[sarcmembers] Re: IC-736 and the TE Systems amplifier...
- From: "Avery Wright KD4GBA" <kd4gba@xxxxxxx>
- To: <sarcmembers@xxxxxxxxxxxxx>
- Date: Sun, 17 Jan 2010 16:29:41 -0800
John, et.al.,
The MRF-247 is a Motorola Power Transistor that runs about $75 each.
Toshiba makes an exact replacement 2SC2782 and is $31.75 from RF Parts.
Here is a link to the data sheet if you want to check up on it:
http://www.rfparts.com/pdf_docs/2SC/2SC2782.pdf
Avery
From: sarcmembers-bounce@xxxxxxxxxxxxx
[mailto:sarcmembers-bounce@xxxxxxxxxxxxx] On Behalf Of John Maetta
Sent: Sunday, January 17, 2010 3:24 PM
To: sarcmembers@xxxxxxxxxxxxx
Subject: [sarcmembers] Re: IC-736 and the TE Systems amplifier...
Today is a great day to keep dry, listen to the 40 meter swap net on
7.240MHz and work on radio gear.
I opened the TE Systems amp and discovered the T/R and RF Sense relays and
circuits are working, but the amp continues to draw 15amps after PTT is
removed. The o'scope then revealed that the amp starts to oscillate when
PTT is asserted and will not stop oscillating until power is removed. The
amp has turned itself into a 100W noise generator.
The noise emitted by the oscillations drowns out the swap net. :)
Troubleshooting detected that one of the four MRF-247 final transistors, Q3,
is the culprit. With Q3 disabled, the amp keys, draws 2 amps bias current
and then current drops to ~0 when PTT is removed, just like it is supposed
to.
If any of you have one in your junk box, please let me know. Or we can
order one.
Here is what we need to repair the amp:
http://www.datasheetcatalog.org/datasheet/motorola/MRF247.pdf
I also need some wide solder wick (1/4" or wider) to help remove the
transistor from the amp. All I have is the narrow wick.
The other option is to leave Q3 disabled and run the amp at reduced power
out.
73
John
N6VMO
----- Original Message -----
From: John Maetta <mailto:j.maetta@xxxxxxxxxxx>
To: sarcmembers@xxxxxxxxxxxxx
Sent: Sunday, January 17, 2010 12:05 PM
Subject: [sarcmembers] IC-736 and the TE Systems amplifier...
Hello,
Last week a member asked me to have a look at the IC-736 and TE System 2
meter amplifier. It was reported that the antenna(s) were exhibiting a high
SWR and the TE Systems amp would constantly draw 15amps from the Astron
power supply after the PTT was released.
This morning I noted the following:
A quick check of the setup found the IC-736 and amplifier are currently
connected to the 2 meter AZ/EL antenna 'A' via the MFJ-864 SWR/Watt Meter.
Using the IC-736 only and running 25 watts output, I noted antenna 'A' is
also displaying >3:1 SWR on the MFJ-864 SWR/Wattmeter. A more thorough
inspection found antenna 'A' was incorrectly installed on the HF output port
of the swr/watt meter, instead of the UHF output port. After correcting
this, the SWR of antenna 'A' was 1.1:1.
Power up of the TE Systems amp was normal and the preamp appeared to be
working too. Keying the amp and applying 20W of CW drive, the output power
went to ~300 as normal. But, after releasing the key the IC-736 returned to
receive mode as expected, but the S-Meter displayed >S-10+ noise. The
amplifier power output dropped to 0 as normal, but the amp continued to draw
15amps of current. Only after powering off the Astron power supply would
the amp stop drawing current and IC-736 S-Meter return to normal levels of
noise (S-1 or so).
It appears the amplifier is working as far as power output and preamp, but
the TX/RX relay and/or RF sense circuitry may be holding the amp's bias
voltage on and not releasing the amp to RX mode.
Operating the system into the high SWR due to incorrect placement of the
antenna on the swr/watt meter may have caused the amplifier to fail.
I have removed the TE Systems amp and Astron power supply from service and
they are on my workbench at home. The IC-736 is currently available to
operate 'barefoot' by SARC members.
This seems to be a easy problem to fix and I will keep members notified of
my findings.
73
John
N6VMO
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