SX-1250 - no sound on preamp

[ocdecio]

Working on my first Pioneer. I repaired both driver (power amp) cards and that section is working great. Stabilizer card has +-65V.

However when I connect the jumpers on the back, or put a scope on the Pre-Out I get nothing, zilch, on either channel, on any input (FM, AM, Phono, Aux etc). I checked voltages on the flat amplifier and get -25V/+25V no problem. Fuses at the bottom of the unit are OK.

What should I look at first? It is odd to lose both channels, that feels more like a power supply issue but I get the voltages. I also get stereo light and strenght movement when in FM.

 

[Spe@kerBox]

I had that problem on my 1250 and it turned out that when I disabled the tone controls the sound came back. Burned out transistors on the tone board.

 

[avionic]

Are you getting any output from the REC OUT ?

 

[ocdecio]

I had that problem on my 1250 and it turned out that when I disabled the tone controls the sound came back. Burned out transistors on the tone board.

Tried that, nothing. I'll check @avionic 's question.

 

[ocdecio]

Are you getting any output from the REC OUT ?

Yes, loud and clear.

 

[avionic]

With your scope. Check for your signal at pin 2 & 9 of the TONE board AWG-041.

 

[ocdecio]

With your scope. Check for your signal at pin 2 & 9 of the TONE board AWG-041.

In Stereo mode, I see signal (looks faint but it is there) on left channel, nothing on right
In Mono, same signal on both.

Tried with FM and 1kHz on Aux.

 

[avionic]

Flat amp should be your focus.. Have you rebuilt it yet ? AWG-042

 

[ocdecio]

Flat amp should be your focus.. Have you rebuilt it yet ?

No, nothing done yet. It may be one of these cases of "restore to fix" situations. Thank you so much, @avionic !

 

[avionic]

Might just be dirty switches on that board.

 

[ocdecio]

Might just be dirty switches on that board.

Very likely, this unit is filthy.

 

[Spe@kerBox]

Maybe DeoxIt and exercising all the switches is in order.

 

[ocdecio]

Maybe DeoxIt and exercising all the switches is in order.

Doing this now, will recap and check the transistors as I am here anyways, Thanks!

 

[ocdecio]

I am seeing signal at the output of the Flat Amplifier but it is distorted (1kHz). Measuring the -B/+B voltages on it I see +23.18 and -24.41 - would that be enough for the circuit misbehave? I may need to first redo the power supply/stabilizer board before going any further.

Also - I replaced all transistors in the flat amp except for the FET, was wondering if case I need if 2SK30ATM-Y N-Channel 50V 6.5mA would be a proper sub.

 

[ocdecio]

Maybe DeoxIt and exercising all the switches is in order.

I should have known better. After a couple of hours of Deoxit dripping down to the switch shafts I now have a clean signal at the flat amp, one channel out at the preamp but I think I am on the right track. Don't worry, the antenna holder is already broken and I am getting a replacement.

Thank you guys!

 

[Spe@kerBox]

I should have known better. After a couple of hours of Deoxit dripping down to the switch shafts I now have a clean signal at the flat amp, one channel out at the preamp but I think I am on the right track. Don't worry, the antenna holder is already broken and I am getting a replacement.

Thank you guys!

View attachment 2623487

Well done!