[RESOLVED] Up in Smoke (resistor between 44L and 30L)

[Jamier]

I did this on both ends,actually, because the other side connects to the second 130/2W which has the 90 degree bent leads. Just wanted to be clear about that. I found that this approach reduced the complexity of the repair for me. PB does this stuff all the time so his skill set and confidence level may take him a different direction but, yes, get his approval before you do it.
Also, if you connect your components with solid mechanical connection, as Doc advises, this hook approach makes it a lot easier as unsoldering those locked on bent leads is almost impossible, at least for me.

Jamie

[Doc B.]

Our webhost has been fending off DDoS attacks and there has been some discontinuity of service here and there for a few minutes. That may have something to do with messages not getting through.

[Paul Birkeland]

OK, two connections leave terminal 27 to use the high voltage B+ feed.  One goes to the plate choke, the other goes to the C4S board. 

I would start by popping the red wire off terminal 27 that goes to the C4S board, then run the amp with no 2A3.  You can monitor the voltage at terminal 27 as you power the amp up, it should be about 390V and just kind of sit there. 

If you blew that 130 ohm resistor the point where it's open and no longer a resistor, you can temporarily install a piece of wire in its place. 

[WK3K]

PB,

I removed the red wire at 27L and replaced the blown resistor with a piece of wire.

Without the 2A3 tubes installed, I powered the amp on and got 0.0v at 27. Half of each of the 12AT7's lit up (the side closest to the center of the amp).

Thanks, Witold

[WK3K]

PB - looks like the resistor from 30L to 27L is blown too. No resistance or conductivity across it.

[EDIT] That resistor is sitting very close to the terminal strip and it looks like it could have possibly contacted the ground buss at 29L. I don't see contact, but it is tight in there and is the closest place where I could see accidental contact occurring.

W

[Paul Birkeland]

Either something is touching that shouldn't be or that second 130 ohm resistor is blown and open.

[WK3K]

Hey PB. Yes, the second resistor is definitely blown.

[Paul Birkeland]

I don't suppose you have any other resistors sitting around?  Ultimately you'll want to replace the 130 ohm resistors and be sure that no part of those resistors touches a lug where the striped end of a power supply cap mounts, as that will just blow those resistors up immediately.

With fresh resistors, you'll want to power up the amp with no 2A3 in that channel and that red wire pulled to see if those 130 ohm resistors hold, then we can start adding stuff back.

[WK3K]

Sounds good. I don't have any extra, but it sounds like @Jamien is going to send some over. If that doesn't pan out, I'll track some down.

Thanks again for your help. Now that I've pulled things apart I'm suspicious of that resistor between 30 and 27 touching the terminal strip or the black wire from the ground buss going into 29. I'll report back when I can replace the 130ohm resistors and then measure 27 again.

[Jamier]

Witold,

     Just got home after being gone all day dealing with work stuff. I got your email w/ address and I will send you 6, 130/2W
Tomorrow. I live in South Orange County, CA, so they should get there pretty soon.

Jamie

[WK3K]

Hey Jamie, can’t thank you enough! 

This community is the best.

[Jamier]

Witold,
     Your resistors hit the mail about an hour ago. I emailed you the tracking info, etc.
Let me know when you get them.

Jamie

[WK3K]

Hi PB,

I replaced the 130ohm resistors, re-did all the resistance checks (all WNL), removed the red wire at 27U, removed the 2A3's, and powered the amp on with the black lead of my voltage meter to 8U and the red clipped to 27U. I'm getting 460v.

I guess the good news is that nothing popped or smoked this time... I should mention all my power transformer secondary tests were totally normal too.

Will wait further instructions from you before proceeding.

@Jamier — thanks again for the resistors! They got here quick as a wink.

[Paul Birkeland]

Go ahead and plug in the 2A3, then measure the DC voltages on that 4 pin socket.

[WK3K]

Thanks PB.

A1: 63
A2: 375
A3: 0
A4: 63

D1: 63
D2: 375
D3: 0
D4: 63