Under performing bridge rectifier?


By beeker3000

4 months ago


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  • Latest reply 4 months ago by GRUMPY
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There have been 4 images uploaded to this topic. (View topic image gallery).

IMG_1639 (resized).JPG
CN1-CN2Schematic (resized).jpg
BridgeSchematic (resized).png
PsbCn1ConnectorSchematic (resized).png


#1 4 months ago

Greetings, tech wizards. Newbie here. I have a half dead MSF - only general playfield and half the backglass lights come on. I will also occasionally hear a solenoid "kick" and what sounds like an engine slowly turning over. I'm aware of the DMD issues common with this game, but this problem seems larger than that. (By the way, this game was working 100% a month ago.)

After checking all the fuses, I suspected a power issue. I checked the voltage coming into the PSB and found that it was under spec on two of the (cn1) connector pins. The schematic calls for 32VDC, but I'm only reading 28.85VDC. Here's the schematic (it's a little fuzzy, but I'm referring to the 2nd and 3rd lines):
PsbCn1ConnectorSchematic (resized).png

Continuing upstream, I found that the voltage coming from the bridge rectifier (br2) was only 29VDC. Again, this under the spec of 34VDC. Here's the schematic (see green box):
BridgeSchematic (resized).png

Even further upstream, the transformer feeding the bridge rectifier comes from two wires. One measures 44VAC the other 15.6VAC. The schematics show that this should be 25VAC (see blue box in previous schematic). I think this means that my transformer is producing enough juice, but I'm not 100% sure.

So, yesterday I replaced the (br2) bridge rectifier with a new 35A 400V bridge (http://a.co/dOSXEnC). Sadly, no change. Same 29VDC voltage output as before. Frankenstein is still dead. So, my questions are...

1) Am I getting enough AC voltage to my bridge rectifier? If so, what would cause it to output -3VDC under spec?

and, more generally...
2) Am I completely off base with my diagnosis? Maybe 29VDC is actually enough juice and this bridge rectifier thing is a red herring.

Thanks in advance for your help. It's very much appreciated.

#2 4 months ago

Transformer output is relative to your input line voltage. So a 3 volt swing may not be out of line.

Normally, when reading the voltage going to a bridge (from a transformer winding) you do not use "ground" for your black test lead like you would when checking DC voltage. You place your DVM leads across the two transformer wires.

#3 4 months ago

Does your game have 5V?
Check/replace the caps of the power supply (board with the big black cooling block).

Reflow the print header pins.
Espacialy those from the connector you show in the first image.

#4 4 months ago
Quoted from beeker3000:

Even further upstream, the transformer feeding the bridge rectifier comes from two wires. One measures 44VAC the other 15.6VAC.

You're not measuring correctly then. AC is measured across these two wires, one lead of your DMM on each leg. Polarity won't matter.

It's likely the transformer is fine and the rectifier is going.

#5 4 months ago
Quoted from CactusJack:

You place your DVM leads across the two transformer wires.

Thanks for the tip. I wondered about that.

The reading across the two wires is 31 VAC (spec is 25 VAC). The brand new rectifier I installed yesterday is outputting 29 VDC. This is the same output as the old one, which makes me think it must not be the issue either. Still, 5 VDC below spec sounds like a lot to me. (BTW, I'm getting this reading by placing the DMM leads on the + and - terminals of the rectifier.)

I'll take @Inkochnito's advise and start looking at the PSB.

#6 4 months ago
Quoted from beeker3000:

The reading across the two wires is 31 VAC (spec is 25 VAC). The brand new rectifier I installed yesterday is outputting 29 VDC.

Input 25 volts ac output would be 34 volts dc spec.
Your input 31 volts ac output would be 42 volts dc but you are getting 29 volts dc.
This is off by 31%, something wrong here.

#7 4 months ago

Agreed with @grumpy. AC volts in divided by 0.7 aproximates theoretical DC output.

Also bridge rectifiers can give funky voltages with no load. Were you testing it with load disconnected maybe?

If nothing else you could bridge the + and - with a 10k resistor and check the voltage across that.

Other than that, does the game have a good ground?

#8 4 months ago
Quoted from polyacanthus:

Also bridge rectifiers can give funky voltages with no load. Were you testing it with load disconnected maybe?

I tested the old rectifier under load, but the new one with no load. I will recheck the new one under load tonight and report back.

Thanks for your suggestion.

#9 4 months ago
Quoted from beeker3000:

but the new one with no load

Then it would read higher not lower, I have never worked on a Sega board but I think you may need to check your soldering to insure good contact on both sides of the board.

#10 4 months ago

I'm connecting the wires to the rectifier posts with quick connectors. They fit perfectly. Solder instead?

The ground from this and another rectifier go to a separate connector on the PSB board (CN2) :
CN1-CN2Schematic (resized).jpg

Last night I took voltage readings from the questionable rectifier (by setting DMM to read DC voltage and placing the leads on the + and - terminals), both with and without the CN2 connector plugged into the PSB board. Without CN2 connected, I get 29 VDC - same as before. With CN2 connected to the PSB, I get a whopping 48.3 VDC. I also took readings with and without CN1 plugged into the board ("with load"?), but it didn't change my readings.

So, is this user error? Did CN2 need to be connected to the board all along?

By the way, here's what CN2 looks like on the back of the board. The char marks look troubling to me:
IMG_1639 (resized).JPG

#11 4 months ago

Unplug your game from the power outlet.
Open the switch box in the cabinet.
Look is there is a NTC mounted in series with the power supply.
Sometimes these will burn and causing the main voltage to drop.
Just a thought.....

#12 4 months ago
Quoted from beeker3000:

With CN2 connected to the PSB, I get a whopping 48.3 VDC.

That's more like what you should have with the higher 31 volt input.

Quoted from GRUMPY:
I think you may need to check your soldering to insure good contact on both sides of the board.

I was sure something wasn't making contact. It was the ground connector CN2.

Quoted from beeker3000:
By the way, here's what CN2 looks like on the back of the board. The char marks look troubling to me:

That is burnt flux on top of a bad solder job. Clean this up and reflow with fresh solder.

Quoted from beeker3000:
I get a whopping 48.3 VDC.

When you hook everything back up and start a game then this will drop closer to spec as right now you still don't have a load on this circuit.

#13 4 months ago
Quoted from Inkochnito:

Does your game have 5V?

So, here's a dumb question...how do I check this? I have sharp point leads. Can I get a reading if I stick them in the female connector leading to the DMD? When I do this I only get 0.5 volts.

#14 4 months ago
Quoted from beeker3000:

So, here's a dumb question...how do I check this?

No dumb questions here. CN6 pin8 of the power supply should have 5 volts DC when the game is on. All connectors should be connected.

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