Help on car amplifier only one channel works

[mikey5791]

Hi all forum members,
Fyi, i have been working on this old model PCT-4082 (4 channel car amplifier). The owner said only one channel is working.
My initial finding is this amp originally use Toshiba 2SA1943 and 2SC5200 transistor for its output but after opening the top casing i found it used Toshiba A1941 (pnp)and C5198(npn). I desoldered all the 4 pairs transistors and found 1 piece A1941 open(tested "OL" on all legs) and replaced the 1st pair with 2SA1943 and 2SC5200, thats why you can see in the picture it is the bigger size transistor. The other A1941 and C5198 all tested good, so i just soldered them back on board.
When i test run and connect all the 4 channel to speakers, only one channel outputs sound. Then while power still on, i tested voltage on total 6 pieces pin 1 of F4558(op amplifier) and got about 10-12vdc.
I had run out of idea where to check why the other 3 channels has no sound. I will appreciate if some experience member can give me kind insight and guidance on how to solve this missing channel.
Thanking in advance.

[stj]

any hum or anything at all from the dead channels?
did you check whatever drives the base of the output transistors?

[mikey5791]

any hum or anything at all from the dead channels?
No hum or anything from the dead channels.
Is complete silence.
did you check whatever drives the base of the output transistors?

If looking at front side of output transistor, is the base the left side pin, middle pin is collector, the right side pin is emitter?
How do i check what drives the base? By tracing the solder point continuity from base and trace backward...?
Kindly advise.

[mon2]

We are servicing more and more audio equipment weekly for a local music store.

Suggest to create an audio probe. Use any amplified pc portable speaker and place a capacitor of 0.1uf at the tip of the connector. Aligator clips are ok and other side to your meter probe and sharp metal tip. Disconnect the true speakers so you do not exposed to loud sudden audio bursts.

Probe the input source using your new tool and audio on speaker should be clean. Probe onto board till audio stops or is distorted. Be sure to have ground from portable speaker to ground on this amp.

We found a shorted cap with this method in under 15 minutes on a guitar amp last week.

[mikey5791]

mon2,
Thank you for the good tip on making an audio probe.