Motu 16A - Need Help With Repair

[smilan]

I have a Motu 16A DAW.
Today the unit didn't powered up.
I open it and noticed that the four SR360 diodes on the secondary side of the power transformer was faulty.
I replaced the four  diodes with new ones and now the unit powers up and all the inputs / outputs seems to be working .
The only thing is that the volume coming from all 16 outputs is very low and distorted.
I've measured the voltages at all the testing points on the board and everything seems to be alright except for the +10V / -10V  voltages (instead of 10V I have measured 2.2V and instead of -10V I have measured 0.287mV).
The thing is that there's no schematic for the 16A available so I have no idea where to start to look for the faulty component that regulating the +/-10V supplay.
Maybe someone here have some experience fixing those units and can help me?

Those are  the voltages I've measured from the PSU when it was connected to the rest of the circuit:
https://ibb.co/LzZBm9f
And those are the  voltages I've measured from the PSU when it was disconnected to the rest of the circuit:
https://ibb.co/3Y32zrQ

 

[Khron]

How exactly did you determine that all four of those diodes were faulty?

Either way, it's very interesting that the analog voltages (the +/-10v) are lower when unloaded, and yet, they're down to virtually nothing on the  analog board. That would point at whatever's between them having a higher-than-a-plain-wire resistance, which is really weird.

Either way, that looks like the "standard" MOTU switching power supply they've been using for ages now. Even the transformer part number is the same, so they might not have even bothered to alter the windings...

For what it's worth, i fortunately found an older photo out of an 8Pre i used to have, where i labeled the output voltages on that pin header - from top (ie. towards the rear side of the unit), they're ground, 3.8v (the input voltage to that SMD regulator nearby), 3.3v (output of said regulator), ground, +8v, 5.5v, -8v, ground. I'm 98% sure that should be the "good" voltages you should be seeing.

https://4.bp.blogspot.com/-d2dFPRzu5F0/WPTAS03lvsI/AAAAAAAAAIE/MkOufxTcqC49i5QWfzqv04lIwWX7tbDmgCEw/s1600/IMG_9933.JPG

Are the two resistors & (zener?) diodes next to those test-points getting warm / hot at all (D5, D12 and the two 2.4 ohm)?

That being said, none of the electrolytics there are to be trusted (imho), at least the ones in the power supply. If the one on the 5.5v rail's gone out of whack, that'll mess up the whole feedback and throw everything off. The 3.3v rail for the digital stuff's fine, with that separate regulator there, so the thing WILL power up. A while ago i revived a totally dead 828mk3 - the mains filter cap (that 33u/400v) had spilled its guts and corroded the trace / pad going to its positive leg.

 

[smilan]

Thanks Khron!

How exactly did you determine that all four of those diodes were faulty?

I disconnected one tip of  each diode from the PCB and measured it with multimeter in diode check mode and in short circuit mode.
One diode has shorted and conducted current both ways, other showed double amount of forward bias than it should, third diode showed OL in both ways so I decided to replace all 4 diodes with new ones.

Are the two resistors & (zener?) diodes next to those test-points getting warm / hot at all (D5, D12 and the two 2.4 ohm)?

After few minutes the diodes and regulator doesn't seems to get hot. The input voltage on the regulator is 3.76V and the output voltage is 3.1V.

That being said, none of the electrolytics there are to be trusted (imho), at least the ones in the power supply. If the one on the 5.5v rail's gone out of whack, that'll mess up the whole feedback and throw everything off. The 3.3v rail for the digital stuff's fine, with that separate regulator there, so the thing WILL power up. A while ago i revived a totally dead 828mk3 - the mains filter cap (that 33u/400v) had spilled its guts and corroded the trace / pad going to its positive leg.

I replaced now the mains filter cap 33u/400 with new one since the original one was faulty and measured only 1380 pF...
I've also replaced the two 2200/25 caps with new ones.
Now the voltages on the four SR360 are as following (with load):
+19.3V
-27.8V
+5.5V
+3.8V
The voltage at TP +10 is +2.1V, the voltage at TP -10 is +0.7V

 

[Khron]

That still leaves the two caps on the lower-voltage rails (where the feedback for the power supply is taken from).

And what new caps are on the analog power rails?

Are the two resistors & (zener?) diodes next to those test-points getting warm / hot at all (D5, D12 and the two 2.4 ohm)?

Read the above question A LOT more carefully. D5 and D12 are on the lower-right corner of the analog board. I specifically indicated the designators of those two diodes, as well as the value of the two identical resistors right next to them.

Better yet, with the "connector board" removed, diode-check those two diodes - ideally after removing them from the board, and then diode-check their footprints on the board as well.

 

[smilan]

That still leaves the two caps on the lower-voltage rails (where the feedback for the power supply is taken from).

Now I've replaced the two 1000/16 caps.

And what new caps are on the analog power rails?

Here at the local electronics shop there's no big variety to choose from,
the 33/450 is Nichicon vz(m) and  ChongX for the  2200/25 and 1000/16.

Read the above question A LOT more carefully. D5 and D12 are on the lower-right corner of the analog board. I specifically indicated the designators of those two diodes, as well as the value of the two identical resistors right next to them.

Better yet, with the "connector board" removed, diode-check those two diodes - ideally after removing them from the board, and then diode-check their footprints on the board as well.

Both diodes(D5 and D12) tested while disconnected from the board and seems to be fine, same for the footprints (lower one connected to ground and upper side connected to the resistor.
In both resistors I've measured 2.4R with the "connector board" removed.
Now voltages are as following:
Now the voltages on the four SR360 are as following (with load):
+19.2V
-21.45V
+5.5V
+3.8V
The voltage at TP +10 is +2.1V, the voltage at TP -10 is +0.7V

 

[Khron]

At least for my peace of mind, if not yours as well, please do make sure to replace that Chinese garbage with something proper (ie. Japanese low-ESR caps). I wouldn't be surprised if those ChongX aren't even low-ESR, which means they'll throw off the feedback, as well as almost guaranteed to crap out sooner, rather than later.

I've ordered several times from this seller, after reading recommendations over the Badcaps.net forums.
http://www.ebaystores.com/pcmotherboardcapacitorsstore/

Now, with that out of the way... The only thing that can still be at fault, somehow, would seem to be the connector-board.

Are all the pins clean and looking fine? Any traces of soot anywhere? You probably can't do the same check for the sockets, though.

But you CAN do some resistance tests, for example between the diodes for the analog rails (D6 and the one closest to the transformer),  and the +/-10V "test points" on the analog board.

PS: Could you also post a photo of the bottom of the power supply board? There's also a chance you might've broken a trace or torn some vias when removing the original diodes...

 

[smilan]

Now, with that out of the way... The only thing that can still be at fault, somehow, would seem to be the connector-board.

Are all the pins clean and looking fine? Any traces of soot anywhere? You probably can't do the same check for the sockets, though.

All the pins and sockets seems fine, I put little bit of deoxit D5 inside the sockets but it ain't seems to be the problem.
I've noticed that on the back of the connector board there's a couple of diodes missing (D5 and D12) as well as few other unmarked components:
https://ibb.co/bBfj7yt
I haven't remove any components from this board but maybe a technician who serviced this unit at the past removed those ones?
(I'm heard to believe this is the problem since this unit worked for long period without those components).

But you CAN do some resistance tests, for example between the diodes for the analog rails (D6 and the one closest to the transformer),  and the +/-10V "test points" on the analog board.

The ohm meter  shows OL between the +/-10V "test points" and also between the anode of D6 to the cathode of the one closest to the transformer.

PS: Could you also post a photo of the bottom of the power supply board? There's also a chance you might've broken a trace or torn some vias when removing the original diodes...

Here's the picture:
https://ibb.co/N64HTpX

 

[Khron]

Those two diodes, by the way they're pointing, likely played the same role as they do on the analog board - i guess they just moved them there after they had these connector-boards designed and made. No point in throwing these away, or doubling-up on the components.

The resistance tests i meant were between the anode end of D6 in the power supply and the -10V "test point", and the cathode end of whatever-the-+10v-rectifier-diode-is-called and the +10V "test point" (NOT between the +10V and -10V).  Weren't my instructions for that clear enough? I see i wrote "between the diodes for the analog rails (D6 and the one closest to the transformer),  and the +/-10V "test points" on the analog board". Should i have really added a "respectively" as well?

If there's that much voltage drop between the power supply and the analog board, the only thing between them is that connector-board, right? So that's what i was starting to suspect...

Looks like all the "relevant" power traces are on the top of the PSU board, though.

In the mean time, you could also see what voltages you get on the power supply end of the connector board - i've listed the basic pinout of that a couple posts earlier. Actually nevermind, i just noticed that the numbers between the bottoms of the PSU and digital board connectors (the ones right next to each other) are the rounded-to-the-nearest-integer voltages (absolute values, no negatives) you should be expecting.

Second-from-the-top row of pins is around 3.8v, marked as "4", next one's 3.3v marked as "3", and lower down "9", "5" and "9", corresponsing to positive analog rail, 5v for the analog supply of the converters, and the negative analog rail.

[Later edit]

That being said, i CAN be a bit of a doofus sometimes  Only now did it dawn on me that, after looking at the other side of the power-jumper-board, there's a chance the two ceramic caps on it could have failed shorted (PB_C74 and PB_C76). It's unlikely they both suddenly failed in a similar manner, but regardless, i've revived several laptops where the only fault was one crappy little ceramic cap that had failed shorted on a power rail.

So if the PSU-to-analog-board resistance check comes out fine, that's another thing to suspect.

All the pins and sockets seems fine, I put little bit of deoxit D5 inside the sockets but it ain't seems to be the problem.
I've noticed that on the back of the connector board there's a couple of diodes missing (D5 and D12) as well as few other unmarked components:
https://ibb.co/bBfj7yt
I haven't remove any components from this board but maybe a technician who serviced this unit at the past removed those ones?
(I'm heard to believe this is the problem since this unit worked for long period without those components).

The ohm meter  shows OL between the +/-10V "test points" and also between the anode of D6 to the cathode of the one closest to the transformer.
Here's the picture:
https://ibb.co/N64HTpX

 

[smilan]

I found the problem!
The two 0.5Ohm resistors located just after the connector board was faulty and measured approximately 1.5 meg .
I replaced the two SMD resistors temporary with tow 1 Ohm resistors in parallel for each 0.5 Ohm resistor and now everything works
https://ibb.co/K93SCNW
Now I need to find where I can get those two 0.5 Ohm SMD resistors.

Thank you very much Khron  for the guidance

 

[Khron]

Oh... Well, i guess i wasn't too far off; didn't even notice those. Although in my defense, i had no idea what the pinout of that huge connector was, without hands-on measurement.

But hey, glad you tracked down the issue, and glad to have helped, if i did  :

What does one of those retail for, $1.5k-ish? I'll take a mere 25% fee then...  ;D ;D ;D

 

[Donornit]

Hi! I know it has been a while but I have the same problem and I'm wondering what is the power rating on those 0.5ohm resistors? And if you know their purpose?
Thanks

 

[Khron]

Just go to your favorite component distributor's website, and filter the SMD resistors by case size. I doubt there will be more than a couple different power ratings for that size.

As for role, possibly (or most likely) as fuses. Their value is a bit too low to be (significant) dropper resistors, which would be meant to offload some of the power dissipation from the linear voltage regulators.

 

[smilan]

So for the past 3.5 years my 16A worked well and now there is a new issue with the PSU.
When the unit is turned on, the screen flashes for about 1.5 minute before starting the boot process.


Does installing new caps at the PSU will fix this issue?

 

[Khron]

At the very least the little electrolytic on the primary side, yes.

 

[smilan]

At the very least the little electrolytic on the primary side, yes.

Thank you very much!
I replaced this cap and now the 16A has stopped blinking before booting up.
During the disassembly I broke three wires on the tiny ribbon cable that connects the screen to the main board.
After gently repairing this cable everything seems to work fine.