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Topics - Will

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Preamps, Modifications & Upgrades / Rhodes MK I Janus preamp upgrade
« on: June 29, 2020, 01:45:29 PM »

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.

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