MS8-29 blowing R570

[cruzlink2]

Hi guys, long time lurker here and first time posting. Got an MS8-29 from my friends arcade that he got from Yaton in China last year, it worked but was all jumpy and unstable. Recently I decided to recap and reflow any suspect joints on it. The recap went well and I found some dried joints and a patch job on the vertical IC, I took care of the joints and the patch job. Unfortunately now the monitor shows raster but no video, upon investigating I noticed that R570 a 4.7 ohm 1/2 watt fusible had blown. I replaced it only to have it blow again, I checked down the circuit to see if I could find any obvious damage or shorted parts but found none. Anyone ever had an issue like this? Thanks in advanced for the help.

[lilshawn]

hard to say, the ms8-29 schematic has been elusive and the ms8-26 probably isn't close enough to make a determination as to the issue.

double check your caps and make sure the polarities are right. check against other pics online as well as, there have been instances that the are caps screens being printed wrong, leading to you installing it backwards.

a reverse cap will act like a short circuit causing stuff like this resistor to blow up.

[cruzlink2]

Thanks a lot for the reply, so I was measuring against a working chassis and the only thing that was weird was D570. It was .470 the right way and like 2.4v the other way, not sure if this diode is supposed to have bias both ways.

[princess prin prin]

It's blowing because there is a short after D570 (video B+ rectifier). If C570 and C332 (on the neckboard) are good and installed properly, measure resistance of the cathode of D570 to ground (anode of D576). If low disconnect CN303 on the neckboard and check again.

[cruzlink2]

Took some measurements with neckboard and without. The very last set of pictures measuring anode of d72 to the cathode of d570 without neck board and the reading is way higher than with.

[princess prin prin]

If you remove/cut J304 on the neckboard, which side shows the lower resistance to ground? Compare with another neckboard if you have one.

[cruzlink2]

Thanks for the help I looked over all the caps and they were all the right polarity but I made a mistake with c332. I mistakenly grabbed a 220uf 50V cause they are exactly the same size and put it in C332 I found it just now thanks to you. Hoping nothing else took damage, but by swapping out for the propper 10uf 250 all the measurements went to proper levels. By the way would you mind if I post on this thread another ms8-29 I have with a different issue? Or should i just start a new thread for that? Thanks a million.

[cruzlink2]

Problem solved guys, once I swapped the cap the f902 fuse blew but it turned out to be they had the wrong fuse there. They had a 1amp fast blow when it required a 2amp slow blow, I swapped in the proper fuse and it works great now.

[lilshawn]

this is exactly why i hate inheriting someone else's project. you never know what shenanigans they pulled. many a time i've spent hours tracing a problem down to find out they subbed an improper part. at least the fix was easy and it wasn't over fused... then you end up patching up fried traces if something went wrong.