problem with a Fender Rhodes amplifier

[spy]

I have a problem with a Fender Rhodes (Jordan design) amplifier, the schematic can be found here, Figure 11-17. The problem is that I have a soft distortion on the sustain of the notes, especially when I press two or more keys together. The sustain of the notes is not soft and sweet as it should be but a little bit grainy. Hope you can understand my description.

I can't see anything with an oscilloscope and generator but I can hear it when I'm playing. The problem is not on the piano as I have tested it with another amp and other speakers, too.

Is it time for el.caps or is it something else? The caps are the originals but I can see two new drivers over there. The output transistors are the RCAs but the drivers are Motorola MJ4101. I don't know what the originals were as there's nothing on the schematic.

[52 Bill]

I can't see anything with an oscilloscope and generator but I can hear it when I'm playing. The problem is not on the piano as I have tested it with another amp and other speakers, too.

If you can't see it on a scope, did you try the internal amp with different speakers as well?

[soundguruman]

Try adjusting the distance between the pickup and the tine. If it is too close it will distort.

[spy]

Yes I have tested the amp with different speakers, also I have tested the piano with another amp. The problem is definitely at the internal amp.

[52 Bill]

What you describe sounds like crossover distortion to me, but I would think that you'd see that with a scope.

If the drivers have been changed, I'd probably review whatever was done to the amp before and see if there are any problems there.

[J M Fahey]

Also vote for crossover distortion.
Not as easy to "see" on the scope, simply because everybody expects to see clipping or dramatic wave deformation, and this is more subtle.
The amp design is somewhat poor, to begin with , maybe the pre 1969 date explains it.
You must measure Vdc across any of the 0.27 ohm resistors , no speaker, everything on "0".
I bet now you have 0 mV.
Add a 100 ohm preset in series with the 1N816 bias diodes , set it on 0 ohms and slowly raise its value until you measure 5mV across any 0.27 ohm resistor.
Be careful and try to mount it somewhat firm (you can cut a track, add a couple holes and solder it there) because if it opens your amp blows overbiased.
If you dare to mod it a little I may add the 2 or 3 parts it needs to be more "normal", such as a Zobel, some bootstrapping and hum filtering.

[spy]

I do dare! Let's solve the problem with the crossover distortion and then continue with the mods. I'll measure Vdc and if it's 0mV I'll add the preset, I'll let you know! Thanks!

[spy]

Juan, you're right, across each .27R I have 0mV. I added the trimmer and I set it where I measure 5mV on one resistor and 2.8mV at the other. Is this ok or do I have to set it higher?

As the amp is still on my workbech, please let me know how to add these 2-3 parts to make it more normal.

[J M Fahey]

OK.
Referring to the schematic you posted:
1) split the 150k resistor going from Qo/Q9 emitters to -V rail into 2 x 75K ones , you don't need new holes, just put a leg from each resistor into one of the original holes and twist (and solder) the free legs together.
Then connect a 47uFx50V capacitor from that centerpoint to ground.
+ to ground, - to centerpoint.
With this you lower hum *a lot* when clipping, which makes things worse because it intermodulates the signal like a horrible 50 Hz tremolo.
The value is not critical; instead of 75K you can use 68K or even 47K , which usually are more common, or a 100K in series with 47K.
2) similar to (1), split the 4K7 resistor from Q12 base to -V into 2 x 2K2 or 2K7 , same as before.
Now connect another 47uFx50V from the centerpoint (cap minus leg) to the speaker out rail (cap positive).
That's the track which touches Q15 and Q16 collectors, the 22 ohm resistor, the 10 ohm one, etc.
This creates a bootstrapped load for Q11, linearizes the amplifier and, nost important, allows full drive on the negative side, which otherwise was impossible.
3) add a Zobel network.
Classic values: solder a 10 ohm 1/2W (or 1/4W) resistor in series with a 0.1uFx50 or 100V cap.
One free end goes to the speaker out rail; the other to ground.
Preferrably as direct as possible to "real" ground which is the centerpoint of the big filter caps and as far as possible from the signal input ground.

In a nutshell: your amp will have less hum, clip cleaner, and have a little more power, plus being more stable.
Not bad.

[Enzo]

Yeah, but will it still get the sustain I need?

[spy]

Thanks Juan for the detailed mods!

Enzo, what do you mean with the question? That sustain will decrease?

EDIT: I don't have good news as the problem with the "distorted" sustain is still there. The bias mod didn't work.

[oc disorder]

Was just thinking the power amp is also the pre-amp power supply.
Is it possible the 1 watt 1k5 dropping resistor has gone a bit high in it's old age?
Maybe the pre-amp is a bit starved ?
That (I presume 6.4uF 40v) filter cap on the amp input & supply TRS jack too could be a bit tired.
Just trying to throw a different slant on the diagnosis !

[spy]

I have tested the amp with a guitar preamp (with preamp's power supply disconnected from TRS jack) and I have the same results with guitar chords, the notes do not sound separated, there's a slight distortion between them.

[J M Fahey]

Well, all you did is good, so no regrets on that.
But the defect *may* be not electronic but mechanical.
The *other* defect which sounds very much like crossover distortion is a dirty/scratchy/misaligned voice coil.
After all, that speaker is, what, 40 years old?
Plug your Rhodes amp into a new speaker, any guitar type one will do.

[oc disorder]

well that rules out my suggestion .. I'm still trying to consider some simple stupid thing .. well now the experts have left the building
you are still confronted with this...Power supply caps ~dirty connections~ speakers most of these things I'm sure you would have already covered
I had a later model recently which was straight forward- din socket disintegrated at keyboard shorted out regulators one of which exploded
easy fix by comparison ..couldn't help noticing that vintage electrical smell while marveling at the fact that these pieces of vintage gear
despite their shortcomings still sound fantastic in the hands of a good player and are still going !
Their character and sonic signature still in demand.