Lab supply repair help

shzmm · 8th January 2010, 12:28 PM

Hello,
I am trying to repair a laboratory power supply with 2x15V output with adjustable current of 200ma-5A. It's a commercial unit from the late 70's.
The problem is one side is not working. The transformer is fine, I get 23VDC after the diode bridge. I first replaced all the electrolytics thinking that could be the source of the trouble but no luck. Then I swapped the ICs but the problem persists.
The supplies use an opamp UA723CN, transistors bc2378, bc 161 and power transistor 2n3772.
My guess is a faulty transistor but I can't find anything on the bc2378. I have to admit my knowledge of how these large power supplies function is very limited.
Any help would be appreciated.

Enzo · 9th January 2010, 04:33 AM

Any chance it is a BC237B instead of a BC2378?

Regardless of the circuit, transistors are transistors. Pull them and test them with your diode test function on your meter. If you find a dead one, replace it. If the mystery part seems to check like a transistor, it is probably OK.

Verify the resistors in the circuit, especially if you see any fractional ohm resistors like 0.1 ohm or something. An open current sense resistor could shut down the supply.

Look at a data sheet for the 723. No what appears on its pins? How are its needs not being satisfied?

And of course, what is the make and model of your supply?

gootee · 9th January 2010, 05:37 PM

Good news: If the two sides are identical, then you should be able to compare measurements between corresponding points in them, to help find the problem(s).

For example, (with power disconnected) measure the resistance in both directions across each pair of transistor pins on each side. If everything is identical, then it's probably not a transistor. If there is a difference, then it's probably either a transistor or something nearby.

shzmm · 9th January 2010, 07:49 PM

Hello and thank you for the replies. I think the BC161 is the culprit. I'll have to verify it Monday when I can get a new part.
And yes you were correct it was a BC237B, my eyes just aren't what they used to be!
For what it's worth the make and model of the supply is a Dynamic NG-1525 made in W.Germany. I think it has around 30 years of German dust inside.

8th January 2010, 12:28 PM	#1
shzmm diyAudio Member Join Date: Jul 2008	Lab supply repair help Hello, I am trying to repair a laboratory power supply with 2x15V output with adjustable current of 200ma-5A. It's a commercial unit from the late 70's. The problem is one side is not working. The transformer is fine, I get 23VDC after the diode bridge. I first replaced all the electrolytics thinking that could be the source of the trouble but no luck. Then I swapped the ICs but the problem persists. The supplies use an opamp UA723CN, transistors bc2378, bc 161 and power transistor 2n3772. My guess is a faulty transistor but I can't find anything on the bc2378. I have to admit my knowledge of how these large power supplies function is very limited. Any help would be appreciated.

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9th January 2010, 04:33 AM	#2
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	Any chance it is a BC237B instead of a BC2378? Regardless of the circuit, transistors are transistors. Pull them and test them with your diode test function on your meter. If you find a dead one, replace it. If the mystery part seems to check like a transistor, it is probably OK. Verify the resistors in the circuit, especially if you see any fractional ohm resistors like 0.1 ohm or something. An open current sense resistor could shut down the supply. Look at a data sheet for the 723. No what appears on its pins? How are its needs not being satisfied? And of course, what is the make and model of your supply?

9th January 2010, 05:37 PM	#3
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	Good news: If the two sides are identical, then you should be able to compare measurements between corresponding points in them, to help find the problem(s). For example, (with power disconnected) measure the resistance in both directions across each pair of transistor pins on each side. If everything is identical, then it's probably not a transistor. If there is a difference, then it's probably either a transistor or something nearby.

9th January 2010, 07:49 PM	#4
shzmm diyAudio Member Join Date: Jul 2008	Hello and thank you for the replies. I think the BC161 is the culprit. I'll have to verify it Monday when I can get a new part. And yes you were correct it was a BC237B, my eyes just aren't what they used to be! For what it's worth the make and model of the supply is a Dynamic NG-1525 made in W.Germany. I think it has around 30 years of German dust inside.