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SL-120 Replacing Pitch Pots

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SL-120 Replacing Pitch Pots

Postby bbr620 » 06 Jun 2018 11:31

Hi, hope someone with more knowledge than myself can Help here. Needing to replace the pich pots on this deck (the ones next to the speed selector switch, not the V pots).

Image

while getting the 1kohm pot is easy enough, I'm unsure of the power rating I require. I have a choice of 200mW / 400mW or 500mW?

Anyone Help out on this as I've also read that the wrong voltage can blow boards elsewhere if I get the voltage wrong.

Was thinking this but unsure:

Code: Select all
http://uk.farnell.com/citec-te-connectivity/23esa102mmf50nf/potentiometer-lin-1k/dp/1174082?ost=23ESA102MMF50NF&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fsearch#


edit: meant to ask, is there any way to bypass these pots and just use the V pots undeneath? deck is for playing vinyl, not DJ use so I don't really need to adjust pitch?

thanks
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Re: SL-120 Replacing Pitch Pots

Postby p38arover » 07 Jun 2018 01:19

Physical size and shape are all that will matter in that application. It's not a power application - that pot from Farnell would be fine...... Again, it won't affect voltages that could damage anything.

However, you might be struggling to find a pot with a threaded shaft like that.

Re bypassing them, it would be possible. I'd need to look at a circuit diagram.
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Re: SL-120 Replacing Pitch Pots

Postby p38arover » 07 Jun 2018 02:38

Before selecting a replacement pot, what is the diameter of the pot? I just had a look at a photo of the SL120 and there may not be enough clearance for two standard size pots to fit side by side.

See the pics here: viewtopic.php?f=18&t=80215

There is a partial circuit diagram there, too, but it doesn't Help. However, you could try what that poster did - disassemble and clean the speed control pots, or spray inside them with contact cleaner whilst rotating them.
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Re: SL-120 Replacing Pitch Pots

Postby bbr620 » 07 Jun 2018 05:27

@p38arover, thank you , just what I needed. I believe they are 24mm across the base so I'm looking at 24mm pots.

I'm waiting on a can of electrical cleaner coming, hopefully, today but having new pots to solder in woiuld be good for a backup.

I believe the pots are VR 4 & 5 that I'm looking at replacing as the pitch is drifting on 33RPM and when I touch 45RPM, it jumps up and down and I can't get it stable.

From the Service Manual:

SL-1200 Schematics:

Image

SL-1200 PCB:

Image
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Re: SL-120 Replacing Pitch Pots

Postby p38arover » 07 Jun 2018 10:28

Thanks for the schematic.

Yes, you could replace them with fixed resistors of about half the pots' value. However, from your description of the symptoms, it sounds like the pot tracks/wipers are dirty. I'd be tempted to first spray inside with contact cleaner.

See https://www.youtube.com/watch?v=-7guedH2AzY

If you can't get Deoxit, see https://www.youtube.com/watch?v=ygrlO_VacsY
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Re: SL-120 Replacing Pitch Pots

Postby p38arover » 07 Jun 2018 12:29

As you are in Scotland, perhaps Servisol products are easier to find, e.g. Servisol Super 10
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Re: SL-120 Replacing Pitch Pots

Postby bbr620 » 07 Jun 2018 18:30

Yes, I'm in Scotland and it's Servisol Super 10 I'm waiting on. Thank You for your Help, I'll give them a spray first before considering replacing the pots.

Can I assume if I was to fit 0.5Ohm resistors that I could then tweak the pitch via VR2 & 3?
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Re: SL-120 Replacing Pitch Pots

Postby p38arover » 08 Jun 2018 00:29

Spray up into the gap whilst rotating the pot shaft.

No, not 0.5 ohm. Try 470 ohm. 1/4 watt would be more than ample power rating. 1/8th watt would also bo OK.
Of course, you could replace them with miniature trim pots located underneath. That would allow you to adjust the speed although not on a daily basis. :D
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Re: SL-120 Replacing Pitch Pots

Postby bbr620 » 08 Jun 2018 01:42

superb. Thank You once again for your Help.
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Re: SL-120 Replacing Pitch Pots

Postby sylvain999 » 08 Jun 2018 14:48

Clean the speed selector switch as well. Try to clean the pots so you can keep them. Do not try to remove the pitch buttons they probably are stuck and you will break the controls and have two holes in the turntable... If defective, use 470 ohms resistors to bypass the pitch, and if you need to dial the speed to zero,use the sub pitch adjustment pots on the board and clean them as well, they are accessible thru the holes of the platter.
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Re: SL-120 Replacing Pitch Pots

Postby bbr620 » 08 Jun 2018 17:42

@sylvain999 spot on. Thank You for the info. Still waiting on cleaner appearing. Will update how I get on for the purpose of future searches.
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Re: SL-120 Replacing Pitch Pots

Postby sharkey22 » 08 Jun 2018 18:31

As previous contributors suggest it is highly unlikely that the pots need replacing. More than likely they just need a clean.

As a side note to this repair.....i recently replaced all the electrolytics on the motor control board and recalibrated the 20 volt constant voltage output on my SL-120, it is a 40+ year old turntable and the part were very inexpensive.

Since you have access to the motor driver board you might as well check and if necessary adjust 20 volt constant voltage output. When you do this be aware that there is a topographic and typographic error in the SL-120 service manual which states the test points as pin7 and collector of Tr13. This is correct for the SL110 but for the SL120 it should read collector of Tr14. It took me a while to trace this error. I am sure the folks at Technics wrongly cut and past the trouble shooting/adjustment part of the service manual directly from the SL-110 document.

Good luck with your repair. This is quite a decent turntable and well worth the effort.
Attachments
SL-120_SL-1200 test points.jpg
Technics SL120/1200 correct test points 20vdc
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Re: SL-120 Replacing Pitch Pots

Postby bbr620 » 08 Jun 2018 21:02

@sharkey22, Thank You also for this, I'm not too clever on this sort of stuff. Do I use a multimeter at points Pin 7 and TR14 on the C pin?

Do Red / Black leads from Multimeter need to be on specific test points?

If so, I'm assuming it's wired up to the mains and switched on when checking?

If not 20v, can I adjust VR1 with the power running through it until Multimeter reads 20v?
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Re: SL-120 Replacing Pitch Pots

Postby sharkey22 » 08 Jun 2018 22:51

Yes, all of your assumptions are correct. The 2 black plastic motor connectors must be plugged in to the board and measurement taken with the motor running. There are no dangerous voltage on the control board so you will not kill yourself.

If your multimeter (preferably digital) has very small clips this makes things easier. You can clip one probe directly to the centre pin of TR14, be very careful not to contact either of the outer pins when powered up. Attach the other meter probe to pin 7.

If you don't have clips you can temporarily solder a wire to each test point and wrap these round your meter probes.

Use your multimeter in the dc volts position. It does not matter which way round the leads are. Your meter will show either 20v or -20v. If it displays negative voltage you can swap the leads but it really does not matter for this measurement.

If I remember correctly you do not need the platter mounted, that would involve a real balancing act!

With the motor running adjust VR1 until 20.0 volts is shown. That's it.
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Re: SL-120 Replacing Pitch Pots

Postby bbr620 » 09 Jun 2018 08:23

Excellent and informative guidance sharkey22. Thank You once again.
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