wurlie summary

[Haible Juergen]

Thanks again for all who answered to my wurlie questions !

There was some work for me to do, and if anybody is interested, I 
will
give some summary:

(1) 	Changed the noisy transformer with a toroidal one (2x18V, 50W) to 
get rid
     of the mechanical hum. The new one didn't have  a 130V winding. 
So I used
     a tiny 230V/15V, 1.8W print trafo in reverse: just connected the 
15V winding
     to the 18V winding of the toroidal. The voltage at the high side 
is approx 130V.
     Very different to what I would have calculated - guess it't due 
to losses that
     change the calculation of winding numbers ratio in either 
direction.

(2) Changed the bias current of the output amp. The ugly distortion 
dissapeared
      totally. Also decreased compensation capacitors. There were 3 of 
them
      (100p x3). One located at the voltage amp, two located at the 
quasi complementary
      power transistors. Dismissed the last two ones completely; 
decreased the first
      one to 10p. Works fine. Why make it slower than necessary ?? 
This amp
      was so much slowed down that there was barely a negative 
feedback at high
      frequencies anymore, thus the nasty distortion in AB mode.

(3) 	Drilled holes for a line out, a phones out, and a speaker on/off 
switch into the
     lid left of the keyboard. Used a lot of thermal glue (is that the 
word?) to keep
     wires from resonating. This is very important! *Everything* 
inside a Wurlie
     will resonate like mad when the "right" key is struck!

(4) 	Measured the capacitance of the pickup. It's about 240pF. With a 
polarisation
      voltage of  150V (the original value in a 200A) you get about 
100mV when
      several keys are played heavily. No problem for the electronics 
! I am pretty
      sure that *everything* that makes unpleasant noises, comes in 
from the pickup.
      My HUM problem came from a dimmer in my living room. It's always 
full on,
      but  aparently still changes the mains waveform a little. With 
this dimmer switched
      off, hum isn't much worse than on a cheap tape recorder. -40 ... 
-50dB I guess.
      I can live with that. Maybe some time I improove the shielding 
of the pickup.
      Seems that the older models (200) had (and surely needed !) a 
better shield. Isn't
      there a metal plate above the damper mechanism ? That's 
completely missing in
      the 200A.

JH.