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trying to repair FRG-7 receiver

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LAN_403

hsieh

Thu Apr 19 2012, 01:18AM

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

I connect the capacitor and wire to the junction of R101, C101, and Q101,But I still cannot hear any signal.I only can hear some weird sound.

I also measure TP101 with oscilloscope and get a perfect sine wave.and the frequency range seems correct.

Proud Mary

Thu Apr 19 2012, 09:57AM

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

OK Hsieh.

First we will go back to the doubtful waveform at the VFO (Q403) and VFO buffer (Q404).

Go to the junction of C410 and T403 and look for a signal between 2.455 - 3.455 MHz. This signal should vary in frequency across this range when you turn the main tuning dial.

Setting the receiver to the conditions given at the bottom of Table 5, measure the voltages at Emitter, Collector and Base of Q403 and Q404 and compare these test results with the figures given in the voltage chart.

hsieh

Fri Apr 20 2012, 01:37AM

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

I test the junction of C410 and T403 and get a perfect sine wave.The frequency range is correct.I test TP404 again and now I get a stable sine wave too.I didn't do anything.I only clean the circuit board with some sort of cleaner.

I also measure the voltages of Q403 and Q404.They are close to the voltage given in the manual.

Proud Mary

Fri Apr 20 2012, 09:18AM

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

That's good news about TP404!

Do make sure that your probe contacts and crocodile clips are sharp enough to penetrate any surface coatings of varnish, oxidation, or dirt.

Now to some more voltage measurements!

Set the reciever to the condition specified at the bottom of Table 5, measure the voltages on C,B, E of Q101, Q102, Q103, Q104, Q105, Q106, Q107, Q108, Q109 and Q201, and compare with the specifications.

Conundrum

Fri Apr 20 2012, 06:22PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Another thought, view the board on a thermal imager to look for hot spots from bad transistors etc.

-A

Steve Conner

Fri Apr 20 2012, 06:30PM

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

Don't listen to Conundrum! He'll have you improvising heat sinks from Sugru and metal filings.

Where are you going to get a thermal imager anyway? You could buy another dozen FRG-7s for the cost of even renting one.

If it were me, I'd be poking the board with a chopstick and banging on the chassis, to check for loose connections. Oh, and check for dirt between the tuning capacitor vanes, if it has a tuning capacitor.

hsieh

Sat Apr 21 2012, 02:09PM

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

I think probably I can use a thermal imager for free.There is one in our school lab and my teacher once said that I can use it.But will this trick work?

I am curious to know if FRG-7 is a well known model or not.

Proud Mary

Sat Apr 21 2012, 03:01PM

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

hsieh wrote ...

I think probably I can use a thermal imager for free.There is one in our school lab and my teacher once said that I can use it.But will this trick work?

I am curious to know if FRG-7 is a well known model or not.

I can't help you with suggestions about fault finding with thermal imaging devices, as I have no experience in this, and will leave it to Conundrum to tell you what would be the best way of repairing your receiver using this technique. I know only the old slow way of doing this, circuit by circuit, until the fault is found.

But I can tell you that FRG-7 was one of the first high quality receivers built for sale to radio amateurs. It offered triple conversion, frequency synthesis, and Wadley loop stabilization, which had previously only been seen in communications receivers for professional use - such as military and maritime.

A Yaesu Musen FRG-7 in good working order and good cosmetic condition will still sell for a good price on ebay.

Conundrum

Sat Apr 21 2012, 03:49PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Its an accepted technique, industry uses this all the time to locate shorts on multilayer boards.
Its also used for military applications where extreme reliability is essential, when recertifying a repaired board.
I think they compare it with a known good (KG) board and if there are substantial differences then the board gets reworked or scrapped.

-A

hsieh

Mon Apr 23 2012, 01:53AM

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

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?

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