Plexi SE Dead Power Resistor

[turtle441]

Had a weird failure on a new build today, trying to sort out the cause.

I recently assembled the Plexi SE PCB from gapcb.com

Got everything mounted, did initial testing without tubes and voltages looked pretty good.

Installed tubes this AM, heaters lit up, no red plating, so I threw caution to the wind and decided to plug in.  Ended up playing for 5-10 minutes, and it sounded great.  Turned it off, grabbed the multimeter, turned it back on, and started getting weird readings.  Heater voltage is still right, and voltage at the standby switch is 405V (identical to what it had been unloaded).  That same 405 V is present up to the upstream side of the 270R 5W power resistor.  The opposite side of the power resistor has negligible voltage (something like 90 mV).  Turned off the amp, measured the resistor without de-soldering and it reads open.  I'm left assuming that this 5W resistor just died on me. 

As you can see from pics, no evidence of discoloration or other burned components that I see.  It's easy enough to order another resistor from Mouser, but I want to make sure I dig out any hidden culprits before I just replace the victim and drive on.  So... what typically causes the demise of a 5W power resistor?  Any common culprits I should be looking for?     I haven't built that many of these, so this is a new one for me.

[ginger]

You didn't say if the amp still works normally , just that you found odd voltage readings ..... does it work ?... blowing fuses ?

[turtle441]

No, amp is completely non-functional.  Fuse is presumably intact as there’s an appropriate voltage getting through to both the HT and heaters.  All voltages past the resistor in question (including non heater tube pins) are in the mV range.

[ginger]

Do you have any extra resistors in that range... even a 2 watt.. to quickly parallel with the other with jumpers ?

[shooter]

with the amp OFF and standby switch OPEN, OHM across C2.  does the ohmage climb up til it's really big, or does it stay low, how low?

[turtle441]

Thanks for suggestions .  Unfortunately, I had to leave on a business trip today.  I’ll take the measurements when I get back in town later this week. 

[PRR]

The 270r 5W can't burn-up faster than the 470r 5W in the cathode.

My nickle says a bad solder joint, somewhere to/from the 270r.

Failed resistor is theoretically possible, moreso if your kit supplier was buying on lowest-price. But even with crap resistors and skilled solderers, bad solder joints outnumber failed resistors by long odds.

But see this side-chat.

[sluckey]

If C2 shorts, the 270Ω will burn very quickly and the 470Ω cathode resistor will never know what happened.

[turtle441]

I got the pcb loose today.  C2’s resistance settles in at right around 39M, similar to the other PS caps, so it doesn’t seem like the cap failed.  I checked the underside of the board, didn’t see any scorch marks to suggest any unexpected arcing. 

I pulled the 270 ohm resistor off the board, confirmed that the resistor measures open off the board, so I don’t think we can blame a bad solder joint.  I’m ordering a new resistor and cap (just in case).  Could the resistor have gone out solely from the inrush current when I flipped on the standby?

[shooter]

C2’s resistance settles in at right around 39M

that was measured in-circuit at the R-C junction to ground?

Your inrush voltage is more likely to take out a cap than an R, your amp shouldn't be drawing anywhere near enough current, unless you have something shorting, a 27ohm R instead of 270ohm.  order a 10watter, that way whatever caused the increase in current will be easier to identify

[sluckey]

order a 10watter, that way whatever caused the increase in current will be easier to identify

  I've used that technique before!

This may not be a dissipation/heat failure. It is possible that the resistor just some other type failure. Maybe a manufacturing defect?

[turtle441]

That was measured in circuit between the 2 poles (really the 2 solder pads since it was a radial cap mounted pretty flush with the board) of the capacitor.  In retrospect, I supposed I could have just clipped a lead to ground and only had to touch the one solder pad with the meter.  Wasn't thinking that way when I took the measurements.

[shooter]

I've used that technique before!

WOW, I put you in the group of Service engineers that would never use that method of troubleshooting, you are fallible like the rest of us 

[sluckey]

Hey, I've even used the BIG fuse troubleshooting technique too. Sometimes you just gotta see the smoke to get on the right track!  And sometimes downtime is a lot more expensive that barbequed parts. You just gotta turn all the battle-shorts on and see where the fire really is.

[shooter]

downtime is a lot more expensive

Yup, and I was a salaried guy so the sooner it was fixed the more I made and played 
glad to see you're human, I had you pretty high up the ladder , (still do, just a different angle) 

[turtle441]

As a follow-up, I replaced the power resistor today.  Didn't change out the electrolytic cap.  Everything measure out well, voltages are within reasonable tolerance of what was suggested in original build documents.  Plugged it in and cranked it... sounds great.  So, still don't know why the resistor failed... perhaps a bad part?  For the moment, I'm going to enjoy the amp and wait to see what happens.

[shooter]

wait to see

since it's SE, the amp is making max heat at idle, might be worth measuring Vdc on left and right sides of the new R.  then (leftVDC - rightVDC) / New R value = current.  That should give you an indication that things are good, or not