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April 15, 2024, 03:52:04 PM

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Crate BT25 Strange Signal Cut Behavior

Started by Gonflable, June 07, 2023, 03:08:51 PM

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Quote from: g1 on June 08, 2023, 06:45:42 PMMaybe it is Q4 that is responsible for Q5 source drop-out.
Looks like a limiter circuit consisting of Q4,Q5, U8 and U12.
Also, U8 and U12 are the only things running off 15VX (via R82), so maybe one of those IC's could also be responsible for the +15V rail issue.  Do either feel hot?
(edit: that would be the noise gate circuit that tassieviking mentioned, not a limiter)

Q4,Q5, U8 and U12 are all running cool.  65 F


Trying to understand the circuit and figure out where I could run a jumper to bypass the "limiter" feature without losing the "octave" feature.

I am thinking in the Q4 area?

Thoughts?  Thank you


I removed Q4 from the circuit.  Amp works fine now and I do not care to fix the limiter feature.

I bench tested Q4 and found it to be open (not turning on).

My positive rail is now at 12.5 V, still below spec but the amp seems to work fine and sounds great.

Thank you for all the help. 



Q4 was the correct part to remove in order to disable the noise gate.
Glad you got it sounding better.
Q4 would not test like a usual transistor as it is a FET.  It must have been turning on as the noise-gate was doing something.

Did you ever try the octave feature to see if it is working?  That is the only other thing I can think of that might be dragging down the +14V line.  U8 and U12 are for the octave circuit, I was mistaken when I earlier said they were for the noise gate/limiter.


Yes the octave circuit is working great.  Nice feature on this amp.

Thank you


I tested Q4 as a FET switch.  Drain and source were always open, even after charging the gate.  But I would imagine it was partially working in circuit as you stated.