Weird OP amp fault

[Trevor Wilson]

The dual OP amp (LF353) is used as a DC servo and for sending fault
information (at DC) to a protection system in a power amp. I pulled the
OP amp and placed in my test jig (basically an AC tester with a gain of
10). It tested fine (or so I thought), so I re-installed the OP amp and
continued fault-finding. Nothing made sense, so I pulled the OP amp and
compared it to a new one. There is was: The OP exhibited low gain in my
tester (around 20dB too low). Waveform was clean.

Weird.

--
Trevor Wilson
www.rageaudio.com.au

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[Phil Allison]

Trevor Wilson wrote:

The dual OP amp (LF353) is used as a DC servo and for sending fault
information (at DC) to a protection system in a power amp. I pulled the
OP amp and placed in my test jig (basically an AC tester with a gain of
10). It tested fine (or so I thought), so I re-installed the OP amp and
continued fault-finding. Nothing made sense, so I pulled the OP amp and
compared it to a new one. There is was: The OP exhibited low gain in my
tester (around 20dB too low). Waveform was clean.

** Saw something similar a few months back with a pair of Clearcom CP-100 talkback boxes. Such units are worn by crews at a live shows to communicate using headsets. Inside is a mic preamp, a headphone amp and signalling circuits. Headsets connect using a 4 pin XLR while a 3 pin XLR allows linking of units to a master station that also provides 30VDC. These two boxes were pretty old, made in the mid 80s IIRC.

Both gave the same distorted, weak output to the headphones - which was driven by a RC4558 with one half providing gain and the other wired as a voltage follower with the two outputs then linked by 22ohm resistors feeding the mono connected headphones.

Both op-amp halves had severely reduced gain/bandwidth and very low output current ability. This could not be due to damage and must have been age related - an internal current source had likely failed in both ICs.

New (NE5532) op-amps fixed the problem.


.... Phil