Author Topic: Opamp surrounded by transistors circuit?  (Read 803 times)

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Offline mapsTopic starter

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Opamp surrounded by transistors circuit?
« on: May 06, 2019, 07:46:02 pm »
Hey guys,

I have an old used dbx 166a compressor which has a few problems that I am trying to solve. I am quite new to electronics so using this cheap repair as a learning experience.
The issue I am looking at currently deals with the circuit I included, the pot (gate thres) seems to almost be getting VAC (no scope here so not 100% on this) during the first 1/4 turn left before it switches
to a normal value (on the 'working' channel I get -1.6 for the pot fully left, and about -1.8 at max value never but never a + voltage and certainly not a fluctuating one).
I am measuring this on the listed TP in the circuit, which brings me to the topic question...

What exactly is this circuit supposed to do? I have a rough understanding of opamps and transistors but the layout is above my level so any help deciphering it would be appreciated.
I assume it is some know layout with a name so even pointing me in that direction would be great.Additionally, any thoughts on what could be causing the voltage fluctuations would be a bonus.
 

Offline james_s

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Re: Opamp surrounded by transistors circuit?
« Reply #1 on: May 06, 2019, 08:57:22 pm »
Several of those transistors are used as diodes, and it looks like they used a 0 ohm resistor as a test point, likely it was easier for the assembly machine to load a resistor there than a loop of wire. With that in mind it may be a bit easier to follow.
 

Offline mapsTopic starter

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Re: Opamp surrounded by transistors circuit?
« Reply #2 on: May 07, 2019, 09:29:04 pm »
Ok using them as diodes is something I had not considered, thanks
 

Offline T3sl4co1l

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Re: Opamp surrounded by transistors circuit?
« Reply #3 on: May 08, 2019, 03:03:16 am »
Would have to take a closer look at it, but using diode-strapped transistors, especially monolithic arrays (matched transistors, thermally coupled), sounds like a log amp, or a function related to that (like a squarer).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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