buzzing with crack

[mencken82]

Hi--

I built the crack and since then I have a loud 120hz hum in both channels that starts about 5 seconds after powering up and remains at the same volume. I think it is 120 hz by comparing against youtube videos of 120hz tones and note 2hz beat when playing 118hz tones.

The hum does not change with adjusting the volume. It does not change with plugging/unplugging the input or shorting the rca jacks. The resistance and voltage checks are posted below. The hum stops immediately when powering off. So far I have

1) resoldered all joints, twice over
2) tapped each solder connection with a chopstick
3) verified grounding per Grainger49's post http://bottlehead.com/smf/index.php?topic=4812.0 [nofollow]
4) shorted the rca jacks
5) replaced the 6080 tube, cleaned the pins
6) replaced the 220uf 250v capacitors
7) plugged it into different outlets in different rooms

Resistance
1,2,4,5 non-zero
3 0
6 2.48k
7 2.98k
8 0
9 2.95k
10 2.49k
12 0
13 climbs to 30k, then jumps to 380k and continues to climb
14 0
20 0
22 0
B3 2.95
B6 2.97
RCA ground 0
RCA center 101-103

Voltages
If < 5mv then I record as 0 (most of the 0s recorded have a small voltage hovering around 3mv)
1 81
2 175
3 0
4 174
5 79
6 0
7 107
8 0
9 106
10 0
11 0
12 0
13 174
14 0
15 195
16 0
17 0
18 86
19 83
20 0
21 216
22 0
A1 79
A2 0
A3 1.56
A4 0
A5 0
A6 80
A7 0
A8 1.56
A9 0
B1 81
B2 173
B3 106
B4 79
B5 173
B6 105
B7 0
B8 0

Any other thoughts? I'm out of ideas. Thanks!

[Doc B.]

You have done pretty thorough job of working out the possible issues. Have you tried replacing the 12AU7?

[mencken82]

I'll give it a shot and update when it comes in.

[Paul Birkeland]

A buzz in both channels accompanied by perfect voltages generally suggests a loose 220uF/250V capacitor. 

[mencken82]

I actually resoldered the capacitors 4 times over. In fact the soldering gun hit the coating of the capacitors and I replaced them. But anytime I tried resoldering I wouldn't get any changes. It could be that each of my soldering jobs aren't good enough so I included a bunch of pictures to get your judgment. I made sure to touch the incoming capacitor leads/other wires instead of just the solder itself and made sure the solder was shiny when finished.

T14 was very crowded and each time I resoldered I hit a wire or capacitor (you'll see some burn marks on wires). So I diverted all the incoming wires on T14 and soldered them to one wire entering T14. It doesn't look pretty but has continuity and hasn't made things worse.

Any thoughts if the transformer could be the source?

[Paul Birkeland]

In fact the soldering gun

Soldering guns are way too large for this type of work, even a $5 40 Watt iron will work way better.  I'd get the proper tool, then deal with those ground wires in the back (redo them so there's an actual ground buss), then also move the white wire leaving terminal 10 to route as it is in the manual, then have a listen and let us know what you hear.

-PB

[mencken82]

I misspoke--I was actually using a small iron.

I fixed it. On the power transformer I connected T4 to 22L and 14U. I removed the connection to 14U and it works. Don't quite understand why this would do it but I'll take it.

Now I can finally work on the speedball upgrade.

Thanks for everyone's help!

[Doc B.]

Connecting to both 14 and 22 created a ground loop. The idea is to use one or the other.