The hum is probably 50Hz, it remains the same with volume up, but increases with bass adjustment and doubles with loudness on.
The receiver is only partially refurbished. I had one fully refurbished in past, and because of the hum I get rid of it. Now, because it drives my “new” older MonoPulse 42A speakers very nicely, I wonder whether there is a chance get rid of the bass related hum completely, and reduce the natural hum to extremely low level. Thank you for the suggestions.
The receiver is only partially refurbished. I had one fully refurbished in past, and because of the hum I get rid of it. Now, because it drives my “new” older MonoPulse 42A speakers very nicely, I wonder whether there is a chance get rid of the bass related hum completely, and reduce the natural hum to extremely low level. Thank you for the suggestions.
So this is the old original 5000, not the A or X, right?
What do you consider "partially refurbished"? Have capacitors on F-1137 been replaced, as well as C013 (note: 35 V seems too tight, fit 1500 or 2200µ if space permits) and C725, C726 on F-1029? 2SC458s (F-1041, F-1040) replaced as a matter of course? (If you need any subs for 2SC693, they should be quite similar, i.e. 2/KSC945 or KSC1845.) Chassis PCB grounds retightened where used?
I noticed some rather unnerving differences between F-1029 schematic and PCB layout, e.g. the power input part is wrong - the actual circuit is (E) --> R739 --> C726 to ground, rather than first C726 to ground and the rest of the circuit following after R729 (which makes no sense).
Speaking of C726, that would be one of my theories regarding a possible source of the hum - it could use a dedicated power ground return wire back alongside +24V to wherever C013 gets its ground from. Yes, that means lifting one leg of C726 from signal ground. (Going to 470µ 35V wouldn't hurt either.)
Power supply regulation strikes me as pretty minimal in general though, considering in what kind of sensitive circuitry e.g. the +24V supply is being used. (The first stage would seem to receive rather less clean power than the rest, in fact.) I wouldn't expect much more than about 50 dB worth of PSRR from a single transistor zener regulator, and that's not all that great. (I might consider coming up with an LM317 based replacement.) Do you happen to have a lab power supply or similar that you could borrow a nice and clean +24V from?
Oddly enough, I have yet to spot any really severe ground loop or shared ground return issues, although the AM IF and FM MPX boards may be candidates for cutting the surrounding ground trace in a good spot and joining the gap with a small ceramic cap in the nFs.
I had a look at the 5000X schematic, and what do you know, its preamp board (F-1350) supplies its first stage from the main RC filtered +24V (rather than the protection board) but circuitry seems identical to F-1029 otherwise - maybe they noticed that the original setup was problematic. A mod along these lines could easily be accomplished - the wire from input side pin "B" would have to be disconnected (cut) from protection board F-1041 and soldered to somewhere on the C726 / R735 / R736 / R739 junction instead.
Note: The input side is the one with header pinout IL - EL - B - ER - IR.
What do you consider "partially refurbished"? Have capacitors on F-1137 been replaced, as well as C013 (note: 35 V seems too tight, fit 1500 or 2200µ if space permits) and C725, C726 on F-1029? 2SC458s (F-1041, F-1040) replaced as a matter of course? (If you need any subs for 2SC693, they should be quite similar, i.e. 2/KSC945 or KSC1845.) Chassis PCB grounds retightened where used?
I noticed some rather unnerving differences between F-1029 schematic and PCB layout, e.g. the power input part is wrong - the actual circuit is (E) --> R739 --> C726 to ground, rather than first C726 to ground and the rest of the circuit following after R729 (which makes no sense).
Speaking of C726, that would be one of my theories regarding a possible source of the hum - it could use a dedicated power ground return wire back alongside +24V to wherever C013 gets its ground from. Yes, that means lifting one leg of C726 from signal ground. (Going to 470µ 35V wouldn't hurt either.)
Power supply regulation strikes me as pretty minimal in general though, considering in what kind of sensitive circuitry e.g. the +24V supply is being used. (The first stage would seem to receive rather less clean power than the rest, in fact.) I wouldn't expect much more than about 50 dB worth of PSRR from a single transistor zener regulator, and that's not all that great. (I might consider coming up with an LM317 based replacement.) Do you happen to have a lab power supply or similar that you could borrow a nice and clean +24V from?
Oddly enough, I have yet to spot any really severe ground loop or shared ground return issues, although the AM IF and FM MPX boards may be candidates for cutting the surrounding ground trace in a good spot and joining the gap with a small ceramic cap in the nFs.
I had a look at the 5000X schematic, and what do you know, its preamp board (F-1350) supplies its first stage from the main RC filtered +24V (rather than the protection board) but circuitry seems identical to F-1029 otherwise - maybe they noticed that the original setup was problematic. A mod along these lines could easily be accomplished - the wire from input side pin "B" would have to be disconnected (cut) from protection board F-1041 and soldered to somewhere on the C726 / R735 / R736 / R739 junction instead.
Note: The input side is the one with header pinout IL - EL - B - ER - IR.
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