2w Sub Miniature Express

[UR12]

Sure. You might also recall I have a spare MOSFET... I am always happier with doubles & spares... removing it now. Would you like me to install the spare rightaway?

No with it out we can add a jumper and just bypass the HV part of the VVR and run the amp as if it were on full power all the time. But before you get to that I want to see what the voltages on the board look like with the mosfet out and the caps connected.

[MCK]

Hi. Mosfet out and B+ re-connected back to the main board. Ground to B+ reads 97V DC again. Still not 146V DC as it used to.

PS. Any easy way to get those 3 legged buggers out? Despite solder wick taking all solder from the pads they just won't budge.

[UR12]

Hi. Mosfet out and B+ re-connected back to the main board. Ground to B+ reads 97V DC again. Still not 146V DC as it used to.

PS. Any easy way to get those 3 legged buggers out? Despite solder wick taking all solder from the pads they just won't budge.

I use a solder sucker. It is still 97 because we haven't added the jumper to get around the mosfet. If you add a jumper from B+ where the red wire connects to the board and solder the other end to the leg of the 10 ohm resistor (near the mosfet) that is closest to the zener diode. Is it still blowing fuses?

[MCK]

Don't know the answer to the blowing fuse question. Was down to my last so I wasn't risking it and I was keeping each power-up to a few seconds just to get the readout.

Ok. Jumpered B+ and powered it up. Loud crack & arc between the positive leg and ground (I think) of C11, the largest cap. The arc occurred on the lower side of PCB and left a dark stain there. I am guessing that was not a good sign...

Oh, and my last fuse is now gone... Will need to use a single wire strand method to continue testing.

[UR12]

Don't know the answer to the blowing fuse question. Was down to my last so I wasn't risking it and I was keeping each power-up to a few seconds just to get the readout.

Ok. Jumpered B+ and powered it up. Loud crack & arc between the positive leg and ground (I think) of C11, the largest cap. The arc occurred on the lower side of PCB and left a dark stain there. I am guessing that was not a good sign...

Oh, and my last fuse is now gone... Will need to use a single wire strand method to continue testing.

It sounds like you may have had a bad solder joint there and it may have arced. Either that or you may have a bad cap. Try resoldering the cap and then check it again. After you get it resoldered, before you try applying power use you meter on ohms to check for a short on the cap+ to ground. If it kind of charges up and doesn't read a constant short then you can apply power and measure what you have at B+ on the board

[MCK]

Ok. Took C11 out, measured it and found it is still reading 100uF so assume it is good. Then cleaned the contacts, resoldered it paying a lot of attention to the connection. Powered up, same thing. Loud crack & arc. Same place.

[MCK]

Dana, posting photos just to make sure I jumpered the right place and to show you where the arcing occur. Hope these help. Thanks for your strong support.

[UR12]

Try just leaving C11 out temporarilly and see if you still get an arc

[MCK]

Yes.!!! Still arcing there. So PCB has a short or near short. Let me see if I can clean it up as best as I can.

Update : upon close inspection I realize right next to the C11 + terminal solder pad PCB fiberglass area has turned carbon. Trusty Dremel to the rescue... I cleared the extension of the ground copper trace around that pad and opened the gap as much as possible. Cleared the carbon in the PCB. Similar action to a dentist clearing decay out of a tooth. Then comes some nail polish from my daughter to paint over the exposed edge of the ground copper trace. Yikes, turns out she gave me one with metallic specles in it. That won't do. Clean that up and find another one. Black. Waiting for it to dry... Things one get to do in the line of troubleshooting...

[MCK]

YEEEEESSSSS.... Dental surgery over. Black porcelain crown seems to have sealed the decayed area well. Powered on without C11. No arc. Ground to B+ now measures 130V DC ! Feel like we are on solid ground. What next? Install the cap back? It measures 104uF and has about 1MOhm resistance. You think its ok? Below two shots to document the surgery in case anyone else needs to do something like this.

[UR12]

Yes, re install the cap and check your voltage. I have to run out for a while. If you reinstall the cap and the voltage comes back to about normal (147) then you can try removing the jumper and reinstalling the mosfet. Just a note if you decide to try putting the tubes in and firing it up with the jumper. The power control will still be varying the bias voltage so make sure the power is on max so the bias will be at max.

[panrixx]

So what has actually changed since the first sign of trouble? I've got a bit lost with the parts being disconnected, jumpered and reconnected!

[mdd1959]

So what has actually changed since the first sign of trouble? I've got a bit lost with the parts being disconnected, jumpered and reconnected!

I think that MCK as found that there was a shorting path around C11 because of some "build-up." Could be flux or something that caused a short.

[MCK]

Yes, re install the cap and check your voltage. I have to run out for a while. If you reinstall the cap and the voltage comes back to about normal (147) then you can try removing the jumper and reinstalling the mosfet.

Thanks for that. Just woke up and proceeded with the test with cap on. Ground to B+ measured 148VDC !!! Yes! I also left it running for about 30 seconds for good measure to see if the fuse pops and none of that! I think we're good. Proceeding with mosfet reinstallation.

So what has actually changed since the first sign of trouble? I've got a bit lost with the parts being disconnected, jumpered and reconnected!

I think that MCK as found that there was a shorting path around C11 because of some "build-up." Could be flux or something that caused a short.

I think you're absolutely right. I used flux liberally being a newbie and thought I could clean it all up with isopropyl alcohol. Seems like I was not succesful. Carbonized flux creating a conducting path of sorts. I guess this explains the slow fuse blow. Not sure whether mosfet was blown in the first place or not. I hope it wasn't and I hope that had nothing to do with the arcing & carbon buildup because if so we still might have a problem as in needing to findout why the mosfet gave. Anyways, one thing at a time.

[Structo]

Are you saying you used additional flux besides the flux inside the solder wire?