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Wiring a standard female stereo jack to a 7-pin PCB
Old 6 days ago
  #1
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Layman's Avatar
Wiring a standard female stereo jack to a 7-pin PCB

Hello all,
My keyboard got a connector tip stuck in it and I damaged the original jack trying to get it out while not realizing that it was not a simple stereo jack.

Now I have a free gig with my friends coming up and unfortunately a similar 7-pin jack won't arrive in time.
See picture https://ibb.co/MRKfLxG

So I would like to try wiring a standard 3 connection stereo female jack instead to the board. See pic: https://ibb.co/R3Dd0BZ (Side note, this replacement jack plug is labeled as "TS" but also "stereo" so I can assume TRS also?)

I realize that I will have to sacrifice the functionality of some of the 7 pins, (namely the cutting off of the circuit to the on-board speakers when a jack is inserted), but that is totally fine with me for now as long as I can get a clean stereo output.

This is the circuit board wiring diagram, the one labeled M734-CN3M https://ibb.co/PWt9Q85.
You will see 5 connections labeled from the main-board to this circuit board the 7-pin jack is mounted on.
This is a pic of the underside https://ibb.co/N9M9b9p

So with this seen I'm having difficulty figuring out the 3 connections for the replacement jack.

Thanks a mil for any help.

Last edited by Layman; 5 days ago at 08:04 PM..
Old 6 days ago
  #2
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Richard Crowley's Avatar
 

In your PC board photo, the 5-pin input connector at the bottom.
If you number the pins 1 through 5 left to right:
1) to Left speaker
2) Left input to connector board
3) Ground
4) Right input to connector board
5) to Right speaker

Wiring a standard female stereo jack to a 7-pin PCB-lgr.png

I have 98% confidence in identification of these pins.
However I have only 50% confidence in the identification of Left vs. Right
It could be the other way around, can't really tell without seeing the connector.
Attached Thumbnails
Wiring a standard female stereo jack to a 7-pin PCB-lgr.png  
Old 6 days ago
  #3
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Layman's Avatar
Thank you for that very helpful response.

Quote:
Originally Posted by Richard Crowley View Post
I have 98% confidence in identification of these pins.
However I have only 50% confidence in the identification of Left vs. Right
It could be the other way around, can't really tell without seeing the connector.
Did you take a look at this pic also showing the 5 to 7? https://ibb.co/PWt9Q85 This will be a view from above while the actual pic of the PCB that you annotated is from below.

Edit: nevermind, i think you were right the first time. Thanks again ! Great post. Will test it after work and report.
Old 6 days ago
  #4
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According to the schematic diagram I was wrong with Left vs. Right.

If you look carefully on the PC board it looks like there is a "1" to the left of the first pin on the left,
and a "5" to the right of the last pin on the right.

And the schematic shows:
  1. out to Right Speaker
  2. Right signal input
  3. Ground
  4. Left signal input
  5. out to Left speaker
Old 6 days ago
  #5
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The picture of the proper replacement jack appears to have pin numbers inscribed on it, though I can't make out enough to see how they're ordered. The schematic also (and helpfully) has the pin numbers listed. Anyway, you'd wire the sleeve of the temporary jack to where pin 1 of the original jack is (my guess is that's the single contact nearest the "entrance" end, but I could be wrong), the sleeve to pin 3, and the tip to pin 2. You may find that your output isn't the same even so as the speakers will give a different loading to the amplifier than the phones/output alone.

Note that there's more going on with the little board than just the jack; it also has a pair of DC blocking capacitors, what appears to be a voltage divider/load circuit made of a few resistors (I assume to reduce the output level appropriately for headphones), etc. Otherwise, wiring to that five pin connector would let you get your output and disable the speakers.
Old 6 days ago
  #6
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Layman's Avatar
Quote:
Originally Posted by Richard Crowley View Post
According to the schematic diagram I was wrong with Left vs. Right.

If you look carefully on the PC board it looks like there is a "1" to the left of the first pin on the left,
and a "5" to the right of the last pin on the right.

And the schematic shows:
  1. out to Right Speaker
  2. Right signal input
  3. Ground
  4. Left signal input
  5. out to Left speaker
Oh yes, that is true. I also had my bearings wrong from the underside view.

So that settles it for left and right. What would you recommend for the sleeve, should it be connected to one of the 7 points or the 5?
https://ibb.co/R3Dd0BZ

Edit: Ah I see DrewE answered this exactly..

Last edited by Layman; 6 days ago at 07:00 PM.. Reason: answered by other poster
Old 6 days ago
  #7
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Quote:
Originally Posted by DrewE View Post
The picture of the proper replacement jack appears to have pin numbers inscribed on it, though I can't make out enough to see how they're ordered. The schematic also (and helpfully) has the pin numbers listed. Anyway, you'd wire the sleeve of the temporary jack to where pin 1 of the original jack is (my guess is that's the single contact nearest the "entrance" end, but I could be wrong), the sleeve to pin 3, and the tip to pin 2. You may find that your output isn't the same even so as the speakers will give a different loading to the amplifier than the phones/output alone.

Note that there's more going on with the little board than just the jack; it also has a pair of DC blocking capacitors, what appears to be a voltage divider/load circuit made of a few resistors (I assume to reduce the output level appropriately for headphones), etc. Otherwise, wiring to that five pin connector would let you get your output and disable the speakers.
Thanks DrewE! I think the sleeve connection is where I made the biggest mistake. Only managed to dig up that very helpful schematic today.

Also, great eye! I did not see the numbers but assumed the 7-pin was the same design because it has the same pattern as the board.
Old 6 days ago
  #8
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Happy to report that after doing some hard pan tests this configuration gives me the intended stereo connection. Thanks guys!

One problem though, the output is somewhat distorted, most noticeably in the bass frequencies for e.g. distorted kick drum.

What is the culprit in this scenario? Possibly the replacement adapter?

P.S I am using regular speaker wire for this test.
Old 6 days ago
  #9
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What is the destination of this signal?
Assuming you are using TRS plug at the keyboard end?
What connector (s) at the other end? What gear at the destination end? What input(s)?
Is it capable of handling such a "hot" signal?
Old 6 days ago
  #10
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The destination is indeed TRS via my working headphones tested on other sources without distortion.
So the keyboard is feeding directly into the headphones.
Quote:
Originally Posted by Richard Crowley View Post
Is it capable of handling such a "hot" signal?
I'm not certain about that but hopefully now knowing it's headphones should help.
Old 6 days ago
  #11
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Did you tap into the signal BEFORE those series resistors (R339 thru R342) or AFTER them?
Maybe the reason they are there is to knock down a hot signal?

I would tap into the pins right at the old (broken) connector (AFTER the series resistors).
I would not connect directly to the 5-pin input connector.
Old 6 days ago
  #12
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After for sure, tapped into the pins 1-3 (out of the 7), also tried directly on the solder joints/pin ends on the underside with the same result.
Old 6 days ago
  #13
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If you tapped the signal at the same place in the circuit as the broken connector, then it is hard to imagine how that could affect the symptoms you are describing?
Old 5 days ago
  #14
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Agreed, it is mysterious.

I'm wondering if it was the replacement.
It's one similar to this.
https://ibb.co/R3Dd0BZ
A variety is hard to find here locally, but I'll look around to see if I can get another one to test.
Old 5 days ago
  #15
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Richard Crowley's Avatar
 

Unless there is some kind of wiring error, distortion is not something that can be caused by a connector.
Old 5 days ago
  #16
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Layman's Avatar
Ok, what about the pins for the original "box" where I'm tapping into, they are bent up a bit, maybe that? I can get a pic later if needed.
Old 5 days ago
  #17
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Sorry I don't know what you mean by "original box"?
Isn't that board an internal component of the keyboard?
Perhaps it goes without saying, but we are assuming that you connected the ground/sleeve properly at all points?
Old 5 days ago
  #18
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Sorry I should've specified that the original jack (like this one https://ibb.co/MRKfLxG) is what I referred to as the "box". It can open in two pieces which when closed the 7 pins slide into grooves. I have it opened to tap into them, but almost all of the pins are a little bent from when the tip got stuck in it.

Quote:
Originally Posted by Richard Crowley View Post
Perhaps it goes without saying, but we are assuming that you connected the ground/sleeve properly at all points?
I am hoping this is the problem, maybe I didn't tap them properly for the test.
Old 4 days ago
  #19
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No improvement unfortunately.
What about the components like caps, transistors and resistors? Did not notice any burn marks and they don't look out of place.
Old 4 days ago
  #20
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Was this operating properly before that TRS connector broke?
I can't imagine any scenario where replacing the connector could cause those symptoms.

This seems like a completely different problem unrelated to the connector.
Old 4 days ago
  #21
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Noted!
I don't recall it having any problems before the connector broke.

I should state that the last time it worked was many years ago as I never had use for the output since then till this gig arose because I used soft synths through midi.

I suggested the possibility of those components because of all the fiddling trying to get it to work with another connector after I got the broken one out.
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