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LAN_403

Proud Mary

Wed Apr 11 2012, 05:37PM

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

Next test:

Set signal generator to 2.5MHz, AM audio tone, 10mV.

Set the receiver to AM.

Fixing a small capacitor (say 3 - 10pF) to the end of the probe, inject 2.5MHz signal at junction of R445 and the hot end of T401.

Now tune the receiver very slowly across a 1MHz segment.

Stop turning the main tuning dial when - if - you hear the audio tone from the speaker.

What is the frequency reading on the dial when this happens?

Is there any indication on the signal strength meter - "S-Meter" - on the front panel during this process?

Do you have a counter-timer that we can use to measure the frequency of the various oscillators?

hsieh

Thu Apr 12 2012, 01:23AM

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

When doing this,I can sometimes hear a very faint/unclear tone when the main tuning dial is tune to 500 and the S-meter also reach 20dB.

Yes,I have a frequency counter.

Proud Mary

Thu Apr 12 2012, 10:01AM

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

hsieh wrote ...

When doing this,I can sometimes hear a very faint/unclear tone when the main tuning dial is tune to 500 and the S-meter also reach 20dB.

This result strongly suggests that Q401, Q402, Q403, and Q404 are all OK, as the 2.5MHz test signal has appeared where it should at 500 on the main tuning dial - but as you describe the signal as being 'very faint' we must not dismiss the possibility of IF breakthrough.

So we will perform one more test on this 2nD IF/3rd Mixer stage just to be sure.

Set your counter-timer to a range that can detect signals in the range 2.455Mhz - 3.455MHz.

Fix your counter-timer probe to TP404, and observe the counter-timer while tuning the main dial across its full range.

If Q403 and Q404 are working properly, the counter-timer should display 2.455Mhz - 3.455MHz as you turn the main tuning dial across its full range.

hsieh

Mon Apr 16 2012, 01:23AM

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

I measure TP404 with an oscilloscope.The waveform is unstable,especially when my hand is on the dial.

Proud Mary

Mon Apr 16 2012, 01:58PM

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

hsieh wrote ...

I measure TP404 with an oscilloscope.The waveform is unstable,especially when my hand is on the dial.

OK Hsieh. That's because the probe is detuning the circuit. Connect it back to TP404 through a very small capacitance ~2 - 3 pf and try again.

hsieh

Tue Apr 17 2012, 09:45AM

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

Proud Mary wrote ...

hsieh wrote ...

I measure TP404 with an oscilloscope.The waveform is unstable,especially when my hand is on the dial.

OK Hsieh. That's because the probe is detuning the circuit. Connect it back to TP404 through a very small capacitance ~2 - 3 pf and try again.

I did this but the waveform is still unstable and distorted.

Proud Mary

Tue Apr 17 2012, 11:16AM

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

hsieh wrote ...

Proud Mary wrote ...

hsieh wrote ...

I measure TP404 with an oscilloscope.The waveform is unstable,especially when my hand is on the dial.

OK Hsieh. That's because the probe is detuning the circuit. Connect it back to TP404 through a very small capacitance ~2 - 3 pf and try again.

I did this but the waveform is still unstable and distorted.

It's difficult to be 100% certain what is going on, without being able to see it myself, but here is what I think:

"waveform is unstable" - this is an analogue VFO, being operated without the Faraday screen of the equipment case. Moving your hand, for example, near the variable capacitor may alter the frequency.

"distorted" - remember that this is the switching waveform being applied to 3rd mixer. It is not intended to be a sine wave, but don't expect to see good square waves either!

Were you able to measure the frequency range of the signal at TP404, as I suggested? If you haven't got an instrument to do this, we will put this question on one side for the moment, (as you clearly have a signal of some sort) and move on to the frequency synthesizer.

Synthesizer crystal oscillator test: Measure the frequency of the signal at the junction of L301, L302 and C307. This should be 1 MHz.

Patric

Tue Apr 17 2012, 04:25PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 254

I do not want to interfere, but it is very stretching and I hope for a good ending!

Nothing wrong with the speaker?

hsieh

Wed Apr 18 2012, 12:58AM

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

waveform and frequency at junction of L301, L302 and C307:

waveform at TP404(sometimes it is a stabe sine wave):

Proud Mary

Wed Apr 18 2012, 02:46PM

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

The waveform and frequency of the synthesizer oscillator is excellent.

The VFO signal is within the correct frequency range. The waveform does look a bit sickly, so we will leave it as OK Probable? and come back to it later if we cannot otherwise eliminate the fault.

Now we will get into the heart of the RF.

This next test will show us if there is a failure in the the elaborate RF pre-selector switching, one of the most common types of fault in older equipment.

Solder a small capacitor of approximately 50 - 100 pf to the end of a piece of insulated wire about 100cm long. This is your new antenna!

Fix the free end of the capacitor to the junction of R101, C101, and Q101 Gate 1. This is the input of the first RF amplifier.

Now tune across each of the receiver frequency bands and tell me what signals - if any - you can hear. If all is good, there should be plenty of signals (though not all in the right place, as we have by passed the pre-selector!

)

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