Thanks so much for this. I measured Q3=83ohm(drain to source), -11.8v on each BUT 0 volts on the Gate. I measured 0v on Q6 gate as well. Does this mean they are both bad? If I order them, do I just order 2-J175’s?
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IMO, the easiest way to verify that the FET is the cause of the problem is to unsolder or clip the drain and/or source leg out of the circuit temporarily- effectively removing the FET from the circuit. All the FET does is short signal to ground. If you temporarily disconnect it and the amp suddenly works as it should, the FET is likely bad (although it could be a lack of switching at the gate). You don't need the FET for the amp to work."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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By "beeps", I presume you mean it shows "0" resistance? If so, unsolder one end of it and check it out of circuit. Is there a circuit board designation for the diode?"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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It may be by design but my Fluke 83 will beep instead of displaying OL half the time whenever I read a diode in circuit. So I pulled a leg & retested and it resulted in OL both ways.. The diode is SA-22-1 but is listed on the schematic as also SA22. Does SA designation & the symbol make it a Shotky diode? If so, is OL both ways the correct reading? I have some diodes here but not sure what value can it be replaced with? Sorry for all the questions.Last edited by Perkinsman; 11-10-2020, 03:07 AM.
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Originally posted by Perkinsman View PostThe diode is SA-22-1 but is listed on the schematic as also SA22. Does SA designation & the symbol make it a Shotky diode?
Suppressor Diode SA22. DatasheetIt's All Over Now
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If that diode is not shorted, it would not cause your symptoms. It's not a schottky diode, its a suppressor diode and I wouldn't suspect it as bad unless you measured a short."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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I measured Q3=83ohm(drain to source), -11.8v on each BUT 0 volts on the Gate.
The TVS diode is there to protect the FET from transients and is not likely to get blown but it's not impossible.
Also note that TVS diodes may exhibit some capacitance in the nF range so they may affect tone as well.
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Originally posted by Perkinsman View Post3.I decided to replace all V1-V5 since they’re all old original. I’m also replacing the power tubes with a new matched quartet of 6L6gc’s
Just as a car does not test with old wheels, so amplifier not test with old tubes.
Originally posted by Perkinsman View PostThat’s a very good idea, because there is something else going on regarding the volume beside the tubes. Which Fet is typically the problem?
Instead of an endless exchange of posts and vain measurements, it is much easier extract and Q6 and check - hear how Mesa Electradyne works without them.
When Q3 and Q6 is removed it is much easier to test their correctness.
note
Gate Q3 (J175) does not receive any command (hangs in the air) page 6It's All Over Now
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Originally posted by Gregg View PostThat must be a mistake otherwise it doesn't make sense.Originally posted by vintagekiki View PostVoltages on schematics should be observe with reserve because something wrong with the values. Probably is error in writing value for voltage between page2 and page3.
page2 D+ = 366V page3 D+ = 348V
page2 E+ = 365V page3 E+ = 345V
Tube V3B on the anode has 367VDC, no anode resistance, that same anode connected has D+ = 348VDC.It's All Over Now
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