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-   -   120Hz hum in my amp, and here is a scope picture. (https://www.diyaudio.com/forums/equipment-and-tools/335717-120hz-hum-amp-scope-picture.html)

warrjon 30th March 2019 11:05 AM

One other thing is a ground loop because the scope will be grounded at the plug and at the scope probe. I was a bench tech for Marantz Aus many years ago and (did my apprenticeship there) and one of the old techs showed me to leave the probe ground off in some situations to prevent erroneous signals.

What BSST said about the input short and output stage load is correct. 120Hz is the 2nd harmonic of the mains.

Post 6 is that the output directly from the siggen? or the output from the amp.

Hextejas 1st April 2019 02:45 PM

Thanks Warr and I will try and get to this. I have been busy rewiring and relocating stuff. The sine curve that you see displayed is from a signal generator from an IPad.

After I relocated some wires and rewound them, it has gotten quieter so let me try the above suggestions.

BSST 1st April 2019 04:27 PM

Maybe you've already established whether the distorted sine wave is a problem in the scope or in the signal generator. If not, a couple of thoughts:

If the displayed waveform evolves as you manipulate scope range switch and/or position knob, it's likely a scope defect. If the shape remains unchanged as you change ranges, probably the IPad. Yet another idea for an independent test signal is to probe a 60 Hz signal within the amplifier's power supply and see if similar distortion presents.

I hope it's only the app. Good luck!

GuitarFogie 1st April 2019 05:24 PM

Hi Gabdx; Just thinking out loud here. 120 Hz is a harmonic of the 60hz. You may want to try another 120vac outlet. Also check if there is any type of motor or fluorescent lighting operating on the same circuit. If you have an uninterruptable (standby) power supply used for computers try it. A surge protector strip/outlet may help out if it is a 60 hz harmonic. Performing in older venues I have encountered many "ghost" signals that we had to work around. Good luck

Hextejas 2nd April 2019 12:34 AM

Well, I have all the issues sorted out and it was a valuable learning tool. I jumped right in soldering and ordering parts before I had any semblance of a plan. I learned that the location of the ground wiring was very important and I needed to pay attention to how close it passed to other components. I think that it would have been a good idea that I map things out 1st and plan the layout. I re positioned the toroid so as to locate the wires away from the input from the mains

Ah well, lesson learned.

K-amps 28th September 2020 04:22 PM

Bumping this old thread. Built a Dartzeel clone 3 years ago abs it gave good service for those 3 years. Now on one channel I am getting an audible 120hz at the speaker output. It will get a bit worse if I short the input ( as it adds a bit of 60hz). The other channel is fine.

Audio comes out fine....

1audio 28th September 2020 06:01 PM

Look for a bad ground connection on the input side of the noisy channel. make sure all the solder joints and connections are sound. Its possible but unlikely a filter cap has failed. It could also be oscillating if something changed with the load.

K-amps 28th September 2020 06:18 PM

Thanks Demian: I added extra caps on the bad side and it subsided significantly.. just audible when I bring my ear near the speaker.

Its a Dual mono topology, so one side PSU caps probably failing. (caps were old/ 30 year old pull outs form an Adcom 545ii ) Will replace all PSU caps with larger units.... I like the sound of this amp. Worth upgrading PSU

Current ones are 10kuF. Adding 6800uF did nothing, per rail. Adding 27kuF did it. Its a Dartzeel kit, semi class A so a lot of ripple...

CG 28th September 2020 09:22 PM

Those caps are beyond their expected life. You've been fortunate to get as much use as you have!

At least you found the problem.

K-amps 29th September 2020 12:42 AM

Found the issue... open solder joint on the Adcom caps (Rubycons)... They still might have some life. Deliberating on which way to go...


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