Running into a problem on first start. Using dim bulb tester, and on startup, offset quickly climbing toward negative rail voltage. I shut it off around 20v. No smoke or anything. No visual damage. Any specific areas to test? I went over board with magnifying glass, and didn't find anything unusual. Thx
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Using dim bulb tester, and on startup, offset quickly climbing toward negative rail voltage
instead of bulb tester it would be clever to resort to temporary fuses in amp rails, say that 2A5 will do
frankly - I forgot what is level of minimum Iq when trimpot is set for minimum, but - I'm sure that even if bulb tester in circuit practically messing up things, DC Offset can't be a problem
starting with Iq trimpot pre-set to minimum, DC Offset trimpot arbitrary in mid position
both inputs shorted to GND, no load at output
observing both DC offset and Iq, as described in short initial procedure in post #1
Or inverse/rotate then 180 degrees them perhaps?especially check output mosfets that you didn't swap them (been there, done that)
Update: Output mosfets are in correct position and orientation.
Went through whole board for proper parts and values.
Installed 2A5 fuse for each rail and powered up.
Offset again went up to 24v ish, then R24 smoked and I shut it down
Fuses both ok
Figuring I should pull Q1 and 2 and check. Not sure what to do thereafter. Might be useful to swap mosfets from other side to test? Run BD pair without heatsink for a few seconds to see operation? (I did have mica installed on both). Only replacement resistor I have right now for R24 is 47r.
Thanks
Went through whole board for proper parts and values.
Installed 2A5 fuse for each rail and powered up.
Offset again went up to 24v ish, then R24 smoked and I shut it down
Fuses both ok
Figuring I should pull Q1 and 2 and check. Not sure what to do thereafter. Might be useful to swap mosfets from other side to test? Run BD pair without heatsink for a few seconds to see operation? (I did have mica installed on both). Only replacement resistor I have right now for R24 is 47r.
Thanks
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Update:
well, it must be something fishy in assembly process, R24 doesn't go in smoke without a cause
anyhow, besides checking each and every component for placement, value and orientation (where applicable), if R24 gone Dodo ........ you need to check and replace what's gone:
R18, R19, T6, LED1, R21, R22, Q1, Q2, R24
all ref to schematic for full mos, post #1462, linked here:
Found R15 missing! My eyes are sore staring through the magnifying glasses. Later, I found it easier to take a pic and examine on the screen. I'm still amazed how good a phone can capture the micro stuff. Anyway, gonna rest and get on it anew tomorrow.
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