I had a customer outside the Continental USA with an Oberheim Four Voice with a power supply that failed.  Both filter capacitors “popped” and the synthesizer stopped working.

Because shipping the whole synthesizer such a distance would be expensive and risky, we decided to send just the power supply, and the Programmer module which was also in need of repairs before the power supply gave out.  Besides the two large filter capacitors, I also replaced the two tantalum capacitors and the two LM723 voltage regulators.  I also tested the diodes that serve as the bridge rectifier.

While obviously these power supply capacitors need to be replaced, I wasn’t convinced that the root cause of the problem is actually on the power supply board.  Particularly because both capacitors popped, not just one, and the +18.5 and -18.5 rails are regulated independently.  The transformer and the pass transistors for the power supply aren’t on this board.

So when the board arrives back home, it will need to be hooked up in phases, testing each part of the power supply before connecting it to the load.

Step 1 – Test the Transformer & Fuse

The first step will be to check the AC voltages at the transformer secondaries.  If we get the proper AC voltages here, the power wiring, fuse, and transformer are all good.  The transformer has a single primary winding that accept the 120VAC line voltages, and two secondary windings.  I didn’t measure the AC voltage of the secondaries on any of the FVS’s I was working on, but I’d imagine the secondaries would be around 30VAC.

To check this, with our multimeter in AC voltage mode, we first measure across pins 1 and 2, and then 4 and 5 of the Molex connector that plugs into the right side of the power supply PCB.  It doesn’t matter which color lead we put on the pins.  If each winding measures around 30VAC, we’re good to move onto step 2.

Step 2 – Test the Power Supply & Pass Transistors

The LM723 voltage regulator is in a DIP package and by itself cannot regulate large loads.  The maximum current of an LM723 DIP is only 150mA, clearly not enough juice for a massive Oberheim FVS.  So rather than configuring the power supply so all the current to the load goes through the regulator, the supply is designed so the LM723 regulates the pass transistor, through which the current to the load is flowing.  In the FVS the pass transistor is a 2N3054 transistor in a TO-66 package, capable of a continuous collector current of 4A, or a power dissipation of 25W.  These transistors are mounted on the back of the metal synthesizer chassis by the power inlet, with an additional small aluminum heat sink.

Even though I essentially replaced all the components on the power supply PCB, without the rest of the synthesizer I was unable to test the customer’s pass transistors.  So the power supply may still not be working properly.  So the next step will be to plug in the connector for the transformer secondaries, which is color coded white, and the two molex connectors that go to the pass transistors.  These are color coded blue (for the negative power rail) and red (for the positive power rail)

With these connected, the regulated DC voltages can be measured at the black connectors on the left side of the power supply PCB.  We will want to check these voltages first, before connecting anything to those connectors to make sure the power supply PCB and pass transistors are OK.

Let’s check the +18.5V rail first.  With our multimeter in DC voltage mode, we place the black lead on pin 2 or 3 of any one of the black connectors (these are the middle two pins), and we place the red lead on pin 1 (the right-most pin).   The voltage should measure close to 18.5V.  This power rail is adjustable, so if your reading isn’t close to 18.5V, adjust the trimmer (the lower trimmer is the trimmer for the +18.5V rail) until you get about +18.5V.

Next, check the -18.5V rail.  With our multimeter in DC voltage mode, we place the black lead on pin 2 or 3 of any one of the black connectors (these are the middle two pins), and we place the red lead on pin 4 (the left-most pin).   The voltage should measure close to -18.5V.  This power rail is also adjustable, so if your reading isn’t close to -18.5V, adjust the trimmer (the upper trimmer is the trimmer for the -18.5V rail) until you get about -18.5V.

If the supplies can be dialed into around +18.5V and -18.5V, respectively, the power supply is safe to connect to the synthesizer’s modules and we’re good to move onto step 3.

Step 3 – Test the downstream load

Many times people will bring me a synthesizer that they erroneously feel has a power supply problem.  One or more power rails in their synthesizer are lower than they should be (e.g. +15V at +1V), so they figure it has to be a problem with the power supply.  The problem is they are measuring these power rails with the load connected.  A short circuit in the downstream load can cause the voltage regulator to drop out the voltage to prevent an over-current situation.  So now that we’ve tested the TVS/FVS power supply with no load, we now need to test it with the load.

To do this, we will power off the synthesizer, and one module at a time, connect it to the power supply, and recheck our DC voltages as per Step 2.  If you’re confident that your modules are OK, you can plug them all in at once and check.  But the one at a time method is what I would use to check a synthesizer in an unknown status.

If all modules are plugged in and the DC supplies are still around +18.5V and -18.5V, your power supply is good, and you can try to dial in the +18.5V and -18.5V rails a little finer.   The +18.5V and -18.5V are regulated further in each module down to 15V and -15V, which is used for the majority of the circuitry, so this calibration isn’t critical, but still try to get it as close as possible.

Last year I did a three part video series repairing an Oberheim Four Voice. Now I’ve got couple more Four Voices in for repair, and I thought we’d check one of them out together. In this one we repair a problem with the polyphonic keyboard assigner module.