EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: mubase on July 16, 2018, 12:41:37 pm
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Hi guys. My name is Steve. I am currently in the process of diagnosing whats wrong with a sunfire subwoofer (HRS-10). The unit has 2 UF1604FCT ultrafast rectifiers on the main amp board. When I test the outer legs with a multimeter I get continuity. Is this normal?
Can anyone explain the best way to test these 3 pin rectifiers with a multimeter?
Thanks,
Steve. :)
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Check the datasheet, the UF1604FCT has two diodes inside, with the anodes on pins 1 and 3, and a common cathode in pin 2. In many power supplies, the two anodes are tied to the same PCB trace, putting the two diodes in parallel. In that case there will be continuity between the outer legs, but check under the PCB to confirm this.
To test them, put the multimeter between anode and cathode (+ on 1 or 3, - on 2) they should give a forward voltage of around 0.7V.
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Hi!
To test them, put the multimeter between anode and cathode (+ on 1 or 3, - on 2) they should give a forward voltage of around 0.7V.
Only a generalisation that isn't entirely correct I'm afraid!
The diode-test function of most reputable quality DMMs will give the following voltage-drops:-
0.25 to 0.35V:- Small (<5A typically) schottky barrier rectifier diodes
0.3V to 0.4V:- Large current (15A or more) schottky barrier rectifiers, both single and dual;
0.45 - 0.5V:- Fast recovery rectifier diodes;
0.5V - 0.6V:- Standard recovery rectifier diodes;
0.58 - 0.63V:- Small-Signal diodes - the package size/type will determine if it's a power diode or a signal diode - a signal diode will be much smaller;
0.65 - 0.75V:- Zener diode biassed in forward direction!
Chris Williams
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Guys:
The unit has a constant hum when I turn it on and when I turn it off it the speaker returns a discharge thump. Any one want to chime in with some ideas.
Thanks
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Have you checked for absence of DC on the speaker output and measured bias current on the output stage?
Are power supplies symmetrical where intended?
A common problem in equipment where brown (neoprene?) glue is used so secure comments and wires against vibration is that is sometimes corrodes component legs away or creates unwanted paths.
If there's old brown glue involved better scratch it away just to make sure.
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Thanks for your reply will do these checks on the output stage.
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Sometimes the 'outer two' legs of a twin-pak diode
are across the power transformer secondary and if
you measure across those pins it will look shorted
because you are actually measuring the resistance
of the transformer winding which will be very low.
If it has a hum, but not blowing fuses or smoking
look for cracks in solder or circuit board from being
dropped or capacitors that are bad or have vented
or leaking electrolyte. Is the hum going from your
signal source due to a ground loop???