Sony PS-4750 “pulses” - need help

[Rob42]

Hi, I’ve inherited a Sony PS-4750 that’s been fine for a year or so but has now suddenly started pulsing in speed. I try to fine tune it on the pitch control but as I get close there’s an audible whir from the deck (not through an amp, no connection) then the deck speeds up.
Just as it seems to slow to the correct speed it pulses again. Happens every 4-5 seconds.

Hoping someone might have an idea what could be wrong?

Thanks,

Rob

 

[JamVal]

I have a PS-4750. I got it at a yard sale. First time using it did the same kind of thing yours is doing.
The first thing I would do is clean those pitch control potentiometers and the selector switches in front of them.

To do it effectively you will need to remove the headshell/cartridge and the platter so nothing gets damaged. Secure the tonearm, then turn the deck over with the dust cover on to protect the tonearm. Get yourself a can of Deoxit or Electrical Contact Cleaner (non-residue, plastic safe). DO NOT USE WD40.
Remove the bottom of the turntable.
Spray the Deoxit or Contact Cleaner into the speed potentiometers from the bottom and rotate them from the top several times over the full range. Repeat a few times.
Also spray into the selector switches from the bottom and work those back and forth several times.
Let it dry, a few hours and then see if that helps.

Let us know if that corrects the problem.

 

[Rob42]

I have a PS-4750. I got it at a yard sale. First time using it did the same kind of thing yours is doing.
The first thing I would do is clean those pitch control potentiometers and the selector switches in front of them.
View attachment 3031044
To do it effectively you will need to remove the headshell/cartridge and the platter so nothing gets damaged. Secure the tonearm, then turn the deck over with the dust cover on to protect the tonearm. Get yourself a can of Deoxit or Electrical Contact Cleaner (non-residue, plastic safe). DO NOT USE WD40.
Remove the bottom of the turntable.
Spray the Deoxit or Contact Cleaner into the speed potentiometers from the bottom and rotate them from the top several times over the full range. Repeat a few times.
Also spray into the selector switches from the bottom and work those back and forth several times.View attachment 3031049
Let it dry, a few hours and then see if that helps.

Let us know if that corrects the problem.

Hi, thanks for the advice.
I’ve downloaded the service manual you kindly marked up for me.
Gave everything a clean with an air duster first then contact cleaner in the switches & pots.
The PCB trims were a bit stuck at first but seem to run smoothly now & the main pots run nicely, but unfortunately it still chases when running.
It’s like there’s a reset and a pulse or voltage increase which speeds up the deck.
As soon as the strobe shows the speed is ok, the pulse occurs. This happens wherever the pots are set.
There’s a lot of dirt on the speed detecting head so perhaps that’s the issue.

I think I’ll need to put a ‘scope across the motor drive to see what’s happening next & maybe try to touch up a few solder joints.

Any thoughts you have would be greatly appreciated and thanks again for the initial advice!

Rob

 

[Rob42]

Turns out three bouts of cleaning was required. Redid all the pots with isopropyl alcohol spray, air, then contact cleaner. Scrubbed the magnetic head with a toothbrush and isopropyl alcohol and reseated it a tiny bit closer to the platter. Improved the drift but not cured it. Repeated all of the above, retouched a couple of suspect solder joints and it runs great. Just in time for more vinyl for the wife’s birthday.

Thanks again JamVal, so happy to have it back up and running!

 

[JamVal]

Turns out three bouts of cleaning was required. Redid all the pots with isopropyl alcohol spray, air, then contact cleaner. Scrubbed the magnetic head with a toothbrush and isopropyl alcohol and reseated it a tiny bit closer to the platter. Improved the drift but not cured it. Repeated all of the above, retouched a couple of suspect solder joints and it runs great. Just in time for more vinyl for the wife’s birthday.

Thanks again JamVal, so happy to have it back up and running!

Happy that it's working for you now. I haven't used mine in awhile. I prefer an auto-return deck. I tend to fall asleep while listening in the evening and my wife gets annoyed with the repeated end of runout noise. I had forgotten how the speed is controlled on the PS-4750 and neglected to mention being careful handling the magnetic coated edge on the underside of the platter, which if dirty could also cause the problem you had. Sorry about that.