Hi, I´m having trouble with a nintendo 64 with no signal (no audio, no video). Reading voltages on each electrolytic capacitor on the motherboard I've found that 4 caps have voltage readings on both sides. I mean, with black tip (negative) of the multimeter wire connected to ground and the red tip (positive) touching both sides of the capacitors getting almost the same voltage on both sides... is this possible ?? Please see the schematic (attachment) to figure out what I'm talking about. Those 4 capacitors are audio and video precisely.

 

Yes. Capacitors are inherently AC devices. As such, different potentials with respect to ground would be expected. Especially since a multimeter is a REALLY crappy tool for AC measurements.

 

Yes. Capacitors are inherently AC devices. As such, different potentials with respect to ground would be expected. Especially since a multimeter is a REALLY crappy tool for AC measurements.

Thank you for your answer Papabravo. With multimeter in continuity mode, could you tell me if it is possible getting resistance (500 ohms) with the positive tip of the multimeter to ground pin of any IC on the motherboard (+3v rail) ?? when I use negative tip to VCC pin with positive tip to VCC and negative tip to GND the reading is OL. I'm getting same readings with a regulator in same rail and RAM memory between VCC and GND pins. This only happen on 3.3v rail. I've tried with 12v and 5v ICs and I get OL in continuity mode no matter the order of the multimeter tips.

Multimeter positive tip to any GND IC pin and negative tip to VCC pin= 500 ohms (continuity mode, 3.3v rail)
Multimeter negative tip to any GND IC pin and positive tip to VCC pin (3.3v rail) = OL (continuity mode, 3.3v rail)


Thanks in advance.

 

Sorry I was never foolish enough to use a multimeter in continuity mode for that type of testing. Continuity mode involves the intoduction of voltages and currents into unpowered circuits. This is inherently dangerous to certain types of circuits. You're on your own for this one.

 

If you try to measure resistance of a component in-circuit you are measuring the effective resistance of the entire circuit.
Moreover, when there is capacitance and semiconductor devices on board the reading will be affected by leakage currents and any semiconductor on or off currents. In other words, don't try to extra any meaningful information from your readings.

You may be able to infer some useful information depending on if the reading is zero ohms, medium resistance or very high resistance.

For example, I can do a qualitative test of electrolytic capacitors out-of-circuit using an analog ohmmeter. The needle would kick to low resistance reading and then gradually climb up to high resistance reading. Then I would reverse the leads and repeat. From this I can get a crude estimate of the capacitance value (low, medium, high) and also the leakage. If the ohmmeter does not settle down to infinite resistance reading then I know that it is a leaky capacitor and needs to be replaced.

 

Your use of a multimeter to make random measurements of components in-circuit is not going to take you anywhere. Either you use an oscilloscope to observe the circuit in operation and then diagnose any observed failure (but I suppose you don't have a scope) OR you pull components OUT OF CIRCUIT and measure their parameters OR you simply replace likely components in the appropriate circuit section.

 

Sorry I was never foolish enough to use a multimeter in continuity mode for that type of testing. Continuity mode involves the intoduction of voltages and currents into unpowered circuits. This is inherently dangerous to certain types of circuits. You're on your own for this one.

none component on the board are under 3.3v according schematic. Looking for shorts are very basic and gives you a general idea about the basic function of a component. Thank you anyway.

 

none component on the board are under 3.3v according schematic. Looking for shorts are very basic and gives you a general idea about the basic function of a component. Thank you anyway.

No. You misunderstand. A meter applies current and voltage in order to measure resistance. You do not know if that test current or voltage might destroy a sensitive component.