MOTU 828 MKII repair

[Potato Cakes]

Hello, everyone,

I picked up a MOTU 828 MKII for $40 as it was listed for parts/not working. The description just said "will not power on", so for the price it would be worth to see if it could be fixed for not a lot of money. When I opened it up upon receipt, I noticed that the fuse was blown and one of the MCI 4-05-4012 power transformers looked very burned. Also there was brown discoloration around the some of the carbon film power protection resistors, especially a pair of 0.5R's that looked like the outside was burned but it still measured correctly. I also noticed this some discoloration around the LM1117-3.3. For fun I replaced the fuse it and it blew immediately. I managed to find a replacement transformer and ordered new regulators as well as 0.5R resistors for around $50. Hopefully that will get this guy back up and running.

The question I have is how could the power transformer get hot enough to burn and not blow a fuse? It's a 500mA fuse, so it seems that it would have blown long before the transformer would have overheated. I'm hoping to avoid replacing all of the parts and find out there was a super secret problem which recreates the condition of the unit as when received.

Thanks!

Paul

 

[Khron]

The caps go bad. I've encountered more than one where the snap-in electrolytic before the 5V regulator was well bulged. So much for USA-made "Meritek" :

 

[David Kulka]

500ma is about 60 watts. That's more than enough to cook a power transformer, even if some of that power is being dissipated elsewhere in the circuit.

 

[Potato Cakes]

The caps go bad. I've encountered more than one where the snap-in electrolytic before the 5V regulator was well bulged. So much for USA-made "Meritek" :

The caps (the one before the 5V regulator in particular as you mentioned) were the first thing I thought and had hoped would be the issue, but those seem to be fine. I will still replace the 15kuF and 3300uF capacitors, but I don't see what could have failed to draw that much current to burn up the transformer. As I had mentioned I'm going to replace the parts around the PSU and see what comes of the that.

Thanks!

Paul

 

[Potato Cakes]

500ma is about 60 watts. That's more than enough to cook a power transformer, even if some of that power is being dissipated elsewhere in the circuit.

Oh, Ohm's Law... you sneaky elephant.

 

[scott2000]

but I don't see what could have failed to draw that much current to burn up the transformer.

Maybe some kind of power surge?

Curious to hear what you think of the unit when it's fixed. Never really been much of a fan of them but have never had nice gear to pair it with either.....

 

[Potato Cakes]

Maybe some kind of power surge?

Curious to hear what you think of the unit when it's fixed. Never really been much of a fan of them but have never had nice gear to pair it with either.....

No idea. Like I said, it was $40 for the cost of admission. I'm going to change out the coupling caps and the op amps, which are JRC 4580's, with LM4562's. The IC swap is half to continue to practice working on SMD chips and the other half for improvement in sound (hopefully). I looked at doing something from the OPA line, but $1.50 versus $5-6 a chip times thirty was all that I was willing to pay considering the rest of the circuit would probably have to be further modified to make full use of a high end op amp. Plus, being SMD requires a full commitment to component selection.

I'll report back when the parts arrive.

Thanks!

Paul

 

[Khron]

Do note that the two mic preamp opamps are hidden underneith the two little bridge rectifiers, used as clamping diode pairs.

And, due to reasons heretofore unknown, MOTU decided to go with those weird dual antilog 2k / 20k pots for the gain adjustment. Those often go bad, and you're SOL finding a replacement.

 

[Potato Cakes]

I received my replacement transformer, but it is too big. The model number matches the old one (4-05-4012), but the replacement guy is larger. This is the one I ordered:

https://www.surplussales.com/item/_tp/4-05-4012.html

According to the MCI site (https://mcitransformer.com/product/mci-4-05-series/) I received the correct transformer, but the measurements for the what is in the 828 matched the 12VA version (3012) even though the label on the MU shielding for the MOTU transformer is 4-05-4012.

I went looking around on Mouser for a comparable replacement and this is what I found:

https://www.mouser.com/ProductDetail/Triad-Magnetics/FP12-950?qs=%252BfhPKvh1fqN9s2NnGxQc6w%3D%3D

The measurements seem to match but it is 12VA. However the fuse for this unit is 500mA slow blow. Would it stand to reason that the actual VA needed for this unit is 12 and not 24?

Thanks!

Paul

 

[Potato Cakes]

Wound up using the Triad transformer (12VA) and it fit perfectly. I'm pretty sure this is the correct VA rating and that the original transformer is somehow mislabeled. I also replaced the big 15k uF cap along with some others, changed out some of the rectifiers and regulators that had burnt or discoloration by their solder joints. I also replaced the JRC4580s with LM4562s, for partially for better (hopefully) sonic improvement but mostly to work on my SMD chip reworking skills. Everything works now and passes audio with the initial tone generator tests. The LCD is a little dark (don't know if that can be adjusted yet) and I'm missing a knob since there was a broken encoder which I also replaced. Pretty excited as for about $100 plus the initial $40 I now have a slightly upgraded MOTU 828MkII, which is a handy little interface to own. I didn't realize how much noise is audible directly from the two El power transformers (not in the audio path but from the component). This is a twenty year piece of technology, so I guess I shouldn't be surprised. At any rate, I'm pretty happy this wasn't a complete waste of time and money.

Thanks!

Paul

Thanks!

Paul