Infiner

The other schematic, unused and untested, use at your own risk:

Thanks much appreciate that, I'm just here to share what I've learned by poking around with my soldering iron, and multimeter. I only hope to save the next person that works on one of these some time, as I spent quite a while scratching my head.

Well I will think about the other schematic later I'm anxiously waiting for some tools from Stewmac. I really should be sleeping, but I'm tracking my package, and no it hasn't moved.

They actually knew about this problem and offered people a free repair service if you sent it back to the store you bought it from. They would then offer you to trade for a fixed one the store already had, or they would send it back to the manufacturer for you. The trouble is, not many knew about the warranty repair service, so a there are a lot of T-Pros are still out in the wild that have electronics issues. I'm not sure the exact years the T-Pro had these issues, I'm thinking the earlier models when they first came out. So if someone is selling them for a steal, you might know why, because it hasn't been fixed yet. Although it's often cheaper to get a used broken one, and throw in an EMG BQS or Bartolini system, than to buy one new. I won't even get into the neck dive issue, that's been addressed in many articles on the web. Bottom line is, if you want the sound of a T-Bird you are going to have to really want it, and be able to pay for it's flaws. The T-Pro is like the hot chick at the bar, she sounds sweet, but costly to maintain!

Yes that's correct they work in a passive mode, if the red wire was wired to ground, or signal instead of battery, that might remove the hum as well, although they appear to have the capability to switch between active and passive on some other schematics I have, which I can't upload because I have used all my upload space. It is not unusual for bass pickups to switch between active and passive, Bartolini has bass pickups that can do this. The reason they can do this is because the bass string is much larger and generates a stronger magnetic flux than a guitar string is capable of. Switching between preamp and direct pickups is a possibility. It is a simple task of connecting the switch between the pickup and volume at the output, and the pickup and the EQ preamp. I just want to make a note that the scratchy fader pot can be replaced with a 3 position switch, like a Les Paul has, a much cheaper alternative to buying another expensive concentric pot from an overpriced online dealer. If you wire the bass like a Les Paul the balance of the pickups can be adjusted by two separate volume knobs. There a tons of schematics for this configuration, although I might try using 25k pots, more like EMG wiring, or Seymour Duncan Blackout wiring, or if you want to get really weird use the pots from the EQ with the center detents in them for tone pots. It's probably best to wire it like the regular Epiphone Thunderbird, which there are more schematics for anyways.

What I've done so far: 1. I soldered the red T-Pro pickup wires into the main red battery wire along with the preamp brown wire, and hum gone. 2. I removed the plastic output jack that comes stock and soldered in a 3-prong Switchcraft jack in it's place. Tip: Signal, Middle: -Battery (Black), Ring: Ground 3. I tried the parallel 18v battery mod, which didn't increase volume or affect the scratchy blend pot as I hoped it would. The additional battery did not fit along with the preamp in the control cavity, causing the cover to bulge, which over time will permanently warp. If I remove some wood from between the jack area and the extra blank pot area the battery will fit, but also it will remove some shielding paint. The parallel 18v battery mod: I disconnected the red T-Pro pickup wires from the main battery and added another battery. I soldered both pickup wires directly to the red wire on the new battery, then soldered the black wire directly to the middle of the jack, where the other battery connects. The preamp brown wire going directly to the old battery as it did stock. This runs the batteries in a Parallel configuration as opposed to the standard serial battery configuration that is normally done. This Parallel mode allows more current (not voltage) to be available to each circuit simultaneously. (see http://en.wikipedia.org/wiki/Series_and_parallel_circuits ) This improved headroom and dynamics of the pickups, but did not increase volume or fix the scratchiness of the blend pot. What I'm trying next: 1. For the scratchy blend pot: First write down what wires are located where on the blend pot, be as detailed as possible. Remove the blend pot, and de-solder all connections. Spray with contact cleaner and move the wiper around a bit until it feels clean. Set the blend pot aside for now. We are going to need to solder a black ground wire to the pickup grounds, cover the exposed wire with shrink tube, and then solder the pickup ground wire to the main volume ground, they are not going back on the blend pot. Rewire the pot, but do not put the pickup grounds or any other grounds on the blend pot, with the exception of the grounded wire coming from the preamp and going across diagonally. Trying a washer ground instead or no ground at all to the blend pot other than the one that goes to the leads. Here's a nice tutorial on cleaning pots: http://www.michaelshell.org/gadgetsandfixes/fixingscratchypots.html If that doesn't work, time for a new blend pot. 2. For the low volume: This might be fixed by placing the grounds to the pickups onto the volume pot instead of the blend pot. Replacing the main volume pot with higher values from 50k to 250k may increase volume with the preamp installed. I came to this conclusion after looking at many active bass schematics which said a 250k pot will increase volume by about 6 decibels. I find this value a little strange for active pickups, the preamp must be affecting volume somehow. Since I only have 500k pots available I'm going to try that. My chicken scratch math tells me this could make it put out 12db, but we will see. 3. For the extra Battery: Removing some wood to make room for second battery. Will need a small saw or hack saw blade. Also should repaint shielding paint over cut. 4. If none of the above works, remove the preamp and wire them like standard bass actives, will need 25k or 50k or 100k pots for this. Four controls: two volumes, a main tone, and a tone circuit bypass would be my choice here. Not a complete cutoff switch that would be wicked on speakers and amps, a tone circuit bypass eliminates the tone circuit completely, like wiring the pickups straight to the jack and ignoring any pots. 5. Funds and time permitting, EMG Thunderbird Pickups with something like a BQS eq system. The Thunderbird Pro can accommodate 5 control pots, although one hole needs to be drilled, carefully with a router and a good bit. This would be the most wicked and ideal situation but they aren't even out yet, and well... expensive! On second thought, I think I'm just going to chuck the preamp, forget cutting wood, and wire them like standard actives, with two batteries. The only minus I can see here is I will need a pickup selector or a blend.

In the process I made a little color chart for what each color wire does.

I found a push/pull schematic ( RBA04p ) that switches between active and passive. It looks to me the red and orange wires are swapped when switching from active to passive, with the orange wire going to ground in passive. The MJ300-NI in the wiring diagram is a stereo Switchcraft type jack.

The preamp bears very close resemblance to the Duncan Designed BEQ-2, as a matter of fact, they might be the same, just thought I would mention that if anyone searches. I did a search for BEQ-2 and some guy is selling one, I look extremely close at the picture several times and try to make out the lettering. I look closer and see that it says RBA-04m, I found the schematic as a search for RBA04m.

I searched far and wide for these schematics due to the humming problem, note these schematics only apply to the Epiphone Thunderbird Pro IV and V. I re-wired it exactly as outlined in the following steps. 1. The Bass and Treble potentiometers were not grounded at the factory, they should be grounded in series to the volume pot. This is shown in the schematic wiring diagram attached. 2. Both of the T-Pro pickups have an extra red wire on them that should be wired to the battery red cable, as they are indeed active pickups. (I suspect based on Duncan Basslines) 3. Remove the pickup grounds from the blend pot, solder them to a black cable, cover with shrink tubing, and solder the cable to the volume pot. Notes: The volume has a 103 (.1uf) capacitor for treble bleed. The preamp is too big to add another battery in the compartment unless some wood is removed. There is room for another control pot but a hole needs to be drilled, carefully with a sharp bit. Here is the wiring diagram for the Epiphone Thunderbird Pro: