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Restoring A 47 Year Old Piezo Wireless FM Microphone

By Jestine Yong on October 14, 2022

Several weeks ago an old friend brought some used but still useful for parts boards. Including a badly old rusted Wireless FM microphone that no longer worked. It was from a Japanese firm called PIEZO and was manufactured around the year 1975. Following photo shows the inside transmitter board that looked like it was wired wrong as I noticed the wrong blue and red marked tracks. Because the 3 transistors on this PIEZO FM WIRELESS MICROPHONE board all were NPN types which always means collectors to + of the 3V battery voltage and the emitters to ground! Which here was colour wired the opposite because the blue marked tracks and black battery wire must be the plus pole of the 3V voltage. And is normally never blue but a red marking. The 3V comes from 2 penlights in its battery holder. But because the black wire was indeed connected to the plus side of the battery holder and the red wire to the min side of both batteries it correctly solved this odd opposite colour wiring. Maybe something the manufacturer decided to do to prevent their design from easily being copied, or perhaps to prevent unauthorized servicing by unqualified users?

Before I continued examining and testing this device any further I first had to remove all rust. Most likely after the batteries had leaked and also badly contaminated the metal case with its chemical substance causing the brown rusted metal plating all over. And maybe this Microphone had also been stored in a very moisty environment in the past for way too long.

And I already had used my KEMO M128N up to 3.6GHZ working Wifi Spy Detector to check if the Wireless Microphone transmitted any signal. But my Spy detector failed to see any active oscillation. So I started removing all rusted screws, completely disassembled all parts and started cleaning the metal from its badly corroded state with enough washing powder soap.

And by using a toothbrush and a small nail file and more soap managed to remove most of the brown rusted metal surface. After a day most of it was gone and I started spraying black paint over the metal plating to preserve the case from any further future rusting. Next photo shows the solder board side.

And here follows the components side view:

Examining the transmitter gave following connections and next following circuit:

Below the circuit again but now drawn in Tina design suite version 11.

Previous photo showed the correctly wired board under test. Black wire to gnd board (was the originally red marked track) and the red wire to the plus 3V track (was the originally blue marked track).

To test if the primary coil of HF transformer M1 showed any oscillation which also is the collector of T2, I soldered a short piece of solder wire to this track as antenna. And moved my mini HF detector annex spy finder near it. This time it clearly showed an active HF signal by showing its red blinking led.

And to confirm this also my Tektronix oscilloscope showed a perfectly at 100 MHz oscillating sine wave on the collector of T2.

Somehow the HF detector however still didn’t see any oscillation on the antenna output of this FM Microphone. But my scope showed a weaker signal on that track. So I of course also had checked T3 on my Peak Atlas tester which said it was fine. And also both 470 ohm resistors in emitter and collector connections of T3 were fine. When checked on the FM radio band the Microphone transmitted a clear and excellent audio signal through its antenna. And both the grey trimmer capacitor and the HF core of M1 worked excellent in tuning to an empty FM channel. But to rule out that transistor T3_2SC930 was still defect somehow I already ordered a set of those new HF transistors. To see if replacing transistor T3 helps to enhance the transmitted power on the antenna output. Which also confirms if my Peak Atlas DCA75 Pro correctly approved HF transistor T3. But in case the secondary coil of HF transformer M1 plays any part in this is also something that can still be checked.

Anyhow to make this repair complete I now also list here all components, including there positions on the small pcb, and the BOM list. First photo shows the resistors and the transistors.

Following positions of coil L1 and HF core M1

PS: keep in mind that I did not measure the inductance of the coils. But I did measure the size of the coils and core measurements as good as possible. (see next photos for more info on these coils).

Above all capacitors used in this FM transmitter.

Above sideview pcb with M1 and grey trimmer on the left.

Above sideview pcb with coil L1 on the right. Below Mini Spy finder detecting signal on collector of T2.

This Spy finder also detects my working 3GHz phone signal. But it fails to see my 4G mobile phone.

Here follows the BOM list of all parts.

Next last photos show the now restored working about 47 year old Wireless Microphone. First the now nicely restored and repainted battery holder case. New paint, new solder. New screws.

To check and make sure that the last transistor T3 stage in above given Tina circuit was correctly working the circuit was simulated showing a correctly working oscilloscope diagram (see next picture). Although already was clear that the FM Microphone without antenna indeed was not working so T3 obviously was here probably just used as a buffer stage to prevent being a load to the oscillator stage with T2 for frequency stability reasons.

The above shown (in green) oscillator sine wave signal was with more than 6V about 5.8 times stronger than the antenna output below it (in brown) at here just about 1.2V. Because as above scope screen shows the Antenna output is only 1V/Div while the Mixer oscillator wave is 2V/Div. Although the results in this simulation still may differ from the actual results in my working FM Microphone circuit.

But the generated oscillating frequency is here about 100MHz (f =1/T is 1/10nS) like it was on my Tektronix 2465A oscilloscope. I just took the values for L1 and HF core M1 so that my circuit was correctly simulating. Still what the real L1 and M1 coil values were was as said not yet measured only their coil sizes were given. And in Tina the 2SC930 was not available so I therefore had to select a HF BF240 transistor which is an AM/FM transistor. And in Tina it was impossible to find the right M1 core that matched the HF transformer. Apparently the right HF core with adjustable core was not available. And neither was it possible to select a transformer with opposite wired coils. Therefore in the simulation the oscillator wave and the Antenna signals are 180 degrees out of fase with each other. Most likely the HF core in the FM Microphone was differently wired. (Which normally would mean M1 HF primary coil with one dot marking in its top, and secondary coil with one dot marking on its bottom side).

Sadly as legit licensed Tina 11 user my older version also misses on all the new enhancements and many bugfixes that only the new Tina Design Suite version 12 comes with. And there is no lifetime support from DesignSoft for users like me who nevertheless paid over 600 top Euros for a now dated

version that can’t even read the new version 12 .tsc schematic design files! Even the new Tina 12 book with .tsc file examples and training lessons I bought is practically useless since none of the files can be used on my legit older Tina 11 version! Apparently Designsoft doesn’t even care to present buyers with a simple file converter tool so also older Tina version users can still convert the new file formats into older formats. And I sure had already asked them in vain! Not even their 30 days trial version of Tina 12 has the option to save any of the projects to older Tina versions.

Which means that my older Tina version 11 got obsolete and was no longer supported as soon as version 12 came out! In a way DesignSoft is completely degrading its own simulator software this way!

Only new Tina 12 buyers/users are able to convert their files to version 11 or older which is just insane! With my older Tina 11 version I am only able to save .tsc projects to version 7,9,10 and of course also into Tina 11 file formats. And also the circuit design size in Tina was very limited which I noticed when I made the article about the fully in TTL/CMOS designed Exachron DCF receiver circuit. Below are the links:

https://jestineyong.com/exachron-dcf77-time-pulse-receiver-repair/

https://jestineyong.com/exachron-time-pulse-receiver-repair/

That if not in size surely failed in simulation speed! Although it was completely drawn in Tina as circuit, it still was completely dead in the simulation. And the promised multicore cpu support to speed up the program never showed up either. Why I ended up in only simulating several smaller parts of the Exachron circuit.

So be aware of this very annoying Tina file limitation when you are also thinking of buying a new expensive Tina version!

Know that In the meantime a previously very expensive commercial circuit simulator became completely free!

Micro-Cap12! That simulator before they did quit doing business did cost as much as $4500! See here for more info =>https://hackaday.com/2020/01/08/commercial-circuit-simulator-goes-free/

I myself haven’t used that great designer/simulator program yet but if you do not have the Tina Design Suite it sure could be a fantastic start of your career in designing and simulating electronic circuits!

This concludes this small but very complete service manual on the PIEZO Wireless 88.1-106 MHz FM Microphone that after 47 years and a lot of special attention still operates. A manual with circuit and all data that like often couldn’t be found anywhere online.

Some additional info on the PIEZO Wireless FM Microphone:

Although the Tina circuit did show 2SC930 transistors for T2 and T3 in the article, they were in fact just the mentioned AM/FM BF240 transistors in the Tina simulation renamed into the originally used NPN transistors. Because as said the Tina device list on transistors and other parts is very limited!

Afterwards I also experimented with the Tina circuit in the simulation to try if it somehow was possible to enhance the Antenna power output.

And found that changing R8 from the 100k value into a 10k resistor about doubled the amplitude of the Antenna signal on the virtual oscilloscope without decreasing the oscillator output of T2 in any way. And when applied in practice on the Microphone circuit it showed that my HF wifi spy detector finally was able to also detect the RF output on the lower end of the antenna wire. And according to the simulation also changing R10 into a higher 1kohm resistor helps to further increase the output of the Antenna signal.

Odd thing however was that in the Tina simulation the voltmeter still measured a steady 3V on the Antenna output, although the amplitude of the sine wave practically had doubled!?

In theory we know that the dc current of a signal is equivalent to the effective ac (rms) current value but here it stayed completely unchanged even after the amplitude of the Antenna output doubled?

Another uncommon but good thing about this PIEZO Wireless Microphone was that accidentally reversing the battery polarity does nothing bad to this circuit at all! It simply just won’t work in that case. All thanks to the current limiting emitter and collector resistors in the 3 transistor stages! And because of the previously rusted battery contacts the Microphone at first didn’t seem to work at all.

Albert van Bemmelen, Weert, The Netherlands.

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Note: You can read his previous article on Investigating In-& Outdoor DVB-T2 Antennas

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Microphone Repair

13 Comments

Paris Azis

October 15, 2022 at 1:58 am

Interesting article for RF enthusiasts, Albert! many years ago I used to play with circuits like this one...So the article was a travel back in time for me...Every friend of mine wanted to have an FM transmitter for himself at that time, just for having fun by transmitting music for others, no matter that this was not a legally allowed act...Fortunately the transmission range was very limited and we had no troubles with the authorities...!!

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- Albert van Bemmelen
  
  October 15, 2022 at 3:45 pm
  
  Yes that was a great time Paris! In that time in the 80's most of those pirate music stations operated from Belgium. And I heard that also in my home town someone even was caught with a live tv pirate station. I myself however never saw that channel in action on tv, only heard about it and knew who that person was. But I also had built such a working VHF tv transmitter with in its endstage a 2N3866 transistor with only very limited rf output power. It was from an article in Dutch magazine Radio Bulletin.
  And then came the release of the 27 Mc citizen band (Dutch: Bakkies) still at first only for 500mWatt RF power. It was great fun and also could be useful, but all of course before todays mobile phones existed. 27Mc probably was also released at that time because transmitting tv channels over cable instead of air made TV signal interference less likely. Especially in the event someone still used an illegal RF power amplifier to enhance his limited RF output.
  
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  - Albert van Bemmelen
    
    October 16, 2022 at 4:37 pm
    
    PS: I read in old Dutch Radio Bulletin magazines from the 50's that in the beginning of tv reception motors of mopeds frequently caused interference with reception of the Amplitude Modulated (AM) television screens. Which caused disturbing horizontal lines on tv screens in passing by any tv antenna. You probably know that television images are not transmitted as Frequency Modulated (FM) images because of the then much larger required bandwidth, why they are modulated in AM. Which normally only creates 2 in size limited sidebands, and makes it even possible to remove one of the sidebands saving power. Also still used by radio amateurs on Short Wave Bands (SSB modulation). Therefore in those years they obligated by law engine noise filters on mopeds.
    
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Imoudu O

October 15, 2022 at 4:56 am

It's only a genius electronics repair guru like you can carry out this kind of repairs, with on hand tools.It's also surprising some of the components on board are still in good shape after 47yrs of manufacture,which it's not common these days in manufacturing industry.Thanks for sharing sir.

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- Albert van Bemmelen
  
  October 15, 2022 at 3:52 pm
  
  Yes Imoudu, that was also what I thought when I noticed the badly condition of the rusted case compared to the nice looking pcb board. Afterwards all components were fine but not the corroded battery contacts and the case itself after the batteries must have leaked and had deteriorated the inside.
  
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Parasuraman S

October 15, 2022 at 12:44 pm

Vow! It is a very extensive article with full of technical details! There cannot be one to match your patience and perseverance! Hat's off!

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Mark

October 15, 2022 at 2:36 pm

Well done Albert on a successful repair and a very detailed article

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- Albert van Bemmelen
  
  October 16, 2022 at 4:16 pm
  
  Thanks Mark!
  
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Albert van Bemmelen

October 15, 2022 at 3:55 pm

I am very sure you can dear Parasuraman! Particular your many difficult lcd tv repairs are not my cup of tea!

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Albert van Bemmelen

October 15, 2022 at 4:41 pm

PS: This Wireless Microphone likely uses a dynamic microphone. So not the piezo type. PIEZO is here the name of the manufacturer in Japan. And not even the radiomuseum.org website that has a very large database of circuits on old devices had any info on this WX-172 Wireless FM Microphone. That radiomuseum is
online located at => https://www.radiomuseum.org/r/unknown_piezo_fm_wireless_microphone_wx_172.html

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- Albert van Bemmelen
  
  October 15, 2022 at 5:48 pm
  
  By-the-way: The dynamic microphone that was used in this FM PIEZO WX-172 wireless microphone transmitter measured 257 ohm on the 2k scale.
  
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Waleed Rishmawi

October 18, 2022 at 4:12 pm

Every time I encounter a long and detailed article it has to be from you. Again a lot of information and great repair success. Keep up the good work.

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Yogesh Panchal

October 20, 2022 at 5:00 pm

Albert, Thanks! for sharing wonderful information.

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