Finding a short circuit

Talk about non-Acorn classic computers/hardware/software here (including retro consoles)
Post Reply
User avatar
roland
Posts: 2896
Joined: Thu Aug 29, 2013 8:29 pm
Location: Born (NL)
Contact:

Finding a short circuit

Post by roland » Sat Feb 17, 2018 5:19 pm

The computer where I'm working on is a TRS80-M100 portable computer but the basic of the problem goes for many computers: how can I find the defect component that creates a short circuit?

This computer had a failing power supply and I found out which transistor was defect. I replaced it and the power supply is working again. But when I switch the computer on, the power goes down to about 2V and the transistor gets hot. I connected another power supply with current meter and I see the current go over 1.5A. So there is a defective component on the board. Almost all the IC's are soldered on the PCB, just like the Beeb and Elk. What is a good way to find the bad components? Just add power and let it burn?
256K + 6502 Inside
MAN WOMAN :shock:

User avatar
1024MAK
Posts: 7442
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Finding a short circuit

Post by 1024MAK » Sat Feb 17, 2018 5:34 pm

Yep, you can do that. If you have access to a thermal infra red camera, that would pinpoint any hot components. Failing that, it's the finger tip test :mrgreen:

Does the board have any tantalum capacitors? These are known to fail short circuit. Ceramic capacitors if physically damaged can also go short circuit. Cut the legs of any you suspect...

If there is a volt drop along the PSU/GND tracks, you can use a multimeter on the mV range to narrow down the area where the short is. Where current is flowing along a track, there will be a small voltage drop. But on sections where the fault current is not flowing, there will be significantly less voltage drop.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

cmorley
Posts: 474
Joined: Sat Jul 30, 2016 7:11 pm
Location: Oxford
Contact:

Re: Finding a short circuit

Post by cmorley » Sat Feb 17, 2018 6:04 pm

Freeze spray (or upside down air duster) is an option. Where the frost melts quickly there is lots of heat being dissipated. Cheaper than freeze spray is 99% IPA (isopropanol). Spray/pour it on... where it drys out quickly hints at where the short is.

User avatar
roland
Posts: 2896
Joined: Thu Aug 29, 2013 8:29 pm
Location: Born (NL)
Contact:

Re: Finding a short circuit

Post by roland » Sat Feb 17, 2018 6:54 pm

Thanks for the hints, I'll try to remember them.

But I have found the problem. There are two IC's in sockets. And guess what? Yes, both of them were inserted the wrong way. So they got there + and - swapped which caused the short circuit or high current. I hope they aren't damaged.....

One of them is an eprom and has a label on it with AMI BIOS 386 - very curious on a 8085 system. The socket of this one has the text "standard rom" so this might be the system rom. But I'm afraid that this won't work and I'll have a look at the web to find a copy of the original rom. The other one had is a 8042 with a printed text KB-BIOS-VER F from MEGATRENTS so that looks very much like a keyboard controller for an old PC. I think that both of them shouldn't be in this computer.

Edit: I have found a ROM that might work, in the VirtualT emulator 8)
256K + 6502 Inside
MAN WOMAN :shock:

User avatar
1024MAK
Posts: 7442
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Finding a short circuit

Post by 1024MAK » Sat Feb 17, 2018 10:45 pm

EPROMs often don't survive being put in the wrong way round if power has been applied...

It's not unknown for people who don't know what they are doing to incorrectly insert chips. I bought some ZX81s once that apparently came from a college. Apart from the usual broken keyboard membranes, a lot of the BASIC ROMs were inserted incorrectly. Once sorted, they all worked when a spare membrane was connected up.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

User avatar
roland
Posts: 2896
Joined: Thu Aug 29, 2013 8:29 pm
Location: Born (NL)
Contact:

Re: Finding a short circuit

Post by roland » Sat Feb 17, 2018 11:33 pm

Well, the fitted eprom is a 27c256 and the original is a 27c128. So it's definitively the wrong eprom. Some unexperienced person might have thought to upgrade this computer by putting a 386 bios the wrong way on the motherboard and he probably hoped to run Windows for Workgroups 3.11 on it. :lol:
256K + 6502 Inside
MAN WOMAN :shock:

Post Reply