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AuthorTopic: Small resistance between 5V and GND normal?  (Read 110 times)

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Offline efrenmgp

Small resistance between 5V and GND normal?
« on: March 26, 2018, 11:17:18 PM »
Hello!

I am working on an A500 6A motherboard but I noticed something that I don't know if it is normal or not.

Whenever i test for continuity between 5V and GND, I assume the should be none, ever. But what I get is a very brief beep (about a second or so) from my multimeter and then silence. Then I tried to measure resistence and I get something around 60 Ohms between 5V and GND. However the machine works (turns on, reads floppies, makes sound, has video output, reads the mouse...) The only thing that does not work at all is the trapdoor expansion. I put a 512 KB expansion with clock that I know works fine and neither the ram nor the clock are visible to the Amiga. Other that that all appears to be OK.

Are those readings something normal? If not... any advice what to look for?

Thanks!
- Efrén
 

Offline giZmo350

Re: Small resistance between 5V and GND normal?
« Reply #1 on: March 26, 2018, 11:26:19 PM »
After you hear the short beep, try reversing the leads of your DVM and recheck the circuit for continuity. Do you hear another short beep? If so, you are charging and discharging a capacitor in the circuit with the battery in the DVM. Check schematic, there's probably a cap in the circuit somewhere. So, that would be normal.

As far as malfunction, not sure. :(
A500: 2MB Chip, 8MB Fast, IndiECS, MiniMegi, IDE4ZorroII on Z-500, KS1.3/KS3.1, WB3.1&BWB
 
A2000HD: 2MB Chip, 128MB Fast, P5:Blizz 2060@50MHz, PCD-50B/4GBCF, XSurf100, RapidRoad, IndiECS, Matze RTG, MiniMegi, CD-RW, SunRize AD516, WB3.9
 
A1200: 2MB Chip, 64MB Fast, 4GBCF, GVP Typhoon 030 @40MHz w/FPU, Subway USB, EasyNet Ethernet, Indi AGA MKI, FastATA MK-IV, Internal Slim CD/DVD-RW, WB3.5

The internet doesn't weigh anything Jen
 

Offline efrenmgp

Re: Small resistance between 5V and GND normal?
« Reply #2 on: March 26, 2018, 11:58:34 PM »
Hello!

Thank you for your answer :) I tried and yes, that's exactly what happens if I invert and test again, and again, and again... I can spend all day hearing those short beeps hehe :)

About the cap... there are many caps that sit between 5V and GND, decoupling caps I think they are, so ok, If that's expected then I'll need to continue searching somewhere else for the trapdoor issue.

This machine had an A501 whose battery went mad and the acid affected even the motherboard, so I think chances are high some trace or component got damaged, hopefully I'll find the problem soon.

Thanks again!

- Efrén
 

Offline giZmo350

Re: Small resistance between 5V and GND normal?
« Reply #3 on: March 27, 2018, 12:23:09 AM »
Quote from: efrenmgp;837896
I tried and yes, that's exactly what happens if I invert and test again, and again, and again... I can spend all day hearing those short beeps hehe :)


Mesmerizing, ain't it! :laugh1:

Quote from: efrenmgp;837896
About the cap... there are many caps that sit between 5V and GND, decoupling caps I think they are, so ok, If that's expected then I'll need to continue searching somewhere else for the trapdoor issue.


1 cap, many caps.... doesn't mater. What that tells you is that you don't have a short anywhere between the two points you had your leads on. That's a good thing.

Quote from: efrenmgp;837896
This machine had an A501 whose battery went mad and the acid affected even the motherboard, so I think chances are high some trace or component got damaged, hopefully I'll find the problem soon.


BINGO! You'll find it! :) Schematics are your friend.
A500: 2MB Chip, 8MB Fast, IndiECS, MiniMegi, IDE4ZorroII on Z-500, KS1.3/KS3.1, WB3.1&BWB
 
A2000HD: 2MB Chip, 128MB Fast, P5:Blizz 2060@50MHz, PCD-50B/4GBCF, XSurf100, RapidRoad, IndiECS, Matze RTG, MiniMegi, CD-RW, SunRize AD516, WB3.9
 
A1200: 2MB Chip, 64MB Fast, 4GBCF, GVP Typhoon 030 @40MHz w/FPU, Subway USB, EasyNet Ethernet, Indi AGA MKI, FastATA MK-IV, Internal Slim CD/DVD-RW, WB3.5

The internet doesn't weigh anything Jen
 

Offline efrenmgp

Re: Small resistance between 5V and GND normal?
« Reply #4 on: March 27, 2018, 06:22:39 PM »
Well I found the problem and turns out it was a mechanical issue with the connector itself making a poor connection. By just slightly pulling out the memory expansion, like 1 mm or so, it is now recognized. Both RAM and clock. I have tested 3 expansions and they all work when I do that, and they don't if I push the expansion all the way in.

Thanks again gizmo350! :)
 

Offline giZmo350

Re: Small resistance between 5V and GND normal?
« Reply #5 on: March 28, 2018, 02:54:24 AM »
Glad I could help but you did all the work! Good job! :)
A500: 2MB Chip, 8MB Fast, IndiECS, MiniMegi, IDE4ZorroII on Z-500, KS1.3/KS3.1, WB3.1&BWB
 
A2000HD: 2MB Chip, 128MB Fast, P5:Blizz 2060@50MHz, PCD-50B/4GBCF, XSurf100, RapidRoad, IndiECS, Matze RTG, MiniMegi, CD-RW, SunRize AD516, WB3.9
 
A1200: 2MB Chip, 64MB Fast, 4GBCF, GVP Typhoon 030 @40MHz w/FPU, Subway USB, EasyNet Ethernet, Indi AGA MKI, FastATA MK-IV, Internal Slim CD/DVD-RW, WB3.5

The internet doesn't weigh anything Jen