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MZ-721 power supply repair

Posted: Mon Dec 10, 2018 10:39 pm
by tingo
While I was testing stuff on my MZ-721, a small pop came and then smoke started to rise. I quickly turned off the machine, just in case it is more severe than the power supply. A visual inspection shows that one of the capacitors in the psu is dead (leaking) so it is time for some repair. the tantalum and electrolytic capacitors pass a visual inspection, at least.

Luckily the mainboard looks ok.

I have the service manual, and found the "Repair PSU's" article on the original sharpmz.org.
I will be replacing the dead capacitor and any that looks like it.

Should I do more?

Re: MZ-721 power supply repair

Posted: Tue Dec 11, 2018 12:20 am
by hlide
Is that a X2 ?

I had this issue (it is not necessary to work but I dedided to replace while I could only remove it).

https://forum.system-cfg.com/viewtopic. ... 99#p133548 (french language).

Re: MZ-721 power supply repair

Posted: Tue Dec 11, 2018 1:24 pm
by tingo
Not sure it is X2 (how do I know?) but it is X class, at least. The schematic only says 0.047 mikroFarad AC 250V. Here are some pictures:
C2_1.jpg
C2_2.jpg
C2_3.jpg
C2_4.jpg

Re: MZ-721 power supply repair

Posted: Tue Dec 11, 2018 1:32 pm
by tingo
And C1 (the other filter capacitor) has traces of leakage around it, even if it looks ok. Maybe it has been replaced in the past:

Re: MZ-721 power supply repair

Posted: Tue Dec 11, 2018 8:21 pm
by hlide
It's the same as mine. You have a choice: just remove it or replace it with an equivalent one.

It is not necessary as it was just here as a filter which "cleans" the frequencies interferences especially with old CRTs around (so your neighbours may watch their TV set). Nowadays, it is almost useless. It won't prevent your machine from running well.

If you want to replace it, look for .047µF (47nF) - 275V - X2 on eBay: for instance, https://www.ebay.fr/itm/MKP-250V-275V-2 ... :rk:1:pf:0.

Re: MZ-721 power supply repair

Posted: Wed Dec 12, 2018 4:05 pm
by tingo
Yes, I have ordered new capacitors ( I ordered some for C1 as well) on eBay already. They will get here in some weeks time, hopefully. In the mean time my MZ-700 runs unfiltered :-)

Re: MZ-721 power supply repair

Posted: Tue Jan 15, 2019 12:29 pm
by rel2009
3 of the 4 MZ700s I have I've had to replace C2... fairly easy job.

Re: MZ-721 power supply repair

Posted: Wed Jan 16, 2019 7:16 pm
by tingo
Oops, I forgot - I've replaced C2 already. PSU is good as new now :-)