Hello, I'm refurbing my Wurli and looking for some advice on fine tuning the electronics and noise.
I've just given it an overhaul and everything's technically working. It sounds a lot better than it did but there's still room for improvement. I've replaced all preamp components, electrolytic caps and big red caps with a mix of bipolar, decoupling, film caps, metal film resistors and modern transistors, (all matched to original specs).
R58 was removed & R34 increased to 199Ω to achieve +0.57V at TR9 (originally +0.35V), although I still only have ~1mV across R37. All nearby resistors and test points seem to be in spec (with 25.6 & 25.17V at TR11 due to increased rail voltages). Christian at Tines & Reeds suggests this should be 8-10mV, so should I keep increasing R34 to achieve that or look at modifying R36 maybe?
Overall noise and hiss seems better but is still audible. On full volume, loud notes are ~25V peak to peak, quiet notes are ~10V peak to peak, the hum on the scope is around 40mV peak to peak and noise spikes are around 200mV.
Internal voltages differ from spec a bit, but don't seem totally out of range:
+/-25.8V, +142.6V, +14.12V
One thing I've been trying to get rid of is the small 'pop' on the speakers a few seconds after powering off, but this could be bias related. When this happens the +25 rail drops to +0.5V whereas the -25V rail stays at that voltage. It looks like the - cap is still holding charge at this point whereas the + cap is being discharged but not sure if that's normal? Disconnecting the neon lamp doesn't change the hum or pop either.
There's also some occasional HF noise in the signal path which could be new capacitors and transistors settling in, or could be bias related again. It's prominent in the decay of some notes, especially in the bass region but sometimes sounds like it's quietly always there in the background.
C28/29 were changed to high spec Vishay 056 PSM-SI 2200uF caps. These were the best I could find that would physically fit the board correctly and I'm guessing there wouldn't have been a lot of benefit in increasing the value here?
The vibrato peak to peak voltage is 9.76V whereas the schematic suggests it should be 11.5V. If the 15V rail was correct, I think this would still be a bit lower than it should be (assuming the same current draw).
I'm also considering star grounding it to reduce the hum. If I measure the resistance between different ground points across the piano, the value jumps around a bit so I'm assuming there are some ground loops in there. The mains outlet is earthed and it looks like the 0.1uF cap by the neon lamp was never installed or has been removed (assuming this was the 'death cap'?). The mains wires also have their own metal channel behind the heatsink to keep them away from the amplifier.
Another anomaly is the Aux output has a 680Ω resistor in series with C26 - it looks factory spec and I'm assuming it's to limit current/output voltage? (Just wondering as it's not in the schematic)
Regarding my earlier post, I replaced the speaker fuses with fast blow ones and they're working well. (One fuse blew previously as a transistor leg had broken before the refurb, but otherwise was all working ok.)
The only potential issue I can see from my refurb is I put a bipolar cap at C12, but not sure if this would benefit from being a standard electrolytic instead? Also, I've been working without the hum shield on but installing it makes no difference to the noises mentioned above.
I've just given it an overhaul and everything's technically working. It sounds a lot better than it did but there's still room for improvement. I've replaced all preamp components, electrolytic caps and big red caps with a mix of bipolar, decoupling, film caps, metal film resistors and modern transistors, (all matched to original specs).
R58 was removed & R34 increased to 199Ω to achieve +0.57V at TR9 (originally +0.35V), although I still only have ~1mV across R37. All nearby resistors and test points seem to be in spec (with 25.6 & 25.17V at TR11 due to increased rail voltages). Christian at Tines & Reeds suggests this should be 8-10mV, so should I keep increasing R34 to achieve that or look at modifying R36 maybe?
Overall noise and hiss seems better but is still audible. On full volume, loud notes are ~25V peak to peak, quiet notes are ~10V peak to peak, the hum on the scope is around 40mV peak to peak and noise spikes are around 200mV.
Internal voltages differ from spec a bit, but don't seem totally out of range:
+/-25.8V, +142.6V, +14.12V
One thing I've been trying to get rid of is the small 'pop' on the speakers a few seconds after powering off, but this could be bias related. When this happens the +25 rail drops to +0.5V whereas the -25V rail stays at that voltage. It looks like the - cap is still holding charge at this point whereas the + cap is being discharged but not sure if that's normal? Disconnecting the neon lamp doesn't change the hum or pop either.
There's also some occasional HF noise in the signal path which could be new capacitors and transistors settling in, or could be bias related again. It's prominent in the decay of some notes, especially in the bass region but sometimes sounds like it's quietly always there in the background.
C28/29 were changed to high spec Vishay 056 PSM-SI 2200uF caps. These were the best I could find that would physically fit the board correctly and I'm guessing there wouldn't have been a lot of benefit in increasing the value here?
The vibrato peak to peak voltage is 9.76V whereas the schematic suggests it should be 11.5V. If the 15V rail was correct, I think this would still be a bit lower than it should be (assuming the same current draw).
I'm also considering star grounding it to reduce the hum. If I measure the resistance between different ground points across the piano, the value jumps around a bit so I'm assuming there are some ground loops in there. The mains outlet is earthed and it looks like the 0.1uF cap by the neon lamp was never installed or has been removed (assuming this was the 'death cap'?). The mains wires also have their own metal channel behind the heatsink to keep them away from the amplifier.
Another anomaly is the Aux output has a 680Ω resistor in series with C26 - it looks factory spec and I'm assuming it's to limit current/output voltage? (Just wondering as it's not in the schematic)
Regarding my earlier post, I replaced the speaker fuses with fast blow ones and they're working well. (One fuse blew previously as a transistor leg had broken before the refurb, but otherwise was all working ok.)
The only potential issue I can see from my refurb is I put a bipolar cap at C12, but not sure if this would benefit from being a standard electrolytic instead? Also, I've been working without the hum shield on but installing it makes no difference to the noises mentioned above.