Blew VOM fuse

[ed52]

I blew the meter fuse while measuring milliamps on the Sansui 661 receiver.
The bias pot wasn't turning smoothly so I shut the 661 of left it plugged in and spun the pot while leaving the meter connected to the jumper dummy fuse. The meter stayed at 0 when the receiver was turned on. The .5a internal meter fuse blew. Why would spinning the bias adjustment cause a 'current spike'?

 

[petehall347]

you may have had your leads in wrong on your meter ? . this may be a blessing in disguise as your outputs may have survived over current . . maybe best to check the trimpots on ohms setting to see if the are adjusting smoothly .or just replace them with better ones .

 

[LBPete]

Yes, you have to switch the positive lead to the amp jack. If not, you will pop the fuse.

- Pete

 

[ed52]

I left the meter connected to the fuse terminals used for the bias adjustment. The leads were not changed. I just turned off the receiver, spun the pots and turned the receiver on again and that's when I noticed the meter was not responding.

I'm guessing even though the receiver was, off the remaining charge in the caps was fed through my meter as the pots were spun.

Does that seem like a plausible explanation? Is there some other voodoo going on with the electrons?

 

[LBPete]

If you had the meter leads across the fuse terminals with the fuse removed, you had the meter in series with the power supply. That will blow the fuse in a heartbeat. You have to move the lead to the ammeter position.

- Pete

 

[ed52]

If you had the meter leads across the fuse terminals with the fuse removed, you had the meter in series with the power supply. That will blow the fuse in a heartbeat. You have to move the lead to the ammeter position.

- Pete

The setting was on the milli amp...exactly the way it was for measuring bias. I left the meter on, turned off the receiver, spun the pots, put them back to the original rotational setting, turned the receiver on and then the VOM read all 0.
I replced tyhe 1/2 A fuse and all was well including the adjustment.

 

[skippy124]

If the meter only has a 1/2 amp fuse for the ammeter protection, there is a chance a current spike on turn on took out the fuse.

 

[KingBubba]

What LBPete is trying to warn you about is that most meter have a second port on the front. Your red lead stays in the voltage port for volt and ohm tests, but the red lead must be moved to the amperes port on the front of the meter for reading amps. It does not matter where your meter is set, if your red lead is in the wrong place on the front of the meter. This will also cause the fuse to blow; as will not waiting for the filter caps to fully discharge, as you suspected. Check the front of your meter for the second port.