SX-939 worth the effort?
- Thread starter SpinninAgain
- Start date
john stumpf
Super Member
if exact wattage isnt available use a HIGHER wattage as long as they fit metal oxide is fine and space any that run hot off the board about a third of an inch
In the power supply changing to a metal film / metal oxide from carbon is usually fine, IIRC if the wattage isn't specified it is 1/4watt.
Carbon and metal have different thermal characteristics (temp/resistance) so don't universally replace carbon with metal (for example in power amp stages) unless you are certain that the resistance change at temp isn't part of the circuit design, much like changing electrolytic to film caps there are often reasons that the particular construction was selected (or compensated for) and changing wholesale is not what I'd recommend. Carbon resistors do work just fine, if you want to improve the accuracy use lower tolerance resistors.
Carbon and metal have different thermal characteristics (temp/resistance) so don't universally replace carbon with metal (for example in power amp stages) unless you are certain that the resistance change at temp isn't part of the circuit design, much like changing electrolytic to film caps there are often reasons that the particular construction was selected (or compensated for) and changing wholesale is not what I'd recommend. Carbon resistors do work just fine, if you want to improve the accuracy use lower tolerance resistors.
Yes, metal film is simply a better substrate. It's still a resistor.
Be sure to up the wattage rating by at least 1 if they are getting toasty.
Yep! Value and wattage the same. If it has a dual 1/4-1/2w rating for a 1/4w resistor, that is ok too. The one thing you'd want to do is put a crimp in the leads to raise them off the board slightly. Get them in the air and off the board cools them better. See IMG below......
Yes do the Faderlube on the push switches. POWER SUPPLY 1st. Raising the resistors above the board helps keep them cooler, and also keeps the board from cooking to char as there is now air flow (what little there is due to location and orientation.) Adding additional heatsinking to the plain heatsink is beneficial to reducing heat in the area.
Mark the Fixer years ago figured out the BIAS on the 939 is too high per the manual. PIONEER Engineers goofed on the Math (go figure.....College kids!!!?!??!!??!) 25ma is recommended bias for the 1010, 939, and 838. This will decrease the heat somewhat on the power supply.
This is from the SX-939 Schematic.......
Yes do the Faderlube on the push switches. POWER SUPPLY 1st. Raising the resistors above the board helps keep them cooler, and also keeps the board from cooking to char as there is now air flow (what little there is due to location and orientation.) Adding additional heatsinking to the plain heatsink is beneficial to reducing heat in the area.
Mark the Fixer years ago figured out the BIAS on the 939 is too high per the manual. PIONEER Engineers goofed on the Math (go figure.....College kids!!!?!??!!??!) 25ma is recommended bias for the 1010, 939, and 838. This will decrease the heat somewhat on the power supply.
This is from the SX-939 Schematic.......
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Larry thanks a lot! I will follow your advice. I will update as I progress.
I checked the bias it was set for 20 mV for both channels. I did not change it. Should I change to 25 mV?
Larry: Do I need 8 ohm dummy load to set the DC offset?
Thanks!
OFFSET NO. Set all controls to Center or OFF, NO SOURCE (Disconnect anything on Aux or Tape Mon to make sure)connected. Set Volume to Minimum, and Select AUX Function. Hook up the Meter, then turn on unit. Adjust per the manual.
The 8Ω dummy load is a left over artifact for the line people who did the initial alignment adjustments at the factory.
BIAS. Up to You. There may or may not be more distortion @ 20mv than 25mv(You might see it on a scope,, but probably wouldn't notice it by ear. If it was mine, I'd set it for 25mv and call it a day
The 8Ω dummy load is a left over artifact for the line people who did the initial alignment adjustments at the factory.
BIAS. Up to You. There may or may not be more distortion @ 20mv than 25mv(You might see it on a scope,, but probably wouldn't notice it by ear. If it was mine, I'd set it for 25mv and call it a day
Thank you Larry!
Hi Larry
Is it possible to take out the power amp boards without messing with any wires? Do you know any way to do that?
Bothering you too much!...
Others might chime in but the typical method is to de-solder the pins on the board that have the wires wrapped on them, take lots of photos first.
Jeff: Thanks
Disconnect the amp board from the heat sinks, and frame. The heat sinks for the output's will be disconnected at the same time. Then the whole shebang can be flipped up and on it's back on a towel or other padding, over the tuner board. The service manual has exploded drawings that can help locating screws to undo.
I fully recapped the control amplifier. After recapping I am having a problem. With tone toggle switch off same sound on both channels. When I engage the tone the right side volume muted by almost 15 db! But no effect on volume for left side with tone control on or off. I have replaced the capacitors of both sides with same one. I did not change those few green mylar capacitors.
Please help!!
Please help!!
1st place I'd look is underneath the board for COLD solder joints around the Tone Switch, and Solder bridges in the areas of repair. Use a magnifying glass.
Larry you are spot on! Thanks a lot and learning a lot from you! Meanwhile I recapped the power supply and heat is bit less. Next I will recap the protection board and will change the heat sink transistors in PS board. Do you think I need to change all the diodes and other capacitors in PS board?
Electrolytics, YES. Ceramics, NO! You can lift one leg of each diode and test. If they test ok, you can leave them in. If not, 1n4006 and 1n5406 are good replacements. UF4006 and UF5406 are faster, & quieter with reference to Switching noise. I can't hear any difference between those and 1n4006 and 1n5406 but your hearing may be better than mine.