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-   -   Help with Bridge rectifier (http://www.pinballnirvana.com/forums/showthread.php?t=13659)

Screaming01L 03-10-2017 02:05 AM

Help with Bridge rectifier
 
I'm past what was being posted in the last thread and have found what I think is the final issue with this machine> Copy and pasted a post below

Hello. I'm working on a Street Fighter 2 machine that I picked up with a house purchase. The machine was working, but in disrepair. I've brought it to 2 different shops and they cant get it right.

So far I've replaced some fuses, <ACRONYM title="Dot Matrix Display" class="bbc ipSeoAcronym">DMD</ACRONYM> board and driver board. but I keep blowing the F25 fuse, left and right lift gate, and keep melting the coils, mostly the left side because it stays open longer. I originally though it was the wrong coil, The previous repairman used a 20095 which is a super flipper coil very very strong I replaced it with a 30468 and did both sides because the right lift gate was also way to strong, and showed signs of heat (browning paper) I got 2 white coils 30468 but found that the q8 and q9 mosfet on the second rottendog board had failed. Replaced those and kept blowing the F25 fuse for those 2 coils, It was a 1 amp slow blow, so I went to a 2 amp. The coil worked for all of 30 minutes, but the 2nd time it went to open (usually stays open for 5 min.) it got so hot it smoked, took out the fuse and mosfet. I unhooked it and now went looking for more issues and found one that's probably been there the whole time. High voltage

The coils are supposed to run on 48 volts, my DMM is reading 68 volts and drops to 61 when any other coil energizes. It hasn't hurt any of the other coils yet because they are momentary for the most part, but the ramps stay up for extended periods. I went and tested my voltages all the way back to the bridge rectifier. I have the proper voltage coming in, 50 VAC. But my outlet voltage is 68 VDC.

My question is, is the rectifier the culprit?? Ive read that an open diode will drop voltage and shorted diode with make it fluctuate, but the only fluctuation I see is during voltage draw.

I've ordered 3 new Bridge rectifier - lug 35a 400v and want to do the 12 and the 20 along with the 48. Is there anything after the rectifier that can raise the voltage?
I'm certain that the additional voltage to the coils are cooking them and the mosfets on the driver board, but want to make sure theres nothing else in the system.

When I'm done with this machine I want it perfect

Any help appreciated

Coil_Smoke 03-15-2017 12:05 PM

Coil Smoke, You say...? Capacitors charge to full not RMS value of applied direct current pulses. That means a capacitor will achieve almost 30% higher charge over applied rectified value read by average DVM/VOM type meter. 68 volts seems a bit high but it is conceivable. I would expect more like 62-65 VDC but you got that 68 volts to explain. I'd say your 50V regulator is shorted( or conducting full ON due to faulty reference/driving circuit. Is there any voltage drop across the main transistor ?) and passing what ever is on your filter capacitor(s). That voltage fluctuation you mention under load backs this notion up.
The only other thing that comes to mind is... Somehow transformer wiring or another pathway is interacting with the coils power supply by adding to B+ side or lowering the ground potential.

Screaming01L 03-15-2017 12:31 PM

Here is what I did, and I did it because the machine has a history of issues. When I got it last year, it had the original driver board and someone pulled the mosfets out so I'm thinking its been an issue. Ok. Couldn't get the voltage to drop, even with a new bridge. BUT I found an EOS on the left ramp that looked almost like it was glue together. It tested fine with no power using it by hand, BUT when using the coil, it let voltage through. I took apart the switch and cleaned off what looked like Elmer's glue and noticed that the fin the contact was on was getting hung up on the plastic plunger so I trimmed just a bit off so the top doesn't hit. To lower the voltage draw, because the new bridge was 69.5, I installed a second bridge for the 2 ramp coils, since theyre the only coils that actually stay open for long periods of time depending on game play. I also added a ground from the bridges to the chassis. Now my momentary coils are in the mid to high 50's , dropping to the low 50's when energized, and the 2 ramps are at 50 volts, dropping to 44 to 46 when open. Also, the F25 fues, for the 2 ramps, is solely connected to the new bridge

Coil_Smoke 03-15-2017 11:37 PM

That's great! It still sounds like no regulation on your 50VDC rail, but maybe it is not regulated? It's kinda hard making diagnoses without a schematic. That EOS contact sounds real important on a widget energized for extended times. Did someone deliberately monkey with that switch? That's a good tip you are on to there. Look for the existence & proper operation of end of stroke switches in unusual locations. Thanx... C_S


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