Prophet 600 Refurb – Part 1

P600 as bought

Overview In June 2020 I rescued a rather forlorn and broken Prophet 600 for way too much money, but I decided it was worth the time and effort to get it back it to perfect working order. Although financially it does not make sense, as I will probably spend more on it than its currently worth, once I have fixed the P600, I plan to implement a number of upgrades.

• Gligli OS upgrade
• New power supply
• Stereo chorus and voice panning
• New Fatar keybed
• New walnut end cheeks

The Prophet 600 was announced at NAMM 1983 and shipped a month earlier in December 1982 with the first MIDI implementation ever (thanks Dave!). It was a reasonably successful product with 6000 (approx) sold until mid 1985, against the heavy competition from the DX7. The innovative use of digital envelope generators and a digital LFO pushed the capability of the original Z80 microprocessor, the slow envelopes and obviously stepped pots put customers off. The budget nature of the P600 was a bit too apparent!

CPU board

Part 1: Getting The P600 Working The first stage is to get P600 tested and working again with no modifications, although dusty the circuits look to be in good condition and better than I expected. I downloaded the Technical Service manual from July 1983, as I am sure this is going to be a constant companion for the next few weeks! My Prophet 600, serial number 1406, arrived at the workshop on 12th June and I logged these initial visible faults:

• Large number (11) of keys missing
• The keybed is in very poor condition, needs to be replaced
• Slightly damaged Mylar control panel
• No control knobs
• One pot missing
• One pot with spindle cut
• OS ROM missing, all other chips in place
• All cabling intact but disconnected
• Pitch and Mod wheels stiff
• Battery disconnected

The front panel was cleaned and found to be a very good condition with only one visible chip down to the metal which will be carefully infilled with black paint. The rear and underside of the casing have not survived as well and there are numerous areas of black paint missing. The white silk screen lettering has survived, so it needs a black repaint feathered into the original. The rubber feet are in place but the wooden end panels will be replaced. All the intact pots rotate ok and need a clean, and the toggle switches work fine.

Power On Whilst a set of OS (Version 8) and Diagnostic ROM’s arrive from Germany, I can test some basics; is the power correct and the microprocessor being clocked. There are no visible signs of power failure, such as blown caps or regulator heat damage. The first step is to power on the transformer at the correct 240V but disconnected from the CPU PCB. I then checked the AC output voltage rails which should be 18 and 36 VAC. Measurement shows all ok at 22 and 44 VAC with slight transformer whine.

Onto powering up the CPU and Voice PCB’s (they are hard wired together) and checking the analog and digital DC rails. I left the keyboard, bender and panel PCB’s disconnected at this stage. The power rails measured good (+4.97V, +15.07V, -4.96V,-14.95V) and there is a 4 MHz clock at pin 6 of the Z80 chip. I ordered new OS ROM and Diagnostic ROM chips from a German eBay seller and they arrived very quickly.

Initial Repairs Two new potentiometers were ordered from Wine Country in the US and the wooden sides measured, so that new American walnut sides could be made by MintCase. The piano hinge for the front panel is missing the six machine screws that attach it to the rear casing. I ordered a pack of ten M3 x 8mm black posidrive machine screws, and they were sometimes a loose fit requiring a M3 nut to be fitted as well. More progress in Part II to follow, when we try and boot up the P600.