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Old 10th Jul 2016, 11:58 am   #1
Malcolm G6ANZ
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Default VHF Radio mic

I'm not sure where to put this request for assistance, so mods please move as required.
Anyway, I have a Toa VHF radio mic type WM370 and the associated rx WT770. when I acquired them ,as a pair, they were intermittent with the sound coming distorted and of low volume. New battery in the TX had no effect. There were no frequency markings on either the TX or RX so to fine out there operating frequency I took them both apart and looked at the crystals.
For the TX the crystal was marked as 14.483. Multiply this by 12 and you get 173.796MHz close to 173.8 which is one of the allowed channels.
For the RX the LO crystal is 54.366MHz multiply this by 3 and add 10.7MHz and yo get 173.798MHz. So they are a matched pair.
But when I measure the tx crystal frequency with a counter I get ~16.5MHz. Viewed on a 50MHz scope the wave form looks clean.
Is it possible for the crystal to be that far off? Also as i have no information and get find nothing on the web does anyone have a circuit or board layout? I don't want to start moving coils/trimmers without knowing what they do.
I am only 'guessing' that the multiplication factors are correct and that 10.7 MHz is the RX IF, but they do give sensible answers.
Malcolm
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Old 10th Jul 2016, 1:06 pm   #2
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Default Re: VHF Radio mic

Some possibilities:

1) never trust frequency counters until you've played about a bit with their triggering. They can miss edges or count ripples if dv/dt is low as the signal crosses the threshold.

2) Don't trust the number stamped on crystal cans. Crystals have a number of spurious modes and oscillators can go off on these. A lot of basic crystal oscillators don't have a tuned circuit to force operation onto the wanted mode and they just rely on the crystal manufacturer getting the thing to be most active on the wanted mode. This isn't too bad with fundamental operation, but it's risky with overtone units.

3) Crystals can suffer if dropped.

The proper mode can't have shifted that far. 1000 ppm pulling is possible with care on a fundamental crystal (pullability falls as the square of the overtone number)

4) Some crystal oscillators will oscillate free running with a (series mode)crystal shorted.


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Old 10th Jul 2016, 2:08 pm   #3
Herald1360
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Default Re: VHF Radio mic

Have you got a sig gen you could use to check the receiver's receiving on the nominal channel then inject 14.483 MHz into the TX Xtal socket and see what happens?
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Old 10th Jul 2016, 2:25 pm   #4
m0cemdave
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Default Re: VHF Radio mic

Where are you attaching the counter probe? I wonder if it is stopping the crystal and allowing the oscillator to free-run.
Also check the soldering around the crystal, the same thing may happen if the crystal isn't connected.
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Old 10th Jul 2016, 3:19 pm   #5
Malcolm G6ANZ
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Default Re: VHF Radio mic

David,
I'm fairly sure that the counter is correct. It shows the right frequency when measuring from the sig gen. The crystal may be running on an overtone .I was guessing at x12 to give 174MHz, It could be meant to be running at 28.996 and x6.

Herald,
I have tried injecting a signal from a sig gen direct but nothing happens. This may be because there is +10v on the aerial socket. Presumably to drive and active aerial so this upsets the generator.
The tx crystal is soldered in, and the board is full of surface mount bits.

Dave,
The crystal stopping is a possibility, but as I don't have a board layout its hard to know where to probe. I have tried probing, with the scope, at random points around the crystal looking for oscillation but nothing shows.

The pair have worked together at times but not reliably. This is what has led me to investigate the TX as it's more likely t have been subject to knocks etc.
As suggested I'll have a close look around the crystal and its soldering.

Malcolm
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Old 10th Jul 2016, 4:37 pm   #6
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Default Re: VHF Radio mic

Hi Malcolm.

Perfectly fine frequency counters work on one thing and erratically trigger on others, sometimes.

The radio mike will be simple FM so using a low freq transmit crystal allows them to use a phase modulator at umpteen MHz, then multiply by x4 and x3 to get to the TX freq. This process multiplies the FM deviation as well, so the phase modulator isn't worked too hard which would spoil the linearity.

A coax adaptor with a blocking capacitor in it is a useful accessory to any sig gen, scope, spectrum analyser.

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Old 10th Jul 2016, 7:43 pm   #7
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Default Re: VHF Radio mic

I'm not sure if it's relevant but these have a PLL pilot tone that rides above the audio, that only lets the receiver unmute once it's received the pilot. Hopefully probing the receiver will tell you if the pilot is being received properly irrespective of the audio. If that's ok then troubleshooting on the audio side might be the next step.
If the fault is on the 'portable' side I would look for drop-damage eg intermittent coils, cracked ferrites, dry joints etc. It it's a lavalier system try a new mic, bearing in mind the bias wont ride through a bad plug so well as audio!
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Old 11th Jul 2016, 10:30 am   #8
Malcolm G6ANZ
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Default Re: VHF Radio mic

Update. Having looked on the net for information I emailed Toa directly asking for circuits and board layout. Within the hour I had a reply with attachments for full service manuals for the Tx and Rx.
Full marks to Toa

Malcolm
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Old 11th Jul 2016, 3:49 pm   #9
Malcolm G6ANZ
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Default Re: VHF Radio mic

Update 2.
It appears that the TX crystal is dead. the oscillator appears to be free running at ~16MHz. I have contacted Toa and asked if they can supply a new crystal.
The TX does have a pilot tone of 32.768 KHz which is running on the TX, Waveform shoes on the scope. I have modulated the carrier from the sig gen with 32.768KHz and the RF present led comes on on the RX with the generator set to 173.8 MHz, so it seems that its a u/s tx
Malcolm
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