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October 06, 2022, 07:26:23 PM

Author Topic: Resistors burned [resolved]  (Read 405 times)

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Offline Yahhmuns

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Resistors burned [resolved]
« on: January 06, 2022, 07:50:59 AM »
After completing the resistor check on my S.E.X amp I began the voltage checks but the moment I plugged the amp into the wall I saw a bright light and immediately unplugged it but found these two resistors burnt (one on each of the tube ports) which connect 1L to A12 and 15L to B12 on page 39. They're supposed to be 220ohm but now measure 105 ohm and I can only find 4 of the 6 220ohm resistors which leads me to believe I used the right ones. I'm wondering what had caused these resistors to burn up.
« Last Edit: January 06, 2022, 06:46:59 PM by Paul Birkeland »

Online Paul Birkeland

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Re: Resistors burned
« Reply #1 on: January 06, 2022, 09:13:58 AM »
 I would post some build photos to start with.  I have had this happen to me when I have missed soldering a ground wire elsewhere in the circuit, though I have no idea why that caused those resistors in particular to burn up.
Paul "PB" Birkeland

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Offline Yahhmuns

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Re: Resistors burned
« Reply #2 on: January 06, 2022, 10:27:06 AM »
I tried to post photos in my original post but kept getting errors so heres an imgur links.
imgur.com/a/2IXfYuF

Online Paul Birkeland

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Re: Resistors burned
« Reply #3 on: January 06, 2022, 10:50:06 AM »
If either connection going to pin 11 on each socket happens to make metal-to-metal contact with either of those 220 ohm resistors, you'll get this issue.  Was the glow test successful earlier in the build?

Paul "PB" Birkeland

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Offline Doc B.

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Re: Resistors burned
« Reply #4 on: January 06, 2022, 11:00:38 AM »
Strikes me that 105 ohms is about what you get when you measure the resistance of two 220 ohm resistors in parallel. That is possible as the resistors are connected in parallel thru the tube heaters. Try pulling the tubes and measuring each 220 ohm resistor. They may be reading 220. In which case you need to focus on why they are drawing excess current, i.e., what is dumping current thru them that is not supposed to.
Dan "Doc B." Schmalle
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Offline Yahhmuns

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Re: Resistors burned
« Reply #5 on: January 06, 2022, 01:19:16 PM »
If either connection going to pin 11 on each socket happens to make metal-to-metal contact with either of those 220 ohm resistors, you'll get this issue.  Was the glow test successful earlier in the build?

Yes the glow test worked earlier, and only the heat shrunk wire is connected to pin 11 and makes no exposed contact with the resistor.

Offline Yahhmuns

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Re: Resistors burned
« Reply #6 on: January 06, 2022, 01:26:47 PM »
Strikes me that 105 ohms is about what you get when you measure the resistance of two 220 ohm resistors in parallel. That is possible as the resistors are connected in parallel thru the tube heaters. Try pulling the tubes and measuring each 220 ohm resistor. They may be reading 220. In which case you need to focus on why they are drawing excess current, i.e., what is dumping current thru them that is not supposed to.

The resistors now measure 235 on side A and 195 on side B after removing the tubes, I will replace the resistors since they appear quite damaged. Where should I begin searching for what is dumping the excess current, also how can I check that the transformer and other expensive components are not damaged.

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Online Paul Birkeland

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Re: Resistors burned
« Reply #7 on: January 06, 2022, 02:31:01 PM »
I would take the resistors out and check the DC resistance between pins 1 and 12 on the 12 pin sockets and the following places (you don't need the amp on for this):

1.  The chassis plate.
2.  The blue wire on one plate choke.
3.  The red wire on one plate choke.

This may reveal a short that we can help you debug while you await more resistors.

-PB
Paul "PB" Birkeland

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Offline Yahhmuns

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Re: Resistors burned
« Reply #8 on: January 06, 2022, 03:28:04 PM »
I would take the resistors out and check the DC resistance between pins 1 and 12 on the 12 pin sockets and the following places (you don't need the amp on for this):

1.  The chassis plate.
2.  The blue wire on one plate choke.
3.  The red wire on one plate choke.

This may reveal a short that we can help you debug while you await more resistors.

-PB

From A12 to blue or red choke infinite ohms A1 to blue choke 1.47kohm and to red choke 665ohms, B12  to blue choke is 1.471kohms and to red choke is 664 ohms B1 to blue and red are infinite. A1 and B12 to chassis is infinite and A12 and B1 to chassis varies in the millions. A1 to A12 and B1 to B12 also vary in the millions of ohms. Also I already got replacement resistors.

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Online Paul Birkeland

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Re: Resistors burned
« Reply #9 on: January 06, 2022, 03:36:08 PM »
Whatever you do, don't just put new resistors in and fire things up.  Stand by for a few mins and I'll follow up with some additional instructions.
Paul "PB" Birkeland

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Online Paul Birkeland

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Re: Resistors burned
« Reply #10 on: January 06, 2022, 03:46:54 PM »
OK, so we see 665 ohms between the red choke wire and A1, this is no bueno.  There are four 680 ohm resistors in the kit, so this resistance reading is unlikely to be an accident. 

I would remove the black Solen cap on the A side and post a picture of the wiring under there.

To see about 680 ohms between A1 and the red choke lead, A1 would somehow have to be connected to the high voltage DC power supply rather than the 6.3V power supply.  Where the 10,000uF cap mounts there is an adjacent node that sits at around 400V DC, and those two terminals may be touching or have debris between them (terminals 32/33).  This would give you the resistance issue on A1 but not A12 if you had no tubes in the socket.
Paul "PB" Birkeland

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Offline Yahhmuns

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Re: Resistors burned
« Reply #11 on: January 06, 2022, 04:40:07 PM »
OK, so we see 665 ohms between the red choke wire and A1, this is no bueno.  There are four 680 ohm resistors in the kit, so this resistance reading is unlikely to be an accident. 

I would remove the black Solen cap on the A side and post a picture of the wiring under there.

To see about 680 ohms between A1 and the red choke lead, A1 would somehow have to be connected to the high voltage DC power supply rather than the 6.3V power supply.  Where the 10,000uF cap mounts there is an adjacent node that sits at around 400V DC, and those two terminals may be touching or have debris between them (terminals 32/33).  This would give you the resistance issue on A1 but not A12 if you had no tubes in the socket.

There seemed to be a long lead that I forgot to trim that was jumping 32 to 33, I trimmed it and A1 to red is now reading infinite. Should I put in the new resistors and test now?

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Online Paul Birkeland

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Re: Resistors burned
« Reply #12 on: January 06, 2022, 05:23:03 PM »
Yes, give it a shot!
Paul "PB" Birkeland

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Offline Yahhmuns

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Re: Resistors burned
« Reply #13 on: January 06, 2022, 06:04:33 PM »
It works now but for some reason the left channel is having static even when nothing is playing but music does play through. If I switch the rca jacks it remains static in the left channel and if I switch the speaker cables it goes to the right speaker.
Edit - After trying with headphones it seems does not go out the headphones and after unplugging the headphones the static seems to have gone away
« Last Edit: January 06, 2022, 06:06:41 PM by Yahhmuns »

Online Paul Birkeland

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Re: Resistors burned
« Reply #14 on: January 06, 2022, 06:15:50 PM »
Static is frequently from loose connections, so it if comes back let us know and we can provide some pointers on how to narrow things down.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man