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PAS 3 Hum
PAS 3 Hum
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Old 12th June 2018, 01:09 PM   #1
Marksd is offline Marksd  United States
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Default PAS 3 Hum

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.
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Old 12th June 2018, 01:54 PM   #2
kstagger is offline kstagger  United States
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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.
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Old 12th June 2018, 02:45 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by Marksd View Post
the 120hz hum is in the phono, tape and special selector.
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?

Last edited by rayma; 12th June 2018 at 02:47 PM.
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Old 12th June 2018, 05:14 PM   #4
Marksd is offline Marksd  United States
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I replaced the filament supply with the silicon diodes and new capacitors. I actually bought a neat little circuit card and it has worked flawlessly.
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Old 12th June 2018, 05:50 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by Marksd View Post
I replaced the filament supply with the silicon diodes and new capacitors.
I actually bought a neat little circuit card and it has worked flawlessly.
Good, can you post the schematics of the new audio pcbs, and some photos?
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Old 12th June 2018, 08:50 PM   #6
Marksd is offline Marksd  United States
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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.
Attached Images
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Attached Files
File Type: pdf PAS 3 Schematic.pdf (304.6 KB, 24 views)
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Old 12th June 2018, 08:55 PM   #7
Marksd is offline Marksd  United States
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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.
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Old 12th June 2018, 09:04 PM   #8
Marksd is offline Marksd  United States
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This is another schematic that I think explains things a little better.
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File Type: pdf Another PAS Schematic.pdf (62.6 KB, 36 views)
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Old 12th June 2018, 09:26 PM   #9
petertub is online now petertub  Sweden
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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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Last edited by petertub; 12th June 2018 at 09:26 PM. Reason: Spelling
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Old 12th June 2018, 11:07 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by petertub View Post
The only thing changed is the can cap. Still hum remains. Then one must check the new cap
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?

Last edited by rayma; 12th June 2018 at 11:12 PM.
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