Vintage Synth Repair

Hi

 

I would like to help, but there seems to be some inconsistencies in your
text.  The drawing on page 28 is a PCB layout drawing and not a schematic;
the schematics are much later in the PDF and show the actual circuit.
Secondly cathode is positive i.e. + and anode negative i.e. -.  I have found
difficulty in finding the components you mention on the circuit diagrams so
could you let us know which page(s) these components are on, we will then be
in a better position to advise you.

 

Regards

Brian G3OYU

 

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of earth2willi
Sent: 25 May 2010 10:19
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] Wurlitzer 200 amplifier noise quirks - filter
cap/volume pot maddness

 

  

Hi,

Recently I've been working on a couple of Wurlitzers, some that I own and
another that belongs to a friend. The two units that I'm currently working
on are 200 series units (NOT 200a-type units). Both units were suffering
from large amounts of noise in the amplifier. I've set about rebuilding the
amps, referencing this PDF:
http://www.vintagevibe.com/misc/Electronic%20Pianos%20Series%20200%20and%202
00%20A%20Service%20Manual.pdf This is a compiled service manuals document
available at Vintage Vibe. Thanks, VV!

As a matter of practice, I first replaced all electrolytic capacitors. All
transistors have also been replaced. TR-1 was replaced with a NTE123AP which
is a cross referenced part for one of the replacement transistors suggested
on page 80, in troubleshooting item 'G'. Neither of these major steps in
rebuilding the amp - transistors or caps - made any real significant
difference in the output; it's still fairly noisey. There is a lot of static
or hiss, and the unit will modulate this noise with the tremolo. I do
realize that the reed bar/pickup shields introduced in the 200a series would
help minimize interference and noise, and they have been suggested to my
friend. My current running Wurlitzer has 200a type pickup shields installed.

On page 32 in the PDF, there are some troubleshooting suggestings listed. In
particular, I have added all the extra filter caps suggested:

Additional Filter Capacitors per pages 31-32

Amplifier Hiss; Voltage regulator oscillating;
*470PF Cap from base to collector of TR-1
*.01MFD capacitor from collector of TR-5 to ground

RF Interference Suppression
*750PFD capacitor from base of TR-1 to ground
[i]*750PFD capacitor from base of TR-2 to ground[/i]
*750PFD capacitor from base of TR-3 to ground

Resistors; page 31, 80
R-10, 2.7 ohm 1 watt
R-14, 200 ohm 5 watt
R-36, 8.2k ohm
R-40, 12k
R-45,15k
R-66 22k

These filter caps started to make a more noticeable difference in the noise
levels. However, my unit picked up a quirk where the last bit of travel in
the volume knob causes noticeably excessive hum and noise, and lowered
signal level. I was a bit dismayed to note that this problem began on my
friends Wurlitzer as well, indicating it's unlikely to be a problem with my
specific pot. In fact, it was when I added the 750pf cap from the base of
TR-2 connected to Ground, that the noise level really dropped significantly
BUT this problem also started on his Wurli!

Replacing the resistors shown above, as recommended in the service
documents, did improve the noise in the last bit of travel, but it is still
a problem. I have yet to try a lower value cap, such as 470pf, in this TR-2
location. I'm a bit fearful of trying a higher value cap without
understanding the circuit better. I see there is an adjustment pot, R48, but
I have not touched it yet in case the adjustment is very fine. I only have a
basic voltmeter to test with at the moment.

What is it about this filter cap from the base of TR-2 to Ground that causes
both a big drop in noise, but also this problem with the volume pot? It's a
problem.. in performance you don't want to have to try to remember not to
turn it up too much! 

I noticed that the schematic on page 28 of the PDF has a couple of
inconsistencies with the hardware in the amplifier. In particular, the
polarity of D5 is indicated opposite of what was already installed and
functional in the unit. I suspect this is merely an error in the schematic,
since the pin closest to the tremolo control lines is connected to Ground
and in the installed hardware this pin is the D5 Cathode (-). Is this
correct or should this diode be reversed to match the schematic (Anode (+)
to Ground, as indicated in the schematic)? Replacing this part with the
existing polarity seemed to clean up a little bit of noise in the upper
(troublesome) range of the volume pot, so I suspect it is just an error in
the schematic. Which leads us to D6...

Upon further inspection I had a hard time locating D6, but it appears to be
in parallel with R57...? Is the polarity indicated as being Cathode pointed
towards TR-1? I initially suspected this was incorrect and placed the diode
with the Cathode connected to Ground. The amp would then turn on and the
full range of volume from the output transistors and speakers was there (the
remaining, lowered noise level was still audible), AND the issue with the
volume pot being noisy at the end of it's travel disappeared! But
unfortunately, the input signal also disappeared! In hindsight, connecting
the Cathode to Ground effectively shorts around R57 and directly to Ground.
So, is the correct polarity for D6 Anode to Ground, and is this intended to
prevent feedback or leakage of some sort coming from the Ground plane? 

Finally, the manual seems to indicate that tremolo speed is determined by
R32. Is it possible to replace R32 with a pot, wired to act as a variable
resistor, in order to change the tremolo speed? Or will this damage the
circuit somehow? 

THANK YOU in advance, so very much, for your help with any and all of these
issues... and probably more to come! ;)