Dummy needs help

[davidlzimmer]

Sorry if this is off topic but, I figured you gouys would know. Is there any way to check a pair of Darlingtons with a DMM?

Thanks,

[myhrrhleine]

Check what?

[anatech]

Hi David,
The only thing you could reliably check would be the lack of shorts and the base to collector reverse diode. You an use a current source to check the base to emitter diode drop (2x diode drop). Most meters do not have enough voltage compliance on diode test to do this.

-Chris

[slowhands]

Originally posted by davidlzimmer
Sorry if this is off topic but, I figured you guys would know. Is there any way to check a pair of Darlingtons with a DMM?

Thanks,

Well, there is a way to check Darlingtons. It's crude, inaccurate -- but quick.

With most digital multimeters, there is a diode setting for measuring resistance. Set it to that. Remove the device from the circuit for testing.

Measure for shorts between any two pins first. If there are shorts (the most common failure) it is defective.

Next measure the forward and reverse resistance or voltage drop from base to collector (forward drop should be about 0.6-0.7v, reverse very high resistance in the megohms). As you know, you may have to reverse leads to get a forward reading, depending on whether it is an NPN or PNP.

Next measure the forward and reverse resistance or voltage drop from base to emitter. Darlingtons usually have internal base emitter resistors, so you won't see quite as high a voltage drop. It might read 50-100 ohms forward, but typically higher reversed. In reverse, you are just measuring the base emitter resistor, typically 100 ohms on power Darlingtons and higher on small signal darlingtons.

This method catches hard failures like shorts and opens, and gives you a pretty good idea of the CB junction quality, but it is flakey on the BE junction because of the built-in BE resistors.

This method is not intended as a perfect test, just a quick and dirty method that works most of the time to find blown devices, but may miss a subtle leakage problem. If you really are not sure, test it on a curve tracer or just replace it.

[davidlzimmer]

Ok Thanks guys. I just need a quicky check to see if they are trashed or not.

Trying, without a schematic, to troubleshoot an old bass amp that suddenly died.

I presently am without a soldering iron but expecting shipment tomorrow.

Thanks!

[anatech]

Hi slowhands,
Not all devices have base to emitter (speedup) resistors. Some do. You need to look at the spec sheet for each device you are testing.

Mind you, shorts are always bad. You can measure a false short testing a mosfet.

-Chris

[davidlzimmer]

Yea, I'm probably going to end up replacing them anyway. They come from an old Peavey amp and, frome what I've searched. the numbers on them are Peavey numbers (SJE2152 and 2153)no spec sheet to be found. But they can be replaced with TP-106 from NTE.

I was just curious if you could make a crude test like you can with a transistor.

Thanks

[anatech]

Hi David,
Don't use NTE parts, get real parts. It might take longer to find them, but real parts are less expensive and more reliable. New heatsink grease and insulators are mandatory.

Can you take a picture of the output stage? What's the model number?

-Chris

[davidlzimmer]

Its a Peavy Minx 30 watt Bass Amp.

The out put stage is just the two transistors on a heat sink. On the transistor it says:

.....................................S..........J
....................................... E2152

IGNORE THE DOTS. I just ust the to positin the digits. Of course the other one says E2153. Then there is what looks like a Motorola Icon on the lower left corner.

I've searched Motorola and came up with nothing.

Oh, might be important. The Power Supply has 30v rails.

[anatech]

Hi David,
Motorola will not have any public information on it's house numbered devices (SJ). It's an old tradition that still exists. (house numbers)

Is that +- 30V rails? What package are these parts in?

-Chris