Power supply replacement part 2

So, I finally finished building the replacement PSU yesterday by adding the bias voltage parts. The bias voltages are now buffered. I did use a different OP-amp from the schematic since I could not find a TL072, so I used some other part I had, and I might add series resistors on the OP-amp outputs for short circuit protection later on.

Power supply replacement

At last, some work has been done on the power supply. I decided to redesign the whole module, but keeping the panel design and functionality intact. The +/-5V bias voltage sources are not yet done, but the +/-9V and +18V sources are working fine. I will post the schematic for this later on.

This is the new PSU board. Still missing the parts for the bias voltages.

 As you can see, there is a toggle switch instead of the original slide switch. The reason for this is that I wanted to avoid routing mains power to the panel, and for that I needed a two pole switch to be able to turn both the positive and negative rails off with the same switch. I couldn't find a two pole slide switch in the same size as the original, so I will have to fix this later. For now, the toggle switch will do.

I put a fuse holder at the side of the box so that I could use a fuse and access it from the outside. This goes against my intention to keep the look identical but I think it is worth it, and the fuse holder is not visible when the case is open. I attached the transformer using straps to avoid drilling another hole through the box.

Trying to keep the wiring tidy and separate the mains power from the low voltage stuff.

Cheap and dirty power distribution busboard (part 2)

A quick update about the power distribution busboards. I removed all the modules from the case, and removed the old power rails.

The pictures above show a case with the old power rails, and won with them removed. I left the reverb tank in place for now.

I used the old mounting holes to attach plastic cable tie mounts, and fastened the busboards using cable ties. Not the prettiest method, but the result is quite stable.

PAiA 4700 resources

Some internet resources that I have found useful:

PAiAs own forum: http://paia.com/talk/

Another 4700 blog: http://obsolete-synthesis.blogspot.com

Obsolete synthesis document links: http://obsolete-synthesis.blogspot.com/2010/11/paia-documentation.html

 A collection of synth documentation: http://sonic.net/mjones/archive/docs/index.html

4750 Control oscillator/Noise

A quick exploration of the LFO/noise module shows some issues:

• The LFO output is clipping, making the bottom part of the sine waveform flat
• The LFO amplitude varies considerably over the frequency range

In addition to the issues R20, the resistor in series with the +18V supply voltage, has been changed from 100Ohm to 4.7kOhm. This dampens the LFO output voltage to approximately 5V p-p. I'm guessing there is some other issue causing this that this change kind of fixes... I will have to get back to this later.

The frequency ranges indicated on the panel are quite accurate.

4771 Power supply

Just a quick note about the power supply module. The unit in my synth has been heavily modified. From the beginning I noticed that the power was wired so that the power switch did not switch off the 18 volt line, just the positive and negative 9 volt lines. A quick look under the panel shows that the thing is quite ugly below the surface. The kludge mounted directly on the transformer seems to be an 18 volt regulator.

Apart from looking ugly, and the power switch issue, the unit actually delivers clean voltages on all lines. That being said, I still plan to replace the unit with my own design. with the following design goals:

• The front panel controls shall function as the original unit
• Keep the original face plate and transformer
• Use a fuse
• Use a voltage regulator for +18V
• Add OP-amp buffers on the control voltage outputs

To be continued!

4740 ADSR

On ADSR modules the manual trigger was unreliable. Sometimes the envelope would not start when the button was pushed, sometimes there was multiple triggers. I replaced the trigger push-buttons with new ones. The Elfa number 35-080-42 is  the same or very similar. When replaced, the behaviour was a lot better but I still get the occasional double trigger. I guess this kind of switch is kind of low quality and have some contact bounce.

One of the modules was missing the attack LED, so I replaced both the Attack and Decay LEDS with nes ones. I didn't find exactly the right type, but the ones I used fits in the panel holes and looks OK from the outside. Unfortunately I forgot the exact part I used.

I attempted to trim the modules. It was easy to set the peak level to 5 volts, but I could not get the sustain to go all the way down to zero. Also, when setting the sustain near maximum, the sustain voltage was not stable but slowly rising above 5 volts. I will have to get back to this later.