Potato Cakes

MOTU 828 MKII repair
« on: March 26, 2021, 06:34:18 PM »
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

Re: MOTU 828 MKII repair
« Reply #1 on: March 26, 2021, 07:42:30 PM »
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" ::)

Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

David Kulka

Re: MOTU 828 MKII repair
« Reply #2 on: March 27, 2021, 06:31:29 AM »
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

Re: MOTU 828 MKII repair
« Reply #3 on: March 28, 2021, 10:19:01 AM »
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

Re: MOTU 828 MKII repair
« Reply #4 on: March 28, 2021, 10:20:19 AM »
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

Re: MOTU 828 MKII repair
« Reply #5 on: March 28, 2021, 10:35:12 AM »
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

Re: MOTU 828 MKII repair
« Reply #6 on: March 28, 2021, 12:48:57 PM »
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

Re: MOTU 828 MKII repair
« Reply #7 on: March 28, 2021, 02:46:28 PM »
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.
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Potato Cakes

Re: MOTU 828 MKII repair
« Reply #8 on: April 05, 2021, 02:46:10 PM »
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



 

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