Crack: hum and pops, twice the voltage readings of the manual

[ffivaz]

I finished building the crack on Friday. It worked fine for first few hours. Then it began to hum and crack stochastically on the left channel, especially when I gently knock the side of the enclosure. The hum and pops only appear after a few minutes, or it seems when the tubes get hot.

I checked the voltage : I get around twice the readings of the manual : around 200 volts for 1 and 350 for 2. I am not an electrician and thought it was normal since I live in a 230V country But maybe it is not.

I checked all solder joints and reflowed the ones that looked bad. I checked the resistance readings, ok. The diodes are working. And the sound is great, but the hums and pops keep coming...

Could you help me ?

[ffivaz]

Here is a picture of the board !

[Paul Birkeland]

There is a 120V power transformer and a 240V power transformer, which sticker does your power transformer have (or which did you order)?

Your voltages would indicate that you have a 120V transformer on 240V, though I would want to double check that it's not a measuring error.  To do so, measure the AC voltage between power transformer terminals 4 and 5 (black probe to 4, red probe to 5).

-PB

[ffivaz]

Well, I kind of screwed up my measurements (AC...).

6.2 AC between T4 and T5.

In DC, voltage readouts are like that (from the manual). Bold = more than 15% over the "normal" values.

1 - 93 (90)
2 - 199 (170)
3 - 0 (0)
4 - 199 (170)
5 - 93 (90)
6 - 0 (0)
7 - 124 (100)
8 - 0 (0)
9 - 117 (100)
10 - 0 (0)
11 - 0 (0)
12 - 0 (0)
13 - 202 (170)
14 - 0 (0)
15 - 227 (185)
20 - 0 (0)
21 - 252 (206)

A1 - 93 (90)
A2 - 0 (0)
A3 - 1.5 (1.5)
A4 - 0 (0)
A5 - 0 (0)
A6 - 94 (90)
A7 - 0 (0)
A8 - 1.5 (1.5)
A9 - 0 (0)

B1 - 93 (90)
B2 - 200 (170)
B3 - 126 (100)
B4 - 90 (90)
B5 - 200 (170)
B6 - 118 (100)
B7 - 0 (0)
B8 - 0 (0)

Seems to be in order then... I'll try again to see if hum and pops is still there.

[ffivaz]

Sadly, pops are still there 

It takes about 5 minutes, or the tubes to get hot, to begin popping and screeching....

Could it be the tubes ? I sadly have to seek a replacement pair and cannot test it now.

[BNAL]

It could be A cold solder joint. I had this happen to me with my S.E.X. amp. It drove me nuts for a longtime. The amp played fine until it was fully warmed up. I would re heat all your connections.

[Paul Birkeland]

I'd double check the orientation of the caps at the output (the 100uF caps).

This is most likely a bad solder joint, and I'd recommend the chopstick test to localize where you are having issues (poke around with a wood stick, not a pencil, until you can force the popping to happen, then reheat the offending joint).

If you measure the AC voltage coming out of the wall, I would guess that it is closer to 125V than 120V.

-PB

[ffivaz]

Sorry, I didn't answer one of the questions : I am from Switzerland where voltage from the mains is 230 V. The PT3 reads 240 V, as I have ordered, and is thus correct.

As you can see on the picture in post #2, the 100 uF caps are correctly oriented, with striped side attached to Terminal 6 and 10.

Now I'll try to reflow all joints and try the chopstick test.

Thanks for your feedbacks

[ffivaz]

Well, I checked and reflowed all joints. It is still screeching awfully.

I did a chopstick test. It looks like everything around the octal (big) tube socket makes the noise, from touching the 100 uF caps, the 3K resistances to every cable around there.

[BNAL]

Have you checked to make sure that the power tube is seated in the socket securely? From your picture all the connections and solder joints look good.

[Grainger49]

ffivaz,

To assist those helping you, you might want to modify your above post with the voltage readings. 

What is needed are those voltages greater than +/- 15% from the specifications.  Post your voltage and the expected along with the point where you are reading the voltage. 

This keeps the community from having to check every voltage you posted against the expected in the manual.

[Doc B.]

Try cleaning the tube pins.

[ffivaz]

Thanks so much for all your help.

If you look at post #4, I put in bold all the voltages 15% above the values from the manual. About all the "higher" voltages are above the limit, by 20 to 25%.

I checked the sealing of the power tube. Everything seems in order. In cleaned the tube pins. Screeching and popping is still there. Inside the power tube, I saw something strange: one of the pins inside is "larger", as if metal has melted and flowed on it. I posted a picture above. Could it be related?

[Grainger49]

Screeching usually comes from tubes.  Inductors, resistors and capacitors don't screech.  It might be the tube itself, or the contact between the tube and socket.  The only other thing that I see possible is a bad connection.  The chopstick test should have found that.

So we are back to the tube.  If it has run in for 50 hours try replacing it.  The output tube, it is the one with odd readings.

[Doc B.]

Rock the tube slightly in the socket and see if the screeching comes and goes. Then try tapping on it with a chopstick or pencil and see if the screeching changes at all.