Author Topic: Passive Electronics  (Read 77 times)

Offline Dr. Monster

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Passive Electronics
« on: September 11, 2019, 02:55:03 PM »
Hi all!

I just purchased my first Rhodes piano and I am extremely excited, but it needs some work. I have a few questions:

The passive electronics are not working correctly. The bass boost knob does nothing, and the volume knob only “works” past about 3 ‘O clock. Before this, there is no signal, then a bit of static. At 3 ‘O clock it pops into working at 100% volume and sounds identical to plugging directly into the harp with a 1/4” RCA adapter. I’ve seen the post detailing the original wiring diagram here: https://ep-forum.com/smf/index.php?topic=4650.msg22605#msg22605 . Everything looks wired correctly on mine.

Are there any good ways to troubleshoot this problem with the existing passive electronics on my piano, or should I look into replacing the pots/cap entirely? Should I buy a new RCA cable somewhere? I’d love suggestions on parts sourcing if this is a good way to go.

If I decide to replace the pots, what are some options other than the original specs? I saw a post from user “pnoboy” that said “Fender chose to use a 10k pot for the volume control, which does remove some of the sparkle from the sound.  A nice change can be made by changing the volume pot to a 50k audio taper, the bass pot to a 250k reverse-audio taper, and the capacitor to 10 nF”


I’ve attached a picture of the Harp. It looks like the worn out grommets should be replaced. Some of the springs look rusted/broken as well as screwed in extremely tight at the bottom end. should I replace those too?

Thanks!

My overall restoration plan is as follows:

1. Replace grommets/springs
2. Clean interior of case, polish tines, tone bars, etc.
3. Replace passive electronics
4. Diagnose strike line, key leveling/dipping
5. Tune & voice as needed
6. Re-Tolex or paint exterior

Let me know if there is anything else I should be thinking about!


Offline Alan Lenhoff

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Re: Passive Electronics
« Reply #1 on: September 11, 2019, 03:46:06 PM »
Keep that excitement going! Some comments:

The "bass boost" knob is actually a bass cut control.  So, at it's maximum setting, you won't hear any more bass than what you'd get taking a signal directly from the harp. Given that, is it possible it's working properly?

Maybe try cleaning the volume pot before replacing it?

When you replace the grommets, replace the screws, too.  Nice straight screws are important in allowing you to get everything aligned correctly.

If you decide to save some or all of your tonebar springs, be aware that different springs are used in different regions of the piano, so don't get them mixed up.

The really tight bass springs probably resulted from someone trying to reduce the escapement in that area (the distance from the hammer tip with the key depressed and the bottom of the tine).  Setting the escapement should be on your list -- a critical part of being able to play the piano with both a light and hard touch.  The Rhodes service manual (and the archives of this group) offer good suggestions for how you can get this right  for your style of playing. Generally, you'll do this by adjusting the number of shims on the bass and treble harp supports.  (Although sometimes that's not enough.) The three Rhodes pianos I've owned all came with the escapement set too high from the factory. Removing some shims made the pianos much easier to play softly.

And speaking of the service manual, if you haven't given it a good read, this would be a great time to do so.

One more hint:  Don't replace parts without having a good reason to do so. It's not just a question of saving money: The replacement parts available today are of varying quality. You can lose some of the original character of the piano by needlessly substituting modern parts.

Good luck!

Alan

1974 Rhodes Mark I Stage 73
1972 Rhodes Silver Sparkletop Piano Bass
1978 Hohner Clavinet D6
1968 Hohner Pianet N II
1966 Wurlitzer 140B
1967 Gibson G101 combo organ
1965 UK Vox Continental
1980 Minimoog Model D
1954 Hammond B2; Leslie 21H; Leslie 31H
1977 Fender Twin Reverb
Vox AC30CC2X amp
(SEE THE COLLECTION: https://vintagerockkeyboards.com/ )