Amplifier powers up. low humming, and music is barely heard at full volume.

First thing is to determine if sound is from the speaker or possibly the cabinet. If from the cabinet it is likely the power transformer is vibrating the chassis. This sometimes can be cured by adjusting the mounting bolts.
If the sound is from the speaker, then an electronic "cure" is needed. Please note that a very quiet hum is normal but if you can hear it 5 foot away, that is un-normal.

The hum would probably be due to failing component in the power supply. Unles you are adept at electronic repair, best to take it in for servicing.

Yes it almost certain to be the big Electrolytic Capacitors in the power supply. What you are hearing is the 50/60hz mains hum they are supposed to take out. Don't know the amp in question, but this is common problem with all amps. You can't miss these. But be care full they can hold a charge! There might be just one big one or more. At the least 2,000UF. Replace all.

If at full volume with the balance cutting out the right channel you can hear some music on the left (very weak), then the pre-amp is working and it's the power amp that's gone. Vice-versa if you can't anything, but say hum.
If it is the power amp check out the devices on the heat sink.
Check too for broken or burnt parts too!

You need to determine if the power is getting to the audio amp section. For instance with the volume full up is it humming etc?
As a 'rule' it's unlikely that both left and right channels will fail at the same time (unless you have done something silly with the amp) you are looking for something that is common to both channels. Which is why the power section is first suspect. The exception to the rule (above) is where amps use say an IC power amp for both channels. You will find this IC on the big heat sink. Check whatever is on that anyway for damage or burning.
The only other area could be the pre-amp part. Again this could be a single IC controling both channels, so it's failure could cause a loss of sound. By the way if you can hear music/sound coming through when you put it up to full volume the pre-amp will be generaly good.

You need to determine if the power is getting to the audio amp section. For instance with the volume full up is it humming etc?
As a 'rule' it's unlikely that both left and right channels will fail at the same time (unless you have done something silly with the amp) you are looking for something that is common to both channels. Which is why the power section is first suspect. The exception to the rule (above) is where amps use say an IC power amp for both channels. You will find this IC on the big heat sink. Check whatever is on that anyway for damage or burning.
The only other area could be the pre-amp part. Again this could be a single IC controling both channels, so it's failure could cause a loss of sound. By the way if you can hear music/sound coming through when you put it up to full volume the pre-amp will be generaly good.

it sounds like you have the polarity of the speakers hooked up wrong on one of them, try unhooking the new speakers you added and hook them back up one by one, when the music gets fuzzy then reverse the wires for that speaker, this should cure your problem.

I just fixed my bass amp with a similar problem last week: the problem is related to a loose connection on the PCB board (likely).
A bass amp is a rough place for circuitry: the low rumble and power shake a bass amp more than a guitar amp.
In my case, the power transistors needed to be re-soldered and then the grounding bolts and screws needed to be removed and cleaned because the connection was bad. Remember that when two different metals needed for an electric connection are toughing, there is the likelyhood that corrosion WILL happen.
My bass amp did basically the same thing yours did: sounded good at low volumes, but at higher volumes it would act up.
In short, there is nothing you can do (unless you are a repair person): you need to take it to a local music shop and have a tech fix the intermittant contact problem: someone who knows what they are doing should only take an hour at most.

Hope that helps!
-mike

Replace C21 (10uF/50V) and C38 (2.2uF/50V) and it will work great.

The frequencies do not sound right, unless you are in Europe.
Low frequency hum almost always results from:

1) stray input pickup = Damaged cables, poor shielding,
Missing ground connections,
or ground loops.

This pickup occurs at the power line frequency:
50 Hz in Europe and Asia
60 Hz in North America

Harmonics (multiples) may occur as Fourier components
from signal distortion:
50, 100, 150, 200 .... in Europe
60, 120, 180, 240 .... in North America

Sub-harmonics such as 25 Hz are mathematically
impossible to derive from power line frequencies unless
there is some kind of weird inter-modulation, mixing
or frequency beating going on.

2) Defective power supply most often occurs at twice the
power-line frequency because of full wave rectification.

100, 200, 300... In Europe
120, 240, 360... In North America

Except for switching supplies, like those in a computer,
which can produce a high freq whistle at any frequency.

3) Amplifier feed back and other malfunctions can produce
other low frequencies, often by rectifying and detecting
the envelope of very high frequencies in the tens or
hundreds of megahertz. This is called motor boating
and it sounds more like a revving motor boat than hum.

(This kind of behavior is unlikely in a well designed amplifier)

By correctly identifying the noise frequency and
the environment, it is possible to narrow down the culprit:

1) Have you actually measured or observed the frequency
on an oscilloscope ?

If not, do you have the equipment to do so?


2) Where are you, and what is your power line frequency?

=======

If you have line frequency noise, possibly stray pickup,
check the cables, shields, make sure the turntable is
grounded and check for ground loops created by long
runs of parallel shielded cables running multiple grounds
between two auxiliary devices.

If you have twice the line frequency noise, check the
power supply filter capacitors and voltage regulator
chips and/or transistors within the power supply.

Power supply filter capacitors (electrolytic type) can dry out,
degrade and die during long periods of not being used.
They need to be periodically charged to regenerate the
dielectric insulation coating.

Also if you are actually getting line frequency noise
from the power supply, then check the rectifiers.

Full wave rectifier => twice the line frequency
Half wave rectifier => line frequency

A full wave rectifier with a blown diode behaves like half-wave.

From the description of your symptoms, hum is not affected
by input switches nor controls, I would suspect the the
problem is in the power supply feeding the final amplifier,

or perhaps in a broken feed-back loop within the output
amp, causing excessive gain and motor-boating.

A burned out power transistor in the output amplifier
can also cause bizzare oscillations, as the negative
feed-back circuit is trying to balance the DC operating
point of the amplifier but it can't.

Forget the voltmeter. If you have an oscilloscope, check
the power supply ripple, and scope key points in the output
amplifier.

Martin

check faulty or old connections esp rca