You could replace the IC
https://www.ti.com/lit/ds/symlink/lm1875.pdf or repair the pins. The DC offset voltage would be an indicator if it's working
Coupling caps like C1 in the above woud be prime suspects. They really should be metalized polyester caps, but for cost reasons electrolytic are used.
Caps like C2 will also affect the gain. It sets the low frequency response.
2.2. met film 5% is $1.23 USD
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There's a big difference in price and tolerence:
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I see a missing tube - that's probably intentional.
Freeze spray may help in this situation.
So can the component tester with an ESR function. Sometimes the capacitance can be way off. You can use your new meter for that/
I'd look at the < 2.2 uf electrolytic caps first after replacing/repairing the AMP IC. Inspect carefully for bulges.
The smaller IC's may be OP amps. They may not have real numbers on them. Sometimes they are hard to identify. The difference between + and - should be close to zero unles s used for other functions. The output offset is something else to check.
Helping someone service something usually doesn't turn out to well. These are general recommendations.