Fender Rhodes Peterson 4 pin preamp channel imbalance

[marloubow]

Hi all.

First posting here and pretty much a hobbyist at best.
Most of my tinkering has been audio gear, but a friend of my brother's wants to restore a Fender Rhodes suitcase 73 and ask me give the electrical a go.
I picked up rebuild kits for both amp modules, power supply and preamp from vintage vibe.
The amp modules and power supply are good to go. The preamp is functioning properly (including the vibrato), except there is a channel imbalance.
It's about half volume. The gain on both amp modules is pretty well matched.
Headphones and external speakers show no indication of channel imbalance from the power supply to speakers.

I've replaced all the supplied caps and acquired new bulbs for the vibrato. I had a broken .22uf 250v cap that I replaced with a .22uf 100v.

The preamp rebuild kit came with 2 diodes and 4 transistors (no values listed on component), but no instruction on replacement locations.

My supplied voltage at pin 1 is 24.09V
Measurement at pin 2 is a steady 4.9mv
Measurement at pin 3 starts at about 1 volt and after about 20 minutes, come down to about 55mv and continues to drop. (revised...see below, reply #10)

That's what I have so far. I've read what I could find here and online.

Any help in the right direction would be greatly appreciated.

Thanks
Martin

[pianotuner steveo]

Are you sure it is safe to use that 100v cap in place of a 250v?

[marloubow]

That's a really good question.
I've often asked the same question, why put a 250v capacitor in a circuit that has supplied current of only 25 volts?
Like I said, "I'm a hobbyist at best", and open to an explanation as it applies to this circuit.

Thanks for watching out for me.

Martin

[pianotuner steveo]

As I always say, I am not an electronics expert, but I do know that you should use the same or higher voltage cap, not a lower voltage cap.

If this circuit really is only 25v, then it shouldnt matter, but, why was the original rated at 250v if so?

[voltergeist]

So, the outputs are balanced with headphones, but not with the cabinet speakers?

Seems to point to speaker cables, the speakers themselves, or bad contact in the headphone jack.  There is a switch inside the headphone jack that disconnects the speakers when headphones are plugged in.   If the contacts on one of these switches is corroded and not making good contact it could account for the imbalance.  Try cleaning the contacts and/or bypassing the jack and see where that gets you. 

Your 100V cap should be OK, but Steveo is correct in that replacing with equal or higher ratings is the general rule.  There may be a good reason that a 250V part was used, but 10X overrating does seem like overkill.  100V is 4X overrating, which I think is sufficient margin.

[marloubow]

Voltergiest
Thanks for the reply.

First I suppose I should let you know that I don't have the suitcase and keyboard to test the electrical system as it should be. They are located about 100 miles from me.
All my rebuilding has been bench testing with a ipod as a signal source.

I when I plug the ipod source into the "external amp"  1 & 2 on the power supply face plate, I have no imbalance in the speakers or the head phones...no problems...sound is reproduced properly.
But when I plug the source ipod into the keyboard jack on the preamp , I get the channel imbalance.

Maybe I'm approaching this from the wrong angle.
Because this is my first time to work on a electro-musical instrument, it may well be that I'm thinking too much in terms of stereo equipment and not as a single musical instrument.
I hope that makes sense.

So, If am going to bench test this using a source other than the keyboard, how should I be hooking it up? What jacks should I be using?

Is the keyboard considered a "mono" source?

Many thanks for any help.

Martin

[voltergeist]

What are you calling the "keyboard jack on the preamp"?  One of the accessory inputs on the namerail?

[Tim W]

Martin,

The preamp does expect a mono source.  Since it sounds like you have the power supply/amps sorted out we can assume that is OK.  If you have an imbalance coming from the preamp, it could be due to:

a) bad or incorrect bulb in one channel
b) bad opto in one channel
c) opto sensor/bulb proximity issue
d) metal can missing from one of the opto/bulb pairs
e) bad circuitry that drives the bulbs, leading to different brightnesses
f) bad circuitry in one of the channels once the signal is 'split' to stereo, pre or post opto
g) bad connector or cable from preamp to power supply module
h) some type of loading on one channel between the preamp and power amp.  You will need an ohmmeter to make sure both impedances are about equal.

Power up the preamp with the lid off, make sure both bulbs are the same brightness.  Then you can narrow it down from there.  You can use a simple AC voltmeter instead of a scope to feed a signal into the preamp and  find where in the chain the signal becomes weak.  Just use a sine wave test tone at about 200Hz from your ipod.  You should be able to make up a test file for you ipod on a PC or find a test tone on the web somewhere.

Good luck,
Tim

[voltergeist]

Apparently I misunderstood - I thought you were saying you had ruled out the preamp. 

[marloubow]

Voltergiest,

The preamp has a single RCA that I, assume, plugs into the keyboard. Where on the keyboard?
Don't know, I don't have the physical piano here. But that is the cable that I have been using to supply a source to the preamp for testing.

thanks
martin

[marloubow]

Martin,

The preamp does expect a mono source.  Since it sounds like you have the power supply/amps sorted out we can assume that is OK.  If you have an imbalance coming from the preamp, it could be due to:

a) bad or incorrect bulb in one channel
b) bad opto in one channel
c) opto sensor/bulb proximity issue
d) metal can missing from one of the opto/bulb pairs
e) bad circuitry that drives the bulbs, leading to different brightnesses
f) bad circuitry in one of the channels once the signal is 'split' to stereo, pre or post opto
g) bad connector or cable from preamp to power supply module
h) some type of loading on one channel between the preamp and power amp.  You will need an ohmmeter to make sure both impedances are about equal.

Power up the preamp with the lid off, make sure both bulbs are the same brightness.  Then you can narrow it down from there.  You can use a simple AC voltmeter instead of a scope to feed a signal into the preamp and  find where in the chain the signal becomes weak.  Just use a sine wave test tone at about 200Hz from your ipod.  You should be able to make up a test file for you ipod on a PC or find a test tone on the web somewhere.

Good luck,
Tim

timengr 1

`Thank you for your detailed response. It'll take me a bit of time to go through your list.

   I have a 200Hz sine wave somewhere on my computer already. But won't that still be split into a stereo signal leaving the ipod? I think there is something to this stereo/mono input thing.

  I have new #19 bulbs installed, but the proximity of the LDR's I haven't checked. Good point.
  Both bulbs appear to be of the same intensity and do function (alternate flashing) when  the tremolo/vibrato is engaged.
  Both cans are present.

F. G, and H will take a little time to go through.

One important note...I had been testing the power supply and amp modules separately thru one speaker. I discovered that this is the reason I was getting erratic measurements on my pin 3.
Once I hooked up both speakers I'm getting about 4.9mv on both pins 2 and 3. So I feel better about that.


Your help is greatly appreciated!

Martin

[voltergeist]

You just need to get an adapter or two to break out the stereo signals.  I.e. a 1/8" trs to 2x rca adapter.  How have you been connecting it?

[marloubow]

 Like this.

[Tim W]

That cable config will work, but you are effectively shorting the L and R channels of the iPod together.  It can probably handle that (there is likely some level of protection), but I would only connect either the left (white) or right (red) to the Y connector and leave the other disconnected.  This is for the iPods sake, not the preamp.  The preamp has a signal input which it is OK with, as long as it isn't too hot.  Keep the volume on your iPod low.  You don't want to clip the gain stage in the preamp.

And, to agree with voltergeist, we always have problems with the old headphone jacks on the Peterson power supplies.  We always replace them and the 330 ohm resistors too.  We have also had bad 3A speaker fuses on the power supply regulator board cause imbalances.

Tim

[marloubow]

 After some further searching, it seems that I have a faulty transistor the is in that particular signal path.
Went ahead and ordered up a full board's worth.
And I also ordered up 4 - .22uf  250v dipped ceramics so we can all sleep better at night.
It's been known that manufactures will buy higher than spec'ed components just because the supplier has a boat load and therefor a better price than the spec'ed components price.

By the way...Mouser carries the Switchcraft connectors for preamp to power supply cabling. Just type in "switchcraft"

Will let you all know if the transistors resolve the imbalance.

Thanks
Martin

[marloubow]

Well that was it, the faulty signal transistor that I had found.

This one is done. Woot, woot!

Thanks for all the help!

Martin

[Olaf]

Does anyone know the value of the capacitors that are paralelled with the 10uF vibrato capacitors, as seen in the picture above? I haven't had any success with VV's thump mod, maybe this one is better...

Thanks
Olav