Hello,
New to this forum!
I’ve been trying to fix my Poly 61- the transfomer gets hot when switched on (i heard a bang near the power supply side one time when i was fitting a Midi kit) i have descovered this only happens when under load when other boards are connected, and the bridge rectifier gets very hot, so i’m convinced its this that is at fault. After connecting another board to the supply and taking chips off to lower the current load, the transformer cools and the bridge rectifier still gets very hot. I could’nt find any shorts on the other boards.
In the schematic, you can see this part is connected on the secondry side, and is labelled D3. I will attach.
Please can someone advise me which replacement bridge rectifier part to get? I’m unsure what the numbers mean, and the schematic power supply page is unreadable, it just states the part as 4B4B41?
I’m not sure how to test these components also, and if it is at fault, then it might be tricky to buy another seen as this one is at fault? Will a generic bridge rect diode part work as a replacement? I’m cautious of buying another part and using as it could cause more faults (and obviously dangerous), i’ve spent so much time trying to get to the bottom of this and I can see light (maybe) any help here to advise would be appreciated, thank you
Kind regards
Connor
New to this forum!
I’ve been trying to fix my Poly 61- the transfomer gets hot when switched on (i heard a bang near the power supply side one time when i was fitting a Midi kit) i have descovered this only happens when under load when other boards are connected, and the bridge rectifier gets very hot, so i’m convinced its this that is at fault. After connecting another board to the supply and taking chips off to lower the current load, the transformer cools and the bridge rectifier still gets very hot. I could’nt find any shorts on the other boards.
In the schematic, you can see this part is connected on the secondry side, and is labelled D3. I will attach.
Please can someone advise me which replacement bridge rectifier part to get? I’m unsure what the numbers mean, and the schematic power supply page is unreadable, it just states the part as 4B4B41?
I’m not sure how to test these components also, and if it is at fault, then it might be tricky to buy another seen as this one is at fault? Will a generic bridge rect diode part work as a replacement? I’m cautious of buying another part and using as it could cause more faults (and obviously dangerous), i’ve spent so much time trying to get to the bottom of this and I can see light (maybe) any help here to advise would be appreciated, thank you
Kind regards
Connor
Attachments
Here's the datasheet for the bridge: https://datasheet.octopart.com/4B4B41-Toshiba-datasheet-181389253.pdf
That should help to find a replacement.
That should help to find a replacement.
Here's an example for a replacement: https://www.digikey.ca/en/products/detail/genesic-semiconductor/BR62/3927701
It doesn't hurt (or is imperative...) to figure out why the original part failed...
It doesn't hurt (or is imperative...) to figure out why the original part failed...
Korg Poly 61 ?
if something got shorted when you "where trying to fit a midi module" you need to determine just what got shorted before jumping to the conclusion that the rectifier is bad, it could be a filter cap or a regulator or something else.
checking diodes even in bridge form is dirt simple, but you should look at some form of current limiting so as to not damage more with subsequent attempts at troubleshooting.
if something got shorted when you "where trying to fit a midi module" you need to determine just what got shorted before jumping to the conclusion that the rectifier is bad, it could be a filter cap or a regulator or something else.
checking diodes even in bridge form is dirt simple, but you should look at some form of current limiting so as to not damage more with subsequent attempts at troubleshooting.
Thanks Schiirrn! Really appreciate this. I’m interested in why, not sure how to figure that out as my skills are limited. I’ve seen fixing this synth as a learning tool in electronics, and i’m pretty sure this is the issue, obviously i could be wrong.
Turk thanks for the suggestion, Korg yes. I replaced the filter caps but not the regulators, so this would be the next thing to replace, i suppose. Perhpas I’ll replace aswell as the bridge.
Thank you
Turk thanks for the suggestion, Korg yes. I replaced the filter caps but not the regulators, so this would be the next thing to replace, i suppose. Perhpas I’ll replace aswell as the bridge.
Thank you
The original bridge is rated for 100V and 4A current draw. In order for it to roast either the voltage rating got exceeded or the current draw. The only way I see for the voltage rating to get exceeded would have been the power transformer windings getting shorted in a way that would pass primary AC on to the secondary. That would be odd and would have blown up a bunch of other parts but still check that: unhook the transformer secondary and check with the resistance setting of your multimeter, if there is any continuity from the primary to the secondary. If there is not, hook it to mains and read the secondary voltage.
Other, more likely cause would have been a short somewhere after the rectifier that would have drawn excessive current. This would also have potentially damaged the power transformer.
Other, more likely cause would have been a short somewhere after the rectifier that would have drawn excessive current. This would also have potentially damaged the power transformer.
Next: check the old elkos. Have any of them deteriorated so badly that the presented a short? The Korg Poly 61 has a battery. In what shape is that?
Thanks for this detailed and helpful response Schiirrn, I appreciate you taking the time here to help! When i'm back in my space i’ll troubleshoot following these steps.
I replaced the big filter caps on the PSU, but not the others on the power supply side, this could be the short issue, I will investigate this along with the potential transformer short! The transformer has been buzzing since this happened, so this could be the/an issue for sure.
I've done lots of restoration on the other boards to find the short, as the battery leaked, so i had to clean this up and swap components - I also swapped the original battery for a CR2032 (with added diode etc), the battery might have cause this excess current to blow I'm guessing, perhaps also my tinkering.
I measured the transformer voltages on the secondary taps and there seemed to be a V difference on the furthest right Blue taps (one goes straight into the bridge rectifier) Blue 1 (left tap) is 5V+ the Blue (right tap hitting the rectifier) is about 4V - I'm guessing these have to be the same (5V) and that the 4V one is either shorted in the windings or the bridge rectifier is the issue drawing the current/not functioning. Ordering some more today, will keep you posted. Thanks again for this help!!
I replaced the big filter caps on the PSU, but not the others on the power supply side, this could be the short issue, I will investigate this along with the potential transformer short! The transformer has been buzzing since this happened, so this could be the/an issue for sure.
I've done lots of restoration on the other boards to find the short, as the battery leaked, so i had to clean this up and swap components - I also swapped the original battery for a CR2032 (with added diode etc), the battery might have cause this excess current to blow I'm guessing, perhaps also my tinkering.
I measured the transformer voltages on the secondary taps and there seemed to be a V difference on the furthest right Blue taps (one goes straight into the bridge rectifier) Blue 1 (left tap) is 5V+ the Blue (right tap hitting the rectifier) is about 4V - I'm guessing these have to be the same (5V) and that the 4V one is either shorted in the windings or the bridge rectifier is the issue drawing the current/not functioning. Ordering some more today, will keep you posted. Thanks again for this help!!
Maybe just the lighting in your photo but it does look like the board under the bridge is burned and there is some corrosion on the + pin.
So the bridge soldering could be fractrured and open.
I would remove the bridge and test it for open and shorts with the diode function of a DVM.
If good, you can re-use it and perhaps add a small heatsink on top, especially if the board is cooked.
If bad, I would replace it with a bridge with higher current capacity.
So the bridge soldering could be fractrured and open.
I would remove the bridge and test it for open and shorts with the diode function of a DVM.
If good, you can re-use it and perhaps add a small heatsink on top, especially if the board is cooked.
If bad, I would replace it with a bridge with higher current capacity.
Is this with the rectifier/filters hooked up still? And is this before rectification??? There shouldn't be any positive or negative voltages on the transformer primary, only AC! Please make sure there is no mixup between AC and DC.
The negative side of the supply being 20% lower is a sign for more current draw. Investigate what's going on on the negative side of the supply.