SX-1250 Phono1 and Phono2 No Sound in One Channel

[Flip69]

I am working on an SX-1250. When source is set to Phone1 or Phono2 there is no sound in the right channel. All other source settings work as they should. The turntable and cables work fine when running through another amp. What should I check first?

BTW, all electrolytic caps and transistors on the AWF-021 board have been replaced. All switches, pots, and jacks have been cleaned with deoxit. I double-checked to make sure there were no backwards caps, and that the transistors were in properly. Seems like a potential bad wire connection issue. However, all the wires from the board appear to be properly connected.

 

[zebulon1]

Did you use the "Pioneer SX-1250" restoration list?
Was it working before you restored the board?

With the DMM in ACmV's, with the black probe grounded to the chassis you can test with the red probe - the outputs of the EQ board, Pins 1 and 10. Receiver power on; A turntable with a album running and connected, you should see the meter jumping to the beat/music on both pins. This would check the EQ board for output.
Look for it/signal at the input of the EQ board, Pins 4 and 7. It will be really faint - a cheap meter might not see it.
Don't forget to check the wires to their other connections.

 

[petehall347]

one way to find out is first check dc voltages from phono amp .if ok bridge left right outputs phono stage . or better use scope or signal tracer .
or use wet finger test if you are that way inclined .

 

[zebulon1]

Wet finger on pins 1 and 10 then 4 and 7.
Good idea Pete. I use it all the time but never advise it only because I forget about the technique.

 

[petehall347]

Wet finger on pins 1 and 10 then 4 and 7.
Good idea Pete. I use it all the time but never advise it only because I forget about the technique.

has it limitations and place . can be a good go no go starting point .

 

[Flip69]

When I do the testing with the turntable connected and a record playing, should I have speakers connected or disconnected? Volume up or down. Does it matter one way or the other?

 

[petehall347]

using your meter speakers not needed . moderate vol level should suffice . you will see after trying it out on good side .

 

[Flip69]

I ask because I initially took readings with speakers attached and volume at a low level. Touching pin 4 caused a loud hum. The the other pins did not. Because of the hum, I did not feel comfortable leaving the probe on pin 4 long enough to get a reading. Pin 7 had a very low reading, fluctuating in the .001-.002v AC range. Pin 10 was a flat 0. Pin 1 had fluctuating readings in the .010-.040v AC range. Hard to tell on my DMM if the voltage was pulsing with the beat of the record, but it was fluctuating.

If I read the schematic right, pin 4 is the input to pin 1’s output. So, that must be the side that is working. The hum when I touch pin 4 seems inconsistent with that. Is it supposed to be there?

I will check the readings again with no speakers attached.

 

[markthefixer]

AWF-021 phono amp pins 4(lL) and 7(R) are very sensitive inputs
AWF-021 phono amp pins 1(L) and 10(R) are the outputs of the phono amp.

Your finger is injecting signal to locate the "break". Noise is good and indicates that sound will pass through from here.

We are trying to get you to find where the dead area starts. Readings aren't important now. A quiet pin 7 is.

That says there is a problem ON the phono amp board.

Recheck all your transistor placements, and capacitor polarities.

Post pictures of the phono amp so we can check your transistor installations.

 

[Flip69]

Found the problem. The Q4 transistor was not installed correctly. Made the correction and everything works fine.

How did you know from the symptoms and other data that the problem was on the phono board? Because of the apparent lack of signal at the 7 pin, I would have guessed the problem was before the board, i.e., no signal reaching the board.

 

[markthefixer]

Found the problem. The Q4 transistor was not installed correctly. Made the correction and everything works fine.

How did you know from the symptoms and other data that the problem was on the phono board? Because of the apparent lack of signal at the 7 pin, I would have guessed the problem was before the board, i.e., no signal reaching the board.

I ask because I initially took readings with speakers attached and volume at a low level. Touching pin 4 caused a loud hum. The the other pins did not.

Your finger injected signal that when injected at pin 4 caused the loud hum, if the board had been working, touching pin 7 would have had similar results in the other channel.

How did I know? Experience? Training?

In troubleshooting I usually end up taking more time locating accessible probe points than actually figuring out the problem.

 

[Flip69]

So, my finger was injecting signal when I touched pin 7, but it was not causing an audible hum because the bad transistor broke the signal/interfered with the output. Makes sense. Thank you for the explanation.