troubleshooting sex build gone bad

[Bad@DIY]

Finsihed build, went rhtough resistance checks everything was good, plugged in, turened on, smoke.

now resistance checks are bad at 3 (301 instead of 880) and 11, (93 instead of 680)

Any ideas on what to replace, rewire, rethink, reconsider or revisit?

 

[Paul Birkeland]

Do you know what smoked?  Can you see anything that's burnt?

 

[Bad@DIY]

Nothing is obvious.  The smell / smoke seemed like it was coming from the 34 / 35 neighborhood, but I had a solid exhaust hood running so it is difficult to say with any degree of certainty.

 

[Paul Birkeland]

Could you shoot a picture showing how the UF4007 diodes are installed in your kit?

 

[Bad@DIY]

here they are.  tough to get a good angle on them.

 

[Paul Birkeland]

The problematic resistances you're describing would likely blow the fuse and cause things to get hot.  There's a 680 ohm resistor that goes across from terminal 5 to terminal 11, then from there the primary of the output transformer connects and feeds over to terminal 3.  You are seeing the difference in resistance between 3 and 11 that looks mostly OK, but terminal 11 being 93 ohms means that there's something wrong around that terminal most likely.  I would carefully inspect for debris that would allow for conduction that could throw off the resistance values like this.  If there isn't any debris or an unclipped wire lead causing this, I can give you some guidance on pulling a thing or two apart to better isolate where the problem may be.

-PB

 

[Bad@DIY]

Darned if I can see anything, let's start tearing it apart.

I'm a knucklehead but it seems like since both of my suspect values are tied to the a side output transformer that I might want to warm up my checkbook)

 

[Paul Birkeland]

I would start by taking the 1000uF cap out of the problematic side, then recheck the resistances and let us know if they change.  Maybe also post some photos of the area with that cap out.

 

[Bad@DIY]

resistance stay the same.

 

[Bad@DIY]

also, do you guys actually work at a location?  cause if you do next time I'm on the peninsula you're all getting doughnuts.

 

[Paul Birkeland]

If you are local, I'm in Renton and I can help you IRL.

 

[Paul Birkeland]

You took the 100uF cap off, not the 1000uF cap.  You can leave the 100uF cap off for now if you like, but take off the 1000uF/35V cap and see if that makes a difference.

 

[Bad@DIY]

sigh.  Thank you for your patience.  sorry about that.

resistances are now 34 m-ohmon both 3 and 11

 

[Paul Birkeland]

resistances are now 34 m-ohmon both 3 and 11

This tells me that one end of the 680 ohm resistor crossing between those terminal strips isn't well connected.  Since it was "missing" when you powered it up, a lot of voltage built up across that 1000uF cap until it failed, then your fuse blew.  You will need to resolder both ends of that resistor and be sure that the black wire feeding ground to it is well connected on both ends.  The 1000uF cap you removed can go in the bin, definitely don't put that one back in.

 

[Bad@DIY]

Replaced blown cap.  Resoldered the suspect resistor.  Reinstalled the 100 ohm cap I removed in error.

All checks passed.

Plugged in, all checks passed.

headphones plugged in(sacrificial grados), MUSIC! 

Thank you Paul.  Pretty excited. 

Channel imbalance at low levels addressable by upgrading volume pot? 

 

[Paul Birkeland]

You can change the pot out or do the mod outlined in #3 here:
https://forum.bottlehead.com/index.php?topic=4295.0

 

[Bad@DIY]

New volume pot installed, much more useable range AND. No channel imbalance.  Pretty happy.  Still getting faint hum in left channel even with new pot so may need to do the resistor mod for sensitive headphones.  But I wanna hook this up to a speaker first!