Babelfish XA252 / Babelfish XA252 SIT / Babelfish XA252 SET

[Zen Mod]

But, you just did!

Mighty ZM is above Rules

 

[Zen Mod]

yeah.........

 

[flocchini]

Somebody once told me laws are binding rules are subject to interpretation

 

[Zen Mod]

 

[flocchini]

How true !!!

 

[cwlo]

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

 

[cwlo]

To clarify, offset was -20v and climbing when I shut it down.

 

[Zen Mod]

regular checks - every part - type, value and position

better pics welcome

especially check output mosfets that you didn't swap them (been there, done that)

PSU checked prior to wiring amp pcb?

problem with first channel already, or same with both?

 

[Zen Mod]

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

 

[cwlo]

PSU checked and working properly. Problem with first channel. Have not powered 2nd channel. I will get do an exhaustive look over everything, and then do your prescribed fuse startup next. Thank you!

 

[andynor]

especially check output mosfets that you didn't swap them (been there, done that)

Or inverse/rotate then 180 degrees them perhaps?

 

[Zen Mod]

that too

 

[cwlo]

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

 

[Mikerodrig27]

As a note, It is probably a good idea to always raise R24 on all XA252 builds. It seems as though when there is a fault, R24 is subject to blowing. That will save the finish of the board, traces etc.

 

[tonyEE]

Aaah, yes... that R24...

It doesn't blow up, it SMOKES....

 

[Zen Mod]

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:

 

[cwlo]

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.

 

[Zen Mod]

 

[Russellc]

Well, in the words of Vincent Van gopher, "What happended, what happened???"

 

[Zen Mod]

R15 didn't happened!!!