burnt out resistors

coors · 4555

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Offline coors

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on: March 20, 2013, 04:45:59 PM
Hi
I started to experience microphonics through the headphone plug then some loud popping sounds.
When I checked the unit the 2 resistors , 2.49 k value ones, where brown and burned up.  I did not see the message to connect the unused terminal tips to ground until recently.
Do all I need to do is replace those resistors and then ground the unused terminal tips or am I missing something else.
The music still plays fine through it before I turned it off to troubleshoot
Thanks
Sam


Offline Paul Birkeland

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Reply #1 on: March 20, 2013, 05:33:09 PM
That is a very, very bad sign.  How do the 100uF capacitors on 6/7 and 9/10 look? 

I'd do a quick voltage check and let us know if there are any anomalies. 

Paul "PB" Birkeland

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Offline coors

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Reply #2 on: March 21, 2013, 03:40:02 PM
The Capacitors look fine, no bulging or lengthening.

Voltage check, everything checks out fine
The higher voltages were
T15=207 volts
T21=234 volts
I suspect this is fine

On the ring terminal to the TRS headphone jack I get 0V-12V-0
Whats next?


Offline Paul Birkeland

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Reply #3 on: March 21, 2013, 04:09:48 PM
Quote from: coors on March 21, 2013, 03:40:02 PM

On the ring terminal to the TRS headphone jack I get 0V-12V-0


Can you clarify those measurements a bit?

Paul "PB" Birkeland

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Offline coors

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Reply #4 on: March 21, 2013, 05:32:33 PM


Final check


Offline Paul Birkeland

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Reply #5 on: March 21, 2013, 05:35:25 PM
Hmm, it sounds like the amplifier is operating properly, can you post a pic of the resistors?  They really shouldn't turn brown unless the coupling caps short, the 2.49K resistor dissipates a paltry 0.06 Watts for less than a second during startup, then much less than that.

Paul "PB" Birkeland

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Offline coors

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Reply #6 on: March 21, 2013, 06:21:27 PM
I did measure them and the top one measures
2.45 and the bottom 2.44


Offline coors

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Reply #7 on: March 21, 2013, 06:23:17 PM
I forgot to add that the top resistor, if you look closely has a ellipiitcal defect in it. This was caused by me gently probing it and a piece chipped off

Sam


Offline Paul Birkeland

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Reply #8 on: March 22, 2013, 06:34:38 AM
Having burned a few of those 2.49K resistors in non-Crack related experiments, I can attest that those resistors are not burned, though they do have a strange oxidized look to them.

I wouldn't worry about it.

-PB

Paul "PB" Birkeland

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Offline adamct

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Reply #9 on: March 22, 2013, 07:30:37 AM
I suspect this is what happens when you smoke your Crack...  ;D ::) :P :-X :'(

[Ducks to avoid flying pieces of rotten vegetables]


Offline coors

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Reply #10 on: March 22, 2013, 09:14:58 AM
Any thoughts on what would then be causing
The problem?   I know it is not the headphones since I also have a SEX amp and do not have the same problem


Offline Paul Birkeland

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Reply #11 on: March 22, 2013, 01:28:34 PM
It looks more like a chemical deposition, like solvents from paint or super glue.

Paul "PB" Birkeland

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Offline Doc B.

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Reply #12 on: March 22, 2013, 01:34:18 PM
I agree with PB, they look like they got coated with flux smoke or super glue vapor rather than looking burnt. The joint where the two resistors join together at the jack terminal is saturated with flux. I would be inclined to reflow that joint and get a good shiny solder fillet in the joint. It could have something to do with the filmy coating on the resistors.

Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline adamct

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Reply #13 on: March 22, 2013, 03:21:31 PM
If the problem is microphonics and popping sounds, isn't it more likely the problem is the tube, rather than anything with the amp? (Though I agree about the caked-on flux.)


Offline Grainger49

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Reply #14 on: March 23, 2013, 01:30:56 AM
Coors,

The resistances you read on the headphone jack are good.  What solder are you using?  Is it a Kester or a "designer" solder?


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