Search found 208 matches

by stephen_usher
15 Jan 2017 11:22
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

Re: MTX512 fault: Blowing fuse.

Thanks. At some point I really must get the MTX500 fixed, but I think I'd need a good oscilloscope before I can do that.
by stephen_usher
14 Jan 2017 21:07
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

Re: MTX512 fault: Blowing fuse.

OK. What an idiot! I decided to bring out my MTX500 (which has a fault so bad that it doesn't even reset the video chip) to test that as a reference to determine what the amperage through the fuse should be... The current was even higher than on the MTX512! Then I checked the fuse side by side with ...
by stephen_usher
14 Jan 2017 18:32
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

Re: MTX512 fault: Blowing fuse.

So, the capacitors have now been changed and there's no difference. I decided to run a longer powered test to see if I could find and chips which were getting warmer than normal: http://www.lingula.org.uk/~steve/images/Memotech-on-test.jpg As it turns out the video chip and all the video memory chip...
by stephen_usher
12 Jan 2017 00:22
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

Re: MTX512 fault: Blowing fuse.

Removing C57 made no difference. Oh well, the tricky C11 will be next. I'll get replacement capacitors from Maplin's at the weekend and replace them both anyway.
by stephen_usher
11 Jan 2017 22:58
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

Re: MTX512 fault: Blowing fuse.

OK, so after replacing the power transistor and the zenor diode (which is what let out the magic smoke) I'm back to where I was. It still blows the fuse. But... with a meter where the fuse should be I'm measuring 0.86 amps and the machine is operating seemingly normally. As you can imagine, the heat...
by stephen_usher
04 Dec 2016 14:36
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

Re: MTX512 fault: Blowing fuse.

Hi Stephen, I guess that you don't need any comments on the "wisdom" of bypassing the fuse :D The burning smell is not a good sign, the problem could be pretty much anywhere on the board. A good, easy anyway, starting point would be to remove all of the socketed chips and try that first. After that...
by stephen_usher
04 Dec 2016 14:32
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

Re: MTX512 fault: Blowing fuse.

The 7805 voltage regulator and the TIP transistor work together to regulate the voltage for the +5V line. If the fuse is blowing, too much current is flowing, which indicates that something is breaking down and drawing too much current. But first, if your multimeter has a diode test function, test ...
by stephen_usher
04 Dec 2016 13:06
Forum: HARDWARE
Topic: MTX512 fault: Blowing fuse.
Replies: 11
Views: 3477

MTX512 fault: Blowing fuse.

I recently bought an MTX512 on eBay which the vendor had previously tested but arrived in a non-working state as the internal fuse had blown. Having replaced C39 and C40 just in case I could get the machine operational by bridging the fuse, though the rectifying diodes got a bit warm. I replaced the...