[sarcmembers] Re: IC-736 and the TE Systems amplifier...
- From: "John Maetta" <j.maetta@xxxxxxxxxxx>
- To: <sarcmembers@xxxxxxxxxxxxx>
- Date: Sun, 17 Jan 2010 15:24:16 -0800
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
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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