Trouble with Hum

[captouch]

Hi All,

I'm having some trouble with hum on my Crack w/Speedball.

After the Crack warms up, more often than not I get a low (but audible) hum on both channels.  It doesn't increase in volume as I turn up the volume knob, and I can sometimes get it to go away momentarily when I gently hit the chassis.  Sometimes it will come back on its own, or if I unplug and replug the headphones, many times it will come back.  It's not the tubes as it happens with completely different sets of tubes.

Judging by the fact that physical movement (gentle hits) can make it come and go, a couple of days ago, I tried to retouch all the solder joints that looked like they could have been marginal, but it didn't help.

I don't know if this is related or not, but when I have the cans plugged in and hit the power switch, I get a loud crack through the cans.

So I'm wondering if I have a bad power switch?

Any ideas or suggestions?  I'd truly appreciate it as it's greatly affecting my enjoyment level of listening to the Crack.

Thanks.

 

[Grainger49]

I see this is your first post, welcome!  Hang with us, there are hundreds of Cracks out there that are "dead silent." 

No change with volume clears you all the way to the grid of the input tube.  That also clears your sources, good.  Also, since it is in both channels it is probably a grounding problem.  Ground is a long and winding road.  I posted it sometime back and I'll look for that post to bring all the connections over here.

Going away when you hit the chassis means there is a connection, probably solder joint, that is not 100%.  Still, rewetting is most likely the answer.  I'm going to look for the ground points and get back.

For the "Crack" through the cans look at FAQ #1 here:

http://www.bottlehead.com/smf/index.php/topic,2408.0.html

The power switch is Ok.  Just unplug your cans till the Crack has warmed up 10-30 seconds.

 

[captouch]

Thanks for your support and reply Grainger.  I agree with your line of thinking and grounding likely being the issue.  Would appreciate any info on those grounding points when you can find it. 

 

[Grainger49]

Well, crap!  I can't find it.  So here goes from scratch:

The start of the grounding points is terminal 3.  This jumps to the 2 left lugs of the volume pot and back from there to the RCA jacks.  From the top left lug of the volume pot there is a grounding jumper to the two bottom lugs of the headphone jack.  This jack may be different than what is being delivered today.

Also from terminal 3 you go to the center lug of the 9 pin tube socket.  This is the ground route for the LEDs in the cathode circuits.

The power supply ground comes from those bottom headphone jack terminals to terminal 12.  From there it jumpers to terminal 14 and ends at terminal 20.

The heater (AC) supply is a ground wire from transformer terminal 4 to terminal 22.

Ok, so all this means you should clip on to the chassis (which is screwed to terminal 3 the source of all the grounds).  Tracing from the plate outward you should read zero to T3, Volume 2 left lugs, both RCA outer conductors, headphone jack bottom terminals, T12, T14, T20, center pin of the 9 pin socket, and to T22.

Other grounds that should be solid are pin 8 of the large tube, pin 4 and 5 of the small tube, T8, T11, T14, T16, T17, T20, T21 & T22.

 

[captouch]

Thanks Grainger!  I'll try this out Monday and see if I can track down the problem.  Very much appreciate the effort on your part.  Will post results!

 

[captouch]

No luck, unfortunately. 

Powered off, I used an ommeter to check all the listed points against GND and all seemed to have good connections (zero resistance) - except T21, which I wasn't certain was really supposed to be GND based on the reading and manual. 

I also wasn't sure if I should actually be powering up and then taking the readings using voltage mode instead of verifying zero resistance between points - it's a bit physically awkward to keep the tubes and power cord plugged in and have safe access to all those points.  But perhaps that's what's next.  And I guess I should have the HP's plugged in as well so I can be listening for the hum to see what's funky that's going on when the hum is actually occurring.

Still the same symptoms though - low level hum.  When I can get it to go away with the gentle tapping, unplugging and replugging in the HP's almost a sure way to get the hum back.  Not sure whether it's physically jostling what's unstable back into the unstable state or what. 

 

[Grainger49]

While doing the resistance check the Crack should be off and the tubes out.  That probably doesn't make much difference on most readings, the tubes that is.

The Crack must be on to get the voltage readings right.  The fact it goes away with tapping indicates the tubes are probably not at fault.  Maybe retouch the ground wires at the headphone jack.  If that doesn't do it then retouch all the solder joints I listed.

The next thing is to swap tubes if you have them.  But first maybe unplug and reseat the tubes 5 or 6 times.

 

[captouch]

It's been awhile - I took some time off of trying to kill the hum as I didn't seem to be getting anywhere before and was getting a little annoyed at my lack of progress. 

Yesterday, I put back the stock tubes which enabled me to flip the amp/top plate upside-down and still use the wood case as a stand while I poked around with everything powered up.

I took voltage readings and all grounds looked solid.  Then I tried a different approach (don't know if it was dumb or not), but I took a wire, grounded one end, andwith cheap headphones on so I could hear when the hum came and went, I touched wire to solidify/short to GND at all the points in the circuit that were supposed to be solid GND to see if hum disappeared when I shorted any to GND through the wire.  But no dice.

Finally, with headphones still on, I re-probed all GND points again with the DMM.  Finally!  When I semi-accidentally touched the cap lead directly that was supposed to be connected to T12, the hum disappeared.  The terminal itself was solid GND, but there wasn't a solid connection between the cap lead and T12.

After soldering it on well, the hum now appears to be gone and tapping on the amp/case no longer produces any change in sound, pops, etc.  So now a happy camper.

I wanted to thank you Grainger for taking the time and helping me with this.  I really appreciate it sir.

 

[Grainger49]

Excellent!  We have a name for what you did, the chop stick test.  But you now have the meter probe test.  I haven't suggested the chop stick test in quite a while.  I wish it had come to mind earlier in your trouble shooting process.

Now sit back and enjoy the music!