SX 727 Preamp Troubles

[ivandezande]

Fresh recap and replacement of the two 2SA720 transistors on the preamp board and now one channel is out on all sources, and the amp is fine. At zeroed volume, you can hear a little bit out of the dead channel and it goes away as the volume is turned up. Tone controls don't work for either channel strangely.

I have resoldered all joints as it is a double sided board, the two transistors changed are in correctly, and there are no solder bridges or shorted components (which always seems to be my problem).

 

[EastPoint]

Double check to make sure you didn’t snap any wires while working on the board.

 

[ivandezande]

Double check to make sure you didn’t snap any wires while working on the board.

Thank you, forgot to mention I did that. The FM board connections did but go those back on. The tone thing makes me really think the boards being weird.

 

[EastPoint]

Also triple check that your stuff is in correctly.

 

[john stumpf]

if your talking about the AF unit awk-012,thats a double sided board with plated through hole's.you have trouble getting those parts removed? as there are solder runs both sides do you have a solder connection on BOTH sides?

 

[ivandezande]

if your talking about the AF unit awk-012,thats a double sided board with plated through hole's.you have trouble getting those parts removed? as there are solder runs both sides do you have a solder connection on BOTH sides?

AWK-010, yes it's a double sided board and no I replaced all parts just fine and reflowed all joints that I replaced as well as visually inspecting I could see the solder connection to the top side trace.

 

[john stumpf]

just trying to be of assistance

 

[rcs16]

what did you use as a 2sa720 replacement? 50V,500mA,626mW
check bias voltages, compare good/bad channels, inject signals probe with scope, std stuff.
I'll have to look at the design to offer more input.

 

[ivandezande]

just trying to be of assistance

Sorry my post sounded snippy, I seriously appreciate the help! I'm just trying to be specific so others know what I've gone and checked. Thanks again.

 

[ivandezande]

what did you use as a 2sa720 replacement? 50V,500mA,626mW
check bias voltages, compare good/bad channels, inject signals probe with scope, std stuff.
I'll have to look at the design to offer more input.

I tossed in the good ol KSA992. Sounds good, I'll get out the scope and start tracing, I'm just lazy and hoped it wouldn't come to that :/ Thank you.

 

[EastPoint]

As long as you're in there, you might post some zoomed-in pics of the board, both front and back. That was the only thing you worked on, right? So the issue is most likely with that board then?

 

[ivandezande]

 

[ivandezande]

As long as you're in there, you might post some zoomed-in pics of the board, both front and back. That was the only thing you worked on, right? So the issue is most likely with that board then?

I've redone everything, but narrowed it down to this board.

 

[ivandezande]

Deleted

 

[ivandezande]

So.
It's actually just the radio and I'm dumb. On AM, FM, and FM mono the signal is not there on the left channel, just a little on the AM. So what the heck? It's not the selector switch, I cleaned it twice and moving it a little doesn't do anything.