EEVblog Electronics Community Forum

EEVblog => News/Suggestions/Help => Topic started by: Tacheon on March 04, 2011, 08:45:53 pm

Title: Better way to find shorted power output transistors.
Post by: Tacheon on March 04, 2011, 08:45:53 pm
Greetings from up above Dave...

I was honored by Robert Pease publishing a question I emailed to him on how to more efficiently find shorted power transistors in audio amplifiers that are paralleled in circuit. Below is the link for the letter and comments by people:

http://electronicdesign.com/article/analog-and-mixed-signal/Bob-s-Mailbox-Audio-Power-Amps-DAC-Stuff-Spice-Suggestions-And-Lunar-Engineering.aspx (http://electronicdesign.com/article/analog-and-mixed-signal/Bob-s-Mailbox-Audio-Power-Amps-DAC-Stuff-Spice-Suggestions-And-Lunar-Engineering.aspx)


I would be very thankful and grateful if you would comment, or preferably do a program on creative ways to do this.

Aram
Title: Re: Better way to find shorted power output transistors.
Post by: Fraser on March 05, 2011, 09:50:05 am
Aram,

I use a 'ToneOhm 850' by POLAR instruments for such situations. This is no ordinary 'simple short tracer' or milliOhm meter

It can measure voltage drop along a short copper conductor. It can also ac stimulate a circuit that contains a short and trace the current to the lowest impedance using it's built in non-contact flux probe.

The models to look for are the ones with the flux probe capability....namely 700 / 850 / 850A / 950. Suggest you avoid the simpler 500 / 550 models.

Take a look here:

http://www.polarinstruments.com/ (http://www.polarinstruments.com/)

http://www.polarinstruments.com/pdf/brochures/Toneohm950.pdf (http://www.polarinstruments.com/pdf/brochures/Toneohm950.pdf)   - the latest version but expensive :-(

http://www.polarinstruments.com/support/pcb_troubleshooting/AP201.html (http://www.polarinstruments.com/support/pcb_troubleshooting/AP201.html)

Manuals -

http://www.polarinstruments.com/de/manuals/manuals.html (http://www.polarinstruments.com/de/manuals/manuals.html)

http://www.tv.quuq.org/forum/index.php?topic=3982.0 (http://www.tv.quuq.org/forum/index.php?topic=3982.0)

http://www.artisan-scientific.com/70430.htm (http://www.artisan-scientific.com/70430.htm)


I picked up an 'as-new' ToneOhm 850 on e*ay for only GBP50 but I was very lucky as they normally retail for much more than that on the used market. The unit has proved itself perfect for finding shorted or low impedance tantalum capacitors across power supply rails. There are often many such capacitors and other components on those rails yet it copes fine even when multiple capacitor failures are present :-)  The ToneOhm is much reveered by repairers of Racal equipment as tantalum cap shorts are commonplace on older Racal equipment. It would hopefully work in your scenario as well.
Title: Re: Better way to find shorted power output transistors.
Post by: BobBeBob on March 01, 2021, 09:50:23 pm
 hello, I came across one of your posts about polar toneohm 850 tear down. now I cannot find it. I am trying to make my probes for a toneohm 950 and and the info you had in there would have been very useful to me. can you help to me find that post again please.
Title: Re: Better way to find shorted power output transistors.
Post by: Brumby on March 01, 2021, 11:32:32 pm
This has to be a record - or close to...........

This thread is 10 years old.  Necropost Gold.