Negative Idle current?

M.Yu

In the Vintage Vanguard
Subscriber
Hello! I have recently repaired/restored a Luxman L-100 to the best of my very limited ability. It has been running fine for a several months of intermittent use though not sounding as good as it should IMO. Yesterday something shorted out on it. Using a DBT I determined it was something in one of the power modules.(I think)

Since I have two of these units I took a module from the spare (also in the process of being rebuilt) and put it in. It passed the DBT test. I decided to adjust the DC offset and idle current on this module because I had never done that since adding new parts. I could set the DC offset very low and went to check the idle current and it read a negative 36mA! I thought that was weird so I checked the other one that has been working and it read same!

I am very confused by this to say the least! Is my DMM bad? I checked the fuse and its OK. I am sure the readings were not like this when I finished working on it the last time. If I use the idle current adjuster it barely changes the reading at all.

This is a total mystery to me now and I'm not sure how to proceed. The unit has been working OK up until yesterday. Can there even be a negative idle current at the test point? If so how could the unit be working as it was? Is the power supply board not working properly?

Any input into this is greatly appreciated because with my limited understanding I am stumped at this point.

Mark
 
Don't like the negative.. Swap your test leads around. Just go with absolute value.
 
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The procedure was written for and analog meter. To get the correct meter movement. The test lead polarity was important. Today with digital multimeters polarity of leads is not so important.Unless.. Your measuring amperes and milli-amperes.
 
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When I swapped the leads around I got no reading. I'll check that again. I think I must have other power supply problems then. The adjuster doesn't change the reading much.
This unit is a real bear to get dialed in. Thanks for the quick response, I appreciate it! :thumbsup:
 
BE CAREFUL. Lots of threads here at AK about blown amplifiers while adjusting idle current.

Fit and fix your probes with the unit unplugged. Make sure they are making good connection. Read resistance, to be sure the resistor where you are measuring is OK and not open.

If you are measuring amperes, not volts, do the same, make sure you have good connection, and make sure the meter is set to read mA (you should measure open circuit if you measure resistance in this case). If you have your meter set to mA, be positively sure you need to use an Ammeter and not a DC Voltmeter at that test point, since an Ammeter is a short circuit to the board.

Then , turn the amplifier ON while looking at the meter, without poking inside the unit with your fingers or the probes. Just read the meter without touching anything. If needed, power off, move the probes , turn on again.

Do you know where are you measuring the Idle current (I mean at which test point or resistor), and what value you should expect?
 
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In the amps/milliamps function of my Flukes. It does not auto-range. You have to manually select DC milliamps/amps and current range. As well as moving the test probe to the appropriate jack on the meter. Always start out at the highest amperage range. And down range as needed without over-ranging. Over-ranging tends to blow expensive fuses.
 
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