Identifying a diode value, conductive glue strikes again!

[mikay786]

Working on a Monitor Audio 370HD sub which is blowing fuses.

Nasty yellow glue on board again which looks like its turned conductive and evidenced by the burnt nature of it around the big cap and this diode.

Anyhow need help identifying diode value. In diode mode is reading 0L both ways, and it isn't shorted so that means its open?

Any idea what value it is and a suitable replacement?

[redwire]

Re: Identifying a diode value, conductive glue strikes again!

Not a plain diode, no cathode marking, so it's a bidirectional something.
The only thing I know that looks like that is a Diac, like a BR100 or Asian DB3.
The brown bar on the lead is the voltage category I think.

They read open until you exceed their trigger voltage (in either direction) 28V-45V depending on the part.
You'd have to scrape the glue off the silkscreen for more info, or trace what it connects to.

That electrolytic cap looks pooched too, like it overheated.

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

Have scraped the glue but its just says D24 which is placement number.

Tested big cap and it reads ok 330uf @ 200v, reads 334uf 0.2esr.

hmm might check the rest of the caps first as they are the usual suspects but given the burnt glue and what I assumed was a dead diode since it was reading 0L I thought it was that.

[budm]

Re: Identifying a diode value, conductive glue strikes again!

Scrape off the glue and let us see how the diode is connected in the circuit, so, we will need good pictures of top and bottom side of the board.

[Behemot]

Re: Identifying a diode value, conductive glue strikes again!

Not a plain diode, no cathode marking, so it's a bidirectional something.

It is visible it was there before, second pic left side. I think it's the thing you call brown, it is just scraped off.

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

On the board its got a cathode marking. Anyhow budm some pics







the transitor is a Toshiba k4107 the its directly below and in line with

[rievax_60]

Re: Identifying a diode value, conductive glue strikes again!

If the primary side is a self oscillating design, the diode is likely to be a diac to produce starting pulses. It would be normal to read open on diode test. Diacs trigger at some high voltage.

[budm]

Re: Identifying a diode value, conductive glue strikes again!

If the primary side is a self oscillating design, the diode is likely to be a diac to produce starting pulses. It would be normal to read open on diode test. Diacs trigger at some high voltage.

I think you are correct, it looks like half bridge drive with two large lytics caps and two power MOSFETs drive setup to drive the power transformer.

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

All the caps on the main board have tested ok so far, might just put it together and test without the glue.

On the Toshiba k4107 is it normal for all the pins to have continuity between each other?

[eccerr0r]

Re: Identifying a diode value, conductive glue strikes again!

A k4107 is a mosfet, and normally if all pins have been shorted together and then taken out to test for a short moment, all pins should be open with respect to one another.

Be careful with handling mosfets, they are easily damaged by static, sometimes static weaker than can be felt.

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

Ta, is 2.8 ohm esr too high on a 10uf 50v cap?

My Panasonics of same value only read like 0.3 esr

[Behemot]

Re: Identifying a diode value, conductive glue strikes again!

If it is some very basic GP cap, it may be OK.

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

I replaced them as have some replacements on hand plus will be keeping this one for my self.

Am I ok to leave the mosfets (K4107s) out while testing just to see it doesn't blow a fuse again?

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

found a couple of caps on another smaller part separate from the main board. It has a couple of 4.7uf 400v caps with esr @ 7 ohms.

These are toast right?

[Behemot]

Re: Identifying a diode value, conductive glue strikes again!

With that small capacity I'd say they may be OK. Consult with datasheet.

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

So put it all back together without the 2x K4107 mosfet and it powered up fine. Red light on etc.

Put the K4107 mosfets back in and blew the fuse, so likely error is around that area. Tested the two diodes directly behind them and are fine. Not sure how I can test the Diac thing which is also very close by.

edit: nevermind, remembered how to do it by a youtube video!

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

So tested mosfets and they are shorted, should I check anything before ordering replacements as don't want to blow the new ones!

[CapLeaker]

Re: Identifying a diode value, conductive glue strikes again!

sorry. I just saw the diac. It's self oscillating ... But I'd change some caps, while I have it open.

[mikay786]

Re: Identifying a diode value, conductive glue strikes again!

Yea I've changed most of the smaller caps, just left the big filter ones.

Can that diac cause the mosfets to blow?

[PeteS in CA]

Re: Identifying a diode value, conductive glue strikes again!

So tested mosfets and they are shorted, should I check anything before ordering replacements as don't want to blow the new ones!

Since you need new MOSFETs, "K4107" is not the full part number. Those are 2SK4107s. In that (Japanese) part numbering system, the part number is owned by the vendor (Toshiba), and no other source can use it. Unless you have good access to Toshiba parts, I'd suggest looking up the datasheet for its parameters (Voltage, current, R(DS On), rise and fall times, gate capacitance, package type) and get a similar part from another manufacturer.