Sir samuel phillips . . . . .
Since you now say that full familiarity with your equipment’s normalcy in the past, now confirms that the present volume is 1/2 or less than it used to be, therefore you remember that level as a reference of its prior capabilities.
I just prepped up the block diagram, as it seems to have all referencing output to the audio flow thru the system.
Originating, all the way from its point of origin over at pins 11/12 and 9/10 as the analog R and L audio outputs at the Digital Analog Converter IC.
Initially, can you confirm for us, that you have the full operational DC supply voltage at pin 5 of the IC551 AF out?
Also, can you confirm that IC 502 is properly receiving a proper +6 vdc at its pin 8?
You’re telling us of having
NO volume control action is certainly suggestive of your problem being in the AF output stage.
Your headphone jack merely switches audio out between either the headphones or the speakers, with no intermediary lower drive level going exclusively to the headphones.
Did you get the identical decreased audio level effect with the old IC551 AF chip in circuit ?
An initial test requires a wire leaded ceramic-poly-paper capacitor of 0.01 thru 0.1 ufd range, such that its lead ends can bridge from pin 1 over to pin 12 of the IC551.
There is a 50/50 chance of a good result . . . . . . . . it’s all being in accordance to phase relationships.
If the unit is powered up and in normal working settings and conditions, when that cap is bridged between those points .
If the moon is in the right solar positioning . . .and you are LUCKY ! you then just might get the loudest tone coming from that unit that you have heard since all was last working normally.
If no luck, think about your having additional audio equipment , on which, we need a line level audio output from something that typically does that outputting thru a female RCA jack. Also we will need a shielded, male at both ends, RCA patch cord and that prior isolation capacitor that was being used.
AF sources . . .? . . . . . .CD player - - - - - a DVD players R or L audio output - - - - --VHS or BETA tape players and their AF output - - - - - - - another component system with Tuner AF output - - - - -tape output . .etc.
Standing by to further fill in and trade info . . . . .
Relevant Techno Referencing:
73’s de Edd