White noise...

[jogle]

Hi everyone!

I just closed an old thread about my acoustic 370 with humming outputs. That problem is as god as solved I hope... :^\ But in my eager to solve that I must have created a shortcut or something, because now there is another fault! I've isolated it to the last stage by disconnecting the preamp. The symptom is as follows...

20 sec after the amp is switched on a very strong white noise is heard.

Could a transistor behave different when it is hot or is it the big choke that give up?

Grateful for any ideas!

//Glenn

[Robert M. Martinelli]

How about freezing spray?

Hi,
if you' re concerned about a failing semiconductor, there is a very simple way to track down thermal breakdowns, you should be able to find some sort of "freezing spray", repairers all over the world use such thing to spot such failures; just spray some of it on the component under test and see if the thing starts back to work. By checking the transistors/ics/diodes this way one by one you should be able to identify the defective part.

Hope this helps

Best regards

Bob

[jogle]

Thanks for the tip Bob!!!

I ordered freezing spray today, couldn't find it in my small hometown. Hopefully I'll get it tomorrow.

To be continued...

//Glenn

[Robert M. Martinelli]

oops - I forgot....

Hi Glenn,
A thing I forgot to mention - thermal failures can also occur on "cold" ( bad ) solder joints, so if you don' t manage to spot a defective component, try to carefully inspect solder joints for crackles or imperfections - if in doubt, rework all the solder joints "by hand" I know it' s boring and time-consuming, but it' s often the only way to eliminate such issues - PCBs are usually wave-soldered and this method, though fast and inexpensive, leaves room for imperfections.

Hope this helps

Best regards

Bob

[jogle]

Freezing spray - God's gift to mankind!!!

Thanks again Bob. It took me 30 seconds to locate whats causing the white noise. It's the "Field Effext Transistor", 2N4360. I can't find it here in Sweden but in the 370 Service Manual the 2N5462 can be used which i did find and ordered. Hope it works.

Talking about the Olympics, how about cooling beer with this spray?

//Glenn

[Robert M. Martinelli]

Hi Glenn,
I' m Very,very glad I've been helpful in solving your problem, I' m not SO old , but I started tinkering with electronics at the age of eight, and sometimes found myself stuck with "intermittent" problems, ( one thing is to track down and replace a "for good" defective component, one completely different thing is to deal with something that comes and goes ( like hearing a noise from your engine which, when you go to the repair shop and try to have the repairer hear it, is no longer there....kinda frustrating ).

So, it' s a good thing to have that spray around, but I think it won't do good to your beer....at least taste-wise....you never know, though, so if you try it let me know the results ( just kidding, of course, don' t even think about that, bet that spray' s poisonous... ).....

Let us know if the replacement FET does the job....

Regards

Bob

[Rick Erickson]

Be sure to check the pinout of the replacement device to be sure the Gate, Source and Drain leads match their pc board locations. This is important to check with any replacement device.

RE

[Amp Kat]

Using a variac or lightbulb limiter is a lifesaver for these. I always put my meter on the rails and make sure they come up smooth before giving them the juice.

[jogle]

Hi again!

I replaced the faulty FET today and the white noise is gone, almost...
One error remain however. It's when I turn off the amp THEN it start to generate white noise. I don't know if it's worth tracking down or if it doesn't affect the overall function.

Since I couldn't get a hold on the original FET 2N4360, I'm not sure about if this replacement FET 2N5462 is pinned out the same way. I could't find any marking on either of them, so I placed the D-shaped head in the same direction.

Confused...

[Enzo]

JFETs are simple to determine pinout.

By its nature, the JFET is ON until turned OFF at its gate. Check it while out of circuit. SO there are three legs. There will be a relatively low resistance between two of them - maybe 200 ohms or something. Those are the source and drain. The remaining leg is the gate. The gate will measure like a diode to either the source or drain legs.

In circuit, put the gate where it needs to be, and then the source and drain are more or less interchangeable.