Spark/Safety Capacitor Across Mute Switch

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jdjohn
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Spark/Safety Capacitor Across Mute Switch

Post by jdjohn » 06 Dec 2019 04:51

I think it is a common issue to have a loud thump from mute switches on Technics linear trackers. I found this thread: viewtopic.php?t=27530 where Ansis unsolders a few wires from the mute switch PCB in order to disable the switch. I wonder if placing a safety/spark capacitor across those wires would work. I've seen that done in Dual turntable switches.

Has anyone tried such a thing, or tried the disabling of the wires from that thread?

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Re: Spark/Safety Capacitor Across Mute Switch

Post by jdjohn » 10 Dec 2019 03:56

The responses have been overwhelming, but I have decided to order this guy: https://www.digikey.com/product-detail/ ... QifQ%3D%3D and bridge it across the mute switch on the PCB. Pics to follow.

I will be installing this X2 safety cap across the mute switch on an SL-J2 linear tracker. This 'table sounds really good with the original Stanton cartridge/stylus, but curious about other P-mount carts as well.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by rewfew » 10 Dec 2019 16:11

Yeaaaa! I think your right. It's been so long ago, but my preamp I built from a schematic has a ceramic cap across the terminals for such a reason. Did you just do a general search online for such principals? Shouldn't cause any harm any who.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by jdjohn » 10 Dec 2019 22:14

I've seen them on Dual turntables for basically the same purpose (I've had to replace a few), so that's what got me thinking about it for this situation as well.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by jdjohn » 14 Dec 2019 02:33

I installed the safety/spark/X2 capacitor across the mute switch/relay leads on my SL-J2, with a fairly successful result :)

The mute switch is circled here, with an arrow pointing to the wires.
SLJ2SwWires.jpg
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The two blue wires enter the underside of the little PCB here:
SLJ2below.jpg
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Topside view of where the two wires terminate (this is where the spark cap need to go):
SLJRSw.jpg
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Cap installed:
SLJ2wCap.jpg
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I added some clear heat-shrink tubing on the cap leads in order to isolate them from touching any other circuit points.

The capacitor has reduced the 'thump' sound quite a bit. It still has an occasional 'bump', but much less noise than before. I cannot claim that it is 100% successful in eliminating the thump, but it is better than before. Perhaps a different value cap would be even better??

P.S. The previous mute switch noise problem is reduced such that I don't feel the need to tend to the volume control at the beginning or end of an LP side. There is still an audible bump at playing volume, but not enough to be concerned about IMO.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by BMRR » 14 Dec 2019 16:33

I've had a few Technics linear trackers, including a couple of J2s, and none of them had that problem. I wonder why it happens to some but not all of them.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by UsableThought » 14 Dec 2019 18:16

jdjohn wrote:
10 Dec 2019 03:56
I have decided to order this guy: https://www.digikey.com/product-detail/ ... QifQ%3D%3D and bridge it across the mute switch on the PCB.
So, you installed a 0.022uF film cap rated 630V DC.
jdjohn wrote:
14 Dec 2019 02:33
I cannot claim that it is 100% successful in eliminating the thump, but it is better than before. Perhaps a different value cap would be even better??
A higher-capacitance cap seems worth trying. See this similar thread from 2015: Technics SL1710MK auto muting funciton. If you scroll down you'll come to a comment by @analogaudio in which he says this about choosing a cap for this application:
A suitable capacitor is approximately 0.1uF to 1uF and 10V to 25V
I'm wondering if the extra capacitance of what he's suggesting, compared to your 0.022uF cap, might take out more of the thump? Could be worth trying. And if you read his full comment, I also like the suggestion he makes for temporarily clipping a cap in w/out any need to solder - this would let you try a couple different cap values (e.g. you could get a 0.1uF, a 1uF, and maybe one in between) and try them all out to see if one works best.

And you certainly don't need a cap rated for 630Vdc, either - a small voltage cap as he suggests would be fine.

P.S. Of some interest to you might be this additional explanation of the phenomenon over at another forum: https://forums.stevehoffman.tv/threads/ ... ps.763614/

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Re: Spark/Safety Capacitor Across Mute Switch

Post by circularvibes » 14 Dec 2019 19:52

After looking at the service manual, I think you may be barking p the wrong tree. PC board 601 is the arm sensor and the switch is only stopping arm movement. The muting circuit is on PC board 701. There is a relay that may be getting noisy or the cap on that board may be faulty. I could be wrong but it is only a suggestion.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by UsableThought » 14 Dec 2019 20:21

circularvibes wrote:
14 Dec 2019 19:52
After looking at the service manual, I think you may be barking p the wrong tree. PC board 601 is the arm sensor and the switch is only stopping arm movement. The muting circuit is on PC board 701. There is a relay that may be getting noisy or the cap on that board may be faulty. I could be wrong but it is only a suggestion.
That seems a good catch - the relevant boards are at top of p. 19 of the PDF of the service manual.

https://www.vinylengine.com/ve_download ... ervice.pdf

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Re: Spark/Safety Capacitor Across Mute Switch

Post by jdjohn » 15 Dec 2019 00:36

Well, this is embarrassing. Thanks for the information...I'll check it out.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by circularvibes » 15 Dec 2019 02:10

Hope it works out for you.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by jdjohn » 15 Dec 2019 05:01

I found the relay, but not sure if it is serviceable. It's almost like an I/C. Interesting on pg. 17 of the service manual in the lower right, it mentions that the relay part used was changed during production. Hmmm...

Panasonic still makes the relay, but looks like a 2-week backorder.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by UsableThought » 15 Dec 2019 06:40

I think @circularvibes was right in that you were putting your cap across the wrong circuit; however I’m not sure that a noisy relay is the culprit. Remember, @circularvibes proposed two possible reasons for the noise:
There is a relay that may be getting noisy or the cap on that board may be faulty.
So maybe it's cap C701 on the muting relay board that's gone bad? See the muting board on p. 19 and the schematic for same on p. 20 of the service manual PDF. The parts list on p. 15 gives the value of that cap as "100" . . . I forget what the convention is when omitting the unit. But you can figure it out.

As for why a cap would be needed, you can look at those two threads I linked to previously - see for example the thread from forum.stevehoffman.tv - read the opening post - here’s an excerpt:
After doing some research I found that the output muting feature operates by shorting out the output of the cartridge every time the tonearm is lifted. If you purchase a preamp that has a DC bias voltage applied to the input (like the CP1), this voltage will be shorted out through the output muting feature making a pop. This was confirmed with Cambridge's tech support who determined the turntable and pre-amp are "incompatible".
Along similar lines, here’s the link again to the thread with @analogaudio’s comments - worth reading all the way through, as he suggests several times that the simple way to diagnose a pop problem like this is just to swap preamps:

viewtopic.php?f=16&t=78348

This all seems rather murky to me - is it truly a design issue w/ thes Technics turntables? What about the cap on the relay board, then? Etc. etc. In any case, you might try switching preamps to see if the pop persists.

And also I'm wondering - you say you have experience with replacing similar caps on Dual turntables? I've come across threads on other forums which say the same thing - including the notion that failure of such a cap can result in a "pop". But then again, many other threads (including on this forum) seem to suggest Dual doesn't use such caps on their muting switches. My own Dual is a cheapie - CS 530 - and the muting switch is of course mechanical, no relay involved. No idea if there's a cap in the circuit anywhere; my service manual doesn't show a schematic for the switch as far as I can see.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by BMRR » 15 Dec 2019 17:07

I actually did have this issue with a late 1970s Sanyo turntable (just a regular turntable, not a linear tracker). There is a very loud pop that happens one or two seconds after pressing the Start button, and the pop happens again when the turntable shuts off (the second pop coincides with the various lights on the turntable shutting off, which happens after the tonearm has returned to the tonearm rest). This was with a U-Turn Pluto preamp, which is currently the only preamp I own.

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Re: Spark/Safety Capacitor Across Mute Switch

Post by jdjohn » 15 Dec 2019 20:28

Going back in, here is where the tonearm wiring resides - under this cover.
SLJ2armcover.jpg
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Cover removed and PCB exposed. The thin blue/white/red/green/black wires come from the tonearm itself, and the pink and white wires go to the outputs. The thin colored wires were covered with a piece of cloth-like tape.
SLJ2armPCB.jpg
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Underneath the PCB. The relays (one for each channel) are inside the box, which as you can see, is covered with a metal shield. Clearly you can see the capacitor as well. It's not swollen or bad or in any obvious way, but I can remove it and test it.
SLJ2armcap.jpg
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