[sarcmembers] Re: IC-736 and the TE Systems amplifier...
- From: "Eric Lemmon" <wb6fly@xxxxxxxxxxx>
- To: <sarcmembers@xxxxxxxxxxxxx>
- Date: Sun, 17 Jan 2010 16:29:24 -0800
John,
I checked with several suppliers, including RF Parts, and could not find one
that stocks the MRF-247. Since I assume(?) that the amplifier has four
75-watt transistors in parallel, it seems prudent to use a genuine MRF-247
transistor for replacement, if one can be found. If a substitute transistor
is used, such as the 2SC2782 or NTE478, it might be advisable to replace all
four transistors to avoid mismatching issues.
Since the 2SC2782 costs about $32 each, this is a significant expense. I
hope we can find an MRF-247 rather than risking further damage with a
mismatched component. Is TE Systems still in business?
73, Eric Lemmon WB6FLY
-----Original Message-----
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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