From don@till.com Mon Mar 18 13:24:46 1996 18 Mar 96 16:24:36 +0500 18 Mar 96 14:03:32 +0500 id KAA05346; Mon, 18 Mar 1996 10:12:28 -0800 Date: Mon, 18 Mar 1996 10:12:28 -0800 Subject: Nuclear powered Chroma! From: Don Tillman Organization: Don's house, Palo Alto, California I just replaced the power supply on my Rhodes Chroma with a modern switching supply. Why would a sane man undertake such a project? No reason on earth. Why did I undertake such a project? The original Chroma supply is a lame design. It's sensitive to line transients (turning off a fan in the room plays a random chord!) It's heavy, and it's location makes the Chroma very heavy on one side. It generates heat which detunes oscillators. It generates ugly magnetic fields that affect nearby guitar pickups. My particular unit happens to be acoustically noisy (it makes an annoying buzzing sound.) According to the service manual a Chroma needs: analog +5.0v 0.25 A digital +5.0v 3.50 A +12v 0.70 A -12v 0.70 A (The manual doesn't mention the extra voltage needed for the tapper. The tapper is a solenoid that makes a thump when the player presses the membrane switches thus providing tactile feedback.) The supply is also responsible for a TTL power-fail signal that hardwarily disables the battery-backed-up CMOS patch store RAM so it won't get random crap written into it when powering up and down. The replacement supply I choose is a PowerOne MAP 80-4000. It's an 80 watt supply that puts out: +5.0v 13.0 A +12v 4.0 A -12v 1.0 A -5.0V 1.0 A Very important: This particular unit features a TTL power-fail signal. The supply doesn't give me an analog +5.0v output, so I have to make that myself from the +12v line with an outboard regulator chip. What about the tapper supply? It's possible to tap off 14V from the PowerOne right before the 12V regulator circuit. First time I powered-up things didn't work because the power-fail line didn't have enough drive current, so some parts of the Chrama were stuck in reset and other's weren't, which is a very interesting situation, so I also had to build an an active buffer circuit for that. Results: It works great. The Chroma has now slimmed down from 72 lbs to a svelte 65 lbs. At this weight, this difference is important. The tapper voltage is a little lower, which I think makes for a much better feel. I should point out that this was a *lot* of work. It's real important to check the wiring many, many times since one bad connection can literally fry hundreds of chips. Mechanically I had to disassemble the beast down to every bolt. You have been warned. (And since I had the Chroma disassembled so completely, I refinished the woodwork, so it looks beautiful too.) -- Don