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How to swap the original power supply with a readily available switching unit

by Sandro Sfregola [21010294] <>

The opportunity to replace the Chroma PSU is an old topic: see Donald Tillman's Nuclear-Powered Chroma [first published 1996]. There is little more to say: if you really want to use your Chroma as a day-by-day, reliable instrument, you have to replace the PSU; so pull the unit and throw it out the window (well...not really, as you could kill somebody!); maybe you could have a new employ for this heavy friend, as a door-stop in windy days for example....

Remember, in the case of a PSU catastrophic failure of the dreaded 5 volts digital regulator your Chroma will be history (or a several hundreds dollars service bill, if you like). The scope of this page is to give you an easy way to adapt one of the dozens of similar, general-purpose Switching Power Supply you can find on the market.

So, go to your local electronic store... and look for a SPSU with the following features:

Output voltage Output continuous current
+5 V 2.5 A
+12 V 1 A (0.7 + 0.3) * note 1
-12 V 0.7 A

*(1) The sum of 0.7 A at 12 V for the analog circuits plus about 0.3 A for the external +5 analog regulator

I found a Mean Well T-60B for the equivalent of about 60 US$ (TAX included); this is a well built, compact 60 Watts unit; see the data-sheet. There are many units of different brands on the market that are suitable for this application; if you can't find one of these at your local store see RS Components for example; I have seen some interesting Astec SPSU on their site.

Do not try to power up your new supply without load; you could damage it! Almost any SPSU has a minimum load specification, see the relative data.

The second step is to build the PSU aux board; this is described in the following section.

The SPSU aux board

The aux board includes the necessary additional circuits to interface the SPSU to your Chroma; there are three different blocks:

Here is the circuit diagram of that board; it's better to click for the larger version and print it at 100% for max detail.

The +5 analog regulator is a classic application of the LM317T i.c.; the output voltage can be fine adjusted by VR101, a 10 turns trimmer. LD101 provide a visual indication of the output and a minimum load for the regulator during setup.

The reset circuit is based on a couple of MC34064P; here is the data sheet of this little, cheap but useful i.c. The outputs of U201 and U202 (open collector) are connected in a wired or to the pull-up resistor R203 and to the push-pull output buffer Q201, Q202; the input (the +5 V digital supply line) is directly monitored by U201 and through a delay network, R201 and C201, by U202; during the power-on transition, the input goes up very quickly and when its voltage cross the typical (increasing) threshold voltage of 4.61 V, U201 release its output while U202 still pull the output to ground (reset signal active or low) for several milliseconds then release it (reset signal inactive or high); at power-off, the input goes down and when its voltage cross the typical (decreasing) threshold of 4.59 V, U201 immediately pull the output to ground (reset signal active or low).

The result of this arrangement is: a delayed power on reset to allow all the i.c. and clocks to stabilize and an instant power off reset while the voltage is still high, to prevent memory corruption caused by improper microprocessor operation. The output buffer give a great current capability to the reset signal.

There is an optional manual reset: pushing S1 (normally open) adds R202 in parallel to C201 causing the voltage at the input of U202 to drop well below the threshold; this simulate a power-off condition and thus a reset-active signal. This feature is very useful for me during software debugging on the Chroma (lots of resets...): for this purpose it is better to put a 2 pins connector instead of S1 on board; so you can plug-in a relative long cable with a pushbutton when you need it.

The tapper supply is known as the old "cash-register" drawer trick: since the tapper is activated for a very small time you need a high current pulse but for a short time; the analog +12 V supply constantly charge C301 thru R301 until the Chroma circuit almost discharge it to the tapper (thump..); this meet our goal: to have the tapper functional without disturbing the analog supply.

So, imagine the tapper as the unlock solenoid of a drawer..... There is a problem here: the voltage is a bit low so the tapper is a little soft; I like the result but maybe you don't. An alternative solution exist but I have to test it.

Construction details

As you can see in the pictures, I screwed the SPSU and the aux board on an aluminium plate about 2 mm thick; the size is a fraction of the original supply circuit board; the plate provide additional heat dissipation for the SPSU and U101; the latter must be mounted to the plate (the power dissipation of the LM317T is about (12-5)*0.3 = 2.1 Watts) with an insulation kit to avoid short circuits (check with a multimeter); in the case of the Mean Well unit, the metal box is internally connected with the mains EARTH connection, not the output ground; so you can chose to have separated earth and ground circuits (or "ground lift") or an internal connection. Put a good quantity of silicone grease between the SPSU and the plate, some on the mica insulator of U101 too.

Now some details on the aux board: all resistor are 1% metal film 1/4 watt with the exception of R301, wire wound 7 watt (it is oversized in the event of a failure or short circuit); C102 and C201 must be 10 uF tantalum drop, 16V or more (be careful while soldering tantalum, they are very good but develop long term reliability problems when overheated); C101 is a bypass multilayer ceramic (known as Z5U type) 0.1 uF 50V, C103 is 25 uF 16V or more, 105° standard electrolytic; mount both capacitors very close to the regulator; to improve regulation connect VR101 (10 turns, cermet trimmer) directly to U101 pin 2; see also "How to build high reliability prototype circuit boards...".

The ground paths from the circuit sections to the terminal block must follow the schematics.

Aux board testing and setup

Before to connect the finished aux board with the SPSU and the Molex connectors, a test is needed.

Ready to go ...

For details of the wiring from the new power supply to the relative Molex connectors see Donald Tillman's Nuclear-Powered Chroma; use the same wire colors of the Chroma and check your work several times ! After the final check, you are ready to powering up the beast; your Chroma must work immediately. Measure all the supply voltages and readjust the +5.05 analog if needed (the SPSU voltages are adjusted at the factory). You will found the assembly slightly warm after a hour or more; this is normal.

Well that's all for the moment; enjoy your Chroma: no more buzz, hot, instability, blown fuses etc.