Sansui P-L45 does not start, no spin

M

Marsupial

New Member
I just acquired this linear Sansui P-L45 - I have never used those tangential turntable so I may be running on wrong assumptions...

I think these need to detect a disc to start action, and does so using an infrared led and a sensor under the platter.
If I am right, hitting start should make the disc spin and arm move, and if a record is detected begin the process of reading it.
And until this process is started left/right/updown buttons are pretty much useless.

Mine does not start. Like, no spin. The speed selector switch changes what is displayed up front (45/33) but that's it.

I openned it up to look inside. The infrared led do emit something visible on camera. I don't see the arm motor move, nor the platter motor move.

I am not sure what to test at this point. Should I probe the CPU to see if there's activity? Is there something to check on the motor board? Should I check voltage?
This the first time I look at a turntable that's more electronics than mechanical.

I have a stylus and a replacement for the arm belt on order but I'm assuming there should be some motion even without that?

Any pointers welcomed - thx


Here's a few pics. Unsure what's of interest, but could post more. Thanks for looking.

20260304_191813.jpg20260304_191612.jpg20260304_190056.jpg20260304_140753.jpg
 
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You're right, the P-L45 has a slot on the platter/mat to sense record size.

My experience suggests the most common issue is the slot being obstructed or the led not functioning. However the platter would normally spin, biggest danger being it thinks theres a record there when not. Unlike the P-L50 there is no sensor in the tonearm/cart (one reason you can use t4p carts).

The electronics on these are, erm, complicated. But my guess is either the led is not providing any signal or there's a motor ps fault along the way.

You may be already aware but there is a service manual on the internet archive -
Manual: PL45 SANSUI : Free Download, Borrow, and Streaming : Internet Archive
 
I have been looking at the same service manual. It sadly doesn't say much.
My understanding is that P-L45 and P-L55 share most of the components but not all.
The good : there's schematics. And block diagrams.

I'd think if the motor was the issue, the other motor would still have some action, maybe I'm wrong?

I guess I'll start by the basics and measure voltages, see where it leads. Not having much information and no other unit to compare won't make it easier
 
If there's no sensor in the tonearm, does that mean it won't know how to go to next track? I was hoping to see that feature in action...
 
On the good side, I received the stylus and can confirm the cartridge do output strongly.
Now I just need the rest of that turntable to work...

On the not so good side I started to take measurements. The psu seems toast. 20vac gets to the psu and the rectifier does its thing but I get pretty much nothing out of the 15V rail.
I will replace parts of the psu and hopefully will get the unit going.

At least one zener is dead, I'll replace both. Not sure if the power transistors can be trusted.
 
Progress!
It's not dead!
At least not completely.

I changed the two zener diodes (although one tests good outside circuit) and one filter cap and finally got my 15V rail back.

Behaviour instantly changed.

When I plug the unit, the tone arm lifter goes down.
Start button still does nothing. Up/down and left either. But the "manual right" button does lift the arm and start the motor responsible for arm positioning - without its belt it just makes noise until I unplug the unit.

With this I learn
- tonearm motors both work
- CPU works (at least partially)

Am I correct in thinking that it won't start spinning the platter until it knows the arm is at home position?
I think it's right ennough to trigger a physical switch that I assumed served as home position detector. I'll test again soon.

I'm still waiting on the replacement belt for the tonearm position.
 
I've been trying to diagnose the unit and so far it seem every specific details I test are relatively working. Except I cannot get it to accept "start" and begin action.

If I probe cpu pins for buttons and/or sensors, I see input signal changing on the chip when they activate - as they should. Not quite sure why it refuses to start.

I've seen the unit in correct setup of many pins (according to the service manual's operation timing) where the start should trigger the record playing sequence... but it still is not accepting start when I tought it should.

Perhaps I don't understand what specific conditions are required for a successful unit boot. Or what renders the unit functional.

I'm thinking the initialization fails and I may only get more information once I have the tone arm strap to let the cpu get proper timings
 
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I am currently testing with a temporary rubber band for the missing belt.

Had good results. I can't understand what conditions are needed but I could manage to start the platter. Both 45 and 33 rpm speeds are good. But the tonearm does not get to position.

I need to ask anyone with an p-l45 or p-l55 - if you manually move the tonearm left/right, does that trigger a movement of the tonearm cradle? Because it does that to the right, but not to the left. Normal?

Currently, start/stop and up/down buttons don't react. Left and right either but I think this is normal as long as the tonearm is "home"

If I measure directly at CPU pin 24 for start stop, I get a good 5v when not pressing, but "only" 3.5v when pressing. Is that normal on this CPU?
I'd think it should get closer to 0v.
 
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