If I may, on the HF-85 as designed, the output coupling cap is required to prevent DC from appearing on the output. If you move the input side of the .25 uF output coupling cap to the plate of the tone control amp (allowing the .1 uF there to then be a separate cap to drive the tone control feedback loop), you have now effectively tapped the main output into a point outside the feedback loop, which opens the door to response errors under different loading conditions. This same problem exists at the tape output connection, causing the response at that jack most often to have a bass boosted signal.
The proper fix for the circuit requires the addition of a coupling cap between the line and tone amp circuits, allowing the main output cap to be dispensed with altogether. Besides the removal of the output cap, this fix also eliminates DC on the tone controls, which causes potentially noisy operation.
At the tape out jack, the .02 cap there should be sourced from the output side of the .1 uF cap at the output of the line amp stage. This will then allow the tape output jack to in fact have a flat response as it should have.
Other design flaws exist in the power supply, where the original design provided inadequate filtering for the B+, allowing 120 Hz hum to be audible with sensitive power amplifiers and speakers.
The loudness circuit should not be removed, as that will only exacerbate the already excessive gain built into the line stages. That can be addressed as well, but to do so really requires a new line stage design.
These are areas you should potentially look at -- particularly if you are going to do a ground up rebuild. As I said, there are numerous posts available on these issues.
Dave
