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Registered Member #1412
Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278
I bought this several days ago.When it arrived I found it is in bad condition.The case is rusted.
When I turn it on and switch to AM band to test it,it cannot receive anything.Sometimes when I'm turning the dial, the signal meter reached 20dB and it start to make some weird sound(PO-PO-PO....)
manual: schematic:
I have never repair something like this before so I have no idea how to repair it.Should I undergo the calibration process described in the manual?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
No, don't attempt to recalibrate it! Do not fiddle with any of the dust cores in the IF transformers!
First clean off loose mould and debris with a dry paint brush.
1.) Is the power supply reaching the boards? test with meter. Does the front panel display light up?
2.) If power is reaching the boards, inject a ~20mV audio test tone at pin 7 on Q410. If no tone comes from the loudspeaker, it means that the audio output amplifier circuit has failed. If the audio output circuit is good, inject the test tone at the hot end of VR1, the volume control. If the tone still comes from the loudspeaker, and can be varied by varying VR1, then VR1 is good. If no loudspeaker output, then VR1 has failed.
Work backwards along the signal path in this way, until you find the fault. When you get to the detector, and IF stages, you will need an HF signal generator, or function generator to inject your test signals.
The good news is that your receiver was manufactured in a way that makes fault-finding and repair very easy. There are even test points! (marked "TP" on the board) so you will be able to make it work again if you don't rush into it in a hurry.
Registered Member #1412
Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278
I did what you said and I found that the audio amplifier and VR1 is OK.So now I have to check the detector and RF part of the receiver.But I don't know how to do it.Can you explain this further or are there any websites to reference?
I will borrow a RF signal generator after this weekend.Should I inject a modulated test signal to every stage of the receiver?What amplitude and frequency should I set?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
hsieh wrote ...
I did what you said and I found that the audio amplifier and VR1 is OK.So now I have to check the detector and RF part of the receiver.But I don't know how to do it.Can you explain this further or are there any websites to reference?
I will borrow a RF signal generator after this weekend.Should I inject a modulated test signal to every stage of the receiver?What amplitude and frequency should I set?
Hi Hsieh, I will study the circuit diagram this evening, and tell you where to inject an AM RF signal to test basic detector operation, and the IF amplifier stages. Stella
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Set the signal generator to 455kHz 500mV and AM modulation 500Hz.
With S3 on the receiver set to AM, inject a 500mV signal onto the anode of D402, where it is connected to C428 and L402.
If you hear the audio tone from the loudspeaker, the AM balanced detector and all the circuits after it to the loudspeaker are good.
If NO audio tone comes from the loudpseaker in the AM test, switch S3 to USB/CW and apply the same signal with NO modulation to the junction of D404, D406, and R433. If you can now hear a whistle when you adjust the BFO it means that the demodulator and all the circuits after it are good.
If BOTH of these tests are OK, set S3 back to AM, and inject a 455kHz 50mV AM tone signal onto the hot end of the secondary of T404 where it is joined to C422.
If you can hear the tone in the loudspeaker, Q406 - the final IF amp - is good.
If NO tone comes through, check voltages on Q406 which should be close to Emitter 1.4V, Collector 9.1V and Base 2.0V.
That is enough to be getting along with for now.
Tell me how these tests perform, and we can decide what to do next.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
hsieh wrote ...
All the test you described are OK.I can hear audio tone come from the speaker in all tests.
My receiver also make a lot of weird sound when I turn it on.Does this mean something?
It means receiver not working properly! In triple-conversion superhet like FRG-7, it is best to work systematically, and not to work on 'hunches' and guesses about what is wrong.
Next:
Set signal generator to 455kHz, with AM audio tone, RF output level 10mV.
With S3 set to AM, inject signal at start of ceramic IF filter - the point where FL-1 is connected to R420 and C417.
If you hear tone from loudspeaker it means IF filter and everything after is good.
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trying to repair FRG-7 receiver
