I have a couple of the 1980s Radio Shack/Realistic PZMs that I would like to use as room mics. I love how the mics pick up sounds, but I find the frequency response to be unbearable - particularly in the upper midrange around 5-6k.

I heard that the capsules and basic hardware on these early models are of much higher quality than the rest of the components, being held back by the transformer, unshielded 1/4" cable, lack of power, etc. I wonder if scrapping the transformer and building my own preamp will seriously improve the sound quality of the mic- with the aims of making it a lot more transparent and less nasally.

Anyone have any experience?

You should post this over at www.tapeop.com. Full of mic geeks who have probably written a book on it.

I used to have some web articles tucked away on this. I'll see if I can scare them up. You can convert readily to balanced output and phantom power or higher voltage batteries, then you can upgrade all the electronics if you want. IME, they still sound very much the same after all that. Been there, done that, got real Crown ones and am much happier. That said, I did make some pretty cool recordings with the Realistic ones too though.

Actual Crown PZMs (especially PZM-6Ds) run around $100 used on eBay if you look hard enough, and your Radio Shack ones should bring $50-60 ea, last I looked. I'd start there, get real Crown ones. But before I did that, I'd be sure you're using the mikes you have correctly. There's a nice application guide on the Crown website.

Edit: start here: http://www.itrstudio.com/pzm.txt
http://www.gweep.net/~rocko/PZM/
http://www.uneeda-audio.com/pzm/pzm_ch.htm
http://www.itrstudio.com/pzmmods.html