Fixing up an Ibanez TK999HT

[Stickman393]

I've got this pedal on my bench right now. Picked it up cheap because it was broken.

Someone clearly used the wrong power supply on this thing at some point: there's a little step-up, dual voltage transformer in there, and the primary reads as a short. Like...more of a short than would be typical. Barely any voltage on the secondary.

Soooooo...I need to figure out how to replace this little thing. Easy enough if I can figure out what voltages it originally operated at...but I need to use the schematic in order to find some clues.

The original power supply was a 12vac. From there it goes directly to the primary coil of this little "TUBE-A FEDERAL" transformer. I do not believe I'm going to be able to find a direct replacement for this thing, so I have to improvise.

Looking at the schematic, and from what I've read online: it's a step up transformer with a slightly higher voltage tap and a much higher voltage tap.

The lower voltage tap goes off to a 33 ohm resistor to the tube heater filaments (I imagine in series, though I'll double check) and a 78L12 12V regulator. The higher voltage tap goes off to some kind of poor man's half wave rectifier/ 96vdc regulator. So...my first assumption here is that the first tap is in the 15-18V range, and my second tap is in the 100+ volt range. Could one of the electrical gurus here double check me on that?

IF THAT IS THE CASE...then I have the following that I'm kicking around as an idea to get this thing working again:

Let's just say I run a single 15-16VAC wall wart transformer, probably around 2VA, into the pedal, using that for the low voltage section.

THEN, I would use something like a 120/20VAC transformer wired in in back-to-back configuration, with one leg on the primary and secondary jumped to create a common leg between them. That *should* induce a roughly 100vac potential on the primary, right?

It seems like this should work? Sharing a common leg certainly works for autotransformers, but maybe I'm missing something...after all, it it was that simple why wouldn't ibanez have just done that to begin with? That said: I see 120Vac/24Vac all the time at work with one leg of the secondary bonded to ground...by code, neutral is bonded to ground as well. Wouldn't this be essentially the same thing?

My goal here is to recreate this dual-voltage step-up transformer with as few parts as possible.

Before the warnings, yes I understand the dangers of working with these kinds of voltages and yes I understand how to do so safely. I fuck around with 460Vac on the regular at work. But...I would appreciate some educated feedback, as this is starting to get into an area where I have questions.

Schematic attached:

 

[VoivoD]

Hey, did you menaged to replace the transformer? Still no info on the web.
I have two of these, use them in stereo, yet would like to replace the stock transformers to Uber toroidal shielded and epoxy potted to reduce the noise of the unit, yet local company require me to give them values.
I might hire someone to measure it if you don't have them either.
Wondering if going with power banks with a step up converter and an inverter would reduce the noise even further.

 

[Stickman393]

Haven't gotten around to this yet...but got more information.

This kinda required me to learn a bit more about power supplies...which is actually helping out a bit with my concurrent CNC build.

So...as far as I can tell, this thing is operating with two voltages: 96vdc, and 12vdc. The tube heaters are from an unregulated power supply, straight off the smoothing capacitors. The majority of the circuit operates off the 78l12 regulated 12v, and the plates are fed by the 96v circuit.

So...I'll need to do some calculations RE: ripple, but it seems feasible that one tap of the transformer is basically 1:1, the other is...well...don't quite know yet. I can probably have a better idea with a little math. It's certainly going to be below 160v, given that's the rating of the caps in that portion of the circuit.

I haven't been able to actually use this thing, so I can't comment re: noise, but it certainly seems that there are a few things one could do to reduce noise in this thing...if one was to move to an external power supply.

I've thought of doing this and using a circular, aircraft style screw-on multi-conductor plug, doing all the rectification, regulation, and most of the filtering in an external box.

Granted, this is just my current assessment; I'm trying to broaden my knowledge. I've already had to revisit a few faulty conclusions on this one.

 

[Stickman393]

Well, I modeled the power supply for the tube plates in an Android spice-like-program.



This screen capture confirms my suspicions that the plates operate at 95ish vdc, and supplying somewhere around 115vac to that portion of the circuit should do the trick. Best to do this off an isolation transformer or in back-to-back configuration, though. Current limit that shit. Dangerous voltages ahoy.



The second part of the circuit...is likely in the 16vac range. Just judging by my little mock up circuit, and using a 150ma current source as a stand in for the tube heater. That's assuming that the heaters are in series, something I haven't confirmed yet.

The heater circuit seems like a clear spot where one could make an improvement. Lots of ripple there. Hell, I could be making assumptions here though...this is my first rodeo with this program.

Buttt...I'm thinking that I've got an idea of what I ought a be seeing on the secondary now. The conductors on the transformer should be 16vac, common 0v, 120vac.

I could get a custom transformer made...but I'm thinking I'm more likely to rip out the internal PS and use an external module with a 4 conductor plug. I'll be able to filter and regulate the power I feed into this thing and deliver much smoother DC.

 

[jayemac75]

Hey, did you ever find out a fix to this? I have one too, looks like maybe same problem. Someone used a 12v dc input.