All, It was a busy day at the Repeater Builder shop. Scott Z and I were working on a Mastr II mobile conversion for a customer on 224.000 -1.6 megs and couldn't get the thing to duplex as it self quieted as soon as the transmitter was activated. We both knew about the problem on UHF that happened around 443.400 give or take, but the 220 thing had us baffled. We decided to load the whole scenario into Microflop XL and see if we could see anything that could be producing the problem. Sure enough, lots of data was produced, studied, and the problem found. We took this one step further and made a prediction spreadsheet that shows the "good" and "bad" repeater pairs on both the 220 and 440 ham bands.
If you would like to download the XL files to view to research that shows why certain frequencies using LSI are a no-no, go here: <http://www.repeater-builder.com/ge/mastrII/MII-Injection-Calc.xls> <http://www.repeater-builder.com/ge/mastrII/MII-OK-Calc.xls> BTW, all fields are editable with your own entries to prove to yourself why things are as they are on (your?) frequencies... The Good and Bad fields on the UHF *may* be a bit liberal as an differential frequency of 15 kilohertz was used as the criteria. This field can also be edited if you think 15 Kc is too liberal in the judgment. Moving on, the data also shows that using High Side Injection will eliminate the problem, however, using a popular 220 mod that doesn't change the LO multiplication factor results in a HSI fundamental of of nearly 26 megahertz; something way out of the original range of the ICOM. I wanted a LO fundamental down in the 17.XXX range so we decided to look at other modifications on the internet and found two that indeed used the formula of LO=RX-11.2/12 instead of the original divide by 9. We weren't extremely happy with the tuning of the modifications found on the net, so we decided to do our own research to perfect the Mastr II to 220 receiver mods. Not only did we stabilize the conversion, but we gained several dB of sensitivity in the process. I have never been satisfied with the 220 receiver conversion on the Mastr II, until now. Our bench research showed a lack of LO injection to be the problem with the current mods on the internet. We converted 2 receivers in the shop today just to be sure our conversion can be duplicated. Both turned out to be around -117 dBm for 12 dB sinad with no preamplifier; slightly better than book spec as determined by the HP 8920 service monitor. This modification allows the 220 LO fundamental to be close to the original High-Band fundamental. At some point in time I'll write up the new mods for the 220 Mastr II receiver and post them on the Mastr II Dot Com website. Of course, simple HSI seems to fix the problem in the UHF band with no ill effects, and the X12 LO multiplier scheme seems to fix the problem totally on the 220 band, even with LSI. Hope this helps, Comments welcome. Kevin Custer - M. Scott Zimmerman Repeater - Builder (the company) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
