remembered the adage "try the simple stuff first" so wiggled volume control near F hole to test. Nope. Then the standard 3 way selector, which I could get through the smallish F hole. Fiddle, fiddle, clean the contacts and viola! back in business.

I was not looking forward to pulling the whole harness out through pickup holes, but I was ready: got new dental floss dispenser and everything. There were also a couple mini cable ties inside, which would have been tad tricky to cut. Very neat work done in there, which may be nice when building (they put back cap on last) but as far as getting in there it would be like working under a Japanese car's dashboard...

Thanks for all the support and suggestions

[Jubbathehog]

Thank you very much for posting this. I have just bought a secondhand Vox with exactly the same problem. I took the pickups apart and measured the resistance and all looked ok so will try cleaning and jiggling switches.

[Emory]

On 8/13/2020 at 2:11 PM, Jubbathehog said:

Thank you very much for posting this. I have just bought a secondhand Vox with exactly the same problem. I took the pickups apart and measured the resistance and all looked ok so will try cleaning and jiggling switches.

A bit late, but I used some regular typing paper slide back and forth between the contacts to clean out the carbon buildup. Used to do same with Aurora HO cars back (way back) when I was young.