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EAR834p clone with hum. Would appreciate some help

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#86
If what is showing the multimeter is real then the power supply is not good at all , maybe the transistor is damaged , wrongly connected , so on ...
All parts should be checked . What is the DC voltage before the regulator ? On AC is showing the same crap ?
 
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#89
Should be elevated above ground , The schematic is on internet
Anyway the power supply is not good , even if the preamp is oscillating itself , the B+ on the output cap filter after the regulator should be clean . Can't draw so much current ... 🙂
 
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#90
I don't know why but when I read AC on auto I get what you saw in the video, but if I read the AC from the manual ranging instead as Depanatoru suggested I get 0V AC on B+ and 0.2V AC on heaters. So maybe not so bad after all.

I changed all the diodes and the transistor (had extra). No changes regarding hum. Old diodes measured fine too. I measured all resistors still mounted on the board. All measured fine expect the two small ones R22 but that probably because they are still soldered to the board. I don't know how to test the shoke or caps.

I'm using the same PSU as everyone else and all is mounted as it should as far as I can see. Mods on the main board are well known and tested by others. Same goes for the grounding scheme.

This is all above my paygrade. I just thought that following Skunkies videos and Leocoheavens instructions would make everything world fine. Do not have the knowledge to fix this issue. It's not to bad so I can live with it but I am prepared to try out different suggestions.
 
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#91
I don't like how grounds are connected there in back with that screw . The star center should be at the filter caps on the board .
If you have mains ground connected there try without it .
Remember that for no audible hum everything must be perfect , every detail is important , RIAA is very sensitive 🙂
 
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#92
The mains ground is mounted on it's own bolt next to where the star ground is connected to the chassis. I did have the star ground where you suggested at first (that was also the recommended place in the Lencoheaven guide) but I moved it since it was so crowded by the PSU and also inconvenient while moving around stuff searching for the cause of the hum. I also had hum then too. But of course, that was before I fixed the shielding problem.

I will move it back to that place. 🙂 May I ask why this is recommended? It is good to learn the proper ways.
 
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#93
Because a wire or trace on PCB is not a perfect short , there is a little minuscule voltage drop , If diffrent parts of the circuit share a single wire , like you have now from that screw to capacitors ground , can easily influence each other . The most sensitive part can amplify that voltage drop from other currents Or that voltage drop is like a positive feedback and would oscillate .
So wires must go like a star or tree branchesr from filter capacitors positive and negative , that are like shorts for AC , to diffrent separate parts of the circuit that perform diffrent functions and can influence each other
 
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#95
The heaters are lifted.


EAR834 Clone Schematic.jpg
 
#96
There are some common practices against hum:
  • Grid resistors, series 1k resistor directly on the socket, to prevent RF oscillation. Applied at each tube.
  • Ground loop breaker. This is a high current antiparallel diode between signal GND and metal chassis. 10R resistor parallel with the bridge. The earth symbol on the leftmost should be removed.
  • Input ground loop breaker. This is a 10R resistor between the signal GND of the 1st stage (included the RCA common) and the rest of other stages' signal GND.
  • The first 2200uF filter capacitor in the heating supply is too high, resulting in high peak charge current. I'd replace it with a 100uF.
  • Snubber across the output of the rectifier bridges.
  • 2M2 and 3M3 grid resistors seem extreme. I know the 2M2 is part of the RIAA ckt. I would divide the resistors by 10 and multiply the capacitors by 10.
 
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