Attempting repair on Sunfire sub.. how do I test ultrafast rectifiers? - Page 1
Forum section: electronics / repair

author: mubase
author_rank: Newbie
timestamp: July 16, 2018, 12:41:37 pm
content: 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.

author: samnmax
author_rank: Regular Contributor
timestamp: July 16, 2018, 02:31:21 pm
content: 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.

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 19, 2018, 09:20:44 am
content: Hi!QuoteOnly 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 diodes0.3V 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

author: gyounger
author_rank: Newbie
timestamp: December 25, 2020, 03:03:49 pm
content: 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

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 25, 2020, 09:52:11 pm
content: 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.

author: gyounger
author_rank: Newbie
timestamp: December 26, 2020, 02:48:30 pm
content: Thanks for your reply will do these checks on the output stage.

author: CaptDon
author_rank: Super Contributor
timestamp: December 27, 2020, 01:40:39 pm
content: Sometimes the 'outer two' legs of a twin-pak diodeare across the power transformer secondary and ifyou measure across those pins it will look shortedbecause you are actually measuring the resistanceof the transformer winding which will be very low.If it has a hum, but not blowing fuses or smokinglook for cracks in solder or circuit board from beingdropped or capacitors that are bad or have ventedor leaking electrolyte. Is the hum going from yoursignal source due to a ground loop???