Issue with resistance check terminals 7, 9, B3, B6 [resolved]

[jamesad1994]

Hello all,

I have a new issue with my stock Crack that has been up and running since ~2017. This past week I powered on the amp and within a few seconds, the tubes dimmed - ever since the amp has been unable to operate.

I did a resistance check and noted that terminals 7, 9, B3, and B6 read 0 ohms (should be ~2.9K ohms), so of course I refrained from proceeding to the voltage check. I checked connections and they seemed OK, but I went ahead and removed each of the joints with a solder sucker and created new ones. However, I am still reading 0 ohms at 7, 9, B3, and B6. The other connections in the resistance check are within normal limits.

Feels like bad news. Any thoughts or ideas? I would appreciate the help!

-James

 

[Paul Birkeland]

Could you post some build photos?

 

[jamesad1994]

Hopefully these photos help!

 

[Paul Birkeland]

I would push the red jacket on the wire going to B6 up against B6 so it can't short out.

Can you show me what your meter is reading for terminal 7? I would suspect this is a meter/measuring issue rather than an actual issue with the amp.

 

[jamesad1994]

I pushed up the jacket like you suggested. After doing so, I noticed a spot where the wire was actually touching the adjacent terminal B7 and melted the rubber. Could this be the source of a short? Picture attached.

Here is the reading for terminal 7, which is identical to the other terminals that are supposed to be 0 ohm. In the past I have gotten the appropriate 2.9k ohm reading using the same equipment.

 

[Paul Birkeland]

With the amp off, are you able to measure the resistance of the 22.1K resistors between 1/2 and 4/5?

 

[jamesad1994]

The resistor between 1/2 was 22.38K ohms and the other between 4/5 was 22.48K ohms.

 

[Paul Birkeland]

Now try measuring each 3K resistor.

 

[jamesad1994]

Each of those are measuring about 2.9K.

 

[Paul Birkeland]

Now recheck 7 and 9, as that resistance check is just measuring that 2.9K resistance that your meter does seem able to resolve.

 

[jamesad1994]

I can't say I understand what just happened, but now I am passing the resistance checks.

I moved on to the voltage check, results as follows:

1. 246.1 V
2. 247.0 V
3. 0 V
4. 246.1 V
5. 245.6 V

6. 0 V
7. 1.0 mV
8. 0
9. 0.4 mV
10. 0 V

I could also hear a high pitched humming/buzzing while having the power on. Values seem all over the place. What do you think?

By the way - thank you so much for helping me, PB!

 

[Paul Birkeland]

Are the tubes glowing? Neither the 6080 nor the 12AU7 are conducting, which is generally a problem with heating. This is the twisted pair of wires going to B7/B8 and up to A4/5/9. The fact that neither tube is conducting would point more toward the wiring going to B7/B8 from the power transformer.

 

[jamesad1994]

Correct, no flow in either tube. I went ahead and tied to reflow each of the connections of the twisted wire between the transformer and B7/B8. However, my voltages read the same.
I attached a closer picture of my questionable soldering work at B7/B8. To my eye it looks a bit burned. What do you think?

 

[Paul Birkeland]

B7 is not soldered well enough. More solder and more heat will help.

 

[jamesad1994]

That was it! B7 was the culprit. Thank you PB for your insight and patience while troubleshooting for me. Maybe now I will finally get around to installing speedball now that I have the tools out!