JX8P - low 5V on the mainboard !

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Pugris
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JX8P - low 5V on the mainboard !

Post by Pugris » Sun Feb 14, 2021 7:59 pm

Hello everyone !

I'm here to get some help, or at least some leads. A month ago I bough a jx8p with several small problems (one voice louder than the other, blinking screen, dead sliders ...) but still sounding, not hopeless then...

I first found a battery in an advanced state of oxidation: installation of a holder and a new 2032 rechargeable battery : done !

Then I checked the power supply. And instead of 15v it came out of 18v. Not catastrophic, but not normal. On Florian Anwander's website, I saw that this excessive voltage could come from a faulty M5230L. So I changed it to a new one.

I also changed the 7805 regulator, the small capacitors on the power supply board (because I had some, so why not), and the main board regulators (B507, D313) because they were well yellowed by heat (the fact that they received 18V should not have been for nothing). I have also added radiators on the new ones.

Once all this was reassembled, I tested the power supply alone : +14.85, -14.85 for 15 volts, and a precise 5V for 5 volts. I am happy.

I reconnect everything. The synth does not turn on: / Just a barely perceptible short leds blink when ON, nothing more. I measured the voltage at the leg of an IC: 1.7 volts. And even at the input connector, 1.7 volts everywhere. For the synthesis part, no problem, however, the ICs receive their power well, it is only on the "assign" side that there is a problem.

So when it is unplugged the power supply well deliver 5V, but once connected to the main board, the voltage drops to 1.7V. Obviously the ICs and the machine do not start ...

So there is a problem ... but I'm not sure where to look, I haven't faced this kind of problem so far (I’m an autodidact... and, abviously, it’s not my job, but I like to learn).

If that is useful, when I measure the resistance between the + 5V circuit and the ground, I have about 600 ohms, everywhere on the board, even disconnected from the power board.

I also tried to connect only the main board to the power supply, without the other boards. Same drop in voltage, so it seems localized on this one.

I inspected with a good light, to see any contacts, components that would have leaked, blackened ... but I do not see anything abnormal, the board is overall in very good condition.

Could it be that some components have "used" to operate at too high voltage, and became resistant to lower voltage? Is there usual suspects ? Is there a way to find those, a better way to proceed ?

If anyone has any clue, I’m listening. I can of course complete the informations.

Thank you in advance ! And maybe it can also help someone who finds the same problem in the future

Dam (cheers from France !)

Ps: picture of the old battery, the horror ! New one, and old regulators too on the main board. Replaced now.
Attachments
Better...
Better...
Changes by new ones now.
Changes by new ones now.
Seems to be radioactive ^^
Seems to be radioactive ^^

Pugris
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Re: JX8P - low 5V on the mainboard !

Post by Pugris » Mon Feb 15, 2021 3:57 pm

No one ? Any idea ?

I dont know where to shearch for... i tried a forum in France but no one respond heather. So sad :despair:

Please please please :hail:

Countcowden
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Re: JX8P - low 5V on the mainboard !

Post by Countcowden » Mon Feb 15, 2021 7:21 pm

Wrong forum section - try the music tech diy forum - this is where the people with solder irons hang out

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Jim the Oldbie
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Re: JX8P - low 5V on the mainboard !

Post by Jim the Oldbie » Mon Feb 15, 2021 8:03 pm

Hello,

When a power supply voltage sags under load like this, either the load is too high (some kind of partial short circuit somewhere), or the supply's output is impaired somehow (defective component in the voltage regulator circuit).

To determine which is happening, find a safe place where you can temporarily attach a "dummy load" resistor to ground on the unconnected 5 volt power supply output. In your case, a 10 ohm, 5 watt resistor or similar would work (5V / 10 ohm = 500 milliamps). This should draw enough current to expose a weakness in the power supply circuit - If the resistor causes a sag (which may be a different lower voltage than what you've been seeing), then the problem lies somewhere in the power supply. If your 5V is still solid with the resistor in place, you know the supply is OK. Then you'll have to look downstream (likely the main board) for something that's drawing too much current when voltage is applied.

Good luck!

EDIT: Yikes, I had some of that completely backwards up there for a few minutes, sorry. The current version is correct. Time for another cup of coffee, heh.

Pugris
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Re: JX8P - low 5V on the mainboard !

Post by Pugris » Tue Feb 16, 2021 6:26 am

Hi !

Thank you for your help ! :cloud: It seems that i'm not on the good section, so thanks a lot to answer anyway.

So i tried to run your test, first with a 10ohm restistor, but it burn pretty fast (it was supposed to be 4w, but i dont think so... that smell :waah: )

Then i tried with a little motor, and a car ligh bulb, it worked fine and the power only drop to 4,95V in each case, so i think the PSU is ok, is it ?

So the problem may probably come from the main board. I will investigate. Is there a better way to go ? Remove some strategic components in the circuit before testing for exemple ?

I've got a multimeter with a continuity fonction (Fluke 789, way to big for me but a friend give it to me...) it can help to find a short, i suppose.

I take any advices :)

Thanks again !
Dam

stirliTZ
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Re: JX8P - low 5V on the mainboard !

Post by stirliTZ » Sat Feb 20, 2021 9:27 am

Just got a similar problem with my newly acquired JX8P. Recapped, changed rectifiers and the 7805. Still no signs of life.
That's what I've measured:
330 Ohms between 5V and GND on the mainboard (same for connected and unconnected)
4.89V from unconnected PSU
3.45V with the mainboard connected

Let me join, advices welcome!

Cheers,
Andrey

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Jim the Oldbie
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Re: JX8P - low 5V on the mainboard !

Post by Jim the Oldbie » Sat Feb 20, 2021 6:51 pm

Sorry for the delay in replying.

Dam, also sorry for "that smell" I should've suggestested a higher-wattage resistor.

Andrey, you also may want to go through rhe motions of isolating the problem as described above, just to verify. Assuming at this point you've checked for the obvious stuff like capacitor polarity, solder bridges etc.

But it sounds like in both cases the more probable culprit is some sort of abnormal load on the main board. This may be tricky to hunt down. Best to concentrate first on any areas of recent repair work: caps, batteries/holders, etc. Also you've likely figured this out, but remember that resistance readings on the power rails can be misleading due to the way circuits behave differently under power and other factors.

Most times, abnormal loads are usually accompanied by abnormal heat at the scene of the crime. Check carefully for overheating parts (be careful with fingers, and take static electricity precautions). It's been ages since I've seen the inside of a JX-anything, so I don't remember if they used tantalum capacitors? Those are prime suspects in this scenario.

I wish I could be more help...

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Re: JX8P - low 5V on the mainboard !

Post by Pugris » Sun Feb 21, 2021 10:16 am

Hi Jim, thanks again for your reply.

So... after a very deep investigation on the main board, with O-scope and multimeter, i didn't found any problems. I even changed some IC's with new ones on supports, as i get some parts from my previous JX3P repair. But nothing better.

So i tried something else. I disconected the 5V from the power supply, and plugged it directly to a trustable 5V alimentation. And the synth starts perfectly well... sounds very good also. So... the problem IS in the power supply.

I changed the diode bridge, no change.

So... i don't know. Do you have any idea of what can cause this issue in a power supply ?

Thanks :)
Dam :cloud:

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Jim the Oldbie
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Re: JX8P - low 5V on the mainboard !

Post by Jim the Oldbie » Mon Feb 22, 2021 2:02 am

OK, returning to the power supply. With it connected to the main board as normal, and power turned on: When you scope the input to the 7805 regulator IC (pin 1), are you able to see a reasonably straight line, perhaps with some ripple but remaining up in the range of 8VDC? Or are there large dips in that waveform?

Pugris
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Re: JX8P - low 5V on the mainboard !

Post by Pugris » Mon Feb 22, 2021 3:52 pm

Hello,

Here are the results, in and out of the regulator (attached pictures). There is not a lot of fluctuations in the signal, even if it’s a bit noisy.

Is there a possibility that the big smoothing capacitor cause the problem ? (6800uF) I will change it anyway. And I remember to read somewhere that the glue used to fix it to the board can become conductive after years, maybe a possibility ?

Thanks. Good night !
Damien :cloud:
Attachments
F0DDCD8F-C689-4277-B15C-1463F7033D4F.jpeg
45AA8BF3-CE31-4322-95F9-D68F3228AFAB.jpeg

stirliTZ
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Re: JX8P - low 5V on the mainboard !

Post by stirliTZ » Tue Feb 23, 2021 7:06 am

Hi, thanx Jim for reply. Damien, it seems like the problem is anywhere else than in C3 (6800). I've changed all electrolytic caps, same result. The only unchanged component in 5V circuit is C10 (0.1).

Pugris
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Re: JX8P - low 5V on the mainboard !

Post by Pugris » Tue Feb 23, 2021 12:45 pm

Ok, brand new 6800uF, nothing new. You’re right ;) Here is the mesure of the o-scope from the +5V when plugged into the main board. There it’s wobbling, yes... (I post 2 pictures, because sometime the digits are not well readable as they are changing).

Hum... I think more and more you’re right about the 7805 as suspect. But it’s weird that it works fine unplugged, but not when feeding the main board, isn’t it ?
Attachments
B42F8A55-6BEA-4476-A441-B393A95A7A79.jpeg
BDCD2A5E-8CF6-4023-A5CC-3664CAC5343A.jpeg

stirliTZ
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Re: JX8P - low 5V on the mainboard !

Post by stirliTZ » Wed Feb 24, 2021 6:18 am

What is really crazy that I have changed the 7805, it did NOT solve the problem. I still think that the culprit resides somewhere on the mainboard.

Pugris
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Re: JX8P - low 5V on the mainboard !

Post by Pugris » Wed Feb 24, 2021 11:27 am

I’m not sure... I change the 7805 two, that’s the first part I changed actually, I was pretty sure it was the problem. But as the synth start with an external power supply, I don’t think that the problem is on the main board... by the way, my external power supply show that the main board drain around 1A of electricity. That’s ok I think... I’m almost thinking of implementing a separate power supply of 5V/1,5A inside the JX... so I will work ^^ but I also want to understand the issue. So... :hmm:

stirliTZ
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Re: JX8P - low 5V on the mainboard !

Post by stirliTZ » Wed Feb 24, 2021 4:57 pm

1A seems OK but it also means that the 5V circuit cannot provide current that high. If all components are changed to brand new, that's quite strange.
Could you please tell me how did you connect your external 5V PSU? In particular, have you physically connected its ground to the common ground of the JX?

Pugris
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Re: JX8P - low 5V on the mainboard !

Post by Pugris » Wed Feb 24, 2021 6:36 pm

It’s my solder station PSU, adjustable. I use +5V and ground directly from this one. So yes, different ground than the synth. Can it be a ground issue ?

stirliTZ
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Re: JX8P - low 5V on the mainboard !

Post by stirliTZ » Thu Feb 25, 2021 9:46 am

Let me think more... it might be the key. I have to find a decent 5V source to experiment with.

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