Pioneer PL-71 over speed issue

[crashden]

Hi all,
First time poster so I’ll start with a hello and some quick background.

I’ve only recently been bitten by the vinyl/vintage audio bug thanks to a neighbour and have been having a great time over the past few months bringing a Dual 1209 back from the dead and getting a little Sansui AU-101 sounding sweet through a set of B&W DM110’s. This has given me hours of joy and all for the grand total of around $150 (Aussie that is) to date.

Now onto my issue/question, I’ve just acquired a mint condition Pioneer PL-71 which has a serious over speed problem. When set to 33 I’d estimate it spinning close to 45 and when set to 45 it’s spinning faster again!

I’ve done a heap of googling and reading and know that the PL-71 have speed stability issues typically related to switches, trim pots and PSU caps so the below is what I have done to fault find to date.

1. Bypassed the 2 x power switches (yes mine is the three switch version) and speed select switch to eliminate these as a source.
2. Measured the 1kohm trim pots (both ok) and also interchanged them to eliminate as a source.
3. Measured the PSU output at 19.5 DC stable and with surprisingly very little noise given the original caps are still place.
4. Used a DC bench power supply (using a range of voltages) as an alternative to the internal PSU to be sure it wasn’t the issue.

All this kind of leaves me with just the motor, both the external and on board trims alter the speed its just that overall its spinning way to fast.
I’m wondering if anyone has seen this before, has any ideas or possibly has a schematic diagram for the motor control board (FL-EGP018).

Any help would be greatly appreciated as I’m desperate to get this, in my humble opinion beautiful table back up and singing.

Cheers, Crashden

[synsei]

Hi Cameron,

Just did a wee bit of Google'ing and I found this which may be of help:


Well from my experience so far it just needs capacitors changing. On the small psu board change the 330mfd/35v - 100mfd/25v - 33mfd/35v if that fails to sort it out then you have to go onto the motor board (circular one under the motor cap) and change electrolytic caps there. You could try cleaning all the pots first. The oxidation accumulated over years is enough to interfere with the contact. For small open pots, slide a piece of paper soaked in contact cleaner between the contact and the strip.

Regards

Dave...

[crashden]

Thanks Dave,

I did find that thread which I think was more to do with an issue on speed stability whereas mine appears to be quiet stable just well over speed.
I might try recapping the motor board anyway in the absence of knowing what else to try.
Unfortunately attempting to interpret the circuit function without (probably even with!) a schematic is getting a bit beyond my limited capabilities.

Thanks again, Cameron

[synsei]

The day shift will be along in a minute or two with some ideas of their own I'm sure, so hold tight Cam ;)

[John]

Speedy Steve might be your man, he sometimes uses the forum

[crashden]

Well I recapped the drive board last night and unfortunately no difference :-(

Bit depressing to have such a cool looking unusable

Cam

[Marco]

Hi Cameron,

Welcome to AoS :)

I'd also speak with Neal (NRG), who posts here, as he knows the PL-71 inside out, and so I'm sure he could help.

Do me a favour, though, please and pop into the Welcome area and introduce yourself properly, telling people what system you've got and what music you like, as that's the correct procedure to follow, when joining the forum.

In the meantime, I'll move this thread into the DIY room, as the discussion is essentially DIY-related.

Marco.

[crashden]

Thanks Marco,

Apologies for the newbie indiscretion, Intro now posted.

Cam

[sq225917]

Dc Motors are voltage locked and the 71 doesn't use a PLL so all you should need to do is drop the voltage by the same % as it is currently running over speed. A motor psu diagram would help greatly.

[Marco]

Thanks for that, Cameron - much appreciated.

I've also just received this message from someone who knows:

"He is running a 120v set PL71 on 220v. It is very easy to change voltage taps, he just needs to download the circuit diagram from the web."

Hope that helps :)

Marco.

[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 :scratch:, 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 :thumbsup:

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?

[crashden]

Neal,

Yep I was a little worried that it was putting out 19.5v against the service manuals 18v so the first thing I did was run it directly from a DC bench supply (taking the external PSU completely out of the picture) and varying the voltage from 16 – 20v made absolutely no difference to the speed of the motor.

When I get a bit more spare time I’m going to have a good look at that reference generator circuit, there is a zener in there that could be the issue. Problem is that it is near impossible to measure anything while running the unit.

I placed the resistor in the brown (common leg of the speed change switch) wire of the external pitch control, seems to be doing the trick at the moment.

The main speed controls work perfectly.

Cam