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Rhodes MK I Janus preamp upgrade

Started by Will, June 29, 2020, 01:45:29 PM

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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.


I did the upgrade last january, as described in the previous post. I kept the original polypropylic caps. I changed the ceramic caps to WIMA MKP types (except C2 120p which I forgot to order). I decided to change the resistors on the signal path but I am not sure it is that relevant (I tend to overdo it sometimes). I also thought of changing the cables but decided not to do it (yet). I cleaned the board carefully with acetone and put everything back in place.
How does it sound now ? Yes, it sounds good, clean and loud. I am satisfied.
Some pictures :


No tremolo. 

The transistor is missing and the connections for emitter and collector are jumpered together.   And also no R26.  Hmmm...   Why?  Are the LDRs healthy?

Or has this been resolved since January?



OH!  I just noticed

That explains the jumper.

Since you installed an IC socket for A2 (the op amp in the oscillator), why not try a new LM1458?

Also, have you been able to test the LDRs?



Hello Sean,
Well spotted. Yes I should have mentionned that the right side of the PCD has not the original layout. You got the right link that explains why.
I did basic tests of the LDRs : 0.6V at led pins and resistance at opposite side, without unsoldering them. I saw nothing wrong here. They seem to work fine however, as the vibrato is working fine now, with front panel led disconnected.
I keep in mind your OpAmp advice for the next time I will put my hands into it. But not soon ; my next task is power supply and amp upgrade.
I will update this thread if I have further advance.