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Author Topic: Sony C37FET or C38 9V Battery overheating  (Read 750 times)

hekho

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Sony C37FET or C38 9V Battery overheating
« on: May 18, 2024, 06:25:30 AM »

Hello,

I bought online what I thought to be a Sony C38B and it is something else. I have two C-38B and the grill are different. Also this MIC does not have High-cut no 8DB Pad.
The Mic has a lot of issues:
-Phantom Power does not work
-9V Batterie power it but overheat after a few minutes
-With 9V Battery,  sometime the MIC does not capture any signal, sometime it does but with a regular and fast paced repetitive noise like a metronome

I am posting this message in a first attempt to make sense of all the information available for those MIC. For a start, I would like to identify the MIC and also figure out if it can work with P48 because mine does not power on in presence of 48V.

I have read that Sony C38 (C37Fet) can only be powered by 9V battery or external power supply but the MIC has only 3 PIN.
And in the following link (in German) it says that it can be powered by P48.

http://www.vintageaudiorentals.com/de/artikel/vintageaudio-445.html

Thanks for your input and idea how I can troubleshoot this.

Best regard from a French lost in Germany.

Hekho

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Kai

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Re: Sony C37FET or C38 9V Battery overheating
« Reply #1 on: May 18, 2024, 06:00:06 PM »

Whether the mic can be repaired depends on the capsule's health.

Obviously (“heating up”) there’s a defect in the electronics.
Could be a capacitor that stopped working or anything.

Find the part that gets hot: this part, or something close to this part, is defective.

Further on disconnect the capsule, and feed a generator signal to the amp, through a 47 pF cap.
Then trace the signal with an oscilloscope.
Also check the internal DC voltages.


Of course, identifying the model and finding a schematic would ease the repair.
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klaus

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Re: Sony C37FET or C38 9V Battery overheating
« Reply #2 on: May 18, 2024, 11:06:47 PM »

All good advice.

What initially helps me when I encounter overheating: I look for charred/discolored components. I also use my nose and sniff around the circuit to identify heat-stressed components.

Any decent tech with troubleshooting experience will have an easy time analyzing the mic system's faults through metering.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

hekho

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Re: Sony C37FET or C38 9V Battery overheating
« Reply #3 on: May 19, 2024, 02:14:57 PM »

Thank you Kai and Klaus for your help and suggestions.

I found the service manual and it's indeed the C-37FET (named C-38 in Japan).
https://www.coutant.org/sonyc37fet/index.html

It's possible to power it with an external power supply (Sony AC-109), but it would require re-soldering the cable to a 5 pin Cannon male plug.
I found an old link to the external power supply for those who are interested in the topic:
https://www.worthpoint.com/worthopedia/vintage-pin-microphone-phantom-power-1848657180

The battery itself is overheating, I could not find any other parts that is heating. There is a "Operation check meter" that remains permanently red. According to the service manual, it indicates a problem in the power supply...

And finally the XLR termination is also one of the problem maker. Just pushing the cable near the XLR plug is enough to get output from the microphone. I will replace the plug by a new XLR male. The current plug is a female, which is a pain to be honest.

Here's a recording test where we can hear the strange noise. Without it, the microphone would sound decent to my ears.
https://www.dropbox.com/scl/fi/86a36d3xb3aqzx3dwbauy/Sony-C37FET-noise.mp3?rlkey=qi7ivx7tom1n7l57jkv4jjta3&dl=0



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hekho

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Re: Sony C37FET or C38 9V Battery overheating
« Reply #4 on: May 24, 2024, 05:04:15 PM »

Just a quick report on the repair. The power issue and battery overheating were both related to the cable. I cut a good 10cm of the cable and replaced the XLR connector. Now there is a consistent signal coming from the mic, the "only" remaining problem is the "helicopter" noise.
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Kai

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Re: Sony C37FET or C38 9V Battery overheating
« Reply #5 on: May 29, 2024, 07:15:16 AM »

Once fixed, according to the information given in the linked technical manual, the Sony C37fet should be easily rewired/modded for standard P48 Phantom Power supply.

What’s needed:
A pair of resistors 9100 Ohm 1/4W, value-matched with a tolerance of 1% or better.
A Zener diode 10V 1/2W for protection.
This fits inside the XLR plug.


The original, 5-wire cable should be terminated to an normal 3-pin XLR male plug like this:

XLR Pin 1 and XLR case: Screen and white wire, Zener diode unmarked side.
XLR Pin 2: Red wire and resistor 1 one side.
XLR Pin 3: Black wire and resistor 2 one side.
Green wire to junction of: resistors 1+2 other side and Zener diode black ring side.

Check the voltage at the green wire to screen / pin 1, it should be ca. 8-10 V.
If you see just 0.7 V the Zener diode is reversed.


Explanation:
C37fet is already terminated for use with an external power supply.
The factory original cable has two extra wires for this purpose:
White for the negative PSU polarity.
Green for the positive PSU polarity.

The mic is drawing a current of 5 mA, which is in the range the P48 Phantom Power can supply.
So it’s just about dimensioning the series resistance to achieve the correct 9 V supply voltage.
P48 is made to be exactly equally loaded from both hot and cold signal wires, therefore two resistors matched for the same value are used to feed the mic.
Absolute resistor value is not important.
The C37fet is designed to work from 7-10 V battery voltage.


The 10V Zener diode is for over-voltage protection in case the mic draws less than the specified 5 mA of current, or in case the standard P48 supply works out of standard, to prevent the capsule bias going too high.
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hekho

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Re: Sony C37FET or C38 9V Battery overheating
« Reply #6 on: June 05, 2024, 02:47:13 PM »

Thank you for the detailed answer. For sure the mod will challenge my soldering abilities, but I will give it a try :-)
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