Electronics > Repair

Quick Repair Guide

(1/2) > >>

pablofg:
Hi,

I have a lot of PCBs and although I've been able to repair some, I still have lots to be repaired.

It would be nice to have some kind of list of what to test in order to find the broken part. The goal is to quickly finding the problematic part, and very specially avoid desoldering.

These are my two cents:

* Visual inspection for cut tracks
* Visual inspection for blown, sweting elements
* I have no thermal camera, so I just power it up and start touching all chips. A hot one, means in most cases BAD.
* Push the chips one by one and see if it boots, that may rule out a bad solder.
* If the board boots up, try the test menu or use the information provided on display.
* If there is a processor, check the Clock signal and if it works, look for activity in the address and data bus. No activity, normally means BAD. Check RESET, CE, HALT signals, etc. in the processor to make sure the lack of activity is not comming from somewhere else.
* Check the Eproms /Roms /Rams and see if there is activity. If you have a programmer, extract the Eproms and verify them. Rams unfortunately normally require esoldering.

I haven't found a way to test 74s without desoldering other than mentioned above.
Also from Dave's video, checking the resistance between +5V or +3.3 and GND should give valuable information. Can someone elaborate on what are the expected values?

Any suggestion / corrections are very welcome.

@Dave we need more repairing tutorials. We love them.

Tita:
Some very interesting videos:
https://www.youtube.com/playlist?list=PL958FF32927823D12

https://www.youtube.com/channel/UCf-EZoze_1mWNMi0XOf5amQ

c4757p:

--- Quote from: pablofg on July 25, 2013, 07:44:29 pm ---* If there is a processor, check the Clock signal and if it works, look for activity in the address and data bus. No activity, normally means BAD.

--- End quote ---

Not always all-caps BAD. The older-style circuits that actually have external address and data buses often have many reasonably benign things that could shut them down. For example, I recently repaired a multimeter that used an external power-on reset circuit for the main processor, and this POR circuit had a tantalum timing capacitor. If that capacitor failed short (somewhat common for tantalums), the processor would be held in reset and the symptoms would resemble a dead CPU unless you thought to check the reset line.

c4757p:

--- Quote from: pablofg on July 25, 2013, 07:44:29 pm ---Also from Dave's video, checking the resistance between +5V or +3.3 and GND should give valuable information. Can someone elaborate on what are the expected values?

--- End quote ---

Dead short = bad. It's hard to say, otherwise. I remember working on a board in my oscilloscope that had ECL logic, with multiple divider-type terminators. This ended up in the board having just under 50 ohms straight across the 5V rail. Other boards will read well in excess of 100k, some "infinity".

pablofg:

--- Quote from: c4757p on July 25, 2013, 09:01:01 pm ---
--- Quote from: pablofg on July 25, 2013, 07:44:29 pm ---* If there is a processor, check the Clock signal and if it works, look for activity in the address and data bus. No activity, normally means BAD.

--- End quote ---

Not always all-caps BAD. The older-style circuits that actually have external address and data buses often have many reasonably benign things that could shut them down. For example, I recently repaired a multimeter that used an external power-on reset circuit for the main processor, and this POR circuit had a tantalum timing capacitor. If that capacitor failed short (somewhat common for tantalums), the processor would be held in reset and the symptoms would resemble a dead CPU unless you thought to check the reset line.

--- End quote ---

OK. I'll add to the list. Check RESET, CE, HALT signals, etc. in the processor.

Navigation

[0] Message Index

[#] Next page