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Topics - Will

A few months ago my piano amp was not working at all. I assumed the power supply was faulty. I ordered parts to restore the power supply and amp modules and made the change. It is working fine now. Here is a report of what I did, for the record.

My piano is a 73 suitcase Rhodes Mark I piano ref. 3077. No upgrades have been done on this power supply or on the amplifier modules so far. It was time to change all the electrolytic capacitors. I also changed the twisted wires to suitable 0.5ohm resistors. I did not change the transistors.

You will find a picture of the finished product here. Zoom in for details :

The part list. At the end of each line, what I ended up choosing.

Amp   capacitor   Electrolytic   C1   4,7uF / 25V NP       AXIAL    >>> Mundorf BG50 4.7uF 75V      
Amp   capacitor   Electrolytic   C2   1uF / 16V NP          AXIAL   >>> Mundorf BG50 1uF 75V   
Amp   resistor   Wirewound   R18-R19 R20-R21   0,47 ohm  -  5W      >>> Royal 0,47 ohm 5W
PSU   capacitor   Electrolytic   C1 C2   1000uF – 35V - 60 mm AXIAL   >>> Jamicon 2200uF 35V
PSU   capacitor   Electrolytic   C3 C4   500uF  -  15V - 40 mm AXIAL   >>> Vishay BC-021 470uF 25V
PSU   capacitor   Electrolytic   C1 C2   11000uF – 40V – diam 50 mm    >>> F+T GMB 33000uF 40V

It costed me 52 euros inc. VAT + 8 euros delivery = 60 euros in total ie. US$ 69 in jan 2022.

The Vishay 1000 uF were not available at the time of purchase so I ended up putting Jamicon spare cap I had from a PSU kit. Not the best choice. I will change them next time.
I left a gap between the resistors and the PCB when I soldered them. Those components are likely the get hot so they need space to dissipate heat.
I soldered an insulated terminal to the wires connected to the 33 000 uF capacitors. They have bolts.
I cleaned the PCB with 95% alcohol, a toothbrush and a tissue to remove flux.
The amp +/- rail shows 28.6V and the preamp rail shows +/- 14.6 V. Good !

Feel free to ask if you need more details.



I have the early version of Janus preamp ref. PART NO.013487 which I intend to upgrade. You can find the schematic diagram here.
These are my thoughts and questions about its upgrade which I submit to your comments.
You can find the part list here to follow the subject.

The LM1458 OpAmp on signal path can be upgraded. I have a pair of Burr Brown OPA2134PA which I intend to use. They have the same pinout config. And add their package.

The electrolytic capacitors need replacement after more than 40 years of services.  I will choose Panasonic (FC series), Nichicon (KA, FG, KW), ELNA (not Silmic II)... The relevant capacitors are C8 C20 C12 C13 C14 C15 C3  and non-polarised C4 C6 C16.

Black and round shaped capacitors seem to be polypropylic. If that is the case, they are good quality caps and age well so they must be kept in place, right ? Those are C11 C1 C9 C5 C7 C10.

Small and flat caps are ceramic. From what I know, they are not suitable on audio signal path. On the schematic, they seem to help stabilise the OpAmp (C2) and to suppress unwanted high frequencies (C17 C18 C19). Am I right ? Are they worth to be changed ? For what type ?

I don't know the quality of the present resistors and I am wondering if a change to modern low noise metal oxyde resistors would be beneficial. I have an assortment of them.

The transistor and diodes are not in the signal path and don't need to be changed, unless faulty. Same for the vibrato speed pot.

Linear bass and treble pots and vibrato intensity pot are on the signal path. I don't plan to change them as they make no crack.

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.