Bad DC offset on my PC-4002m.

[frisouille]

About all it will tell you is its a n-channel or p-channel jfet. Or if its shorted or unidentifiable component

If i undertand, it may be faulty but not shorted otherwise the protection will go on? And that not the case.

 

[frisouille]

I've desoldering it and test with the mk tester. Here what it said in pics.

 

[frisouille]

What sould i do. Find a new N-Fet?

 

[avionic]

Patrice(ciniic audio) might be able to help you out.He is in Bosquentin france.

 

[brutal]

I believe for the J-FET, the GM328 pattern testers are only going to show you the minimum turn on gate current and flow. Those look OK in that regard.

I picked up an MTester (LCR4-T4 rev) up just last year.

I just checked an SK68A with it and it matches the datasheet minimums to switch on @.5mA gate, and forward voltage @350mV nominally (avg both ways)



Make sure you test each SGD and DGS (both ways) and they are similar. I believe the tester can't really tell which is source and which is drain when the gate is the center pin.

Again that should mean it's not open or shorted... but not much more than that.

I wish I had gotten this before I tossed my small bin of scrap parts. It would have been fun to check a few things out. :-(

I stuck two known bad transistors I had sitting here reminding me to order parts. One shows ars two resistors (dead short), the other as a diode (one leg open)

FYI, anyone that's wondering what tester was used, check out this monster thread and this review.
http://www.eevblog.com/forum/testge...0/?PHPSESSID=7b40e4d14581c39b9ddd7f63f97d02c0
http://hackaday.com/2015/04/24/review-transistor-tester/

They're modding these kits to do all sorts of stuff.

Best $20 I ever spent. Too bad I haven't found the time to finish up some of my own projects: dead MX-2000 and noisy C-4 (after recap) among them.

 

[frisouille]

Test SGD-DGS both have same ratings. About I=0.82mA and Vgs=570mV

 

[zaibatsu]

You could try swapping Q201 between left/right channels and seeing if the offset remains worse on the right channel - if so, the problem is elsewhere.

 

[frisouille]

Zaibatsu, hat is what i think to do.

 

[frisouille]

Avionic, what is your second suspect?

 

[frisouille]

I swapped the sk389bl from left to right= the same pb so it's not the faulty way. Any idea?

 

[avionic]

Can you easily disconnect the input cable to the problem side amplifier ? And recheck your offset ?

 

[avionic]

CN201...

 

[frisouille]

Yes i can easily disconnect this cable. What i'm searching for?

 

[avionic]

disconnect the input cable to the problem side amplifier ? And recheck your offset ?

 

[avionic]

Recheck your 245mv issue...After disconnecting the channel input cable.

 

[avionic]

I need to eliminate that part of the circuit.

 

[frisouille]

I've done what you said Avionic. On a DBT, the offset stay about -24v, without the protection relay kick all the time and the voltage is about -40v..

 

[frisouille]

And now the offset on the left side is about -66mv.

 

[frisouille]

The right side, is now about 330mv. Big troubles....

 

[frisouille]

I've found a office clip that was on signal board. It touched diodes-resistors on the meter area. Could it be the faulty way?