AMZ Mosfet boost isn't working...

HeIsAll · February 23, 2010, 02:55:19 PM

I've built two and they are both doing the same thing...VERY hot output with little change in boost when Gain knob is turned. Voltage at Q1's Drain is 5.3 and everything looks ok solder wise. I used the same pot for both circuits...bad pot maybe?

JFX09 · February 23, 2010, 03:20:59 PM

Quote from: HeIsAll on February 23, 2010, 02:55:19 PM
everything looks ok solder wise.

I have debugged (on and off) a rat for 18 months. It had the loveliest solder bridge you could imagine, it looked perfectly legit

make sure you trace it out with pcb layout image in hand.

HeIsAll · February 23, 2010, 03:37:40 PM

but the same bridge two times? I'll check again though. thanks

JFX09 · February 23, 2010, 04:09:43 PM

Actually, it could well be possible: after desoldering my bridge I resoldered the part and it made the bridge again, exactly the same, redone it 3 times. I finally got fed up and soldered it with my left hand and it was fine. One of the things I've learned doing electronics is to never assume something must be fine even if you feel it is, it might not. It's maddening at first, but then you realise there is no problem but the ones you create, which i find very soothing and reassuring

80k · February 23, 2010, 04:19:56 PM

even with pot at minimum there is a slight volume boost. for much of the pot sweep there is little change in gain until the last quarter turn. I'm guessing yours is too loud even at the lowest setting?

bean · February 23, 2010, 04:24:19 PM

Sounds like the wiper on the pot isn't doing its job. It should be wired to one of the outer lugs.

Also, a 5kC pot improves the sweep of the boost across the knob a lot.

80k · February 23, 2010, 04:25:30 PM

maybe try to remove the pot from the circuit and see what happens? it should lower the output signal dramatically (to roughly unity gain). If so, then something weird with that pot not giving you the proper range of 5k to 0k to ground.

HeIsAll · February 23, 2010, 04:51:41 PM

Quote from: 80k on February 23, 2010, 04:19:56 PM
even with pot at minimum there is a slight volume boost. for much of the pot sweep there is little change in gain until the last quarter turn. I'm guessing yours is too loud even at the lowest setting?

Yes with the pot all the way down it is way too loud and doesn't change much all the way cranked. I've built one successfully before and this isn't correct. The center lug is grounded to the outer lug and the other outer lug runs to the PCB as it's supposed to...I'll check the pot...

HeIsAll · February 24, 2010, 08:36:11 PM

well it isn't the pot and i dont see any solder bridges - looks good. anybody have a guess based on the original post?

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AMZ Mosfet boost isn't working...