Thanks for the feedback Gents... (Eric, I will get the serial number to you for
your info).
... Interestingly this repeater was operating on a very lightly loaded channel,
through a TX combiner, so was seeing a nice 50Z load. It was running at 50
watts but out of one hour there would have been only 10 minutes of that time in
TX. Of the 15 Watts @ 7 Amps, the signal was so very spurious that for the
health of the site it was removed from service immediately.
As for the RX sensitivity, I know the Tech who commissioned the -750 and he
is pretty pedantic, often bordering on the extreme, so I can only summarise
that it was set up correctly and according to the book. (As was the VXR-9000).
What alerted us to the fault was not the complaint of low TX power, as 9
times out of 10 the subscriber would not be able to recognise the difference
between 15 or 50 watts, but the incredible amount of crap that this thing was
spewing out all over the band, and into other channels on site when keyed. Once
on site Spec-An proved the culprit pretty quick :-)
Regards
_________________________________________________________________
Gareth Bennett
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please return it to the sender and destroy any copies.
Thank you.
----- Original Message -----
From: Eric Lemmon
To: Repeater-Builder@yahoogroups.com
Sent: Monday, March 19, 2007 10:04 AM
Subject: RE: [Repeater-Builder] TKR-750 P/A Failures
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: Repeater-Builder@yahoogroups.com
[mailto:[EMAIL PROTECTED] On Behalf Of Yahoo
Sent: Sunday, March 18, 2007 11:01 AM
To: Repeater-Builder@yahoogroups.com
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: Repeater-Builder@yahoogroups.com
<mailto:Repeater-Builder%40yahoogroups.com>
[mailto:Repeater-Builder@yahoogroups.com
<mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Eric Lemmon
Sent: Sunday, March 18, 2007 9:51 AM
To: Repeater-Builder@yahoogroups.com
<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: Repeater-Builder@yahoogroups.com
<mailto:Repeater-Builder%40yahoogroups.com>
[mailto:Repeater-Builder@yahoogroups.com
<mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Gareth Bennett
Sent: Sunday, March 18, 2007 4:09 AM
To: Repeater-Builder@yahoogroups.com
<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
