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trying to repair FRG-7 receiver

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Proud Mary

Mon Apr 23 2012, 08:27AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

hsieh wrote ...

Today I measured the voltages and find some transistors that might be bad.

How much error is acceptable?I f the manual says 9V but I get 10V,Is this ok?

That would depend on which transistors they were, Hsieh, but mostly a ~10% deviation will not stop the circuit working.

Very generally, the collector or drain voltage should be a bit less than the supply voltage in normal operation. If the collector or drain voltage is the same as the supply voltage, it usually means that the device has gone open circuit.

hsieh

Tue Apr 24 2012, 01:03AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

I suspect that Q101 is dead.But it seems to be hard to find or expensive(10USD on ebay).Are there any replacements?

Japanese or American transistors are easier to find in Taiwan.

Proud Mary

Tue Apr 24 2012, 09:52AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

hsieh wrote ...

I suspect that Q101 is dead.But it seems to be hard to find or expensive(10USD on ebay).Are there any replacements?

Japanese or American transistors are easier to find in Taiwan.

3SK40 has no ESD protective diode so is vulnerable in position of first RF.

3SK40 is obsolete, so NTE454 is a good substitute, which also has ESD protection on gate 1.

If NTE454 is also hard to get try BF988.

When substituting 3SK40 with a more modern dual gate FET, make sure you re-organize the pin outs!

If the substitute has a higher gain than 3SK40, I would suggest putting a ferrite bead on the drain lead to stop oscillation.

You will need to go through the RF board alignment procedure once you have replaced Q101.

hsieh

Mon Apr 30 2012, 03:13AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

I replace Q101 already.I replace it with 3SK40I.It cost me about 4USD each.

After I replaced it,the voltage still not don't match and I cannot receive anything.

Do I need some sort of antenna or I can just connect a long wire to the terminal block on the back?

What to do next?My friend is now laugh at me that I spent a lot of time and still cannot fix it.

Proud Mary

Mon Apr 30 2012, 12:31PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

hsieh wrote ...

I replace Q101 already.I replace it with 3SK40I.It cost me about 4USD each.

After I replaced it,the voltage still not don't match and I cannot receive anything.

Do I need some sort of antenna or I can just connect a long wire to the terminal block on the back?

What to do next?My friend is now laugh at me that I spent a lot of time and still cannot fix it.

Antenna: The FRG-7 has a sensitivity of better than 1 Î¼V, so with only 1 meter of wire as an antenna, you will be able to hear many shortwave broadcast stations when the receiver is working normally. With a really good antenna, FRG-7 can detect very weak, distant stations just as good as any modern receiver.

Ignorant friends: You are restoring a vintage triple conversion analogue communications receiver that has a soul and spirit, and you should thank the fault for teaching you how to think about analogue receiver construction and repair.

How you should proceed: We know that most of the sub-circuits are working correctly, so start the 'maintenance and alignment' process on page 11 of the manual and keep going until you find a procedure that doesn't work. You may need to make alignment tools - like plastic screw drivers - by filing down a plastic knitting needle, or other hard plastic rod.

I am going to replace all the electrolytics on the IF/AF board in my own FRG-7 in the next two or three weeks - when I can find the time - and will post up pictures of the work here in case you want to make the same improvements to yours.

Steve Conner

Mon Apr 30 2012, 12:35PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The Wadley loop synthesizer is a complicated beast. The Pop-pop noises and bad waveforms are kind of what I would expect from a synthesizer that was failing to lock.

Maybe it would be a good idea to replace the electrolytics before starting to twiddle the IF transformer cores.

Also, dual-gate FETs like the 3SK140 can be extremely static sensitive, so take great care when replacing them. The Art Of Electronics recommends winding wire around all 4 leads and the metal can (if it has one) to short everything together, and not removing it until the device is soldered into the board.

Proud Mary

Mon Apr 30 2012, 03:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi Hsieh, you may find it helpful to join Petra's FRG-7 Group on Yahoo

She also runs groups for other makes of classic communication receiver.

hsieh

Tue May 01 2012, 07:25AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

Some suggest that I should measure TP109.I measure it but I cannot get any waveform.So I replaced Q107.Q108.Q109. But I still cannot get any waveform.

I also measures TP107 and didn't get any waveform.

Proud Mary

Wed May 02 2012, 10:27AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

hsieh wrote ...

Some suggest that I should measure TP109.I measure it but I cannot get any waveform.So I replaced Q107.Q108.Q109. But I still cannot get any waveform.

I also measures TP107 and didn't get any waveform.

I am going to take my FRG-7 (pictured below) out of its case next weekend to estimate the work and parts needs to replace all the electrolytics and introduce a narrower CW filter, and a crystal calibrator. If you can wait till then, I'll look at TP107 and TP109 and see what signals and voltages are on them in normal operation. I don't have a signal generator that can output a calibrated 1 Î¼V, but last night I listened to the CW segment of 40 metres (7.000 - 7.040 Mhz) while I was doing the ironing, and could hear stations from east Europe and Russia, which is good performance for the receiver with just an indoor antenna. (I also heard a very loud WT station called GB60VUL, a 'special event' station celebrating 60 years of the Vulcan nuclear bomber, an airborne horror of the 'Atom Bomb' era.) Anyway, Hsieh, this is the sort of performance you will get from your FRG-7 when you have finished restoring it.

hsieh

Thu May 03 2012, 02:22AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

Today I tried to send 1mV(lowest voltage on my signal generator) amplitude modulated signal to the antenna terminal of my FRG-7.

First I set my signal generator to 1.2MHz and tune my FRG-7 to this frequency.I can hear the sound but the sound is very faint.

Then I tried to send 10.5MHz signal and tune to this signal.Now the sound is very clear.

Did this mean that I need to calibrate my receiver?

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