• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Hum on SX-1250

qguy

qguy

Super Member
Got a humming sound on my SX-1250
Low frequency hum is present on both channels
Low frequency Hum is present even when volume is at zero.
Hum is affected by Bass controls, increasing the bass, increases the hum
Weird sound is experienced when touching the pre-power jumpers
Hum is present even if no source is selected
Balance Control does not affect hum
Hum is removed when pre-power jumpers is removed (so were looking at flat, amp or tone control board)
Hum is present even if Tone controls is off (does that mean the problem is flat amp board?)


EDIT :
Nothing connected except speakers
Upon turn on, no hum, hum starts after about 5 minutes...
Thanks....
 
Last edited:
Register to hide this ad
The ±25V power supplies to the flat amp and other circuits might have AC ripple. The power amps get ±70V supplies and a 65V supply. Failing caps or diodes on the 85V power supply AWR-107 may be at fault.
 
Watthour said:
The ±25V power supplies to the flat amp and other circuits might have AC ripple. The power amps get ±70V supplies and a 65V supply. Failing caps or diodes on the 85V power supply AWR-107 may be at fault.
Click to expand...
+1. check the main filter caps and the caps on the power supply. i seen once with a Pioneer SA-9800 that had a 120Hz hum. replacing the power supply caps fixed it.
 
PS caps maybe?
Regardless the power supply (Stabilizer) should be restored.
Further troubleshooting is needed.
 
The main caps are probably not that bad (yet) because the main amp is silent. The only power feeding the flat/tone/preamps are the ±25V supplies, and they do not touch the main power amp boards.

That would be the first place I would check, anyway.

Zeb is right on it. A complete rebuild of ALL the power supply boards AND regulator board is probably in order. A few dollars in parts would solve that quickly.
 
These are SUGGESTIONS and OBSERVATIONS, I cannot devote the time to manage any repair threads right now.

qguy, re tone control off: well, sort of, it means the tone control is out of circuit.
So it cannot contribute - EXCEPT for wiring problems that may cause hum as the signal passes through. lol sorry...

Interesting set of symptoms, and congratulations for a good write up of the symptoms.

The fact that zero volume does not quash it,. exonerates the portion of the flat amp between the balance and volume controls - which BTW is powered FROM THE PHONO AMP BOARD!!! (+/-65v to +/-49v regulators on phono amp for powering the phono amp and that balance control buffer amplifier stage)

with removing pre/power jumpers killing it points to the preamp YET the +/-65v and +/-25v regulators ALL use the same zener references, then the +/-65v smooth power feeds the +/-25v regulators. Thus hum is darn hard to push through the regulated supply to the preamps, without being at the power amp.It is smoothed once by the 65v regulator, then smoothed AGAIN by the 25v regulator.

edit - if +/-25v is not balanced, pins 8 and 9 of the awg-042 flat amp will NOT be zero volts - there is NO coupling capacitor - and hum could sneak through there with a bad imbalance.

And that power amp design may not be blessed with much regulated power supply noise rejection, so crap on the +/-65v line should get into the power amp signal.

FIRST: have you deoxited all the switches and pots?
Second: are you testing with NOTHING ELSE plugged into the 1250? Only the power cord and speaker cables?
Third: power supply voltage readings - plus you could use a blocking cap and measure any AC present on the regulated DC power lines.
 
Thanks everyone....

markthefixer said:
Interesting set of symptoms, and congratulations for a good write up of the symptoms.
Click to expand...

Thanks MTF

Nothing connected except speakers
Upon turn on, no hum, hum starts after about 5 minutes...
Will clean pots and switches
and get voltage readings....
 
Last edited:
BTW...my unit is 220v

AWR 107 measurements

Pin 1 0.00v
pin 2 0.00v
Pin 3 -0.1v
Pin 4 -0.1v
Pin 5 0v
Pin 6 0v
Pin 7 0v
Pin 8 21.8v
Pin 9 80.5v
Pin 10 -82.5v
Pin 11 0.00v

since you mentioned pin 8 and 9 of the Flat amp
Pin 8 -0.04v
Pin 9 -0.07v

"plus you could use a blocking cap and measure any AC present on the regulated DC power lines." How do i do this ?
 
use your DMM set for ACV (usually <1V) and use a 1uF/ min 100V cap for DC isolation on the positive lead, neg to supply ground, best at the common point ground .
I basically have the same issue with a Sansui G-7500 that uses a sx-1050 PS in it. I need to measure each power supplies ACV ripple.
 
awr-106 voltages (too), my bad, they call the awr-106 a "stabilizer board", and the awr-107 they call the power supply.

Their terminology is inconsistent...

1uf 100v may be hard to find, 0.1uf at 100v will be easier to find and will work.
 
pins Measurements
Pin 1 -72.4v
Pin 2 72.5v
Pin 3 0v
Pin 4 0v
Pin 5 0v
Pin 6 89.7v
Pin 7 24.4v
Pin 8 5.5v
Pin 9 13.8v
Pin 10 24.7v
Pin 11 65.8v
Pin 12 0v
Pin 13 -66.2v
Pin 14 -24.5v
Pin 15 -92.3v

pins With blocking capacitor
Pin 1 0v
Pin 2 0.001v
Pin 3 0.045v
Pin 4 53.6v
Pin 5 53.6v
Pin 6 0.3v
Pin 7 0.3v
Pin 8 0.03v
Pin 9 0.03v
Pin 10 0.06v
Pin 11 0.03v
Pin 12 0.03v
Pin 13 0.03v
Pin 14 0.03v
Pin 15 0.12v

took the board out and saw these..are these due for replacements ?

106%20board.jpg



Thanks for checking !!!
 
Last edited:
Yes, they are due for replacement.

When done, retake the DC and AC voltages like done in post #15 above. The readings seem to be taken properly.
Referenced (black lead) to chassis ground?
Those readings don't have any glaring faults in them, yet the numbers for AC may be higher than expected.
 
Okay thanks. Should I replace all caps on the board or just the ones I encircled ?
Black lead is connected to chassis

Thanks for checkng !!!
 
The board is easy to work as it is removed completely from the set. Do the complete stabilizer list that is posted here in this forum below.
I'm not advocating a complete restoration of the set but the stabilizer should be completely rebuilt with Marks list. This would be the minimum and a good start to troubleshooting the hum issue.

MTF's wonderful 1250 recap and more list

Take your time. Find a good solder sucker and practice on a sacrificial PCB before you work the 1250 boards. Any extra heat will lift the traces on the boards.
The restore of the 1250 is a journey well worth the time.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom