Skip Navigation.
Section 0

Switching Power Supply Unit Replacement Kit

by Luca Sasdelli [21010226]
Sandro Sfregola [21010294]

The purpose of this kit is to replace the existing power supply unit, subject to some component tweaks, with a new one, as detailed previously by Don Tillman (see Upgrading the Rhodes Chroma Power Supply) and Sandro Sfregola [21010294] (see How to swap the original power supply with a readily available switching unit).

This is indeed a schematic from Sandro, and it has been arranged to completely replace the old board and its components, in a very small format, with saving of dissipated power and relevant heat, stray magnetic fields, buzzes and vibrations and overall instrument weight. The new printed circuit board (PCB) has been designed to easily replace the original connections without hassle.

Kit Content

All items are assembled over a single wooden base, to be secured to instrument basement with two screws (supplied).

Wiring Coding

All SPSUs will come with already connected wires, so there is no risk of misconnections. All kits are tested before shipment.


The original PSU board is quite large and it includes the power transformer and a power regulator section, that dissipates heat through the instrument's rear panel. The entire old board will be removed, leaving only AC mains wires in place; in the obtained space the SPSU will be installed by positioning it as per shown images, and using two of the original transformer holes.

How Can I Get an SPSU Kit?

SPSU kits are created in Italy by Luca Sasdelli. To place an order, simply get in touch with Luca (see e-mail address at the top of this article).

It is possible to purchase just the SPSU PCB alone, if you already own a suitable SPSU unit and arrange its mounting inside the instrument' cabinet.

SPSU kit for Chroma is €135.00 (full kit with SPSU, mounting plate and accessories) or €100.00 PCB only. Equivalence in other currencies could be obtained with updated on-line resources e.g.

These prices include all necessary mounting hardware and packing materials.

Shipping is not included in these prices. The method (and hence cost) of shipping can be arranged when the order is placed. The preferred shipping method is via Express Mail Service (EMS), with expected delivery between 2 and 5 working days (saturday and holidays excluded). For faster delivery, an express courier could be chosen accordingly with the user, with all relevant (and higher) costs.

Because this is a build-on-demand project for which there are only expected to be a small number of would-be purchasers, units will only get built upon receipt of an order. There are two implications to this. First, I will need to spend money to build the item, and there will be some delay between when the order is placed and when the order can be shipped.

To allow the purchasing of parts (as well as to help ensure that would-be purchasers don't back out after I have already spent money to procure the raw materials), I will need the payment in advance (after confirmation of the order).

All attempts have been made to choose components which can be procured quickly; however, there will be a non-zero amount of time in order to place the orders, receive the components and to perform the assembly and test. In order to ensure I can provide board at the costs above, I need to order items in batches, assuming at least a quantity of 10 boards are being produced at any one time. When you get in touch, I'll certainly do my best to inform you of the expected timelines for creation.

It is estimated that the initial run of boards (based on an initial number of circuit boards I have on hand) could be made available within a month of order placement.

It is important to remember though that this is not being done as a commercial enterprise. I'm doing this as individual for the group. As individual I may be away from my e-mail for a few days here and there, I may go on vacation or have other duties to take care of (i.e., my family, my paying day-job, etc.) I will contact anyone interested in the SPSU kit as soon as possible but some delays at some times should be expected.

Installation Instructions

  1. Disconnect the Chroma AC mains cable from the instrument.
  2. Open the Chroma top lid, by removing relevant screws as usual.
  3. Note the green ground wire between PSU and chassis; mark it and disconnect it.
  4. Unscrew all green ground cables from chassis in the PSU area.
  5. With cutter pliers, cut the AC mains internal wires from PSU PCB, the transformer wires and the big capacitor (C3) ones.
  6. Unscrew the transformer and the big capacitor from the PCB, saving the screws for later use.
  7. Remove the power transformer and all screws fixing the PCB to the base.
  8. Unscrew the back panel from the basement and save the screws as before.
  9. Please take high care to the back plate wiring!
  10. Carefully lift the back panel and pull it out enough to uncover the power regulators screws.
  11. Locate the power components that hold the PCB to the rear panel, and remove all of them using a screwdriver.
  12. Unsolder the two pins holding the TO-3 regulator to back panel. Now the back panel should be separated from the PCB.
  13. On the PSU board, locate the four power output connectors P1, P2, P3 and P4 and clearly mark them with an indelible pen (care! Finger grease could easily remove the marks and you can get lost afterwards).

Now the PSU area of Chroma should look empty and clean.

  1. Fix the SPSU assembly thru two ex-transformer holes as per above image with supplied Allen screws.

  1. Connect power connectors to the new PCB, in respect with the connector labelling on PCB; for safer handling, the new connectors are ordered in the same fashion of the original PCB. Double check connector labels!

  1. Solder the two AC mains wires to the AC plug/filter and power switch. The AC mains should be connected as follows:
    • AC plug/filter: terminal N (neutral) to blue wire from SPSU
    • AC plug/filter: ground terminal to green/yellow wire from SPSU
    • AC plug/filter: terminal L (live) to one of power switch leads
    • The remainng power switch lead goes to brown wire from SPSU
  2. Fix the back panel as usual.
  3. Restore the ground wiring, by connecting all loosen green cables in a single back panel ground point.
  4. Connect the single black wire coming from the PCB (FRAME) to the single ground point as per above.
  5. Use the supplied rubber spiral to bundle wires, thus keeping assembly as clean as possible.

Test and Calibration

WARNING: The +5V analog calibration requires you to work with the instrument open and powered on. Therefore, pay special attention because you're about to put your hands within the power area of live AC equipment (!). I recommend using the typical technique as the old TV-repair technicians did, i.e. wearing isolated shoes and carrying the whole job (if not left-handed) keeping the left hand all the time in your pocket; in case of accidental contact with AC power, this greatly reduces the risk to involve the heart region.

With this in mind, and with all power connectors in place, connect the Chroma AC mains cable and power it on. Check that both LEDs on new PCB illuminates, and then check +5V analog voltage, between PCB ground (black) wire and the fixing screw of the TO-220 regulator on same PCB; adjust VR101 to read 5.05V. Unless otherwise needed, the +5V regulator inside of the SPSU should not be adjusted further.

The power presence is checked as follows:

IMPORTANT: since the SPSU kit will have a different output voltage than the original one, some instrument adjustments are required. The +5.05V Analog feeds the voltage reference for the DAC, therefore it should be adjusted accordingly as per other reference-sensitive values. This involves SPSU adjustment, DAC null offset and charge-pump VCOs.

Operation requirements:

Just in case you need a scope and you don't own one: there are new pocket instruments with a very very low price, that should be enough for this task (e.g. search for "ARM DSO Nano", 1MHz bandwidth, less than 50US$ on eBay)

  1. Adjust the +5.05V analog as per previous chapter
  2. Refer to Service Manual: Calibration and Checkout for I/O DAC zero and Dual Voice Boards tuning, thus skipping the old PSU references.

Keep in mind that DVBs are temperature-sensitive; therefore, after first tuning, close the instrument' covers, leave it powered on for almost 10 minutes, so that the inner air will warm up, then quickly recheck the DVB settings.

DO NOT ATTEMPT TO REMOVE SPSU METAL CAGE, NOR TO ADJUST ANYTHING INSIDE IT. In case of any doubts, please drop me a line at the address reported on top of this page.

Change Log

Changes on PCB production on September 25, 2013

Changes as per batch on January 4, 2010

Reason: SPSU and PCB are now assembled on a single wooden block, thus simplifying the overall mounting and to enhance electrical safety, by locating the AC terminal blocks as far as possible from the serviced areas.

Changes as per batch on October 10, 2009

Reason: an improved PCB has been created (artwork 2009-01A): this applies only for Chromas and not Expanders, due to space limitations.

(*) = Chroma version only

Changes on second PCB production on 23 April 2008

Changes as per early feedback on 24 March 2008