SOLVED: Roland MC303 No Power On

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SOLVED: Roland MC303 No Power On

Postby coldwar » Wed Jul 25, 2018 11:03 pm

Okay now that you are done laughing or shaking your head here is the issue. I have a proper AC Adapter and plugged it in and bupkus nada zero lights or action or sound. AC Adapter is outside positive and tests fine for proper voltage and amps (and even powers other gear no problemo) so we can rule the adapter out right off the bat.

I tried searching the forums and didn't see another post like this but if I missed something then a link will suffice and I will delete this post.
I have just started probing around and it seems like something early in the circuit. When testing voltage at the adapter input, it fluctuates and comes out with negative 1.45V or less at that point and quickly disappears after that. I went ahead and replaced the couple of caps nearby and reflowed the adapter jack just to make sure. Nothing is really ringing out as a ground fault so far so I am looking next toward the transistors and inductors nearby.

I was wondering if anyone here has any experience with the power supply section of the jack board and could point out a "usual suspect" to check and save some parts swapping/time?
Many thanks.
Last edited by coldwar on Wed Aug 15, 2018 1:36 pm, edited 1 time in total.
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Re: Roland MC303 No Power On

Postby Rasputin » Thu Jul 26, 2018 4:22 am

coldwar wrote:Nothing is really ringing out as a ground fault so far so I am looking next toward the transistors and inductors nearby.


There's really nothing to check with the inductors because with the power switch off, you should see 9V across C56. If the power switch is off and you don't get 9V there then that narrows it down quite precisely.

How do D54/D55/D56 look?

Also: History is relevant. Did this used to work? Is it new to you and you received it knowing that it didn't work / was untested?
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Re: Roland MC303 No Power On

Postby coldwar » Thu Jul 26, 2018 12:21 pm

Thank you, Rasputin :)
It is funny, I have those very components written on my sticky note from the first pass of testing.
I replaced c56 but it still shows a draining voltage to zero volts. The only odd thing is the one inductor L13 that 'looks' like it should be connected to the center pin of the adapter input (gnd) does not ring continuity to the pin. Which now as I type this I am kinda having a duh huh moment. L13 itself tests continuous at its two points. I could have sworn I tried bridging that gap but will do so and get back to you.

As far as history, the beast belonged to a friend of mine that bought it second hand in working condition. He had it for a couple of years but is less knowledgeable about electronics and a townie, so he most likely gigged a couple of house parties so it may have seen incorrect polarity or the adapter gotten bumped. He basically said he doesn't remember how it stopped working smh. He just gave it to me to have/recycle. Nice guy but a little scattered on his best days. I think we all know a person like that lol. To his credit I have not found beer anywhere on or in it yet.

Will try the L13 bridge and get back to you. Thanks :)
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Re: Roland MC303 No Power On

Postby coldwar » Thu Jul 26, 2018 1:45 pm

Okey dokey so I looked super close and there did appear to be a crack between the trace and negative pin pad. I bridged it so L13 is now connected and I am getting 9V across C56 :)
Measuring at D56 there is 9V on the input pin but the center pin is at 0V, so I think we found the culprit.
Looking at the part it is a Schotkky Barrier Diode which appears to just be a beefy protection diode with heat sink meant for high amp protection. I may just try pulling it this afternoon and filling the two holes with another diode and see what happens. I figure it can't get any worse lol. Hopefully that will get juice back into the rest of the circuit.

I really appreciate your help. Power supply sections are not my bag lol. As always, it helps to talk/type it out :)
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Re: Roland MC303 No Power On

Postby Rasputin » Thu Jul 26, 2018 2:18 pm

coldwar wrote:Okey dokey so I looked super close and there did appear to be a crack between the trace and negative pin pad. I bridged it so L13 is now connected and I am getting 9V across C56 :)
Measuring at D56 there is 9V on the input pin but the center pin is at 0V, so I think we found the culprit.
Looking at the part it is a Schotkky Barrier Diode which appears to just be a beefy protection diode with heat sink meant for high amp protection. I may just try pulling it this afternoon and filling the two holes with another diode and see what happens.


Yeah, D56 should have the anode on pin 1 and cathode on pin 2 with pin 3 being not connected. So putting in a large "traditional" barrier diode should suffice at least in the short term. I concur.
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Re: Roland MC303 No Power On

Postby coldwar » Fri Jul 27, 2018 3:50 pm

Hi Rasputin, sorry for the delay. The car died yesterday so I got wrapped up in that and keeping a wife from freaking out. Fingers crossed it comes out okay...

So for the MC303 I thought I would update ya and see what you thought about my next steps.
I tried the diode replacement and it just immediately drains to zero volts when I turn it on and fluctuates at .2V sometimes. I am looking toward C49 (1000uf) which I replaced with a used cap of the same value. Going to try a fresh cap and see how that goes and looking toward the 7805 to swap out as well. I have a bunch of them and have seen them fail on so much gear I figured why not.

It looks like the next couple of things in the chain. Does that look/sound correct to you?

I really appreciate your help. Having someone else to bounce ideas off of is comforting. Hope you have a good Friday and a good weekend :) Will keep ya posted.
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Re: Roland MC303 No Power On

Postby Rasputin » Fri Jul 27, 2018 8:17 pm

coldwar wrote:It looks like the next couple of things in the chain. Does that look/sound correct to you?


I would try lifting the output pin of select regulators (or some some other convenient part) to isolate parts of the circuit. That way you can identify what is dropping the voltage. Basically, I'd isolate the 5V and 8V sections from any other parts of the board, and then see if the voltage comes up. If not, cut it down to just the 5V. No good? Just keep working your way back toward those diodes until the voltage comes back up.

I'll think about it and see if I can come up with any concrete ideas.

EDIT (MORNING THOUGHTS):

First, lift D54 pin 2 and D56 pin 2. 9VDC should almost definitely be measurable at R34 when
the power switch is on.

Reconnect D56 pin 2. If 9VDC not present across C49 then:

Lift IC17 pin 3 -- disconnects circuit from +5VDC digital rail
Lift Q1 pin 1 -- disconnects circuit from +8VDC
Lift IC11 pin 1 -- disconnects circuit from +5VDC analog rail

9VDC should be again present across C49. Reconnect IC17, Q1, and IC11 one at a time
until 9VDC disappears across C49. Whatever one was just reconnected should be closely
examined.

Otherwise, 9VDC *is* across C49 so reconnect D54 pin 2.

Lift IC9 pin 4 -- disconnects circuit from +5VDC digital rail

9VDC now appears across C47? Examine IC9 closely. No 9VDC across C47?
Lift IC13 pin 1. 9VDC on one side of R68? Examine IC13 closely.
Not 9VDC on one side of R68? Examine Q10/Q11 closely.
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Re: Roland MC303 No Power On

Postby coldwar » Sat Jul 28, 2018 4:03 pm

Those are amazing tips, good sire!
I will definitely try those and let you know what I find :)
Many many thanks!
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Re: Roland MC303 No Power On

Postby coldwar » Wed Aug 01, 2018 12:55 am

Sorry it took a while to get back to ya but the good news is that it is pretty much sorted out :)

What I found:
7805 was bad
IC19 is malfunctioning
IC17 is malfunctioning
Q1 was toast.

I am still eyeing one of the diodes as well as it just doesn't seem like it is 100 percent normal. Somehow I didn't order any signal diodes and need to find equivalents of those ICs, so it may be a week or two until I can give a final report. The good news is that I used my benchtop PSU to pump voltage into the board while the rails were isolated and it powered up and worked after a reset :D so that means it is indeed worth some more time and parts to get singing again.

I cannot thank you enough for all of your help-especially that last diagnostic write up you did. I sat there with the post printed out and went thru like you said. It helped shed a lot of light on what was happening with the power supply section, although it definitely makes me realize that I will never be an Electrical Engineer lol as most of it just seemed like kinda ridiculous overkill and weird routing. That being said, I guess it was indeed needed as it saved the rest of the electronics when it failed, so that is a good thing.

You rock Rasputin! I will holler back and mark this thread solved once I get everything in and soldered and zipped back up :) I owe you a beverage of your choosing :)
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Re: SOLVED: Roland MC303 No Power On

Postby coldwar » Wed Aug 15, 2018 1:45 pm

Hi Rasputin thought I would update ya now that I have all the parts installed. IT WORKS!!!
Oh man it took nearly 12 replacement parts but now it powers up and works. I am not really sure if all of them were necessary, I just got tired of chasing the problem down the power rail and did most of the section at once. After that was done it turned on but no screen or sound which, after a quick hear attack, turned out to be one of the ribbon cables had a pin broken off inside the connector smdh. This thing is now back to grooving thanks to your help and advice, Rasputin. I appreciate ya accompanying me on the journey :) Many many thanks for your patience and knowledge. I guess to any future readers of this thread to read his troubleshooting post a couple above as it provides good advice on how to isolate the power rails for better troubleshooting and sussing out which parts are bad. I cannot say what specifically caused such a long chain failure besides the original crack at the power supply point and the possibility of the previous owner trying different adapters as more and more components failed down the chain.
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Re: SOLVED: Roland MC303 No Power On

Postby Rasputin » Wed Aug 15, 2018 8:08 pm

Watch on youtube.com
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