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Kenwood TS-940 Power Supply Voltage Regulation



 
 
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Old September 25th 06, 04:02 AM posted to rec.radio.amateur.homebrew
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Posts: 6
Default Kenwood TS-940 Power Supply Voltage Regulation

Hi,

My 940 was working good for while, but now the S-Meter lights and the
VFO display have started to flicker. They flicker at random times very
quickly (all at once). The LCD display bulbs don't flicker, but they
have both burnt out now. And I just replaced them less than 2 years ago
(with Kenwood factory replacements).

I checked the voltage to the power amp and it reads 28.6 volts. The
S-Meter (lights) read 13.6 volts and when the flicker occurs the
voltage very quickly varies from ~6 to 13 volts - never above 13.6
volts. Is this some kind of voltage regulation issue? I can't get it
to do this by moving wires around and pushing on things. It does only
seem to effect the lights the are controlled by the dimmer switch.

On a side note, I've been reading several posts about the 2N5885 that
regulate the 28v supply for the power amp failing and wiping out the
MRF-485 transistors that are no longer available. Does it make sense to
preemptivly change the 2N5885 transistors to help prevent damage to
those MRF-485? I use my 940 daily - and its on most of the day.

-Scott,WU2X

  #2  
Old September 25th 06, 08:05 AM posted to rec.radio.amateur.homebrew
g. beat
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Posts: 36
Default Kenwood TS-940 Power Supply Voltage Regulation

wrote in message
oups.com...
Hi,

My 940 was working good for while, but now the S-Meter lights and the
VFO display have started to flicker. They flicker at random times very
quickly (all at once). The LCD display bulbs don't flicker, but they
have both burnt out now. And I just replaced them less than 2 years ago
(with Kenwood factory replacements).

I checked the voltage to the power amp and it reads 28.6 volts. The
S-Meter (lights) read 13.6 volts and when the flicker occurs the
voltage very quickly varies from ~6 to 13 volts - never above 13.6
volts. Is this some kind of voltage regulation issue? I can't get it
to do this by moving wires around and pushing on things. It does only
seem to effect the lights the are controlled by the dimmer switch.

On a side note, I've been reading several posts about the 2N5885 that
regulate the 28v supply for the power amp failing and wiping out the
MRF-485 transistors that are no longer available. Does it make sense to
preemptivly change the 2N5885 transistors to help prevent damage to
those MRF-485? I use my 940 daily - and its on most of the day.

-Scott,WU2X


Scott,

May I suggest the TS-940 web page? Jeff King ZL4AI / DU7 (N5IW also has
one)
One stop shopping for almost every issue, mod or change noted for the TS-940
The web page is so big -- it is in 2 parts!
http://homepages.ihug.co.nz/~jaking/TS-940_02.htm
http://www.jking.kol.co.nz/TS-940_02...#_Toc146779962

The power supply, as you have noted, has been an issue for other operators.
http://www.jking.kol.co.nz/TS-940_02...#_Toc146779932
===

Kenwood TS-940s AVR board, by N0XWR on February 28, 2006

You won't find an AVR board for sale new -- you should endeavor to fix the
one you have.

First, check regulator Q103. it is in the rear left corner of the rig as the
rig faces you. it is on the heatsink for the power supply. there are three
regulators side by side. it is the one on the far left. no matter whether
you replace it or it is good, it is imperative that you cut the connector
off of the three wire harness that plugs on to regulator Q103 and solder
directly to it. this harness comes from plug-in #2 on the AVR.

Over time the connector overheats and fails and cannot carry the 20-30 volts
to and from that regulator. I have seen the problem many, many times. when
the connection to that regulator fails, it takes out Q6 on the AVR board, so
check it next.

Clif at AAvid Electronics, (now back in repair business at
http://www.avvid.com) helped me get through the problem. also check D3, D9
and D14 which are zener diodes on the AVR board. they can be checked right
on the board with a multimeter. the board can be removed easily. as you
unplug the wiring harness use a sharpie and number each connector so that
you can reinstall it easily. make a diagram, too. use small long nose
forceps to unplug the connectors to the AVR board.

Incidentally, Q103 is part # NTE377 available at Mouser. Q6 is part # NTE382
also available at Mouser.

I have the part numbers for the zeners if you need them.

73s Jerry N0XWR


 




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