Thought this may be of interest here since this transmitter seems to be a popular choice for AM broadcasting.
The "enchanced" version that costs $100 more from Radio Systems claiming better audio quality is just a modded TH-5.
Did some research and found out the mod is changing the value of just 2 capacitors...C301 and C337. These 2 caps control the bass and treble response of the transmitter. Increasing C301 will increase bass, decreasing C337 will increase highs. Change C301 to .15uf and C337 to 30pf. This can be done on top of circuit board...under $1 in parts and 1/2 hour of time. Not worth $100 more. My new TH-5 is on the way and this will be first thing I do. Thought I'd share that info. with forum readers.
i have the schematic for the enhanced version. it's a little more complicated than just two parts. but you are correct in changing those two parts will give the tx a little better sound but given recent enforcement events it's ill advised to do any mods (even something as simple as changing the input frequency response) to any certified transmitter that are not factory authorized. it will likely void the certification and call into question your entire operation. your better off the plunk the extra $100 down and have the limited (better than nothing) protection of fcc equipment certification. i will likely buy an iAM in the future myself. i'm just waiting on some things.
Everybody I know who has a C.Crane FM transmitter has done the "boost" with the secret output power control that has been widely publicized. Have they voided the certification?
Probably.
But the end user of these products is not judged based on the certification, because the certification has nothing to do with the end user.
The certification is issued to the manufacturer and gives them the legal right to sell their product.
The end user is measured strictly on the particulars of the Part 15 rules, field strength for FM and the usual AM restrictions.
It is extremely unlikely that an FCC agent checks every part inside a transmitter compared to the certified schematic.
all i can say is do the mod at your own risk. guess i am the play it safe type when it comes to these things.