Bought used - Loud Humming@0 volume, seeking troubleshooting help! [resolved]

[q2klepto]

Hey all - so i received my used Crack w/ Speedball from a friend in the headphone community. It was packed meticulously - no signs of damage. It is setup for EU 220v, but included a step down transformer for NA 120v.

Tubes glow and look great - but i get a really loud hum.  At first I thought it was the RCA inputs but even unplugged theres a loud hum.  It is present at 0 volume on the pot.

Any ideas of where i should start troubleshooting?

Going to do the voltage check via the Speedball manual once i get home - althought i dont know where each of the terminals actually are - is there a picture i can go by?

Thanks for any help

[Paul Birkeland]

You'll want to start with the voltage checks.  If the terminals on the amplifier aren't numbered, the terminal numbers are listed in the build manual for the Crack.

[q2klepto]

You'll want to start with the voltage checks.  If the terminals on the amplifier aren't numbered, the terminal numbers are listed in the build manual for the Crack.

Thanks - ive also took a crap ton of pics to see if any nice people would spot the issue. Threw up the album on imgur.com ending in /a/PKrwL (cant link external links yet )

The tube LEDs arent glowing, and i do hear music but an extreme loud hum as well

I have a feeling it has to do with the small tube..not all of the points are connected. Does this look correct?

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimgur.com%2FsIaMqMs.jpg&hash=0b5028872d58d5ac02a75051d93bd8a5cc795c1e)

[fullheadofnothing]

The RCA jacks are incorrectly mounted. There should be a plastic washer between the nuts and the chassis, and your friend mounted a nut, then the washer, then a nut. This will not isolate the jack from the chassis, and will cause hum.

[Doc B.]

Yup, I just repaired one with this same issue a week or two ago.

[q2klepto]

The RCA jacks are incorrectly mounted. There should be a plastic washer between the nuts and the chassis, and your friend mounted a nut, then the washer, then a nut. This will not isolate the jack from the chassis, and will cause hum.

Yup, I just repaired one with this same issue a week or two ago.

Great! Thanks - this is to isolate physical vibration from the RCA jacks? It might not be an electrical problem? What do you make of the LEDs not lighting up - are the LEDs on the tubes themselves? I cant tell if theyre actually part of the crack..i guess it might be on the other (top) side

[Doc B.]

Great! Thanks - this is to isolate physical vibration from the RCA jacks? It might not be an electrical problem? What do you make of the LEDs not lighting up - are the LEDs on the tubes themselves? I cant tell if theyre actually part of the crack..i guess it might be on the other (top) side

No. The ground tabs need to be electrically isolated from the chassis at that point to avoid creating a ground loop. Fix that first, then take voltage readings. Compare them with those in the manual and note any discrepancies here. We won't be able to help much more without both of those things.

[q2klepto]

Update: It was indeed a ground loop from the RCA jacks - thanks fullheadofnothing and Doc.

Ive done a voltage check and theyre all inline, except for terminal 7 (slightly high) and 15 where i didnt get any reading I'll have to try again tonight.

Terminal	Expected	Reading
1	75-90	97
2	170	171.8
4	170	171.8
5	75-90	81.6
7	100	118.8
9	100	108.6
13	170	171.4
15	185	?
21	206	212

Now i get a audible noise floor at all volumes, but im thinking its from the tubes themselves. Id have to guesstimate about a 2-3db increase over background noise volume. I dont notice it once i play audio past 630 on the pot.

Are speedballs supposed to be dead silent at 0 volume?

Before i do another voltage check - i can do a resistance check at Terminal 12 + 15...this should register right?