trying to repair FRG-7 receiver / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

trying to repair FRG-7 receiver

first previous 4 5 6 7 next

Move Thread

LAN_403

Proud Mary

Thu May 03 2012, 09:15AM

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

hsieh wrote ...

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?

Remember that 1 mV is more than 1000 times larger than minimum sensitivity of receiver, which is 0.7 Î¼V. Such a large signal could bleed through a whole defective stage, and so appear at the audio output.

A problem often found with old communications receivers is called 'finger trouble' or 'the man with the golden screwdriver' - bad work of previous owner, who interfere with circuit to 'improve' it, without knowing what they are doing. This can result in the position of dust cores, trimmer capacitors, and pre-set potentiometers being far from optimum position.

I would now suggest - as I suggested above - that you start whole re-alignment procedure given in manual, and tell me how the receiver performs at each stage.

hsieh

Thu May 03 2012, 09:38AM

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

In fact, I am now not very sure how to alignment it.I read page 11 already but cannot understand every step.Especially steps that involve sweep signal generator.

The highest frequency of the signal generator in my school lab is 20MHz.

I also don't know what output voltage should I set my signal generator to.

Proud Mary

Thu May 03 2012, 06:51PM

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

It is possible to align receiver without sweep generator, by plotting frequency response graph on graph paper like Old Times, but you still need signal generator that can output 55 MHz.

So let us leave that problem, and try procedure number (2) on page 11 "Balanced Mixer" - to give us important information.

hsieh

Fri May 04 2012, 12:29PM

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

Did the manual means that connect drain of Q102 and Q103 to the antenna input and tune to 910KHz?

I did this but there seems to be no change on the S meter.I can hear s sound if I switch to CW mode.

Proud Mary

Fri May 04 2012, 01:31PM

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

hsieh wrote ...

Did the manual means that connect drain of Q102 and Q103 to the antenna input and tune to 910KHz?

I did this but there seems to be no change on the S meter.I can hear s sound if I switch to CW mode.

Bad translation Japanese to English! It mean you must connect antenna input terminal to Earth.

Look at back of receiver. Connect wire between SW1, BC, and E. (SW1 mean Short Wave (antenna) 1, and BC mean Broadcast antenna (Medium Frequency) and E mean Earth which Americans call 'Ground')

Now there is no RF signal from outside! Only 910 kHz spurious signal receiver make itself!

Now set BAND to A, and MHz to 0 and do procedure with VR101 and TC105 to make this bad, unwanted signal as small as possible. Slowly adjust VR101 a small amount, and then TC105. Then a little more on VR101. Then a little more on TC105 until you find best combination of R and C to make 910 kHz as small as possible.

hsieh

Mon May 07 2012, 02:35AM

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

I cannot find TC105.And no matter how I adjust VR101,the volume of 910KHz signal don't change.The S-meter didn't move either.

I also notice that when the main tuning dial is near 0 or 1000,my receiver will make a weird sound.

Now I borrowed a signal generator capable of generating up to 1GHz.But I'm still not sure how to do step 1 on the manual.

Proud Mary

Mon May 07 2012, 03:52PM

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

hsieh wrote ...

I also notice that when the main tuning dial is near 0 or 1000,my receiver will make a weird sound.

These are harmonics of 1 MHz synthesizer oscillator Q301. This is normal. It show that most of receiver working correctly.

Today is public holiday in England. I will work on your problem tomorrow.

Proud Mary

Tue May 08 2012, 08:04PM

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

OK.

Band selector position: B - 1.6 - 4.0
Main tuning dial: 500
Pre select: approx 2.5 MHz

Now turn slowly MHz between 2 and 3. Does red Lock light go off and then go on again as you turn knob?

LOCK light go off when frequency locked. When red LOCK light on, circuit NOT locked- like warning! Reverse of what you think!

If LOCK light test OK, try at other frequencies and tell me what you see.

hsieh

Wed May 09 2012, 12:34AM

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

Yes,The red light go on and off when I slowly turn the MHz dial.I tried different frequencies and it works.

I know that the light go off means the circuit is locked.Why they design it this way?why they didn't add an inverter/not gate?

I also replaced all electrolytic capacitor on the IF-AF unit PCB.

Proud Mary

Wed May 09 2012, 10:32AM

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

Red is warning light. Synthesizer not locked!

Set receiver to 2.5 MHz exactly as above. Set MODE to USB CW.

Connect long piece of wire to terminal SW1. 3 meters is OK, 5 m is better.

Turn up volume control.

Check LOCK light is off.

Now turn PRESELECT slowly between 2.3 to 2.6 MHz. Does the noise from speaker get louder while you turn PRESELECT, and then become more quiet?

first previous 4 5 6 7 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob