Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

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Tom_Technics
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Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

Post by Tom_Technics » 21 Sep 2019 19:43

The control circuit in these Technics turntables utilises 2 variable resistors for setting correct voltages after servicing. Correct voltages at C15 and R38 ensures 2 things, 1) that the speed stabilises within a few seconds and 2) that the platter doesn't overshoot on startup i.e. spins faster than it should for a second or so before slowing down to the correct speed.
The service manual states that VR3 sets the voltage at C15 and VR4 at R38. However, adjusting VR4 will slightly affect the voltage at C15 and adjusting VR3 will slightly affect the voltage at R38. This makes these adjustments quite tricky to make, but the task will quickly seem impossible as the service manual states incorrect voltages. You will never get 2.2V at R38 and adjusting for 50mV at C15 is not quite possible when VR4 is set correctly.
After some experimentation, I have found that the correct voltages are 60mV at C15 and 1.3V at R38.
This post is just to share the correct information so people can service their turntables properly in the future.

KentT
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Re: Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

Post by KentT » 21 Sep 2019 22:54

Thanks for this information, very useful for owners and those repairing/restoring their SL-1500 and relatives.

Trio KD1033
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Re: Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

Post by Trio KD1033 » 22 Sep 2019 02:34

Tom_Technics wrote:
21 Sep 2019 19:43
The control circuit in these Technics turntables utilises 2 variable resistors for setting correct voltages after servicing. Correct voltages at C15 and R38 ensures 2 things, 1) that the speed stabilises within a few seconds and 2) that the platter doesn't overshoot on startup i.e. spins faster than it should for a second or so before slowing down to the correct speed.
The service manual states that VR3 sets the voltage at C15 and VR4 at R38. However, adjusting VR4 will slightly affect the voltage at C15 and adjusting VR3 will slightly affect the voltage at R38. This makes these adjustments quite tricky to make, but the task will quickly seem impossible as the service manual states incorrect voltages. You will never get 2.2V at R38 and adjusting for 50mV at C15 is not quite possible when VR4 is set correctly.
After some experimentation, I have found that the correct voltages are 60mV at C15 and 1.3V at R38.
This post is just to share the correct information so people can service their turntables properly in the future.
When you adjust these pots, I guess that the platter is on and spinning. Do you work from underneath like working on a car? What's your technique? At some point I'll probably re cap my SL-1500 and that info would also be useful.

Cheers!

Andy

Tom_Technics
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Re: Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

Post by Tom_Technics » 22 Sep 2019 14:20

Trio KD1033 wrote:
22 Sep 2019 02:34
Tom_Technics wrote:
21 Sep 2019 19:43
The control circuit in these Technics turntables utilises 2 variable resistors for setting correct voltages after servicing. Correct voltages at C15 and R38 ensures 2 things, 1) that the speed stabilises within a few seconds and 2) that the platter doesn't overshoot on startup i.e. spins faster than it should for a second or so before slowing down to the correct speed.
The service manual states that VR3 sets the voltage at C15 and VR4 at R38. However, adjusting VR4 will slightly affect the voltage at C15 and adjusting VR3 will slightly affect the voltage at R38. This makes these adjustments quite tricky to make, but the task will quickly seem impossible as the service manual states incorrect voltages. You will never get 2.2V at R38 and adjusting for 50mV at C15 is not quite possible when VR4 is set correctly.
After some experimentation, I have found that the correct voltages are 60mV at C15 and 1.3V at R38.
This post is just to share the correct information so people can service their turntables properly in the future.
When you adjust these pots, I guess that the platter is on and spinning. Do you work from underneath like working on a car? What's your technique? At some point I'll probably re cap my SL-1500 and that info would also be useful.

Cheers!

Andy
Hi Andy, what you need to do is plug the deck in at the wall with the bottom off and the control PCB still connected to the transformer via the black connector but unplugged from the motor. With the speed selector switch at 'off', measure the DCV at R38 and C15 and adjust the pots accordingly. It's a good idea to clean or replace them as well as, just like the speed pots always causing trouble, VR3 and VR4 can suffer from the same corrosion issues. I always recap these decks when servicing and, in the 1500 I've just done, C2 was actually falling off the board! I have kits available comprised of Nichicon PW and TVX capacitors in case that would help you.

Trio KD1033
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Re: Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

Post by Trio KD1033 » 22 Sep 2019 15:29

Tom_Technics wrote:
22 Sep 2019 14:20
Trio KD1033 wrote:
22 Sep 2019 02:34
Tom_Technics wrote:
21 Sep 2019 19:43
The control circuit in these Technics turntables utilises 2 variable resistors for setting correct voltages after servicing. Correct voltages at C15 and R38 ensures 2 things, 1) that the speed stabilises within a few seconds and 2) that the platter doesn't overshoot on startup i.e. spins faster than it should for a second or so before slowing down to the correct speed.
The service manual states that VR3 sets the voltage at C15 and VR4 at R38. However, adjusting VR4 will slightly affect the voltage at C15 and adjusting VR3 will slightly affect the voltage at R38. This makes these adjustments quite tricky to make, but the task will quickly seem impossible as the service manual states incorrect voltages. You will never get 2.2V at R38 and adjusting for 50mV at C15 is not quite possible when VR4 is set correctly.
After some experimentation, I have found that the correct voltages are 60mV at C15 and 1.3V at R38.
This post is just to share the correct information so people can service their turntables properly in the future.
When you adjust these pots, I guess that the platter is on and spinning. Do you work from underneath like working on a car? What's your technique? At some point I'll probably re cap my SL-1500 and that info would also be useful.

Cheers!

Andy
Hi Andy, what you need to do is plug the deck in at the wall with the bottom off and the control PCB still connected to the transformer via the black connector but unplugged from the motor. With the speed selector switch at 'off', measure the DCV at R38 and C15 and adjust the pots accordingly. It's a good idea to clean or replace them as well as, just like the speed pots always causing trouble, VR3 and VR4 can suffer from the same corrosion issues. I always recap these decks when servicing and, in the 1500 I've just done, C2 was actually falling off the board! I have kits available comprised of Nichicon PW and TVX capacitors in case that would help you.
Many thanks Tom, I may well be in touch in future!

Andy

georgesgiralt
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Re: Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

Post by georgesgiralt » 06 Oct 2019 21:54

Hello Trio KD1033
You wrote :
"Hi Andy, what you need to do is plug the deck in at the wall with the bottom off and the control PCB still connected to the transformer via the black connector but unplugged from the motor. With the speed selector switch at 'off', measure the DCV at R38 and C15 and adjust the pots accordingly. "

Today, I had to open up my SL-150 because I had an intermittent at the phono cable. The turntable was made in 1976. So I thought I could check the voltages driven by VR3 and VR4, given I have a nice overshoot.
You said to have the board only plugged to the little black connector, which I did and have the turntable power OFF. Using this, there is no voltage at either points because the power switch cuts the primary of the power transformer. So No voltage present on the driving board.
I turned it on. And got a reading at C15 or R38. But whatever position of VR3 or VR4, there is no way I can get near the required voltages, being the specified Technics values nor yours.
At R38, I get between 8.48 to 8.58 V VR4 varying from min to max. And at C15 I can get between 38.9mV and 42.4 mV by varying VR3 from stop to stop.
I've the same discrepancies on various voltages on the board.
So I bet one should have the motor plugged in and the platter spinning for the settings to be done properly and not frying the motor nor the board. Or your method does not apply to the SL-150...
Having fixed my cable problem, I put all the covers back in place and returned to enjoy my music... Those settings will be done when things clears up or need arose ;-) I still have that nice overshoot when the regulator does it's job. No big deal.
Have a bright and nice day !

georgesgiralt
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Re: Technics SL-150 SL-1500 SL-1300 Correct adjustments for VR3 and VR4

Post by georgesgiralt » 07 Oct 2019 03:14

Hello,
I just realized I miss quote. It is Tom_Technics which I should have quoted instead of Trio KD1033.
My apologies to both.

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