Have Hitachi Dynaharmony HMA8300 ad HCA8300 pre-amp. When unit has been on for around an hour the sound starts to deteriorate (like you would hear from a really badly tracking stylus on vinyl covered in dust) and eventually the safety overload switch kicks in. Sometimes smells as though it is running too hot. If I leave to cool down the amp/pre-amp is ok again for a little while. Problem started manifesting itself about 9 months ago when I crank the volume up. These amps just keep going till they destroy the speakers without any loss of quality of sound. It would get to about one third power output (75 watts RMS) and would trip out. Leave alone for a few minutes and all was OK. It is 31 years old and I have had it from new so obviously something is getting tired inside. There is still very little that comes close to the sound of this monster in full flight when you can see glass rhythmically bowing to the music in window frames with the volume turned around half way!

No the pre-amp just gives you enough volume to hear it, the main amp takes it higher. However since the sound is normal on earphones. Then it is the headphone connection that is faulty. The socket side, not the headphones themselves. When headphones are placed in a unit, the plug breaks the contact with the internal or external speakers. But when it's removed it should restore the sound to the speakers. But your unit is not doing that, so there is a bad connection on the socket.

If you have disconnected the cartridge and touched the leads in that state and you had no sound out of the right channel, then there is nothing wrong with the cartridge at all. You either have a bad lead from the deck to the amp, or a bad connector to the amp. If replacing or fixing the lead does not correct the problem, then you have to look at the pre-amp in your amp as the cause. The amp will have a dedicated magnetic cartridge pre-amp. It's failure on the right channel will not effect any other function, such as tape or radio. You can easily find the cause, by tracing the phono connector wires back (inside the unit) to the first port of call on the PC board. You will either find a single IC doing most of the work of the pre-amp or two IC's, one for each channel. If you are lucky and a single channel IC does the work, replace the right hand one. Otherwise replace the dual channel one.

If you think it's the cartridge, then remember that putting the left channel connector on the right input of the cartridge should see no sound. But if you hear sound by switching the cartridge connections the cartridge is good.
I have noticed that you have tried another amp with the deck and it made no difference. This implies then a faulty lead between the deck and amps.

I suspect that the pre-amp stage in you amp has failed. What you are hearing is the power amp on full volume. But with no pre-amp to boost the signal it's to low.
What we do know is that the sound is going into the pre-amp and out for you to hear it. This could mean that the power to the pre-amp stage is low, but still present.
Pre-amps these days mostly consist of a single IC. That could have failed, or it is not getting power. Since it would be possible to test the voltages to the IC, one pin should have something on it to the region of 6 to say 20 volts going into it. If the IC has that going to it, it has failed. If the voltages to all the pins are low, then you have a power problem.

The pre-amp has failed. Since you can hear the signal with the deck plugged into the amp that proves the cartridge and deck are working.

Firstly the cartridge could be faulty or incorectly wired. I think what you are refering to is a magnetic cartridge pre-amp. If the turnatable has seperate audio connectors these should be plugged into the "phono" socket. And not an "aux" or "CD" input. If there is no phono socket, you wouldn't be able to plug a pre-amp in anyway. Yes the deck if it is built in should have this pre-amp built in. However these pre-amp can fail!
To test if the pre-amp is working, even if has no phono socket, unclip the leads to the cartridge. With your amp on, touch the end of each wire with your finger. On two you should hear a buzz - one for each channel. If it's not loud then there's a problem with the pre-amp. If you do get a decent buzz the cartridge is faulty.
You can also trace the wiring to the pre-amp inside the unit, if you wish to fix it. It will almost certainly end up at an IC, which should be replaced.

when you said "no sound enough to enjoy listening" did you mean that you could hear something coming thru the speakers, but it isn't loud enough? if so, you may be plugging the turntable into a "ceramic" phono input on your stereo. it may be that your stereo doesn't have a pre-amplifier on the phono inputs. Ceramic tonearm cartridges (the part that holds the stylus - which you may call the needle) have enough output level to drive an amplifier. if your cartridge is magnetic, the output is very low compared o the ceramic. the trade off is that magnetic cartridges have a much wider dynamic range than the ceramic, and when properly pre-amplified, sound MUCH better than a ceramic unit. you can probably find a separate pre-amp at radio shack (i know they sold them several years ago). you plug the turntable into the inputs of the pre-amp, then plug the pre-amps output into your stereo. hope this helps.

It will need a pre-amp to connect it to your newer amp. The old style turntables normally used a magnetic cartridge that older receivers / amplifiers had a special phono input (which incorporated an internal pre-amp) to connect and hear at normal volume settings. Newer receivers / amplifiers normally do not have this type of phono input, thus the need for an external pre amp device.

Check if the audio output on A/V pins have a fine sound output on the audio pins, using a headset. if you dont hear a fent sound from the pin sound coming from the pre amp, then take your tv for a replacement of an audio ic output power amplifier.
thanks.

this would be the pre amp. transistor , audio second pre amp volume control chip or audio amplifier , they will need to be tested to determine which one or ones should be replaced

It definitely has to go through a preamp (a PHONO preamp that is...) something like this: http://www.phonopreamps.com/tc400pp.html