Recently acquired another SX-626 (1973 version). After checking it out I powered it up, there was a noticable hum in the tuner, nowhere else. After I completely recapped the unit (including the AWE-031-0 tuner assembly) and set the bias to 18mV the hum remains. Any ideas?
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If you had a bad diode in the power supply (that supplies the +13v rail) D5,D6 or a blown fuse ( FU on green wire & FU on green wire) that would do this.
1n4004 (generic number) so one could be: 863-1N4004G
1n4004 (generic number) so one could be: 863-1N4004G
I pulled D5 and D6 and checked them. D6 tested fine, at first D5 was unrecognizable according to the Atlas DCA55. Upon retest, it did test fine. I replaced both with new OnSemi 1N4004G diodes. It seems like the hum has diminished some, still there but it is better. Still not as quiet as it should be, however.
D1 to D4 would be possible culprits. might as well check them.
Check Q3 emitter base and collector voltages to be sure the +13v power supply is in regulation range.
then check awr-029 power supply pin 3 and pin 5 voltages to see if the +30v power supply is in regulation range..
Check Q3 emitter base and collector voltages to be sure the +13v power supply is in regulation range.
then check awr-029 power supply pin 3 and pin 5 voltages to see if the +30v power supply is in regulation range..
Replaced D1 - D4 with Vishay UF5408 1000v 3A Rectifiers. Hum is still present. Checked voltages as follows:
Pin 3 should be 57V - tested at 58.31V
Pin 5 should be 30V - tested at 28.52v
Pin7 should be 13.5V - tested at 7.33V
Pin 10 should be 13V - tested at 4.89 V
Pulled D7 Zener Diode WZ-140 tested good, replaced with ON Semi 1N5243B 13V 0.5W Zener Diode. Hum still present.
Both Q1 and Q 3 were replaced with KSC2073TU and Q2 replaced with KSC2373YTA. I pulled Q3 just to double-check, tests fine.
Pin 3 should be 57V - tested at 58.31V
Pin 5 should be 30V - tested at 28.52v
Pin7 should be 13.5V - tested at 7.33V
Pin 10 should be 13V - tested at 4.89 V
Pulled D7 Zener Diode WZ-140 tested good, replaced with ON Semi 1N5243B 13V 0.5W Zener Diode. Hum still present.
Both Q1 and Q 3 were replaced with KSC2073TU and Q2 replaced with KSC2373YTA. I pulled Q3 just to double-check, tests fine.
UF's are going to buzz more, and you need good snubber caps (c1,c2,c3,c4,c7,c8) across them
pin 10 voltage, measure it at R8 C9 connection, check that AC 8 and AC 9 really have AC current flowing through them, pin 7 and pin 10 voltages are low.
pin 10's actual voltage is a draw through R8 a 560 ohm resistor and THAT load drop through R8.
measure voltage across R6 (22 ohms) and voltage across R7 (5.6 ohms). Read volts from one side of the component to the other. Two readings.
the 13v regulator may be being strangled.
pin 10 voltage, measure it at R8 C9 connection, check that AC 8 and AC 9 really have AC current flowing through them, pin 7 and pin 10 voltages are low.
pin 10's actual voltage is a draw through R8 a 560 ohm resistor and THAT load drop through R8.
measure voltage across R6 (22 ohms) and voltage across R7 (5.6 ohms). Read volts from one side of the component to the other. Two readings.
the 13v regulator may be being strangled.
The hum is a bit reduced, I am starting to wonder if this could be a shielding or ground issue? When the bottom cover is off, the Phono settings have a small hum, once the cover is put in place it goes away. I do have another SX-626, I thought I could compare the two. However, the other is a 1971 version which has the tuning capacitor off the circuit board. If anyone has the 1973 version I am wondering if there is a cage or cover over part of the tuning section? It looks like there could have been a cover at one point in time. I will try in the meantime to fabricate something. The receiver is completely done, sounds very nice (except for the tuner hum), and looks great as well.
Appreciate all of the help so far! Stay safe, Happy New Year, 2021 will be better!
Appreciate all of the help so far! Stay safe, Happy New Year, 2021 will be better!