Help me track down this problem

[damadama]

Pulled my amp out of storage and found it to have a hum/buzz in the left channel.  The buzz scales with the pot position and when I put my finger on one of the RCA jacks it gets REALLY loud.

If I short out the left RCA jack to itself the sound disappears.

Now, I looked under the crack and noticed one of the red LEDs is off.  I took voltage measurements per the manual and found Terminal 1 (and everything connected to it) was showing like 160V.  That seems a bit high.

What is next here?  Resistance measurements?  Something else?  It is possible that the box got shaken up during a move a couple years ago, although I've already ruled out what I think are the most fragile components (the tubes).  I replaced those and the problem persists.

HELP!

 

[Doc B]

Sounds like maybe the RCA jack ground is disconnected or the pot wiper is lifted. The first thing to do is carefully inspect all the connections, particularly ground connections, looking for a loose one. It would not hurt to reflow all the solder joints.

 

[Paul Birkeland]

Now, I looked under the crack and noticed one of the red LEDs is off.  I took voltage measurements per the manual and found Terminal 1 (and everything connected to it) was showing like 160V.  That seems a bit high.

If terminal 1 is that high, either 7 or 9 will also be really high.

Look at the 12AU7, do you see glowing dots on both of the internal triodes of hte 12AU7?  If not, the solder joint on A4/A5 is not doing its job.

I suspect that the noise you're getting could be normal, but you're getting it on the working channel but not on the channel which isn't conducting.

(BTW, does this Crack have a Speedball?)

 

[Doc B]

Yup, PB is probably right on this one. The side that makes noise may just be making the typical sound of an open input.

 

[damadama]

I will check the 12AU7 when I get home.

Didn't think that the functioning channel would be the one making the noise, but that does make sense.

This crack does NOT have the speedball.  I was going to order the speedball upgrade this week, just wanted to get my amp back to known good state before I embark down that path.

 

[damadama]

PB,

Looks like the 6680 I have in there has two glowing tips (I assume the triode).

Voltage measurements are as follows:

1- 150
2- 170
3- 0
4- 170
5- 80
6- 0
7- 151
8 - 0
9 - 105
10 - 0
11- 0
12 - 0
13- 170
14 - 0
15 - 190
20 - 0
21 - 215

a1 - 80
a2 - 0
a3 - 1.5
a4 - 0
a5 - 0
a6 - 150
a7 - 0
a8 - 13
a9 - 9

b1 -150
b2 - 170
b3 -150
b4 - 80
b5 - 170
b6 - 105
b7 - 0
b8 - 0


Resistances all check out except for "13 * will climb slowly toward 270K ohms"
Multimeter just reads O/L...

 

[Paul Birkeland]

Looks like the 6680 I have in there has two glowing tips.

How about the 12AU7?

 

[damadama]

yes, same... two glowing points at the top of the tube

 

[Paul Birkeland]

Very carefully measure the voltage at A7.

 

[damadama]

Voltage at A7 is definitely 0.

The voltage at A8 (the LED next to it) still showing like 13V, while the other one (A3?) is showing 1.5V.

 

[Paul Birkeland]

You can wire in a jumper between A3 and A8 (by jumper, I mean a piece of wire).  The LED that isn't lighting is likely damaged, which can happen from excessive heat or tugging on the leads.  Jumpering A3 and A8 together should get you up and running.