Messages

I made some progress today by star grounding the Wurli which has reduced hum quite significantly. You can see in the spectrograms that the 50Hz cycle and harmonics are much quieter though there is still some 50Hz when the volume is turned up (which I expect must be in the preamp section). These tests were done with cheap test hooks for the star ground connections and without the hum shield in place, so hopefully it'll be even quieter once that and the proper earthing cable are installed.

Can anyone comment on whether it would be worth upgrading to 4700uF smoothing caps for the power supply? Nichicon have discontinued a lot of their audio range and the only substitute I can find of the right diameter are these 'Audio Note' ones which are supposed to be good.

I'm also wondering if there's any merit in increasing the value of C30, 31, 34 or swapping any to electrolytics to reduce the 50Hz cycle in the preamp?

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.

Hello, I have a Hohner Electra which I'll be servicing soon. Mine is an early model without the tremolo/vibrato and I was wondering if anyone has any info, photos or a scheamtic for this part of the circuit? You can find the original schematic attached.
Thanks!

So it turns out M5 self tapping screws fit the original U clips perfectly. I used these 35mm ones cut down a bit with a hacksaw: Phillips Hex Washer Self Tapping Screws, M5 x 35mm

I also used rubber washers in place of the plastic washers below the harp which has improved sustain immensely and reduced most of the weird overtones. I put 3 at the front and 4 at the back in the bass section, and replaced the metal washers on top with rubber ones. I also used some below the pickup covers to tame other resonances.

Just waiting on some more washers to arrive to finish the treble section, but it's already singing way more than it ever has done!

Thanks! I'll have a listen to the vibrato circuit once I've finished the acoustic set up and transistor/cap swaps, but I think it's pretty clean at the moment.

One thing I do have is noise when powering off (an audible thump on the speakers and possibly on the DI). Not sure if the recap will fix this, or if something in line with the speakers or a resistor/cap to ground on the speaker line will eliminate this?

Thanks velo-hobo! I've since disassembled further and ordered a few bits to replace the original screws (new M5 self tapping screws and "spire clips" in the post).

The service manual suggests paper keybed shims for the harp to fix dull/quickly decaying notes, and while this works a bit, I've found 1mm rubber washers are a lot better! I've also added these between the harp and pickup shield to remove some acoustic rattles I had. Hopefully the harp will only be ~0.5mm higher after reattaching everything so I won't have to adjust the letoff much.

It's amazing how much the resonance of particular notes are affected by the harp screws though! 1/4 turn makes a huge amount of difference.

This is some amazing work! It would be wonderful to hear what it sounds like

Does anyone know what size/thread these screws are on my Wurli 200A? (The middle/coarse harp screws)
They're killing the sustain of a few notes when tightened and are too short/worn to be shimmed with card as the service manual suggests.
Thanks !

I'm finally getting round to restoring my Wurli and wondering what models/series of electrolytic capacitors would be best for the pre & power amps? It's a bit of a minefield with things like ESR so any help would be greatly appreciated. (I usually end up using Panasonic, Vishay or Nichicons for synths & outboard equipment)

I've already bought replacement transistors and I'm working off this Vintage Vibe schematic.

Below is a list of parts I've put together that may need replacing in future. The caps are noticeably worn so I'll start with those and the preamp transistors to reduce the noise. If there's anything else worth looking at whilst I'm in there, please let me know.
Thanks !


Electrolytic Caps:
5, 6, 8, 26 - 4.7uF
7, 10 - 22uF
12 - 100uF
23 - 1uF
27 - 47uF
28, 29 - 2200uF

Film Caps:
30, 31, 34 - 0.33uF (Panasonic ECW-F2334JAQ)

Metal Film Resistors:
1 - 22k
2 - 1M
3 - 470k
4 - 150k
5 - 33k
6 - 1.8k
7 - 270
8 - 820
9 - 6.8k
10 - 56k

Transistors (spec sheet here)
TR1, TR2, TR15 - 2N5089
TR3, TR4 - 2N5088
TR7, TR8 - 2N5087
TR12 -  MPSA56
TR9, TR10, TR14, TR16 - MPSA06
TR16 -  KSP13BU (later serial # pianos - post #102905)
TR11/TR13 - TIP35C & TIP36C (power output transistors)

I'm restoring my Rhodes and need to remove the hammer cam felts to do the 'miracle mod'.
I was going to leave them on and just apply the bumps but a few are quite worn!

I have available - lighter fluid and methylated spirit. I have a small amount of isopropyl alcohol, but not sure where I can buy more locally.

Supposedly denatured alcohol is the best thing to use, is that the same as methylated spirit in the UK?
Thanks!

I have a Rhodes 88 Mk1 ~1975 that I'd like to get professionally serviced. It's fully working but could use new action pieces - felts, hammer tips, grommets, etc and a nice set up.

A Google search brings up a few companies in the UK who service & restore Rhodes and I'm keen to know if anyone on this forum has experience with any of these companies or recommendations?
I'm based near Oxford, so South England would be preferable.

I'm also considering the 'Miracle Mod' to get a nice quick action.

Many thanks!