I know this is a tough troubleshooting topic. So, here goes! This PAs is used with a Dynaco ST70. During the playback of the unit and phono section one day it developed a hum. I had a leftover electrolytic that I jumpered the pins and the hum went away. This was definitely a bad section. The unit never had much definition as it was all original hand built in about 1966 to 68. I ordered a 30-30-30-30 can that fit right in and also I used the VE replacement boards in my PAS 3. All components channel to channel were matched within 1/2 %. Well, the 120hz hum is in the phono, tape and special selector. the hum increases as the volume is increased on all three selector switch positions. the tape, and all other selector positions are dead quiet. The unit never sounded this quiet and certainly the hiss and other sound artifacts are gone. I have jumpered the electrolytic to test for a bad section, swapped tubes and i have some original Telefunkens and they hum too. All voltages are within tolerance and let's call the voltages right on. I have also tried the PA 3 with my Hafler amps and the hum still exists. I have also swapped a solid state pre amp in the the system with the Dynaco ST70 and there is no hum. I am absolutely at a loss of what to do next. Any help will be appreciated.
Troubleshooting over the 'net can always be difficult. But given the hum is only happening with the phono / tape head (which share the same circuit board) at least you narrow the search to the circuit board closest to the source selector switch.
If it's happening across both channels it points to a power supply or a grounding issue.
If it's happening across both channels it points to a power supply or a grounding issue.
The original circuit built per Dyna has no hum, so carefully check all supply ground connections and wiring
to be exactly the same as in the manual. Have you replaced the filament supply diodes and capacitors?
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Rayma, Yes I will post some pictures and the schematic. If I short the Tape Output the hum goes away. Of Course that kills all of the phone, tape and special inputs also. If i use shorting plugs on all other inputs the hum remains. I have my turntable hooked up the the magnetic phono with the ground loop wire attached. I have an ipod attached to the tape amp so all i do is move the front panel switch from input to tape...that way I do not have to rotate the selector switch. Even without any input the hum remains. I have cleaned up a lot of the wiring and there is no change. The selector switch is delicate so I refrain from soldering on it.
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Rayma, Also the original electrolytic was 30-20-20-10. I purchased one that is 30-30-30-30 and the only thing I can think is that it is too much capacitance and it has change the ripple reduction and has changed the DB of that ripple reduction. Lastly, the only other electrolytic option i have is 20-20-20-20. i know the 12x4 rectifier tube really cannot handle much more capacitance. I thought of adding a choke but i cannot figure that out and it was working great before the electrolytic started failing with the old boards.
The only thing changed is the can cap. Still hum remains. Then one must check the new cap, is it properly ground connected ? Is it properly acting as a can cap ? As a last resort i'd buy a new from dynakitparts. When replacing the can cap it is importent to ground boards and input jacks properly.
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The new can is likely to be fine, so either the wiring or connections are more likely.
Was there no hum after you first replaced the filament supply? When did the hum start, after you replaced the two main boards?
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The capacitor you have is fine. The original power supply design works well enough.
There is a problem somewhere.
Can you put these steps in the right order of what you did?
replace old capcitor
replace phono board
replace line board
replace filament supply
hum started
The hum was audible before I replaced the electrolytic can and was dead quiet with the typical heater hiss at a volume setting of 2:00 o'clock, which is to high volume for me to listen too. I replaced the PC5 and PC6 boards and the definition from the line inputs is dead quiet and very transparent. The music is better then they ever have been. When any of the phono, tape or special is selected there is a hum that is consistent in both channels and I can increase the volume, which increases the hum. The hum is amplified by the line PC5 board. If I use shorting plugs on tape amp it kills the hum but also kills the sound to that channel. I have gone component by component checking values and all are in spec. I rewired he unit exactly as shown in the manual pictorials. the only thing I can think of is that the gain is so high in the phono stage that it is picking up this hum somewhere. I even tried tube shields on the phono PC6 board to no avail. I do not have an oscilloscope to check where the hum is being injected into the phono circuit.
There should be two ground wires to the can from the phono pcb already.
Post close photos of the bottom wiring.
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All, yes there are two grounding wires from the PC6 to the can. I read where a person tied the two grounds together and then connected eyelet 10 the to can. I am very careful to make sure things are the same. I have included pictures of the bottom of the chassis. The capacitors on the bottom side are to block DC from the outputs as used in the pas 3x.
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Looks about normal. Are the can capacitor's mounting lugs twisted to lock it tightly to the chassis?
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