Final Voltage Fail: Blown capacitor; Power Board Smoke

[rsmiii]

Bottlehead S.E.X. 4, Manual Rev. 1.3 @ page 114:
Failure on final voltage check.

The 220uF/63V capacitor between 12U and 15U blew – a second time.
Impressive hiss, then geyser of steam and smoke.
And, smoke which appeared from center of the power supply board.
Quickly terminated power and allowed to cool down.

Resistances on Manual Rev. 1.3 @ 110 checked out;
Noting, I was getting about 1.66KΩ on both 18 and 38 [Specified Range = 1.45K-1.55KΩ]
Noting further, tube socket "A" is missing pins 4 & 6 [unused in present configuration].

I plan to start from the beginning of the manual, and see if anything is mis-wired.
Grateful for any probable-cause suggestions, to focus my inquiry.

 

[Jameson]

I tried to assemble my SEX 4 very fast when I was at Bottleheadquarters since I didn't have my soldering equipment out of storage at the time and only had the weekend to assemble. I blew that same capacitor between 12U and 15U. PB noted I probably lost the ground reference on the 1M grid leak or the ground wire to the toggle switch wasn't 100%. I think I ended up cleaning up the terminals at 12U and 15U because the capacitor and the resistor share those terminals and in my haste I hadn't made a good enough connection.

You could post some photos around that area as well.

 

[rsmiii]

Jameson – thank you – please, what is the "1M grid leak?"

 

[Jameson]

The 1004F resistor connected at the rear of the switch and 26U

 

[rsmiii]

Jameson: thank you. Here are two photographs, of the DPDT Toggle area, as of March 4, 2026:

 

[Jameson]

So I'd probably reach out to replacement parts to get a new cap, then I'd spend some time cleaning up 12U and the back terminals on both sides of that switch. Those switches are a little tricky because you don't want to melt them but I think they could use a little more heat.

Note that I'm trying to get better at providing tech support but that PB might have a different take or some other ways to improve the situation.

 

[Paul Birkeland]

I see a whole lot of solder joints on there that aren't adequately flowed out and won't work properly.
https://www.google.com/search?q=bot...ate=ive&vld=cid:ea380b66,vid:EN16Pi7pcfk,st:0

I see bad solder joints on the 6 lug switch and terminal 12 in particular that will cause this exact issue.

 

[rsmiii]

Thank you, Paul. I will address comprehensively.