Pioneer PL-71 over speed issue

[Buggleskelly]

Hi Cameron,
you can find the PL-71 owners and service manuals here

http://www.vinylengine.com/library/pioneer/pl-71.shtml

The service manual has the basic motor and PSU circuit diagrams, plus an explanation of how it works.

It's the first I've heard of PL71's having over speed issues , but we live and learn

I have two PL 71's ( a UK and a US model) and one of them (the US model) had the typical speed drift problem which I fixed by changing the 1K pots, micro switches and the caps on the motor board. I went for path of least resistance first and used DeOixt on the pots and switches. As that didn't work I swopped the pots and caps out and that improved the speed drift a bit. Then swopped out the caps and that fixed the problem. As a matter of course I also changed the regulator caps too.

Changing the motor board caps was quite fiddly job to do. I know Speedy Steve did his and wasn't too much of a problem; there does seem to be some slight variance in how the models were constructed over the years.

Hope the info helps.

Cheers / Steve

[StanleyB]

I don't know the PL-71 by heart any more, but ALL PIONEER TURNTABLES WITH SPRING LOADED self leveling legs made between 1978 and 1984 had speed stability issues.
Pioneer brought out a mod kit, which consisted of a replacement power switch and a rubber cup that was grey in colour. It replaced the black rubber cup that covered the leg spring close to the power switch.
I did hundreds of these mods when I worked in the service department of Lasky's. No idea why the switch and rubber cup caused a speed issue, but that's how it was.

[Buggleskelly]

That's very interesting info about the Pioneer mod. I'll have to take a look at mine.

Nice one

Cheers / Stevie

[crashden]

Marco,

I'd kind of hoped that was the problem too when the guy I acquired it from explained what it was doing and it is most certainly a US table as it still has the US plug. But the transformer tap was the first thing I checked and it is set to 240V.
Output from the trans is approx 28vac and output from the PSU is 19.5vdc. I even went as far as running the motor directly from a DC bench power supply and varying the motor supply voltage from 17.5 – 19.5vdc with no change, which make me think its an issue on the motor board itself and unfortunately trying to diagnose that board is a bit beyond me (particularly without a schematic which is not part of the service manual).

The good news is I’ve managed to locate a motor, bad news is it’s expensive. I’m weighing up weather to continue trying to fix the existing motor where I’m probably going to have to get someone with better electronic knowledge that myself involved (with potentially no outcome), or just bite the bullet and buy the replacement.

I think it’s worth the effort either way as its such a great looking table and I cannot bear to see it not working.

Cam

[crashden]

Interesting about the switch mod, I know photos I’ve seen and the service manual show two switches, one for power, switching the active leg and one for speed select. My unit has three switches one for speed (33/45) and two for power one switching active and the other neutral.
I wonder if single power switch versions didn’t handle the switching load so well (hence all the switch issues people appear to have) so a second switch was added as a mod and to later versions

[Canetoad]

Mine has the 3 switches too. One controls power and the other two the speed selection.

[Buggleskelly]

Mine has the 3 switches too. One controls power and the other two the speed selection.

Same here with my two.

[crashden]

Ok spent a couple of hours playing around on Saturday.

I figured that if the speed trimmers are just varying the resistance I could add an extra resistor in series and it might do the trick.
Bit of time mucking around finding the right value (22k in the end) and I can now report that she is up and running at correct speed (both 33 and 45) and appears to be stable with an hour or two test.

I’m not 100% happy with this as a fix as to me there is still something not quite right on the motor board, but for the moment this will do and I’ll keep an eye out for a replacement.

Now time to play with the setup and look into some cartridge match ups.

Thanks to everyone who offered input.

Cam

[DSJR]

Just a quickie and to do with the pre-quartz Technics direct drives. On the SL150/1500 mk1 and related models, there are four presets to do with speed, two trimmers on the mainboard and the two top plate ones. ALL of these get contaminated over the years, messing speed stability up no end. I cleaned out the one Alex_UK has before he bought it and two years on it's starting again. The main power micro-switches do similar horrible things too, tweaking the main on-off speed selector messing around with the speed as well. Once they're clean and the deck is running right, it stays absolutely pitch stable and any apparent drift is the mains frequencyaffecting the onboard strobe light (I use an external battery powered strobe unit).

[NRG]

I've not drawn out the motor board schematic yet so can't really help but the first thing I would do is get the PSU voltage down to 18v as per the service manual. Voltage is set by a 19.2v Zener, there is a .7v drop through the series pass transistor so o/p voltage should be no more than 18.5v.

The motor board generates a reference voltage from the supply voltage and it may be causing an issue at 19.5v. Where did you you place the additional resistor? Do the main speed adjustment pots on the deck have any affect on the speed?