njm4558s op amp replacement?

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Re: njm4558s op amp replacement?

Postby Synthetech » Thu Apr 17, 2014 4:02 pm

Steve Jones wrote:Another really useful piece of test gear for situations like this is a capacitor decoupled audio signal probe. Quite easy to build, plenty of schematics on the web.



do you have a link to an example? I searched and could not find much except this?

http://www.talkingelectronics.com/proje ... chAmp.html


I have used a simple Tape cassette adapter in a Boom box to probe signals on this thing before.. it was how I revived it in the first place when it had a digital glitch and no audio issue. I simply just attached leads to the mini jack on the adapter and then G the one lead, probe with the other.

Long story with this guy..
I won it really cheap on ebay. Listing said it had digital glitches and no sound.
I get it and it powers up with all 4 proper voltages checked with just a DMM.. I did not see anything odd about both 5 or 15v supplies.. + or - rails. so I moved on from there.
FIrst time I powered it on, it had one select line for the membrane switches matrix not working.. IE every 4th button would not work. I was able to switch to edit mode and back again. No sound.

Powered it off, closed it up, set it aside on it's end on a wall and left it there until I went back at it later in the evening..
Power it on again. But this time the digital issue has moved! Now it's like every 4th button that doesnt work has shifted by two. And now the Edit mode wont engage.. hmmm....

I poke around again, using a Fluke Trendar 200 logic clip that has Led's to check digital states.
One of the hex inverters was not outputing when I pressed a switch in a group assigned to it.
I found digital issues in a few spots on the board.. one area was the switch matrix and the other was around the DAC.

I get no where. So I close it up and stand it up on a wall, but this time I hear, "clink, clink, rattle.."
I open it back up and find a tiny bit of white key plastic?? not sure.. and a small black bit of ceramic like material..


I take it to my shop to bench it and hook up the recently acquired scope.
I power it on and notice that now all the digital problems are GONE!! every button and mode works correctly!
The small black ceramic material that rattled around inside it must have been conductive and was jammed between the back of the digital board and back of the membrane switches board.. standing it on end a couple times dislodged it, I bet.

I still did not have sound :(
Well not proper sound. I had a very weak signal at the mixed filter outputs that did not get any better afterwards..
The signal was so weak that my scope was gained up so much it was picking up the fluorescent lights pulsing voltages above the bench to see it. I also used my boombox audio tracer to track it. I could hear the osc section steady on and change pitch with keys. But there was no gating whatsoever.. just raw osc output.
I chased and chased for another digital problem and finally after most a day of farting with it, I realize the 15v supply was dead when I figured out the VCA was not working, doh!
I check the fuse, it's open!
It wasnt open before..
I get a new fuse, pop it in and its worked fine since then.. until I began to notice the clipping at higher levels.



So short of it is, there was a short in it at one time that opened the fuse.
I think the 15v short was probably from the digital board near the DAC.
Not sure if the short may have upset something to contribute to clipping.. it's possible I'd think!

Thanks again for all your advice/suggestions.
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Re: njm4558s op amp replacement?

Postby Synthetech » Sun Apr 20, 2014 5:56 pm

Got to the scope last nite.
G is the center horiz. reticule. V/div is stated.


Here's the "9v" Transformer tap. At 10v/div, it looks like it's more like around 5vAC rather than 9V that the schematics said it should be.

Image

this is the "0v" ac transformer tap. seems to be a mirror image of the 1st and not 0v as the schematic says it should be

Image


Here's the a tap for the 15v side. 1st tap. Still 10v/div.

Image

2nd Tap. very different...

Image


Checking the rectifier.

Image


2v/div. The ripple is about .5v high/low. This is the rectified out before the filters?
Image


Here is the upper unmarked corner of the BR. At 2v/div. it looks like it's about 5vAC
Image


Image


This is the 15v side? I could only check the inner leads, out leads were insulated, so I did not check them.
Image

It's 2v/Div.
Image


The actual final outputs going out to the PCB's did not register any AC, I had to put the scope into DC mode to see a change and measured as expected.




If anyone can make sense of these tests, please chime in.
Last edited by Synthetech on Fri Apr 25, 2014 4:57 pm, edited 2 times in total.
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Re: njm4558s op amp replacement?

Postby Synthetech » Sun Apr 20, 2014 6:36 pm

Oh and these are the DC outputs.

Image


Connectors on left..

Blue is -15.08
Red is +14.97

Yellow is 4.99V

Greys are G for the 5v
Blacks are G for the 15v's



I adjusted a "VCA Offset" trim pot while inducing a voice to clip out and it seemed to make no reduction in the clipping at all.
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Re: njm4558s op amp replacement?

Postby cilek » Mon Jul 14, 2014 3:55 pm

@Synthetech: Hi, you really did a very good job and make this device working but I think you are still struggling with the clipping. As I am not good like you in practical troubleshooting but I think if there is op-amp in the circuit it is getting saturated that causes the clipping of the signal and you can increase the power supply to avoid it or reduce the gain of op-amp.
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