Converting a vintage Parasound HCA-1200 from 117VAC to 230VAC

I have posted this information at the end of a somewhat related thread, but thought that it also made sense to put it "on its own" where Google can find it more easily.

I own a Parasound HCA-1200, purchased in the mid 1990s in the USA, which was designed for 117VAC, and is marked that way on the back of the unit. I now live in Germany where the line voltage is 230VAC, and have been operating the amplifier using a voltage conversion transformer. I got tired of that and decided to convert the amplifier to operate directly off 230VAC.

The good news is that the power supply torroidal transformer has two primary windings, which are wired in parallel to run on 120VAC. To convert the amplifier to run on 230VAC is very simple: rewire the primary coils in series. Here are the steps, which closely parallel those above.

WARNING: I ASSUME NO LIABILITY FOR THE INSTRUCTIONS BELOW. IF YOU DO NOT HAVE EXPERIENCE WITH ELECTRICAL WORK AND ELECTRONICS, AND DO NOT KNOW HOW TO WORK SAFELY WITH LETHAL AC VOLTAGES, DO NOT ATTEMPT THIS. FIND SOMEONE QUALIFIED TO HELP YOU.

(0) Remove power from the amplifier and open it up.

(1) Locate the transformer primary wires. These come from the transformer in a bundle of four wires, which are black, red, clear and yellow. (The clear insulation is transparent.) You are not going to touch any other wires than these four! The red and clear wires are connected together to the mains power coming from the front panel power switch. The yellow and black wires are connected to the central (power supply) circuit board near a 10 ohm 30 watt resistor.

(3) You are going to leave the red and black transformer primary wires connected as they are.

(4) Disconnect (unsolder or cut) the clear transformer primary wire from where it is connected to the red wire and the incoming electrical power.

(5) Cut the yellow wire near the power board. Fold over the end of the stub of yellow wire coming from the power board and put some heat shrink tubing over it to prevent it from shorting against anything.

(6) Strip some insulation from the yellow wire coming from the transformer, and use a sharp knife to scrape off the insulation varnish.

(7) Solder the yellow wire to the clear wire. Insulate with heat shrink tubing.

That’s it! If you want to test this in a safe way, I put a 100 - 200W incandescent bulb in series with the power plug when you plug it in. The lamps will flicker from startup current then should dim and the amp front panel lights should light up.

Here is a schematic, the relevant part is the power supply transformer wiring in the bottom left. (The schematic in the forum has the wrong aspect ratio, but if you download it in a separate window it is correctly proportioned.)

Best regards,

Bruce

PS: if someone here can tell me how to bias the output devices, I'd be grateful.  What is the correct bias voltage on the 0.33 ohm emitter resistors?  I have found this online for all the Parasound amplifiers APART from the HCA-1200.