Audio-Technica ATW-R03 VHF radio mic receiver fault

2018-08-20 23:07:08

I'm trying to resurrect one of these VHF radio mics which hasn't been used for many years. All I get out of the output jack is noise, whether or not the mic is turned on. It sounds very much like it's gone off tune (it should be 175MHz).

Inside, the receiver there are 5 ferrite core coils and 2 pots. I guess if I knew how to adjust them I could fix it. No sign of a service manual online that Google could find.

Suggestions, anybody?

Regards - Philip

Re: Audio-Technica ATW-R03 VHF radio mic receiver fault

Reply #1 – 2018-08-21 11:57:57

If it got out of tune, it probably is because the caps dried up. So I'd replace bulging caps first, etc.
Without having a name of the product, it's difficult to give any tips where to start.

Are there any markings next to the pots? Also, are you sure the frequency isn't user-selectable?

What kind of test equipment do you have (multimeter, frequency counter, oscilloscope)?

Re: Audio-Technica ATW-R03 VHF radio mic receiver fault

Reply #2 – 2018-08-21 17:07:53

Thank you, but the sort of caps used in tuning with be in pF range, probably ceramic or polyester, which don't dry out. It's electrolytics that suffer from that problem. And I've checked the on-board linear regulator (7810 - first time I've met one of those!) and it's giving a steady 10V as expected.

The frequency is not user selectable (certainly not with the pots - they'd be nothing like stable enough). It's clearly factory set.

I have another one with a lapel mic, 173.8MHz, which works. Taking another look at them both, they have different crystal frequencies - 61.5 and 61.9MHz. On a first glance I thought they were the same. Now I notice that 61.5 * 3 - 10.7 = 178.3. Also, 61.9 * 3 - 10.7 = 175. So the 3rd harmonic of the crystal is being used as the local oscillator to produce the very common IF frequency for VHF radios of 10.7MHz.

So I swapped the crystals and found that both the receivers work with the 61.5MHz crystal for the 178.3MHz mic. So my problem is actually with the 175MHz mic, not the receiver. This has a 14.573MHz crystal, which is close to 175MHz / 12. So does anyone have any experience with these?

Re: Audio-Technica ATW-R03 VHF radio mic receiver fault

Reply #3 – 2018-08-22 09:52:14

For anyone who stumbles on this thread with similar problems, I couldn't initially understand how the transmitter managed to frequency modulate a crystal oscillator. It turns out that you can pull the crystal frequency by just a small amount with a varicap diode. Which is why it uses a 14.573MHz crystal. This is multiplied by 12 by taking the 3rd harmonic of the 4th harmonic (or vice versa), and in the process, the deviation is also multiplied by 12. My problem must be somewhere in this area as the receiver is not detecting a carrier. I'd need a 200MHz scope or spectrum analyser to diagnose it further.