@Hugoneus: Wonderful video!
I´ve had a working PM6303A for many years and when I was clumsy enough to partially make bad due to forgetting to discharge a 4700 uF electrolytic cap before making a measurement my PM6303A was showing bad measurement values.
I was able to partially fix my PM6303A thanks to your video (had the complete service manual since many years but your video really helped me out
).
My remaining issue now is that when I measure electrolytic caps that have higher capacitance than somewhere around 200-300 uF, I get quite low readings. I do have the DC BIAS set to on (and it does output 2.00 VDC).
A 4700 uF shows up at approx 3500 uF in auto mode and if I turn off automode and select Ls/Cs mode then the value jumps up a little bit to around 4200 uF. If I take the same cap and measure it with my ESR-Micro 4.0 I get almost a spot on value of 4710 uF.
A 470 uF cap shows up at approx 390 uF. Lower caps, for example cheramic caps and polyester caps below 100 uF and down to pF-levels shows up correctly.
I am wondering if something else than the input diodes and resistors are bad. I've checked the 4.7V zeners in the PSU area and replaced one that was showing too low value so both zener voltages there are correct (+4.62V and -4.68V).
Could it be the LF356N (N101) on the input stage being bad or even the 74HC4053 (D101) since this would take a direct hit of a non-discharged cap connected to the front panel? Are the coils or caps on the connector board at the front panel at risk as well (fig 53 on the schematics, unit 3)?
Can anyone of you who also have the PM6303A measure a 4700 uF cap and see what values you get please?
I also realised that my firmware is older than yours, I have 1.2 while your video seems to indicate that you have firmware revision 1.4. Would it be possible to purchase a firmware copy of your chip?