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1178 Channel A "Railing"

Link555 9 June 2021 urei 1178, compressor,

One of favorite units, the 1178 is a very useful tool. This one has an issue when the needle intermittently pins hard right. It took a while to replicate the issue on the bench. It might be a cap, cold solder joint, opamp?

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I don't know yet, I am going to start to trace the problem from the output back to the input.

I added Jensen input transformers to this same unit previously and did not see this issue.  Also had to make discreet JFET replacement for it. So I check those mods out too.

Comments

Link555 Wed, 06/09/2021 - 23:09

Looks like there is a failing opamp on the input of channel A. I replaced it the stock NE5532 with a OPA2134 and the problem went away. I am out of NE5532 so had I had a order a few. Although the new opamp changed the sound more than I would have thought, in a nice way too, more definition . But it's best to go with the stock part, to maintain the wonderful glue that 1178 can provide.

Link555 Thu, 06/10/2021 - 06:13

I replaced the NE5532 opamps and this unit is back to life, but i was curious why it failed a year later... I did some digging to figure out why the first stage opamp failed. 

Originally this unit had a issues with Q7a and I replaced it.  After it was replaced the unit seemed to behave fine for about a year. 

An now the front end opamp goes.  I thoroughly checked the Q7 replacement, it seemed alive and well. Then I started to look at the Caps connected to Q7, I used a in circuit ESR meter I have and all seemed to ok. 

I then probed around the circuit with my scope on normal triggering during power up.  Thats when I noticed a strange charging of the C10, it was charging... but in steps.  I replaced the cap and re checked the waveform behaved like the expected RC wave.   

I am not 100% sure, but I wonder if this failing cap took out the Q7 the first time, and when that happened it worked ICI hard.  The owner keeps this unit powered all the time, so heat would have been an issue. Maybe the impacted IC slowly cooked itself?   I replaced the JFETs as precaution along with C110 and IC3.  Everything seems fine now, but maybe a fan is needed to help prolong this unit longer?

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