Hot U21-U15 270ohm

[henmyr]

Hi.
The U21-U15 270ohm get's really hot.

* I soldered the "transformer 4 to L22" to L15 instead of U15 because it was better space at L15. Could this affect anything?
I had soldered it to L14, wrote wrong. I have now moved it to U14 as the cable had melted a little because of L13-L15 resistor.

From the beginning:
I've built the Crack (withouth the speedball) but have problems with it.
At first it had problems with the LED from A8 to center flickering and the voltages was a bit low. When the LED flickered, the voltage at the headphone socket rose to >20 Volts and back to zero. Then up to >20 V again at the next flicker.

Then suddenly I could listen too it without any problems, just music, and the A8 LED was glowing nicely.
The next day I moved the amp and the flickering LED was back so I reflowed all solder joints. I also found a leg of the U20-U21 capacitor lose, so I soldered it.

Then the voltages was fine, but the A8 LED was stick flickering slowly.

I've reflowed all joints again more carefully and checked all cables and can't find any visual issue.

The current situation:
* Resistance check is perfect
* The A8 LED glows at first but stops after a few seconds and then flickers slowly.
* The U21-U15 270ohm get's really really hot (I can't keep my finger on it) so I turn it of after a maximum of 10 seconds so that it wont break.
Voltages:
* The voltage at terminal 1 starts at 220 V and then rapidly drops to 0, as to the terminal 2. I haven't checked more since I don't want the hot resistor to fail.

I would be very happy to get some help with this! What could be the problem?

[Paul Birkeland]

* The U21-U15 270ohm get's really really hot (I can't keep my finger on it) so I turn it of after a maximum of 10 seconds so that it wont break.

Don't put your finger on that resistor.  If your voltages check out, that resistor is getting as hot as it's supposed to.

* The voltage at terminal 1 starts at 220 V and then rapidly drops to 0, as to the terminal 2. I haven't checked more since I don't want
the hot resistor to fail.

That's quite a problem, does A6 touch A5 or A7?  If it does, clean up the socket wiring to remove that short.

[henmyr]

Thanks for the reply.
First: I don't see any short between A6 and A5/A7. I tested with my DMM also and it doesn't measure a short.
As the U21-U15 270ohm is supposed to get warm (if I put my hand 5 cm above it, it warms my hand! And it smells really hot too.) I measured all voltages, and they are way off:

Location   Should_be   Mine
1    75-90    1,6
2   170   52,9
3    0   0
4    170   52,2
5   75-90    40,7
6   0   0
7   100   0
8   0   0
9   100   40,7
10   0   0
11   0   0
12   0   0
13   170   50,8
14   0   0
15   185   112,7
20   0   0
21   206   178,5
      
A1   90   39
A2   0   0
A4   0   0
A5   0   0
A6   90   1,4
A7   0   0
A9   0   0
      
B1   90   1,4
B2   170   50,5
B3   100   0
B4   90   39,7
B5   170   50,3
B6   100   40,2
B7   0   0
B8   0   0

[Paul Birkeland]

You have a short in your amplifier.  This should appear on the resistance checks.

If all else fails, you can post photos of your build and we might be able to see it.

-PB

[henmyr]

Here are my resistances.
The only one that is suspect is 13 which does not stop at 270K but keeps on climbing to the maximum 2M reading. Maybe the problem could be there?

Position Should_be mine
1   *   ~2M
2   *   ~2M
3   0   0
4   *   ~2M
5   *   ~2M
6   2.4K   2.48K
7   2.9K   2.98K
8   0   0
9   2.9K   2.96K
10   2.4K   2.49K
12   0   0
13   * will climb slowly toward 270K ohms    ~2M
14   0   0
20   0   0
22   0   0
B3   2.9K   2.98K
B6   2.9K   2.95K
      
RCA jacks      
ground   0   0
center pin   90K-100K   95.2K/103.9K

[Paul Birkeland]

Can you post photos of your build? 

[Strikkflypilot]

The manual says 14, not 15! Transformer 4 to 14 not 15. From the first OP post.
It says so on the bottom of page 19

[henmyr]

The manual says 14, not 15! Transformer 4 to 14 not 15. From the first OP post.
It says so on the bottom of page 19

Thanks for your answer!
I had soldered it to L14, wrote wrong. I have now moved it to U14 as the cable had melted a little because of L13-L15 resistor.

[henmyr]

Here is my build:

[Paul Birkeland]

The broken 5 lug strip is a safety hazard and must be replaced.  It could also be the root of the issues you are experiencing.

No more support can be offered on this build until that is resolved.  You can contact replacementparts(at)bottlehead(dot)com to get a replacement 5 lug strip.  Once that's installed, let us know how your voltages and resistances are and we'll continue the debugging.

[henmyr]

Thanks, I'll do that.

[henmyr]

Hi. I've now fixed the 5 lug strip.

One oddity I've found is that terminal 13 climbs into the millions (1.4M) ohms, it doesn't stop at 270K. Can this give a clue?

Regards,
Henmyr

[Paul Birkeland]

That isn't particularly revealing.  Can you run the amp with just the 12AU7 now and see what kind of voltages you see at terminals 1, 2, 4, and 5.

-PB

[henmyr]

I put in only the 12AU7 and measured them all while I had it started. The U21-U15 resistor did not get hot and both LEDs was glowing without problems. I then added the other tube just to check that the problem had not magically gone away, and it had not. One LED stopped glowing and the U21-U15 got hot again.

Anyway: Here is the measurements with only 12AU7:

Location   Should_be   Mine Only 12AU7
1   75-90    96,4
2   170   223
3   0   0
4   170   223
5   75-90    95
6   0   0
7   100   0
8   0   0
9   100   0
10   0   0
11   0   0
12   0   0
13   170   223
14   0   0
15   185   227
20   0   0
21   206   230
      
A1   90   95
A2   0   0
A4   0   0
A5   0   0
A6   90   94
A7   0   0
A9   0   0
      0
B1   90   94
B2   170   223
B3   100   0
B4   90   95
B5   170   223
B6   100   0
B7   0   0
B8   0   0

[Paul Birkeland]

Check your wiring at B3 and B6.  If you have an error here, you may have a gnarled 2.49K resistor or two on your headphone jack (they will turn a very different color as they cook).