Pioneer PL-400X turntable

[Mark Hennessy]

I've not forgotten how a PLL operates.

I was replying to Jeffrey. If I was replying to you, I would have quoted you. I didn't quote Jeffrey's post as his was the most recent.

All I can remember is that the pots were designated 33 & 45 speed. So if these are to lock the PLL adjustment would put it out of lock so no speed accuracy and hence drift, something I never came across once set up!

If folk took just a couple of minutes to skim the reference I posted, they'd see that there are several ways to implement that chipset.

Obviously, using the XTAL to provide a reference to a PLL to lock the speed is the "premium" option. But cheaper turntables don't use the PD1003 chip at all, and instead lock to a reference voltage (all described in that link). So in those instances, yes, those pre-sets set the speed directly, as you describe. But Jeffrey's is different.

Of course, the labels on the schematics could be a carry-over from the cheaper models. But as they effectively change the centre frequency of the VCO (the turntable itself), then perhaps they are technically correct in isolation. But as we now know, they won't change the overall speed while the PLL remains in lock.

It's rather frustrating when replies are fired off before my posts (and references within) clearly haven't been read properly. I don't post often, but when I do, I make the effort to research and provide supporting links, etc where necessary. I am not concerned with post count, but I try my best to maintain post quality. Terse posts made in haste might be fine for social media, but do little to enrich a technical forum.

(For clarity, that last paragraph is not directed at you, Trevor. It's just my general frustration with this place bubbling to the surface again)

So it looks like both approaches are needed. Varying the oscillator frequency to something appropriate for 78, them trimming the resistor on pin 11 to keep the PLL happy.

You're very welcome - don't mention it 

Personally, I think that rather than mess with the original XTAL (and messing up 33 and 45 in the process), I'd be tempted to investigate the RC oscillator included in the PD1003.

They say that vari-speed with this chipset is used with cassette decks rather than turntables, so I'd probably start by finding the schematic of such a cassette deck and look to see what changes are involved (e.g. see page 63 of http://akdatabase.org/AKview/albums/user...ervice.pdf). In theory, it could be quite simple - once suitable oscillator components have been fitted to pins 6, 7 and 8 of the PD1003, it might be as simple as grounding pin 11 to flip Selector III from the XTAL to the RC oscillator (plus the extra preset on pin 11 of the PA2004, naturally).