September 17th, 2011, 01:22 PM
Still touching up a friends Epiphone EA28RVT, and Im currently assessing some hum.
The hum is affected by the volume control, as it gets worse when turned up.
Also, with the input jack/input tube grid wire disconnected, the hum is gone. I'm thinking the problem could be with the input jacks, maybe a bad ground. It could also be a greater grounding issue.
September 17th, 2011, 01:23 PM
Also, I just a scope and Im still learning how to use it properly. Anything I can do with the scope to analyze the hum?
September 18th, 2011, 12:41 AM
Is the hum present with nothing plugged into the input jack? If it's gone when the wire between the jack and the grid is removed but there when everything is connected normally, it is clearly the grounding mechanism of the jack that is the problem, Try an alligator clip between the the jack tip connection and chassis. If the hum goes away, clean the jack ground connections or replace the jack.
If the input wire is long (i.e., greater than 2-3 inches), try a shielded wire from the jack to the first tube input grid. I don't thiink a scope would add much to your troubleshooting in this case. Your ears have already established what is wrong; the first preamp tube input circuit is too noisy. Verify the input ground integrity and make the input wiring as unlike an antenna as you can.
September 18th, 2011, 05:44 AM
Yes you could have a dodgy input jack tip switch
September 18th, 2011, 10:18 PM
Havent had much time to investigate yet, although I will some tonight. One thing I did try was disconnect the grid wire, and the hum went away. Must be the jack. Thanks for the responses
September 20th, 2011, 12:28 AM
Still having issues with the hum. Seems now the hum is coming from one of the electrolytics. I previously replaced all the electrolytics with new Sprague Atoms. Tonight I was messing with it more, and I disconnected the last filter cap for the preamp. When I did, a significant amount of hum went away. I jumpered it back to its connection point, and the hum returned. Even though it was brand new, I tried jumpering another known good cap. Still, the hum persisted with this cap in place, and went away when the cap was removed.
I tried grounding both caps at different points, but got the same results.
So to me this seems backwards. Plenty of hum with the cap in place, and much less hum with the cap removed, albeit one less filter stage.
Whats the reasoning here?
September 20th, 2011, 02:31 AM
That's a strange problem to say the least. If the humming goes away when the last cap is pulled, I'd check the DC resistance of the cap that's pulled. The reason is if the cap has arched and there is an internal short the cap will resemble a resistor. This being said, the cap will start sinking current. This added load to the previous caps will make it more difficult for caps to filter noise. I'm not saying this is the case for this particular situation, but it won't hurt to check. First drain the cap bank, after draining, simply remove the + lead of the cap and measure the DC reaistance from the loose lead ground. The resistance should be extremely high. If not replace the cap. Again DRAIN THE CAP BANKS!
September 20th, 2011, 12:06 PM
Ahh, solved the mystery!
There was a ground loop going on that was creating the hum. When I replaced the electrolytics, I grounded the mains filter cap at a PT bolt, although it was originally grounded elsewhere. I figured this would be better, but it created the problem. All that time searching, testing, measuring...and the problem was right in front of me.
Dead quiet now and sounding great
September 20th, 2011, 01:29 PM
Great job. Make note of that! You'll run into it again =P