I would be setting up to check the freq response at various points in the signal chain, without any GR.
You could use a CRO and a signal gen to do so - traditional or computer based
Make sure obviously, your sig gen is 'flat' response at the input and verify you are indeed getting the hi-boost [or low-cut!]
Then make the same check at a few internal points ... it takes some care and knowledge etc, particularly in a 'computer based test environment' such as
R E W (look it up if not familiar!)
....
Also, check where the state of the locking potentiometer at back and make sure it's up as expected.
I'd be interested to know what's 'going on'
...
[assuming it once worked right - a fair premise for a > 5Kaud unit]
If it really is doing the 'high boost' [same as low cut] with no GR, I would suppose it could be :
- some bad solder maybe, somewhere or anywhere really
- some partial shorting possible
- busted wire connection
or
- something wrong with the input section ax7
or
- a resistor gone bad, perhaps in a 'negative feedback loop' area at the 'output end' possibly on the second AX7
- something gone bad in the 'white cathode follower' line amp
or
- bad wiring in the XLR input and output connectors - one of the hot/cold pair coming unstuck or partially shorting etc
etc
...
If I had it on my bench, I would do response sweeps with my computer+audio_interface with my 'DIY hv probe / attenuator' and R E W analysis suite. About 5 points I would check, including the inputs and outputs.
...
Good luck with it.
