[G0HZU_JMR]

Looking back at my repair notes I may have actually measured the conversion loss in my mixer at some point because I have a gain loss spreadsheet in the folder. I think the conversion loss is frequency dependent because there is an RC pre-emphasis network at the input to the mixer. It looks like I saw 13dB conversion loss here but I was measuring at an RF input frequency of 50MHz where the RC network will be lossy. My RC network shows a 68R resistor and not the 39R in the manual and this is the case for both of my TR4172 analysers. This will degrade the conversion loss at LF.

I looked at my old repair images and I think my 1st mixer device 'must' have been replaced at some point because the gold legs on the quad package still look shiny and clean. Also the soldering around it looks a bit clumsy.

Looking at my old images there are some scary 'scabs' of corrosion all around the insides of this mixer module so I assume this module has absorber stuck in the lid (can't remember). Therefore, it could be that one of the connection legs of your first mixer has finally crumbled and broken possibly due to the simple act of setting the module down which might be enough to finally kill it off. How much corrosion is there on the four legs of the diode mixer?

I suspect that my first TR4172 succumbed to the dreaded absorber glue in the first mixer many years ago so maybe the corrosion happens within the first few years? I think I would remember if it had ever failed at work so it may have had the mixer repair back in 1989/90 before it arrived at the company.