Hohner Cembalet N Revival

[Timentineagain]

Hey all,
Longtime lurker first time poster.  Thanks to the forum for helping me over the years troubleshooting and fixing up various instruments.  I have been working on reviving an early cembalet N back from the  grave.  It was pretty corroded all over so I went ahead and completely stripped the case and took everything apart to clean the rust.   It was missing the power cable and pedal so I never tried to turn it on before I got it.  I have replaced most components  in the amp to the point where I feel good firing it up. This thing still has a LONG way to go.  And finding parts is unicorn status.
(Thanks to Stefan for the pluckers and dampers).  Stefan if your reading this it's Tom

So I have the rebuilt reed bar back together and the amp back in the rebuilt case.  Long story short is it is getting power   but it seems my beyer transformer is shot.  There's a short in the primary.  Secondary is reading 142 ohms.   I can't find any information on the specs of the transformer and the only beyer barrel nowadays seems to be for the clavinet ( 1:15) ratio.  Should be the same input  transformer in a pianet n.   I'm putting it out to the universe.  Can anybody help?  Anybody with a pianet or cembalet n that's handy with a multimeter thanks all.

[theseacowexists]

As far as I know, the Beyer transformer is the same one as the in the Pianet N version 1 and the Clavinet.

Are you getting any voltage on the pickup? Should be somewhere in the neighborhood of 170VDC.

[Timentineagain]

Hey theseacowecists. I was hoping to see what you had to say as I know you have a cembalet. 

The transformer is the same as pianet n version 1 but not the same as the clav.  I have been digging through data sheets and so far my best guess is a 1:5. 

I am getting 180 volts before the transformer.  When I tested it sure enough the primary is open.
The volts pass through the transformer primary into the pickups. So sure enough no volts at the pickup. 

Before I go start slapping random transformers in I figure I'd see what I can find  as it should function as impedance matching and or step up/down before the amp first transistor. Any idea of the voltage that's coming out of the secondary of the beyer. 

Honestly seems later version just used a transistor to serve that purpose and it was a developmental thing like the early cf reed bar.  I have options on how to proceed but it seems an original replacement is far from an option. Any info helps.  Thanks.

[theseacowexists]

I've wondered if the transformer could be bypassed by implementing something similar to the Pianet N v2 preamp. In that case, the signal goes from the pickup, to a resistor, to a DC blocking .1uf cap, into the base of the first transistor. This would replace the transformer and the 25uf electrolytic cap between the pickup and the base of the first transistor. The earlier C and L preamps did not use the transformer either, instead they used a 22nf cap and a zener diode. Since I can't find any specs on that zener diode, the N v2 seems to be the best bet to copy. I recently built a clone of the N v2 preamp, and it works great.

Schematics for both the v1 and v2 preamps are on this page: http://hohner-pianet.com/hohner_pianet/pianet_N.html (v2 is the first one on the page, with the orange background).

[Timentineagain]

That's kinda what I was thinking.  I might try to fiddle with that part of the schematic circuit.  Seems my biggest hurdle  with that is correctly padding the signal after the pickups.  I've been looking at Wurlitzer amps too and they are similar to the N V2 in that way.

I might eventually build a clone of N v2 as it seems like a fun project. 

[theseacowexists]

The 39K resistor does a good job padding the signal down after the pickup in the Nv2. If you go this route, maybe audition some different values before settling on one. You're right about the Wurlitzer amps too, that's what made me decide to clone the Nv2 over the other models.

[Timentineagain]

So I have been playing around with a few different variations and yes I can get sound now. 
I substituted the transformer and 25 uf capacitor.  The v2 schematic has a 5meg resistor after the last .1uf filter cap. I am getting 300v at the diode. 100v after the 20meg resistor. 

The new chain I used is from last filter cap.  2 Meg resistor , pickups , 47k resistor, .1 uf , base of first transistor.  I am getting about 100v at the pickups and -.4 v after the dc blocking .1uf cap. My voltages in the 12v rail seem fine around -15v.  I am getting very low volume out put and I'm suspecting The transistors as I did not change them yet. 

Still a few more configurations to try out but the theory seems to work. Hopefully new transistors will fix my volume issue. 

[theseacowexists]

Why don't you try skipping the 2M resistor after the last filter cap? The V1 doesn't use this one. Removing that should get your pickup voltage higher, and probably your output too.

Before you start replacing transistors, how is your vibrato light bulb? It needs to be shining very bright in order to pass the signal, even when vibrato is off. If it is dim, your output signal will be weak.

[Timentineagain]

My vibrato bulb is shining bright.  I can hear it flickering in my audio signal maybe too much. 
The transistor associated with the vibrato circuit is testing not good so I will have to find a replacement.
My ac 125 transistors need replacing. A107 seems ok. 

I have tried without the 2m and it seems the 2m does not drop my voltage significantly.  I was trying to mimick the resistance of the transformer before the pickup.  Maybe some lesser values would be better suited or none at all. 

When you built the n v2 clone what transistors did you use? I am gathering parts and seems in the states I can't get a bc148 or 149.  Seems plenty of equivalents out there.  Thanks for the help. 

[theseacowexists]

I used BC107 for TR1-3, BC336 for TR4, and BC548 for TR5. BC548 or BC549 should work for TR1-3 as well, but I got the best results with BC107.