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Lab supply repair help
Lab supply repair help
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Old 8th January 2010, 12:28 PM   #1
shzmm is offline shzmm  Germany
diyAudio Member
Join Date: Jul 2008
Default Lab supply repair help

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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Old 9th January 2010, 04:33 AM   #2
Enzo is offline Enzo  United States
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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?
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Old 9th January 2010, 05:37 PM   #3
gootee is offline gootee  United States
diyAudio Member
Join Date: Nov 2006
Location: Indiana
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.
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Old 9th January 2010, 07:49 PM   #4
shzmm is offline shzmm  Germany
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.
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