My first classic PC repair Sharp MZ-700 - Page 1
Forum section: electronics / repair

author: Rikvang
author_rank: Newbie
timestamp: February 11, 2019, 07:56:41 pm
content: Hi Eev'ers,While volunteering at the trift shop this fine piece came in. Its a Sharp MZ-721 personal computer. It belonged to a professor at an university who died recently. The machine was placed on the dismantle and recycle shelf because it would not turn on. I managed to get it for free.I would love to have it working and try some basic commands on the system(that is produced even before my parents met.) So i Tried to get it working. But I stumbled upon an issue. I hope you can help me answer the questions below.Here is how it went down:The actual device:After opening and removing the powersupply:The 500mA fuse was defective.Nothing some aluminium cant fix:It turned on and Beeped!:) The mechanic I am!Before i could check the video output the PSU started to whisper to me. Before i could understand it it gave a big poof and the magic smoke came out the capacitor:It ate too much electricity and blew up on me.I would be delighted to get an answer to the following questions:I think the capacitor blew up because of the age, the rest of the circuit looks fine. What do you think?Can I replace this part with any 0.047uf capacitor that handles 220v? Are there any available?If I should have posted this in the beginners forum my apologies.Thanks! I will keep you posted on the rest of the repair.

author: pinyoro
author_rank: Contributor
timestamp: February 11, 2019, 08:35:40 pm
content: Worst thing you could have done is put aluminium foil instead of a fuse. The fuse blew for a reason. This is when you check capacitors, diodes, transistors, resistors etc before applying power. Substituting a fuse with aluminium will never blow if there is a fault and even damage the equipment further. This is heavily shunned upon. Now that is out of the way, I am sure someone will have a method you can follow...Sent from my WAS-LX1A using Tapatalk

author: pinyoro
author_rank: Contributor
timestamp: February 11, 2019, 08:45:49 pm
content: Quote from: pinyoro on February 11, 2019, 08:35:40 pmHave a look at this Tutorial on YouTubehttps://youtu.be/NoGl7oeh7eUSent from my WAS-LX1A using Tapatalk

author: Rikvang
author_rank: Newbie
timestamp: February 11, 2019, 09:30:37 pm
content: Thanks for the reply. I ordered this part:

author: pinyoro
author_rank: Contributor
timestamp: February 12, 2019, 02:39:21 am
content: Quote from: Rikvang on February 11, 2019, 09:30:37 pmThat part would be fine. You can also get close to the original, not sure of lead spacing. I have had those capacitors go short in the past. Hopefully it just let off it's magic smoke due to you bridging the fuse and it being faulty. There is a safer way to power up equipment for the first time. Google "series test lamp". It prevents excessive current going through the item you are powering up if there is a fault. Just make sure you use the correct type and size light bulb. Link for capacitor similar to your originalhttps://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F390565304736Sent from my WAS-LX1A using Tapatalk

author: Bashstreet
author_rank: Frequent Contributor
timestamp: February 12, 2019, 05:38:58 pm
content: Never short a fuse as fuses do not blow for funFuses are designed to blow when there is over current situation ie. When the device draws too much current as result of fault.Fuse then blows trying to prevent damage to the systemLets say diode has blown and now there is short to ground.(diode lets current flow only one direction)This allows current been drawn and fuse stops this .If you then short the fuse there is nothing stopping the current draw and you can blow every delicate component on the circuit effected.Avoid our mistakesbe smarter than us.

author: Rikvang
author_rank: Newbie
timestamp: February 18, 2019, 09:29:10 am
content: I replaced the capacitor, but not the fuse:It now turns on with a beep(just like they turn on in youtube videos) and stays on.I might change the fuse in the future.Even the data-cassette-deck buttons work.However there is no video output(RCA/Phono) according to my projector; the only device I have which support video out. I removed the 'graphics card' but it seems to look OK. Later this week I will try to use the S-Video output and perhaps an other device/cable for the RCA output to see if that works.I searched a bit on the internet on how to measure video signal. But it seems that on the connector itself you need something like a scope to measure the output, which i do not own.I will post the results of the tests later this week.

author: Bicurico
author_rank: Super Contributor
timestamp: February 18, 2019, 12:23:24 pm
content: That "fuse" hurts to look at it.Don't turn on the device ever again with that aluminium foil acting as a "fuse". Buy yourself a proper fuse, PLEASE!Regards,Vitor

author: CJay
author_rank: Super Contributor
timestamp: February 18, 2019, 12:47:58 pm
content: Quote from: Bicurico on February 18, 2019, 12:23:24 pmI think the point is, it's not 'Acting as a fuse', it's a fire hazard and even if something fails without catching fire then it's highly likely to destroy the rest of the machine.For the cost of the capacitor and a fuse.However, I have a feeling the OP is trolling, I certainly hope they aren't really that stupid.

author: particleman
author_rank: Regular Contributor
timestamp: February 18, 2019, 02:32:56 pm
content: Is that an X2 capacitor that blew?

author: Rikvang
author_rank: Newbie
timestamp: February 18, 2019, 03:23:13 pm
content: Haha I am sorry. I will replace the fuse first when continuing the testing.Letting it blow again while out of sight without a fuse doesn't seem to be a good idea indeed.@CJay thanks for using my proffered pronouns@particleman I have no idea.