Troubleshooting a 5VDC logic Vss issue
- Thread starter jimkk
- Start date
Nice catch!!!! It's been staring at me since the beginning. In fact, when I first starting looking at the schematics, I circled the inductors as a possible place to start, but then got focused on capacitors. Of the 26 chips powered by 5v, the circuit is divided into 12 groups, each which passes the 5v through an inductor. One group includes 6 ICs, a coupe other include 4 or 3 and then there are many that the inductor only feeds one IC. I think I have my next step! Thanks so much. I have long known that extra eyes help be see the obvious.......'cause i am NOT smarter than a 5th grader! Jim
P.S. I'll keep checking in here for additional input and will follow up in a couple days once I start lifting coils.
P.S. I'll keep checking in here for additional input and will follow up in a couple days once I start lifting coils.
Live and learn. Come to AAC first and post your problem before you begin removing components willy nilly.
Hah! You must have teenagers. I never do anything willy nilly. I'm retired and where I have no money, I have plenty of time. And where other might prefer to have someone simply tell them the answer, I prefer to walk as much as I can alone and learn along the way. It's like going into a True Value hardware where somebody immediately knocks you over to find out if they can help you find something. My answer is always no because when I go looking for it myself, I always learn something new along the way. It's the key to basic wisdom. And to that end, you guys were perfect. Was what I did willy nilly? Hardly. Education is never willy nilly......well....maybe with today's high schools, but I digress. Now...as for what I found....good news and bad. It took about 15 minutes, lifted one leg on 5 coils and Vdd was back to normal. Cool. Step 2: connect back the contestant number 1......voltage drop. Disconnected it and hooked everything else back up. Good. Connected coil one again....problem confirmed. Just for fun, put it under the microscope to look for metallic debris and nothing, So......identifying the culprit, thats the good news. The bad news is that the offending IC is a Roland proprietary chip, their Reverb Chip chip p/n MB87126A-006 which isn't available any more but I happen to know Roland used on a bunch of their products so next, I need to start lurking around the Internet and watch for somebody with a dead Roland synth or sound module...... Thanks again to all you guys for your help!!!!!!! Jim
Before we call the custom chip bad ... let's look at the circuit. Which coil did you lift to find the bad player ... you identified them as contestant number 1. Make sure that capacitor is good.
I guess the next question I would have ... are you the original owner and had the problem or is the a hand me down equipment?
If it's hand me down. look to ensure there are no solder bridges on the affected chip.
I guess the next question I would have ... are you the original owner and had the problem or is the a hand me down equipment?
If it's hand me down. look to ensure there are no solder bridges on the affected chip.
The decoupler is not present. The coil is L6. I am the original owner. Have gone over the chip and surrounding area under a microscope looking for bridges or anything that didn't look right. I also verfied continuity to the MB87126A Vdd pins. I would LOVE to find a way that its not the chip!. Thanks again for all your help. Jim
Jimkk, yeah. My cursory view didn't find any other connections to +5V H. I do a second glance tomorrow to be sure.
I just wanted to ensure I was looking at the same area you were. Good job on finding the problem area.
Like you, I'm looking to ensure it is the chip and not some ancillary components.
I just wanted to ensure I was looking at the same area you were. Good job on finding the problem area.
Like you, I'm looking to ensure it is the chip and not some ancillary components.
Update.....test, test and once I'm sure, I'm probably wrong, so test again. New results: Yahoo! I WAS wrong! It's not the proprietary reverb chip. How do I know? Leaving the Reverb chip coil lifted, I put the boards back in and connected everything and tested again. Vdd drop is back. Hmmmmm. Oh, what's this? The array of standard 16x4 DRAMS right next to the reverb chip are heating up....... So, I lifted the single coil that feeds the array (for the second time...as this was one of the first group I tested by lifting the coil leads) and tested again. Normal Vdd. Reconnected the coil and Vdd to the proprietary chip......still normal. Strapped this little rig I built years ago (just a momentary microswitch with two mini-hook test leads) between the DRAM's coil and the Vdd supply and tested back and forth. Yea, so far the short is still the DRAMS. Next, connected all the sub-boards and tested again. Same results.....and just to double check my double checks, left it powered up and with my highly sensitive thermal imaging index finger on the first DRAM......switched power back and forth....confirming that when I put power on the DRAMS, the voltage drops and the DRAMS begin to heat up. Still consistent results....One more test just for fun. Keep it shorted for a minute and check for anything else heating up. Nothing. I'm going to walk away for a while now and let all this sink in. Tonight, I'll test my results one more time and if nothing's changed, pull the first DRAM. This is certainly the best news yet, as I can buy new DRAMS on Ebay or Mouser all day long for a couple bucks each. If you're looking at the schematics, they're the NEC PD41416 which is just a plain 'ol 12ms TMS4416. Used proprietary reverb chips (from parted out synths) have typically sold for between $50 and $175 (when guys have been able to find them).
Note to anyone who might be following this with similar issues (low supply voltages). If I were to start all this again, my first step would be to use "touch" to check for heated up components.
Note to anyone who might be following this with similar issues (low supply voltages). If I were to start all this again, my first step would be to use "touch" to check for heated up components.
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Well.....I'm not sure I'm there, yet.......but I'm still pretty sure the problem is with the DRAMS. There are 5 DRAMS in this bank. I removed one, and Vdd on the whole board rose from 3.76 to about 4.5. Next, I removed the second and with the 3 remaining, Vdd on the whole board returned to normal. Here's the thing......and maybe this is normal, but it's causing me to question what I'm looking at. Is it possible that they're all bad and contributing to the V drop? Next, whether there are 3, 4 or 5 wired into the board, with power applied, the first in line heats up quickly, then the second and so-on. Then, if I apply 5vdc to the removed ICs, they don't pull on the 5v at all.....but heat up just as quick. To me, this is conflicting results.....unless its common, or at least possible, that the whole bank went bad at once and it takes all 5 in the circuit to pull the v down.
One other test I did that only contributes to my confusion.....After I removed the first chip and saw Vdd only partially return, I put power on the removed IC and saw it didn't pull on the supply voltage at all. So I cleaned up the IC and put it back in its slot on the board without soldering. Power up and sure enough, the board Vdd dropped back to 3.76, so with everything sitting like this, I pulled the first chip back out and Vdd rose right back to 4.5. I'm perplexed. Any thoughts? Jim
One other test I did that only contributes to my confusion.....After I removed the first chip and saw Vdd only partially return, I put power on the removed IC and saw it didn't pull on the supply voltage at all. So I cleaned up the IC and put it back in its slot on the board without soldering. Power up and sure enough, the board Vdd dropped back to 3.76, so with everything sitting like this, I pulled the first chip back out and Vdd rose right back to 4.5. I'm perplexed. Any thoughts? Jim
if they heat up when you apply five volts to the chip itself, that maybe the failing. Does it happen with each IC?
At standby, each DRAM draw 28 mW, or 5.6 mA assumes a 5V supply. If you had a 5V supply and a breadboard, that could be an easy test.
The other possibility is to build a test jig to test every possible memory location.
At standby, each DRAM draw 28 mW, or 5.6 mA assumes a 5V supply. If you had a 5V supply and a breadboard, that could be an easy test.
The other possibility is to build a test jig to test every possible memory location.
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Still proceeding with skeptical caution and looking for errors and/or confirmation every way possible. The breadboard results: The worst offender was pulling 310ma. The rest (x5) were pulling between 100 and 280ma. All 6 together, over an amp. Skeptical that all 6 could be bad, after finishing, I started over and tested again. Yea, same results. Also, back when I first started this, the MB (while pulling Vdd low) was drawing over an amp. Without the DRAMS, overall Vdd to the board is a correct 5.00v with a total current draw of a little under 400ma.....which is much more in line by being under the 500ma recommended maximum of the PS regulator. Time to walk away again, but I think the next step will be to order 6 IC's and breadboard them first; checking to see if the measurements confirm my theories, findings and assumptions. I wish I had a good 4416 around here to test (hint to anybody out there reading this). Unfortunately, where I live, the closest source is eBay or Mouser. I'm keeping my fingers crossed. For the first time, all the checks and cross checks seem to be confirming each other. With a little luck, we're going to bring this old dinosaur back to life. Jim
Side note: I seem to remember some time ago, somebody telling me these DRAMS normally run hot and are designed to function properly at up to 70°C (158°F) and that in their early days, some engineers designed in "Ram fans" to blow over them. Anyway, as it was explained to me, because of their high acceptable operating temperature, they've long been susceptible to age related failure. Maybe it's perfectly normal that after 32 years, all 6 would be going bad. Maybe I'll purchase 6 IC sockets to go with the the new chips or clamp a heat sink to them.
Side note: I seem to remember some time ago, somebody telling me these DRAMS normally run hot and are designed to function properly at up to 70°C (158°F) and that in their early days, some engineers designed in "Ram fans" to blow over them. Anyway, as it was explained to me, because of their high acceptable operating temperature, they've long been susceptible to age related failure. Maybe it's perfectly normal that after 32 years, all 6 would be going bad. Maybe I'll purchase 6 IC sockets to go with the the new chips or clamp a heat sink to them.
I might have some 4416 DRAMS laying around. I would have to search hard.
TMS4416-12 should not be taking more than 50mA each.
If it appears that all are bad, I would investigate further. You need to examine the DRAM control signals, in particular /G on pin-1.
Do you have access to an oscilloscope?
TMS4416-12 should not be taking more than 50mA each.
If it appears that all are bad, I would investigate further. You need to examine the DRAM control signals, in particular /G on pin-1.
Do you have access to an oscilloscope?
Thanks, MrChips. I pulled my scope out of storage for this project and once fired up, the image looked more like a "Pong" game from the '70's and still smelled like....uh.....incense.....yea, thats it. .......So for now, no scope but if it becomes necessary, might have to check with one of my ex-students who lives down the road. We're recovering from a blizzard here so nobody's going to be moving for a couple days. More details: At idle, one pulled 12ma, one pulled 13, then 320, 183, 280 and 310....so since checking each individually and finding 2 that weren't dimming the neighborhood lights when power's been applied, I'm feeling a little better about about my conclusions. Asked by a buddy why I felt compelled to socket them (noting that if they die again in 32 more years, I won't be around to replace them again), I referred to Murphy's law.....if I solder the new chips in, I'll certainly have to remove at least one again where if I prepare for it and install sockets, chances are - everything will go well.
Yea.....I admit still being a little worried. That said, I found an app note for 4416's from the 80's saying to be cautious about board layout, commenting that 4416 (from the '80's) will typically run at 150°C, so don't stack them too close and consider a fan or sink for tight applications. I'd like to think that 31 year old chips designed to run hot are going to have problems. Meanwhile, searching for replacements, they're all over out there......mostly with '80's date codes. I did find some Toshibas with 2016 date codes, so bought them (hoping that maybe they've improved the thermal properties). One-two weeks delivery. I will bench them before installing. Also, MrChips mentioned he may have some loose 4416's floating around. If he's able to find them, it would be interesting to see what his idle at.
