Arcade PSU blown resistor ID

[redbaron1007]

I'm working on one of the PSU's in my backlog today that came out of an NBA Jam arcade machine a year or so ago. The PSU is a Pan Yes brand with a model number PY-TV-C. From what I remember the 12v rail wasn't outputting anything. I opened it up and was reminded that the resistor at R28 had blown it's shell off and I have no idea how to ID a replacement for it. (see pictures) The color bands that are still intact a little are brown, black, missing band/bands, and what I'm assuming is silver. When I check it with my DMM it's reading 100ohms exactly but I don't know if that's what it's supposed to be given there's a shit stain down the side of the board.

The other things I've done to this PSU today before writing about it here were to check the ESR on all the caps and I ended up replacing two of them. One that was in circuit with the resistor for the 12v line and another on the 5v rail. The ESR for the one on the 12v rail was a couple ohms higher than I would have liked, especially since the replacement 1000uf 16v Rubycon had an ESR of 0.04ohms.

Other than those two caps all the transistors and diodes are testing fine without any power or load.

Does anyone have any secrets for figuring out what resistor should go there other than guessing and hoping for the best or just plugging it in and seeing if the caps were the issue and the resistor is miraculously ok?

[R_J]

Re: Arcade PSU blown resistor ID

Remove the resistor and check its resistance, I suspect it is ok and just lost the coating due to heat over time. (seen it many times)
I would also replace the electrolytic next to it as it is likely dried out.

You said you had no 12 volts but did you have 5 volts?
The resistor was getting hot it may have also caused a poor solder connection on the trace side (post a picture)

[redbaron1007]

Re: Arcade PSU blown resistor ID

Remove the resistor and check its resistance, I suspect it is ok and just lost the coating due to heat over time. (seen it many times)
I would also replace the electrolytic next to it as it is likely dried out.

You said you had no 12 volts but did you have 5 volts?
The resistor was getting hot it may have also caused a poor solder connection on the trace side (post a picture)

I took out that cap and it was reading open on my esr meter and 28pf on my component tester. I went ahead and replaced it with a new one. The resistor was still reading 100ohms out of circuit so I'm going to say you're right about the coating coming off. I went ahead and double checked the other two caps in the output that I only checked in circuit and they were both fine.

I plugged it up after replacing and checking everything and I got nothing on the +5v, -5v, and 12v lines. I found my notes I scribbled a year or so ago and I wrote down there was no output on any of the lines so that much hasn't changed. I checked all the solder connections under my microscope and found a couple that were a little dodgy so I touched them up before I tried it out.

I'll probably take some measurements farther back in the circuit tomorrow when I get a chance. I'll also remember to post some pictures of the underside and the heat stains on the plastic insulating sheet that goes under the board.

[R_J]

Re: Arcade PSU blown resistor ID

I would likely start with replacing C12 & C13, they seem to be in the primary side and if one is bad, I suspect the switching power supply will not operate.

[redbaron1007]

Re: Arcade PSU blown resistor ID

I would likely start with replacing C12 & C13, they seem to be in the primary side and if one is bad, I suspect the switching power supply will not operate.

I replaced C12, C13, and C8 just for good measure and I'm still getting no outputs. I took some measurements of the TL494 and put them below.

I also checked the transformer on the output side and wasn't getting any voltage on it at the input or output. I checked it out and it's at least cheking out ok with my dmm.

I also checked q1 and q2 for shorts and there weren't any there and the voltage I was reading was 170V and then it would start to drop again before it went back up to 170V. I checked the primary side caps and they were holding their voltage and when tested out of circuit they tested fine.

The numbers on q1 and q2 are SK C2810.

I also threw in a picture of the plastic sheet that was under the PCB just for fun. I'd say it's pretty obvious where that resistor was.

[R_J]

Re: Arcade PSU blown resistor ID

I would check that there are no shorted diodes (including the large one mounted to the heat sink) in the secondary, also make sure none of the +/- 12 and +5 lines are shorted.
I would check that C10 is also ok and not open.
R28 was just a load resistor across the +5 volt output to stabilize it. this voltage is also used to adjust the power supply voltage.

I also checked the transformer on the output side and wasn't getting any voltage on it at the input or output.

You can't check dc voltage on the transformer, you need to check for voltage after the diodes on the secondary side.

[redbaron1007]

Re: Arcade PSU blown resistor ID

I would check that there are no shorted diodes (including the large one mounted to the heat sink) in the secondary, also make sure none of the +/- 12 and +5 lines are shorted.
I would check that C10 is also ok and not open.
R28 was just a load resistor across the +5 volt output to stabilize it. this voltage is also used to adjust the power supply voltage.


You can't check dc voltage on the transformer, you need to check for voltage after the diodes on the secondary side.

I'll take a look at c10 next time I'm in the shop and able to sit down with it. I didn't find any shorts on any of the diodes or transistors and I don't believe there were any on the +/-12v and 5v lines, but I'll have to check those lines again when I'm able to take a look.

I didn't get any voltage on any of the secondary side diodes or anywhere after them.