Error in Crack instructions/ I think it broke something

[lachlanralph]

Hi All,

I have just finished putting my Crack OTL together and i think there is an error in the voltage checking section of the instructions, After i had finished checking out all of the resistances (all good) i moved onto checking voltage where it advises the following

"( )Attach the negative lead (typically black) of a volt-ohm meter to ground. A good spot to do so is at
terminal 12. Use of a clip lead to connect the black test lead to the ground will free one hand, making
testing much easier and safer.
( ) Turn the chassis over and reconnect the negative lead of the volt-ohm meter to the ground buss.
Switch the meter to read DC volts on the 400V or higher scale."

I was a little confused so just left my ground probe connected to U12, i was getting VERY odd readings and then when i checked U21 i got a big spark and so i stopped reassessed the situation and moved my ground probe to the ground bus.

I rechecked all voltages and this time they were all as expected. Great!

I have now used it for about an hour and all of a sudden the music stopped and i got a loud humming perhaps 50Hz, I have rerun the resistance checks and every thing is fine except between U12 and U13 instead of getting "slowly climbing to 270K ohms" i get infinite resistance and all my now all voltages are now messed up as well.

Looking at the new voltages, my guess that the B+ power stage is stuffed  as there is 0 volts at 1,2,4,5 and I am not sure how to fix it.

Please help / have i killed it?

Thanks

Lachlan.

[Laudanum]

From memory, 12 sounds right (- /striped leg of last PS cap I think) but Im assuming here that you are quoting the instructions correctly.  If that's the case,  there is no mistake in the instruction suggesting clipping the negative probe to terminal 12 for resistance and voltage readings.   I think it is suggested to use T12 (rather than the ground lug bolted to the chassis) as that makes sure that 12 is correctly grounded whereas just clipping to the ground lug wouldnt assure that terminal 12 is grounded.  But I am just assuming that is why 12 is suggested.   Personally I found it an inconvienent place to clip so, again from memory, I believe that I tested 12 then clipped to the ground lug.  Is it possible that your clip lead swung and shorted 12 to 13?  If so, and it ran a while, it may be possible that a component was "weakened" and then gave out.  But, I'll let the smarter folks chime in on whether that's possible and what is most likely if so. 

My meter does strange things on some of the "rising" resistance checks.   You may want to check voltages starting back at the supply ... diodes, first PS Cap etc. and see where you are.   1,2,4,5 are further down the line.

[lachlanralph]

Thanks Laudanum for the info, looks like it was just a loose joint on U1  All fixed and working like a champ now lets hope i dont have too many more of them, the soldering iron that i used was less than spectacular!

Thanks

Lachlan

[Laudanum]

Easy fix, once you found it.  Good to hear.   The usual advice from many would be to go back and re-heat your solder joints.  It's sound advice.  Personally, if I have one cold joint I do inspect them all and touch those up that I think may be suspect but I dont re-flow everything.   Sounds like you got it so any more problems then maybe consider some more solder joints suspect.  In the meantime ... enjoy!!!