VIbrato dies at low speeds - output distorted.

[1way2life]

Hi All,

I have an issue I cant seem to solve.
I am restoring my Suitcase Mark 1 with 5pin Janus preamp. I have replaced all electrolytics, diodes and op amps (all were 1458s).

Issue I'm having is when the vibrato is switched on, and I lower the speed to about 40% or lower, the oscillation stops, the LED gets dim and steady, and the output distorts like an overdrive pedal.

I have replaced the 2n3392 driver transistor to no avail.

Any ideas?
Thanks!

[sean]

Have you checked that the potentiometer is not faulty?  Take it out of circuit and make sure that it doesn't become an open circuit at some point in the rotation.

Which Janus preamp do you have?  It should match one of these schematics:
Schematic for Janus preamp with sliders: http://www.fenderrhodes.com/org/ch11/fig11-1.jpg
Schematic for Janus preamp with sliders, version 2: http://www.fenderrhodes.com/pdf/late-mark1-suitcase-janus1.pdf
Schematic for Janus preamp with five knobs: http://www.fenderrhodes.com/pdf/late-mark2-suitcase.pdf


Sean

[1way2life]

Thanks Sean for your reply.

I actually found the issue. Interesting one.

I tested the rails again, and seems one dropped to 7v, while the other increased to 22v. The cause was even more interesting.

I had checked the rails after restoring the PSU and amp boards without speakers plugged and they were fine. I made an assumption that because my 4 speakers looked old, they were the 32ohm variety. Not the case. I measured impedance going to the amp from both speaker plugs, and they both were at 3 ohms. So I pulled the cabinet panels and measured each speaker's impedance: 6.8 ohms each.

Rewired both speaker channels in series (they were in parallel), got both channels up to 14ohms, and gave it a try. All voltage rails were normal again (about -14.7v and +14.7v). Funny thing is, the speakers were wired in parallel for years with the orig components. Once I restored things is when it went wrong.

Plays like a charm now.
Crazy.

Thanks again.

[Will]

Hi,
I have the same vibrato issue. I checked my speakers and they have been changed too to some 8 ohm speakers by previous owner, obviously. The original parallel wiring was kept too giving 3.5 ohm for the PA output. I changed to series config, to no avail to my issue. Still investigating.

By the way, I found this vv document which is getting me confused. In page 4, I don't understand why Janus 8 ohm speakers should be kept in parallel. The page 5 series and phase invert config seems best to me. Which I chose.

[1way2life]

Hi Will,

Yea, series worked great for me, and now the volume spread is more practical on the volume knob. Before between 1 and 2 was very loud.

Check all your rails WHILE the preamp is plugged in.

Have you tested Q1? If you need to replace it, tie the 3rd terminal of the vibrato switch to ground. This will shunt the base of Q1 to ground when the vibrato is off. Otherwise, some weird things may happen noise wise.

[Will]

Hi 1way2life,
Thanks for your reply.

Which wiring did you set up ? Out of phase (p.5)   or   in phase (p.3) as referred to in the vv document ?

I checked the rail without preamp (+14.58V  -14.58V) and with (+16V   -11V). Q1 tested (P>N 0.6V) and nothing wrong. A topic is opened at https://ep-forum.com/smf/index.php?topic=10290.0

"shunt the base of Q1 to ground when the vibrato is off" Which preamp version do you have, as referred to by sean ?

[1way2life]

Will,
Mine are in series.

Ok, you definitely have something pulling your rails off. I would start there, maybe electrolytics and diodes.

I have the early mark 1 Janus preamp, not the late, so first schematic in his list

[Will]

Hi 1way2life,

"shunt the base of Q1 to ground when the vibrato is off"

I drew two schematics to think about it. Which one is suitable to you ?
https://photos.app.goo.gl/n9P43wkdBMC9QpAg6
https://photos.app.goo.gl/c67NybXEV5G5EBRn6
The second one uses a DPDT switch.

[1way2life]

Hi Will,

I would say neither... have a look at how they did in the late mark 1:
http://www.fenderrhodes.com/pdf/late-mark1-suitcase-janus1.pdf

I assumed you had the early mark 1. There is a 3rd terminal on your vibrato switch that is not connected. I simply added  a jumper from that 3rd terminal to ground.
If you have the late version, then its already done, as you can see in the above schematic.

I would look into why you have 16v / -11v

PS, I meant to say previously that I wired my speakers in series (In-phase). :-)

[Will]

I got back to my vibrato issue as I have few days off. I have spent quite some time to know more about the cause and eventually find the cure. To no avail but here is what I have done, just for the record.

I took the preamp out from the piano and I built a separate +/- 15 V lab power supply to see if I could reproduce the problem without the Rhodes power supply. Which I did. The Rhodes power supply is cleared.

I changed all the electrolytic caps which were all faulty. I also changed a non-polarised cap on the oscillator (C8). I removed the vibrato switch and put a shunt in place. With no better result.

One interesting thing to notice : the voltage between the -15V rail and ground drops to few volts when the output of the oscillator (op-amp pin 7) is at low state. This is why the pilote indicator led (LED 4) gets dim. And this is why the sound gets distorted. The signal is clipping. This can be observed only when the oscillator is at a low rate as the the voltage has enough time to drop.

I have the feeling that the op-amp output at pin 7 is short-circuited to ground when at low state, draining the power supply. My guess was that the vibrato indication led or its transistor was the culprit. Disconnecting them from the oscillator did the trick indeed. However I did my best to find a failure there, without success. I even replaced the transistor to a new one with no better result.

I ended up leaving the vibrato indication disconnected from the oscillator output and left the led on : R26 and transistor removed and jumper from R28 to led added. 

Back to the piano, the preamp and oscillator work fine now. Without vibrato indication however.

This initial design was changed to a later one, as Sean stated in Reply #1. More rational indeed. I also connected the third pole of the switch to ground as suggested by 1way2life, who borrows this concept from this later design.

Picture here : https://photos.app.goo.gl/Srw82URnJhFiVtXr9

The issue is bypassed, but not solved and remains a mystery to me.