Author Topic: Help wiring a Pianet T transformer  (Read 83 times)

Offline rjd2rjd2

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Help wiring a Pianet T transformer
« on: November 08, 2017, 09:53:34 AM »
Hi all-I recently lucked into a ragged, beat Pianet T(20 bucks!) that I have almost fully restored. New pads, disassembled and rebuilt the keybed, it's working almost perfectly. My last step is this: everything downstream of the pickup assembly was gone-transformer, jack, gone. I sourced what appears to be a replacement transformer, and have a jack, and I'm a soldering pro. However, I'm having a hard time finding documentation on how to wire the output trans; can anyone help me out there? I've tried brute force jumpering every combination of primary/secondary outputs, and nothing gets a decent signal at all. I'm using this uni, calrad 45-709:

https://www.arcade-electronics.com/Calrad-45-709-p/cad-45-709.htm

An ideas? Thanks!-RJ

Offline OZDOC

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Re: Help wiring a Pianet T transformer
« Reply #1 on: November 08, 2017, 05:51:24 PM »
The schematics I have for a Pianet T do not provide a specification for the transformer.
How have you determined the correct value to purchase?

I also note that there is a single capacitor (2n) in parallel with the secondary output.

Have you checked that the series chain of pickups has no continuity breaks? Do you get an output when you feed the two ends of the pickup circuit to an oscilloscope?

Offline OZDOC

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Re: Help wiring a Pianet T transformer
« Reply #2 on: November 08, 2017, 06:11:12 PM »
I note that the original advice from the Clavinet.com forum was to use:
Calrad Transformer Part Number: 45-703
200K OHM PRIMARY IMPEDANCE to 1000 OHM SECONDARY IMPEDANCE AUDIO TRANSFORMER-PRIMARY RESISTANCE: 2.26K OHMS, SECONDARY RESISTANCE: 43 OHMS-SIZE: 3/4" x 5/8" x 5/8"

You are using:
45-709   
500 OHM PRIMARY IMPEDANCE to 3.2 OHM SECONDARY IMPEDANCE AUDIO TRANSFORMER-PRIMARY RESISTANCE: 35 OHMS, SECONDARY RESISTANCE: 0.27 OHMS-SIZE: 3/4" x 5/8" x 5/8"

This is a significantly different spec.
Are you sure this is suitable?

Offline rjd2rjd2

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Re: Help wiring a Pianet T transformer
« Reply #3 on: November 08, 2017, 07:22:39 PM »
man, i didn't even notice that huge discrepancy...I ordered the 45-703, and the supplier was out of stock, and recommended the 709 as a working sub. dang. thanks-i just sourced a proper 45-703.

for the wiring, should i assume that the primary ties to the pickup output, secondary to the jack, jack ground to chassis ground? are any of the secondary transformer pins tied off to each other, or doesn't matter? also, 2.2n capacitor in line between the trans and jack? thanks for the help!

Offline sean

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Re: Help wiring a Pianet T transformer
« Reply #4 on: November 08, 2017, 07:52:10 PM »


Is there a better schematic than the one halfway down this page?  http://hohner-pianet.com/hohner_pianet/pianet_T.html

It shows the capacitor, but doesn't show a part number for the transformer.

sean

Offline OZDOC

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Re: Help wiring a Pianet T transformer
« Reply #5 on: November 08, 2017, 08:02:28 PM »
I would assume that the primary is connected to the pickup wires - but now that you raise the question I don't know.
While this transformer is being used to present the right impedance to the guitar amp I would assume that it is also being used to step up the signal amplitude.

As far as I know the transformer you have reordered only has two leads for each winding (no centre taps) so there should be no wires that need to either be joined or not included in the circuit.

I've attached a photo of the only circuit I've ever seen for a Pianet T.

However, you could try building the half of the circuit that a Clavinet Pianet Duo uses to actively amplify the Pianet T section of this related instrument.

Offline OZDOC

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Re: Help wiring a Pianet T transformer
« Reply #6 on: November 09, 2017, 04:06:03 PM »
I asked a friend which way round to wire the transformer and here is his reply:
From the photo I can see the individual pickups are wound with relatively few turns, so even though they are all wired in series, together they will still represent a low-ish impedance. So, my view is to connect the ‘secondary’ (lower resistance) side of the transformer to the pickups to achieve impedance matching.
The fact that the transformer will increase the amplitude of the signal is probably a bonus, as the output of those little coils must be tiny.
I can only assume the capacitor was added to address some issue found in practice, it’s not performing any function essential to the circuit operation. At 2 nanofarads it won’t be doing anything drastic, some high frequency bypass only.
A small other clue is that Hohner went to the bother of using shielded cable on the output side, which helps to confirm that as the high impedance side.