Pioneer PL-400X turntable

[ppppenguin]

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.

At the moment this is purely a theoretical exercise, wondering if it's possible. I use a Fons CQ30 as my turntable. This has fully variable speed, which is one of the reasons I bought it. Brand new c1980 from London Sound.

[Murphyv310]

The old Yaesu FT-480R has a PLL board with three separate PLLs, the UK version has a frequency coverage from 144.000 MHz to 145.999 MHz. The frequency can be selected on SSB in 10 Hz steps. The three PLLs control three crystal oscillators that by tuning the overtone crystals which with the correct combination gives the required transmission frequency. These PLLs are a swine to set up plus the crystals have aged (these radios are 43 years old) Which in turn causes drift as the PLL drops out of lock. The point is there is always an input scenario in this case the the frequency knob, and in the turntable the speed switch and it's fine adjustment. It's possible that asking it to run at 78 the PLL will be out of lock so drift will occur, rest a finger on the platter and give it a bit of breaking effect and see if it slows, normally they don't even with a fair bit of pressure.

[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).

[ppppenguin]

If I was doing the mod seriously I might well dig out a CPLD prototyping board and use it to replace the PD1003. No problem with generating any frequencies I might want for this application. I'm sure something similar could be done with a PIC chip. For experiments I'd probably use an external oscillator. This is similar to the approach I took with 405 to 625 conversion experiments where I used an external oscillator in place of the decoder's xtal before synthesising the frequency I needed in the (already existing) FPGA.

The conlcusions I'm reaching, subject to possible future experiment:
* It's very likely to be possible to make the turntable run at 78 or variable speed.
* It shouldn't be too hard.

Thanks to Mark and all who have contributed to this thread.

[Murphyv310]

Hi
I would have read the document if it would open on my phone but unfortunately it didn't!

[Mike Watterson]

For older "78s" you need variable, maybe 65 to 85 rpm. Certainly as high as 82.

[ppppenguin]

I have a number of records marked as speed 80. The older they are, the more likely is the speed to differ from 78. When doing professional remastering it's often necessary to allow for the speed changing throughout a side. If it's a classical work on multiple sides the pitch difference can be very obvious.