Trace Elliot GP12X Preamp hum

[dpm309]

Well, I have another TE product that is experiencing 60cycle hum through all of the outputs (headphones, XLR, 1/4" output jacks). I sent the schematic to Doug's library: trace_elliot_GP12X_series6_preamp.Looks like someone already tried to repair this as you can see the cable anchors have been removed and replaced. Also looks like new e-caps also. Tried shielding the power transformer with no affect on the hum. The ground lift switch does not reduce the hum also. When I plugged this into a power amp, the hum was very loud until I lifted the ground in the preamp. It was still humming after lifting  the ground but not as much. Any other ideas? This is the 3rd TE amp/preamp I have come across that has excessive hum. These amps are at least 30 years old and was wondering if age has anything to do with this issue.

[shooter]

 
you're the expert now....


these are best in RF but i've used them to "narrow down" my search area in audio
use a phone app to determine what freq the hum 60hz, 120, 400....
ground your scope to the amp chassis
set the scope to measure your found frequency (PT = 1/F) (I think:)
walk the probe over the amp, just above the "hole where all the parts are"
the loop is in the horizontal plane (you can look through the loops hole n see electronics below:)
if you get a "lock" or "hot-spot", circle that area with probe rotated ~~20 off horizontal, "facing" the hot-spot


whether that helps or not is sketchy, learning how to sniff signals, might come in handy

[dpm309]

Sold my Tektronix scope a few months ago but am thinking of getting this: https://www.amazon.com/FNIRSI-DSO-TC3-Oscilloscope-Transistor-Transistors/dp/B0BV9X733J/ref=cm_cr_arp_d_product_top?ie=UTF8It's a handheld digital oscilloscope at a good price and good reviews.

[scstill]

Sold my Tektronix scope a few months ago but am thinking of getting this: https://www.amazon.com/FNIRSI-DSO-TC3-Oscilloscope-Transistor-Transistors/dp/B0BV9X733J/ref=cm_cr_arp_d_product_top?ie=UTF8It's a handheld digital oscilloscope at a good price and good reviews.

Pretty cool that you will get the elusive Spuare wave with this device

[sluckey]

 

[dpm309]

Did not see that. I guess this was lost in translation.