From roberts@pharm1.pharm.sunysb.edu Sat Jan 11 19:35:17 1997 11 Jan 97 22:33:03 +0500 11 Jan 97 22:32:37 +0500 Comments: Authenticated sender is From: "Roy Roberts" Date: Sat, 11 Jan 1997 22:32:48 +0000 MIME-Version: 1.0 Content-type: text/plain; charset=US-ASCII Content-transfer-encoding: 7BIT Subject: Priority: normal X-mailer: Pegasus Mail for Windows (v2.30) From: "J.D. McEachin" Subject: Prophet 5 Power Supply Mod MIME-Version: 1.0 I typed this up for Rich, and figured it was one for the archives. Enjoy! The following is from Alan Gary Campbell's "Service Clinic" column in the Nov.87 Electronic Musician: Q: I had my Rev 3.2 Prophet 5 upgraded with the SCI MIDI Retrofit at a local service center. The unit now occasionally "locks up" mostly at random, but more often after it's been on for a while. The service center couldn't duplicate the problem, and they say it's a common sypmtom for MIDI'd Prophets. Is there a cure? A: The Prophet MIDI retrofit demands increased current from the +5V digital supply. The supply design floats its 7805 regulator ground via a diode, producing and output voltage of +5.6V, to allow some headroom (a series diode on the CPU board drops this back to 5V); in some cases, with component aging and high ambient temperatures, the supply voltage dips below acceptable levels, and the instrument locks up. The solution to the problem is to add an additional diode on the power supply board, in series with the existing one. This can be done without disassembling the power supply as follows: unplug the instrument, and remove the bottom panel (refer to the Prophet-5 Technical Manual); disconnect the power supply, audio, and MIDI cables. Locate the 1N4002 diode (D505) at the center of the power supply board, just behind the solder pads for the 7805 regulator. Desolder the left diode lead only; use a small (about 30 Watt) soldering iron with a petite tip, and pry up the lead with a mniature flat-blade screwdriver. Do not leave the tip in contact with the lead and solder pad any longer than you have to; but also do not pull up on the lead until the solder flows, or you might tear the pad away from the board. Take care not to let the iron slip and damage any of the nearby cables. Bend the diode up at about a 70 degree angle, and straighten out the desoldered lead. Prepare a 1N4002 diode (a 1N4003 Radio Shack cat.no. 276-1102 is an acceptable substitute), clean the leads, and cut the anode lead (unbanded end) to a length of about 3/8 inch (9.5mm). Install the new diode at the desoldered location, oriented as shown in Fig. 1 with the cathode (banded end) up. (NOTE: I'm not including the picture - it just shows two diodes sticking up, teepeed together like this: /\). Again, make sure the solder flows before you push the lead through the hole, or you might damage the pad underneath the board; and add a bit of new solder to insure a good joint. Solder the unconnected leads of the two diodes together, and trim any excess lead length. Before re-assembly, use an Ohmmeter to check for continuity between the anode of the new diode and the center pin of the regulator. Visually inspect the underside of the board to be sure the newly installed diode lead does not protrude too far under the board and is not in contact with any of the regulator leads. SCI technician Andrew McGowan advices that this mod should be installed on all MIDI'd Prophets, though it is not mentioned in the MIDI Retrofit Installation Manual. The Prophet can also lock up from less common types of power supply and CPU problems. If the diode mod doesn't fix your glitch, test the power supply under load, and check the clock, batter back-up, and reset circuits. If at all possible, obtain a cassette save of the Prophet's programs before you proceed - if the "disease" doesn't dump them, the cure probably will. If you have to replace the 74C02 (IC309) in the reset circuit, note that it goes on the board backwards relative to the other logic chips nearby.