Troubleshooting Question

[EasyGoing1]

I have this circuit that is powered from AC but on the other side of the transformer, is an old school bridge rectifier. The circuit won't power up, so I took a reading at the low end of the transformer and I got 0 volts



Then, I removed the diodes and took another reading and I got close to 10 volts. and YES, the meter was on AC setting both times.



I tested the diodes and all four of them came back at .56 volts from the Fluke meter.

What could potentially cause this problem?

[petehall347]

Re: Troubleshooting Question

try new diodes

[EasyGoing1]

Re: Troubleshooting Question

try new diodes

Well, I didn't have any like that laying around, but I did have a doner Apple laptop motherboard that had some on it, so I pulled four of them off and soldered them on this circuit, and right away I got 6 volts at the low end of the transformer, so I plugged everything back in and it's working like a champ!

Thank you.

[petehall347]

Re: Troubleshooting Question

thats twice this week i have helped someone with diodes that check ok on a meter and are actually no good . cant remember ever coming across it myself .

[EasyGoing1]

Re: Troubleshooting Question

thats twice this week i have helped someone with diodes that check ok on a meter and are actually no good . cant remember ever coming across it myself .

And what's even weirder, is the diodes that worked, when I checked them in reverse polarity, the voltage kept climbing on my meter until it said OL ... whereas the bad diodes gave me nothing when I reverse-biased them...

[stj]

Re: Troubleshooting Question

well i'v seen semiconductors that pass on a meter.
the meter tests at 1.8v or less with no real current.

also - fluke - again.
had a guy yesterday bitching on the fone for advice.
his fluke287 - soon to be sold now, told him a couple of zener diodes were fine.
his UT-171 said the same diodes had reverse leakage.

and they did, diodes were fucked and the 500$ fluke didnt see it!

[sam_sam_sam]

Re: Troubleshooting Question

That is why you voltage test diodes i do not trust the meter diode testing it does not always tell you the truth about if it good or bad

I have had this issue my self before

[CapLeaker]

Re: Troubleshooting Question

well i'v seen semiconductors that pass on a meter.
the meter tests at 1.8v or less with no real current.

also - fluke - again.
had a guy yesterday bitching on the fone for advice.
his fluke287 - soon to be sold now, told him a couple of zener diodes were fine.
his UT-171 said the same diodes had reverse leakage.

and they did, diodes were fucked and the 500$ fluke didnt see it!

Look at my PS2 power supply repair. Fluke 289 didn’t see the bad cap. Neither did my Fluke 179 and the Agilent DMM. My Agilent ESR meter found it right away.
At the same time, my fluke meters never failed to identify a bad diode. What gives?

[EasyGoing1]

Re: Troubleshooting Question

well i'v seen semiconductors that pass on a meter.
the meter tests at 1.8v or less with no real current.

also - fluke - again.
had a guy yesterday bitching on the fone for advice.
his fluke287 - soon to be sold now, told him a couple of zener diodes were fine.
his UT-171 said the same diodes had reverse leakage.

and they did, diodes were fucked and the 500$ fluke didnt see it!

Chinese knock-off?

[EasyGoing1]

Re: Troubleshooting Question

That is why you voltage test diodes i do not trust the meter diode testing it does not always tell you the truth about if it good or bad

I have had this issue my self before

The diodes I took from the doner board didn't last either. I don't have any new surface mount diodes, so I decided to take some 1n4001's that I have that are brand new ... nice and meaty compared to what was on the board ... I just bent the stems and flush soldered them to the board and they are working fine so far.

Hopefully, the third time is a charm...

[redwire]

Re: Troubleshooting Question

In-circuit testing can lead you on a wild goose chase. I don't agree with the conclusions here.

Multimeter's diode-test has low test current, AN8008 is 1.7mA at 3.2V
If a diode is sort of shorted or in-circuit resistance is around 400 ohms, it will read 0.7V and you think this is OK.
In reverse, a slowly climbing reading is a capacitor charging up.
We don't know the circuit details, but there is a filter capacitor and perhaps more present.

Many times a side-effect of replacing parts is resoldering a bad connection that was the actual root cause of the problem.

If a rectifier diode was shorted, it overloads the power transformer and gives AC to the filter capacitor. Usually the transformer's thermal fuse pops.

If a rectifier diode was open, you would still get 1/2 wave output, but OP got zero. That would mean two diodes were open which is unlikely.

I think the transformer had a bad connection.