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SX-440 problems

choadrocker

choadrocker

Better as a concept
Subscriber
Hi AK,

I have an SX-440 I bought that was labeled non working. When I got it I did some bench tests and it seemed to be fine, so I decided to go through and recap it and replace the known bad transistors as practice for the many, more complicated receivers I have on my pile

I replaced all the 2SC871 and 2SC870's with a matched set of KSC1845's, double checking the the orientation of each
I replaced all the caps a except for the tuner board
I rebuilt the power supply board

After each component I brought it up on the dim bulb to make sure I hadnt shorted anything, testing with FM along the way

I set the bias to 10Mv as per other threads

In the end I was very happy and went to plug in my turntable and the phono is just dead. Nothing. I realized I don't know that it ever did work as it wasn't something I originally tested.
Aux and AM/FM are fine

I rechecked my work and cant find anything immediately suspect and so I'm not sure where to start.

Any ideas?

Thanks in advance!
 
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Both channels are dead, so I'd start by measuring the voltages. Starting with R321, about 25V on one side and about 15V on the other side, from where it goes to both channels of the phono amp.
 
Are you sure about that? Then the preamp itself won't work properly also if there's only 8 volts instead of 25. Otherwise, disconnect the top of R321 and measure again on the side of the resistor that should be 25 volts. If you do get 25 volts after that, there's a short circuit somewhere in the phono amp.
 

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It looks like my sausage fingers just blew up my right channel live checking bias :bye:
So now I'm ordering more 2SD226's from ebay

NEVERMIND, it was just a fuse!
 
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Tracing that low voltage back to the power supply board
Power supply voltages:
Pin 1: 11.71
Pin 2: 40.02
Pin 3: 10.24
Pin 4: -.09
Pin 5: 0
Pin 6: -.09

I replaced the 2SC968's with KSC2383's and the SD-1y with a 1N4004 and the zener with another 12v .5w zener

https://imgur.com/H5WgZ6W
 
Pin 3 should be 25 volts.
So the fault is in the power supply, or there's a short circuit somewhere in the preamp section that's pulling the 25 volt down, but then Q101 should get very hot.

I replaced the 25-volt section with an LM317HVT so the voltage on the preamp stays at 25 volts and doesn't drop when the amplifier starts delivering more power. This isn't necessary of course, just for fun :)
 
MPXNL said:
Pin 3 should be 25 volts.
So the fault is in the power supply, or there's a short circuit somewhere in the preamp section that's pulling the 25 volt down, but then Q101 should get very hot.

I replaced the 25-volt section with an LM317HVT so the voltage on the preamp stays at 25 volts and doesn't drop when the amplifier starts delivering more power. This isn't necessary of course, just for fun :)
Click to expand...
Do you mean swapping the KSC2383 for LM317HVT on the power supply board? Did you use any sort of heat sink?

I'll check if Q101 is getting hot
 
No, you can't just replace the KSC2383 with an LM317. There's certainly no need for that either, it's just a simple mod I did.
First, disconnect R321, R322, and R343 on one side and then check the voltage at point 3 of the PS board.
If it still doesn't return to 25 volts, check all the components around Q101.
 
Thanks for all your help with this @MPXNL

I lifted a leg of R321, 322, and 343 and now pin 3 is at 17.05

I'll start checking around Q101
 
Now I'm finding the left channel has a lot of distortion and either lights the dim bulb or blows a fuse at full volume.
The scope looks horrible at the speakers, but fine at tape out

I pulled the outputs and swapped right for left and the distortion stays on the left channel, tho now the dim bulb is not as bright at full volume

I have not replaced the 2SA561's but I do have some KSA1013's on hand

It looks like Q313 (2SA561) is what feeds Q309 (2SC734) so I may swap that out for a KSA1013 and see if it helps at all

Does it seem like I'm looking in the right spot?

Thanks again for all the help
 
Measure across the bias trimmers VR3 and 4 with the power off. I believe these are 300 ohms so you should get a reading somewhere in the middle. This could be your cause of your problems.
 
Full power on the dim bulb isn't a good idea.
Especially with a low-wattage bulb, the voltage drops significantly, which also causes distortion.

You can easily measure the current per channel using an ampere meter instead of the two fuses on the back.
This should be approximately 40mA in standby mode.
There's a 150Ω resistor across the bias potentiometers; the center should be approximately 75Ω.
 
choadrocker said:
@Bruno Primas I pulled the 2 bias trimmers and swapped them with the new replacements. The old ones tested fine, but still no change.

@MPXNL Ill check the current per channel next, thanks for the direction
Click to expand...


If the old trimmer intermittently failed, a transistor, fuse, or something else could have been damaged. At least now you've taken the trimmers out if the equation being the problem.
 
I checked the current per channel and it was:
Left: 50.8 mA
Right: 37.8 mA

And now, having changed nothing other than the bias pots, everything sounds fine and the signal looks great at the scope at tape out and on the dummy load, so thanks @Bruno Primas

Everything starts to clip at about half volume, but I feel like maybe I should quit while I'm ahead?
 
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