Jeff, You bring up an important point. While my previous post was about tuning the receiver, it is always necessary to avoid either overloading, overheating, or mismatching the PA in the TKR-x50 or in any other radio. That's why I put at least a single ferrite isolator at the TX output to provide a stable 50 ohm load for the PA.
It may come as a surprise to the owners of offshore-produced mobile radios that you can burn up a PA by running the radio at low power! A case in point: My radio club has a Yaesu FT-2500M that is the 2m side of a packet node. Mindful of the high elevation of the packet site, we had the radio set for the lowest of its four power levels. After a year of trouble-free service, it died from a toasted RF module. I replaced the module- which cost about $85 as I recall- and bench-tested the radio afterwards. It seems that the current draw of the radio varied relatively little as the RF output power was changed. In fact, the current draw at the lowest output power of about 6 watts was 85% of the current draw at maximum output power of about 45 watts! Do the numbers, and you'll see that the extra power being consumed was going into heating the PA module instead of being radiated as RF. At full power, the PA runs Class C and is fairly efficient. But at reduced drive, the PA is running Class AB or B (gasp!) and becomes remarkably inefficient. And why is this? Because the radio designers didn't do all of their homework to ensure that the PA would maintain its efficiency throughout the power level adjustment. Thus, an Amateur Radio operator who diligently uses the minimum power necessary for communication may be toasting the PA in his or her radio! Finally, the power efficiency is only part of the issue here. The other part, which Jeff mentioned, is that a PA which is operated at a point below its specified range may become a prolific generator of spurious signals. That's not good! 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Yahoo Sent: Sunday, March 18, 2007 11:01 AM To: [email protected] Subject: RE: [Repeater-Builder] TKR-750 P/A Failures Absolutely correct Eric. The TKR-750 comes factory tuned for wideband base station operation. The TKR-750 does NOT like anything but a good 50ohm load. The transmitter will go spurious. I have never had a problem with a version 1 or 2 with a properly matched transmit path. Maybe just luck. I have also seen transmitters that will go spurious below 10 watts. While the TKR750/850 has some quirks, they have performed fairly well even in high RF environments. Jeff -----Original Message----- From: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> [mailto:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Eric Lemmon Sent: Sunday, March 18, 2007 9:51 AM To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> Subject: RE: [Repeater-Builder] TKR-750 P/A Failures Gareth, Your statement about the receive sensitivity caught my eye. I have to wonder: Was the front end of your repeater's receiver ever aligned for optimum sensitivity on your RX channel? Of the several TKR-750 repeaters I have worked on over the years, only one of them was properly adjusted to optimize the receiver, even though the dealers programmed them before delivery! The instruction manual that is shipped with the repeater makes no mention of this requirement. The fine print under System Setup on page 2 of the service manual states: "The RX RF BPF (L2, L3, L5, L6, L7 on TX-RX unit A/2) must be aligned to obtain the maximum sensitivity at a programmed frequency." Even though the TKR-750 is a low-tier repeater intended for non-demanding applications, it does have helical resonators in the front end. When properly tuned, it should at least match and may out-perform the Vertex VXR-9000 in both sensitivity and IM rejection. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> [mailto:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Gareth Bennett Sent: Sunday, March 18, 2007 4:09 AM To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> Subject: [Repeater-Builder] TKR-750 P/A Failures Hi Group, Can anybody out there enlighten me on the power amp failures on the earlier Kenwood TKR-750 Repeaters? I recall that Kenwood was scrambling around changing these under warranty when the 750 was first released. I have just removed one from service that was only pushing 15 Watts @ 7 Amps (Set for High power). This particular repeater has already suffered dry joints around the PA stage which was not the cause of fault this time. Can any Kenwood dealers shed some light on this common problem and suggest the replacement P/A part number for around 150-160 MHz? Interestingly we replaced the -750 with a Vertex VXR-9000 repeater and commissioning tests revealed about 3 dB better sensitivity than the 750's for 12dB Sinad. Regards __________________________________________________________ Gareth Bennett
