Troubleshooting a 5VDC logic Vss issue

Thread Starter

jimkk

Joined Mar 11, 2019
32
New member first post..... I'm attempting to revive a 35 year old professional synthesizer. One problem I quickly identified was the 5VDC rail was sitting at 3.76. I isolated the PSU and tested.....same result. I replaced the regulator and electrolytics and tested. Perfect. Stayed right at 5vdc even under full rated load. Connecting the PSU to the electronics, still the drop, so isolated the offending board and first did an solid visual. All looked good. Next, I pulled all 25 of the bypass/decoupling caps. As they're cheap and easy to replace, it made more sense than pulling and testing them one by one. Now, I need to wait for a couple days for parts to arrive. Here's my question: To satisfy my impatience, would it be safe to momentarily connect the PSU to the problem board to see if I'm still seeing the drop? Thanks, everyone. Jim
 

danadak

Joined Mar 10, 2018
4,057
Well designed power supplies have short circuit/overload protection.

But no guarantee that was done in your product.

One way of doing this is with a bench top supply that has current limiting control.
You basically set the V you want, then current for max limit, then connect load. If
load tries to take too much current it is limited,and V drops, if not then V stabilizes
at the V you set.

You could put a fuse inline temporarily and try it, but I am guessing, unless there is
power rating on instrument, you do not know what size fuse to try.


Regards, Dana.
 

Thread Starter

jimkk

Joined Mar 11, 2019
32
Sorry, my question wasn't about the PSU. Its about the logic board that is causing the problems.On that board, I've pulled all the decoupling caps location at the Vss of each chip. I question was about powering up THAAT board without the decoupling caps. Thanks. Jim
 

MrChips

Joined Oct 2, 2009
21,620
What was the capacitance value of the decoupling capacitors?
Usually these are 0.1μF and there is no reason to replace them unless there is a dead short across the capacitor.

If the board is basically logic gates there is no harm in turning on the power.

Most people breadboard logic circuits without bothering to install power supply decoupling capacitors.
 

Thread Starter

jimkk

Joined Mar 11, 2019
32
Joe, thanks for responding. Yes, I have the schematics and board layout which are good, but there's no troubleshooting guide. This thing has me perplexed. Its a Roland D-50 which has been with me for over 30 years. Here's a few updates. The core of the problem as I see it: The main board pulls the regulated 5v down to 3.76. The 5v logic supply comes from a PS with a 78MR05 regulator. At first, the PS had a problem and I thought fixing that would solve my problem. I replaced the regulator and recapped it and now, the PS is solid. I've benched it with a dummy load and it works fine. Now the MB: There's 25 ICs on the main board that are powered from the 5v.....all with decoupling caps which I have removed for now. Still the instant drop to 3.76v. The load from the MB is 1.75K so, because that sound be fine, concerns me even more because maybe I'm completely missing the boat. I was thinking next (at the risk of toasting another 78MR05) of hooking it back up and letting it run to see if any of the ICs heat up. Maybe there's a condition besides "short" that would cause such a voltage drop. I've been researching that but haven't found anything. All the ICs are hard wired, so systematically pulling them is not a desirable option. I've considered lifting each of the supply pins, but I have to believe there has to be an easier way.....so wrote to you guys here. If you would like, I'll send you (or anybody else reading this) whatever I have that might help. Just let me know. No matter what, thanks for responding. Jim
 

MrChips

Joined Oct 2, 2009
21,620
Hello Jim,

Presumably, you measured the MB load as 1.75K using an ohmmeter. This is not the same thing as the active load when power is applied.

Can you post a photo of the MB?
 

JoeJester

Joined Apr 26, 2005
4,310
Can I assume you removed the other 5V loads (external of the main board, but connected to the main board).

Did you test C62 and C50, the electrolytics on the main board that are connected to the 5V supply?
 

Thread Starter

jimkk

Joined Mar 11, 2019
32
There is not a capacitor in the C62 location. C50 tests good, but I also tested the board without it and still experienced the drop. I read both the articles referenced above and both are working on the same unit. Unfortunately (for me), in his case, his was partially working. Mine is totally dark. Jim
 

JoeJester

Joined Apr 26, 2005
4,310
The five volts exit the main board to a few sub boards. Look at that diagram. If you remove the connections from the main board, and the five volts remain low, then you have it narrowed down to somewhere on the the main board. Do the disconnection one at a time because if it pops up to normal voltage, you will know which board has a problem.
 

MrChips

Joined Oct 2, 2009
21,620
If you have a bad component pulling the 5V supply line down then you have a difficult task ahead of you trying to locate the bad component on such a complex MB.

My only solutions I can offer is to get a thermal imaging camera to pinpoint the hot spot.

When the power dissipation is high, sometimes using a frost mist might work. The frost melts fastest on the hot spot.
 

Thread Starter

jimkk

Joined Mar 11, 2019
32
Joe, I've already removed the sub-boards. The problem exists on the MB with nothing else attached.

I've thought about thermal imaging but have no experience with this and wqasn't sure if it would even work. I was hoping there might be an option I hadn't considered. Looking on Ebay, there are some pretty cheap (thermal camera) options available. Also, I know an avid hunter who might have a night vision scope or camera(s) in his arsenal.

I'm not going to do anything right away, so if anyone has any ideas, I'd love to hear them.

Thanks, guys! Jim
 

JoeJester

Joined Apr 26, 2005
4,310
JimKK, cool. Did you notice on the schematic all those coils isolating different areas of the main board? If you lifted one end of them, you would narrow down the failed circuit, or group, as well.
 

MrChips

Joined Oct 2, 2009
21,620
JimKK, cool. Did you notice on the schematic all those coils isolating different areas of the main board? If you lifted one end of them, you would narrow down the failed circuit, or group, as well.
Nice. I didn't look closely enough at either the schematic or the mother board.
 
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