VPI PLC Schematics (Partial)

I've put (at least) something here for the VPI PLC.

Having examined the PLC, and having discussed it with forum members, I don't consider it usable without significant modifications.  While it may appear to work for you, it exhibits instability, and its output waveforms are very malformed.  In the case of this particular PLC, I found that the turntable's rotation speed decreases in the first 30 minutes of use, requiring the thumbwheels to be increased by about 2hz.  After 30 minutes of use, its rotation stabilizes, but it still appears to "hunt" (speed bounces a bit).

A consensus is that the PLC's "square wave" output should be converted to sine (perhaps at the 4049) and its drive transistors might be converted to MOSFET devices.

As I plan to begin using the newer model (SDS, Synchronous Drive System) I may not find the time to modify the PLC.  However, if I do, I'll update here and on www.AudioAnnex.com (Vinyl Forum).

Thanks,

Dan (Evergreen)

UPDATE MAY 10, 2006 - I replaced the 2N3055 power transistors with a pair of Fairchild FQP16N25C N-Channel MOSFET transistors (Source to ground, Drain to transformer; the datasheets don't mention it, but be aware the Drain is connected to its heatsink tab).  The existing 2N3904's were removed and reversed on the circuit board (common-emitter changed to common-collector).  The MOSFET change kept all of the transformer reflections from reaching the circuit board (the square waves up to the MOSFET gates are now clean).  However, as predicted, it did nothing to provide a cleaner signal at the PLC output.  In addition, since the MOSFET's are probably conducting most of the time, they run quite hot and require a small fan over their additional heatsinks.

As J.Guilherme (www.AudioAnnex.com) observed, the driver circuit must have a true sine wave for its input.  In addition, the driver circuit should be changed to (possibly) a Class-A power amplifier rather than its existing push-pull design.  While this is possible to do, I don't think I'm going to do it.  Since I plan to get the VPI SDS, I'll have no further need of the PLC, and will probably scrap it.

Thanks,

Dan (Evergreen)