Power down when vibrato switched on

[Will]

Hello,
This is my first post.
I am the lucky owner of a 73 suitcase Rhodes Mark I piano ref. 3077 for two years now. I am a regular reader of this forum ever since and I would like to thank the members of this community for their contribution. I would like to pay tribute to my friend Pierre who offered me to be the second owner of his piano, and who passed away a few months ago. I have been confident enough to restore maintain and adjust my piano so far. But I now have an issue that I cannot fix by myself, which you may have gone through.
On the Janus preamp, I have a drop in power when I switch the vibrato on. The sound fades out and the LEDs light weakens. The switch seems to engage a short circuit. The power comes back to normal when the switch is turned back off. I don't make this test often as I would not like to cause a power supply failure.

I removed the preamp, opened the enclosure and I see nothing wrong. No smelly or burnt component. I tested the transistor and the diodes with a multimeter and they are fine. I sought a short circuit between +15V -15v GND and switch when the switch is on with no results.
One interesting fact is this phenomenon occurs also when I unplug the vibrato indicator led.

I could not find a topic about this. If you know of one please show me the link. Otherwise any advice or solution welcome.

[sean]

Will,

Which preamp board 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

The preamp board will have a part number on it.  What is it?   Something like 015244 or 018017 or 013something.


Sean

[Will]

Hi Sean,
Thanks for your answer. The switch is connected to pin 7 of OpAmp IC A2 on one pole and to R26 resistor on the other pole so I deduce the Fig11-1 applies.
The board reference is
PART NO.013487
ASSY NO.013488
ECI-2
VO
so yes, 013...

[sean]

Will,

Yep, Fig 11-1 goes with your 013488 preamp.  I don't have that version preamp, but.... 

I guess the first thing I would do is make sure that the preamp has no possible way to come in contact with the aluminum enclosure.  (My Janus preamp got fried when the back left corner touched the metal chassis, and that burned off some traces.  Previous owner had to do some major repair.)   You should put a thick piece of plastic, formica, kapton, or cardboard underheath the preamp PC board.  If and circuit board is simply mounted by the front panel controls with no standoffs under the board, it should have an insulator underneath.

Also, take a very close look at the underside of your PC board to make sure there are no traces with solder blobs touching an adjacent trace. 

Check the tremolo/vibrato switch itself - does it short either circuit terminal to the switch body? 

Then I would be curious to know if the tremolo oscillator is actually running.  (I assume you don't have an oscilloscope.)
When you flip the "vibrato" on, do you get tremolo?   

Are you saying that removing the LED causes the symptoms to occur, or are you saying the symptoms will show up with or without the LED?

Sean

[Will]

Hi Sean,
I carefully read your post. I did few things this evening but I will do more this week-end. Some answers however :

A thick piece of plastic is already in place, stuck on the enclosure below the PCB. I don't know if it is from factory or if my friend put it in place but it is done very carefully.

The strips had some oxyde dust on them. They had some flux in some place too. So I cleaned them this evening. They need some more cleaning so I will start again this week-end. A leakage may occur here.

Switch case test still to be done.

The oscillator is actually running. I don't have a scope but at low frequency, my multimeter shows the voltage output swinging from + to - at OpAmp output pin 7. At top speed, the frequency-meter shows 15 Hz.

Removing the "vibrato indicator" LED causes the symptoms to occur, even if the switch is off.

I made further tests with interesting results :
When the vibrato is off :
...The positive power supply rail measurement is +16V (+15V expected from datasheet). The negative rail measurement is -11V (-15V expected from datasheet).

When the vibrato is on (at low frequency) :
...and when the oscillator output at OpAmp pin 7 is hight, the rails remain the same as above (+16V and -11V). The "vibrato indicator" led and the "pilote indicator" led are fully lit. The sound is normal.
...and when the oscillator output at OpAmp pin 7 is low, the power rails change to +26V and -0V. The sound becomes weak.  The "vibrato indicator" led is off which is normal. The "pilote indicator" led weakens too which is unusual. The reason is there is no more potential difference on its poles, between negative rail and ground.

There may be a ground reference issue because the + and - rail difference remain the same. Maybe an OpAmp failure...?

To be continued.

[sean]

Will,

With the preamp disconnected, what do the power feeds coming from the amp cab measure?  (Be careful not to short the pins with your meter leads, it makes an impressive spark.)  The voltage should be very stable, and unwavering, and less than half a volt off the perfect + and - 15 volts.  If they are not close to 15V, then you should replace the 7815 and 7915 regulators (and probably the nearby electrolytic capacitors).   See http://www.fenderrhodes.com/org/ch11/fig11-3.jpg  These regulators create a lot of waste heat, so make sure you get them tight to the heat sink.

[Start making a shopping list of components.  If you are going to pay for shipping or a minimum order, you might as well get extra toys.  Buy yourself a few cheap/disposable solder suckers and some solder braid - oh, I mean solder wick, um solder-removal braid.]

But even if those regulators are bad, that doesn't explain the bad behavior on the preamp board.  Don't give up on your search for a short between traces, and take a good look at the PC board where the switch mounts, and make sure the solder blobs are not making a bridge between traces.

If you suspect that your preamp board is not grounded correctly, check the thin wimpy wires that come from the XLR to the board.  Make sure the connections are clean and tight.  (Also check your 5-pin cable for continuity of the ground terminals from end to end of the cable, and from the board to the far side of the cable.)  Make certain that XLR connector pins are clean and making good contact.

The 2N3392 is a cheap component, and easy to swap out, as are the op amps.  And as long as you are heating up your soldering iron, you should replace the electrolytic capacitors on the preamp board.  If replacing an IC, order a few of the correct size IC sockets, so that future op amp swaps are simple.

When you buy new opamps, get a handful of NE5532 or SE5532 opamps to replace the 1458 opamps that are in the audio path.  You will be able to hear the difference.  When you shop for chips make sure you get the PDIP package, not one of the surface-mount flavors.  PDIP is becoming obsolete on some opamps.  (Leave the oscillator opamp as a slow-slew-rate 1458 chip, unless you are willing to be the guinea pig.)

Even if the oscillator op amp is fried, it shouldn't take down the supply rails.  You should be able to dead-short across pin 8 to pin 4 without making the voltage regulators break a sweat.  Check that the 100Ω current-limiting resistors are still in good shape (see R19, R20, R21, and R22) and close to 100Ω.

Sean

[Will]

Hello,

With the preamp disconnected, what do the power feeds coming from the amp cab measure?

I get +14.58V and -14.58V. The drop in tension is coming from the preamp indeed. I also removed the A2 oscillator IC from its socket and I still have this -11V instead of -15V. So the OpAmp is not faulty.

check (...) continuity of the ground terminals

I checked all the bits with the multimeter and it is showing continuity.

Check that the 100%u03A9 current-limiting resistors are still in good shape (...) and close to 100%u03A9.

Yes to both, they are all fine.

take a good look at the PC board where the switch mounts.

I did and found nothing. However, I am suspicious about the switch, instinctively.

My to-do list :
- Draw up a components list to upgrade the preamp board (I will make a post very soon about this) and order the components
- Remove the components form the board. Remove the solder with a pump. Clean the board with care.
- Populate the board with new components and wires. Clean the flux from board. Look for any bridge between traces.
- Let you know...

[Will]

I got back to my vibrato issue as I had few days off. I have spent quite some time to know more about the cause and eventually find the cure. To this day, the issue is not solved but bypassed. It is OK for me "as is". This 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. I cleaned the board carefully. 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... it did not do the trick.

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

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

I also connected the third pole of the switch to ground.

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

[sean]

Will,

Why don't you replace the op amp in the oscillator?

Sean

[Will]

Hello Sean,
It's a good idea I wish I had tried. I was focused on the led indicator as the oscillator works fine without it.
I may try this the next time the circuit will be on the lab bench but it won't be until... months. I keep my spare time to train my piano skills now... and enjoying my Rhodes. I will update this thread it this case. Many thanks Sean for your helpful replies.
Will