Audio Amp Schematic of Radio Shack/RCA SA-155.

Thread Starter

Flyfisher42

Joined Jul 21, 2015
4
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?
 

JoeJester

Joined Apr 26, 2005
4,310
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.
 

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Last edited:

abrsvc

Joined Jun 16, 2018
39
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.
 

Thread Starter

Flyfisher42

Joined Jul 21, 2015
4
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. This is exactly what I needed. Also your advice. Will follow this up.
 

Thread Starter

Flyfisher42

Joined Jul 21, 2015
4
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.
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.
 

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MrChips

Joined Oct 2, 2009
21,634
Both SA-150 and SA-155 use an LA4440 IC stereo amplifier chip.

SA-155 LA4440 Stereo amp01.jpg

SA-155 LA4440 Stereo amp02.jpg

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.

1597005344303.png

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.

1597006953469.png
 

Audioguru again

Joined Oct 21, 2019
1,733
It produces 6W + 6W with horribly high distortion into 4 ohm speakers. It produces only 3W per channel at low distortion into 4 ohm speakers or about only 1.6W into 8 ohm speakers.
 

JoeJester

Joined Apr 26, 2005
4,310
That 120 Hz signal is typically the output of the full wave bridge and the filter capacitor . I would check the capacitor Mr Chips recommended. If the ripple is NOT present on the filter capacitor, I would proceed elsewhere.

I am curious if you turned the speakers off when you tested the headphones? I find it strange the hum wasn't present in the headset.



You mentioned the hum is in the left channel only.

I would check in the following order.

Remove the speaker cable and measure the output signal (and ground) to see if the left channel still has the ripple voltage. If it is still present, and not present in the right channel, Remove all input cables from the amplifier and see if the hum disappears. If it is absent, look for a connection integrity problem (shield not properly connected).
 
Last edited:

Audioguru again

Joined Oct 21, 2019
1,733
I think the output coupling capacitor (1000uF) on the bad channel is shorted and allows the bad output to draw 6V/4 ohms= 1.5A through the speaker along with a lot of 120Hz ripple. Then the hum is only on that one bad channel speaker but not on the headphones.
 

JoeJester

Joined Apr 26, 2005
4,310
The capacitor on pin 9 is the decoupling capacitor, it, is 100 uF on the schematic and worth checking.
The capacitor on pin 5 is the decoupling capacitor, it is C149, a 470 uF on the schematic.

I still have difficulty believing the headphones were without hum .... Maybe the thread starter can check BOTH channels with the scope to see the DC voltage and Ripple. With the speaker switch on and off.

On edit .... fixed my error.
 
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MrChips

Joined Oct 2, 2009
21,634
I forgot about the part where the hum is only on one channel.
AG has the right idea. The hum is not heard on headphones because the total current draw is low. If the hum is on one channel and not on the other then the problem is not likely to be with C145.
 
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