Members willi Posted May 25, 2010 Members Share Posted May 25, 2010 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 the 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: 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 *750PFD capacitor from base of TR-2 to ground *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? :poke: 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! Link to comment Share on other sites More sharing options...
Members willi Posted May 25, 2010 Author Members Share Posted May 25, 2010 Also, what are the additional components specified on the schematic, outlined in Dashes and labeled parts 18, 77, and 80 (attached to TR3, TR1, and TR2, respectively) and part 65OPT (in parallel with R50)? I notice that in the schematic, R32 is labeled R32SEL. R23 is also labeled R23SEL. What does the SEL mean? THANK YOU!! Link to comment Share on other sites More sharing options...
Members tspit74 Posted May 25, 2010 Members Share Posted May 25, 2010 You might try joining the vintagesynthrepair group on yahoo. Also, fenderrhodes.com has a message board with guys who know about wurlis. Although I have 2 wurlis, I can't help you as I've only replaced reeds. Link to comment Share on other sites More sharing options...
Members willi Posted May 26, 2010 Author Members Share Posted May 26, 2010 Thanks for the suggestions! I reposted the questions on those boards, and I was already a member of the Wurlitzer Yahoo Group, but I guess I was hoping someone here might have some answers sooner. Has anyone else installed those extra filter caps? If so, can you please share your experiences -- what values of caps did you use, how many caps did you install, and on which transistors? I currently have an extra 5 ceramic filter caps added to the board, but it's that filter cap on TR-2 that seems to make the biggest help and hindrance to this project. Any input is appreciated. Thanks! Link to comment Share on other sites More sharing options...
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