I'm trying to fix a hum that appeared on one channel of my Radio Shack/RCA SA-155 integrated stereo amplifier. Does anyone have a schematic for this amp?
Thanks. This is exactly what I needed. Also your advice. Will follow this up.Check the coupling capacitors that should be connected to pins 2 and 6 of the main amp IC (LA4440). I would also check the main power supply capacitors. These may be allowing 60 cycle hum although I would expect both channels to be affectedinthis case.
Thanks Joe. This schematic will help a lot. I just checked the headphones. Good suggestion. The hum is absent! The signal at the speaker is a 120 Hz sawtooth, 0.4 V pk-pk but sounds like 60 Hz hum and is unaffected by the volume control. Pic of waveform attached.You can get the SA-150 schematic and compare the two. You might get lucky.
on edit ... Did you listen using your headphones to see if the hum was still there?
Attached is similar to the SA-150 schematic.
I know this is an old topic, but in the last photo on the left hand side under the ribbon cable there is a blue capacitor and on the schematic that is c125 470p . What type of capacitor is that? There is another blue one at c126 470p and at c121 and c122 that state .0015. I noticed this schematic is for a european market (or somewhere 220 volt 50 hz for power.) I have a sa-155 open ,mine happens to be optimus and looking at that my 4 blue capacitors all have 560J on them and I can't find the exact schematic for mine. So I basicly wonder is the 560J a value marking? And are they polarized? I've never seen a capacitor like those. I haven't worked on alot of stuff but I have worked on the radio shack shortwave receivers some,the DX series stuff 150,160 ,120 ,for instance. I'd like to recap my unit.There are a couple caps that look like the tops are bulging slightly and I also have what I consider a pretty good hum,which for now I'm going to blame on the big power filtering cap. I just thought I'd change out all the electrolytics (not the ones in the preamp section though,i have a different phono pre amp I already use) but I just wonder if those blue ones are something that would need replaced? Thanks in advance (I hope!)Both SA-150 and SA-155 use an LA4440 IC stereo amplifier chip.
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View attachment 214381
Find and replace the equivalent of what is shown as C10, 2200μF/25V in the diagram below connected to pin-11 of the LA4440 IC.
It is labeled as C145 on the board and the schematic.
View attachment 214376
It is the largest capacitor shown here as C145, 2200μF/25V. Check the voltage rating written on the capacitor and replace it with same voltage or higher voltage.
View attachment 214384
by Jake Hertz
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