Sony C37 FET repair

Anyone have information on how to remove the HV converter in a Sony C7 FET?  I've got the manual
with exploded view and it appears to be up on top,  below the capsule. 

Regards,
Jeff

deveng said:

Anyone have information on how to remove the HV converter in a Sony C7 FET?  I've got the manual
with exploded view and it appears to be up on top, below the capsule. 

Regards,
Jeff

Click to expand...

Its in the silver capsule looking housing above the transformer. It also is not the reason that your mic has low output if that's the problem. It is also impossible to measure the voltage conventionally as it is a very high impedance circuit and reqiuires a special meter.  I would not mess with it as it is basically sealed shut. These mics are notorious for their low output. I suppose you could just cut the wires but you would have almost no capsule polarization voltage.

These are not my words but this is how it was explained to me by someone who knows these mics. "The voltage arriving at the capsule travels through 3 high-value resistors, totalling 500 megohms.  If the dc-to-dc converter is operating properly, the voltage at the mic should be 140vdc.  Unless you have a very specialized high impedance voltmeter, you won't be able to measure this voltage directly due to circuit loading caused by the input impedance of the voltmeter.  For example, if your volmeter has an input impedance of 10 megohms, current flow through the high-value resistors would drop the voltage by a factor of  1/50, or down to around 2.8 volts".

I think of this mic (I am fortunate enough to own 3) as a LDC with dynamic mic output and warmth.

This mic has no output at all.  The capsule looks perfect.  Since it has no output, and I can't measure the
polarization voltage, I can only assume the caps have probably gone bad in the HV converter.  I've checked the wiring and all looks good for battery operation. 

How difficult is it (and how) to get the HV converter out to replace capacitors?

Regards,
Jeff

Measure and test. What makes you think you need to change capacitors?

You can tell if you have capsule voltage.  The schematic I found shows a .01uf poly cap at the output of the Dc to DC converter. Do you have a storage scope?  You should be able to see a discharge curve.  RC time constant .01x10-6 X input resistance of measuring device.

What voltages do you get inside the microphone at the jfet drain and source and other nodes?

Are the switches good are the contacts clean?
Is the rheostat before the DC to DC open?

At http://www.coutant.org I found a manual. 

Gus said:

Measure and test. What makes you think you need to change capacitors?

You can tell if you have capsule voltage.  The schematic I found shows a .01uf poly cap at the output of the Dc to DC converter. Do you have a storage scope?  You should be able to see a discharge curve.  RC time constant .01x10-6 X input resistance of measuring device.

What voltages do you get inside the microphone at the jfet drain and source and other nodes?

Are the switches good are the contacts clean?
Is the rheostat before the DC to DC open?

At http://www.coutant.org I found a manual.

Click to expand...

Yes to all Gus said and my apologies for the wrong schematic I put up the c37p not the C37FET manual. Yes there are more moving parts in this mic, and I would look at the caps in the mics impedance conversion circuit first. Can you disconnect capsule and run signal through mic?

I don't have a storage scope.  I have not yet started any dis-assembly of the mic.  Capsule was only visually inspected.  I've done only basic checks for the wiring and measured approximately 7v at the input to the DC to DC converter at R6.  First I was looking for information on how to get the mic apart so I could measure at the head amp.  Since its not difficult to remove the capsule, I think that's a good place to start.  If I can inject a signal there a get good output signal then I can focus on a smaller group of parts including the capsule. 

The mic was supposed to be working when stored but has not been used for years.  It was stored well and the condition of the mic is almost mint. 

Regards,
Jeff

deveng said:

Anyone have information on how to remove the HV converter in a Sony C7 FET?  I've got the manual
with exploded view and it appears to be up on top, below the capsule. 

Regards,
Jeff

Click to expand...

The dc dc converter is actually, at the very bottom of the body, shielded in the can . Those are notorious for failing and often the oscillator transformers get open wiring (right now I have two C37's on my bench and both have the same problem). If that's the case you will need whether rewind that transformer, or make a new 140V inverter. Interesting enough, the oscillation set very low at 20Hz. 

Best, M

Marik,

How did you determine your HV converter was bad?  Also, if my oscillator transformer turns out to be
open, can you rewind for me?  Or is it something I could do?

Regards,
Jeff

deveng said:

How did you determine your HV converter was bad? 

Click to expand...

There was no voltage coming out of it 

Also, if my oscillator transformer turns out to be
open, can you rewind for me?  Or is it something I could do?

Click to expand...

I still need to look at it to see if it is worth rewinding, or easier and faster just to make a new one.

Best, M

Funny guy! 

Keep me posted on the option of rewind or rebuild.

Regards,
Jeff

Gus,

What voltages do you get inside the microphone at the jfet drain and source and other nodes?

Click to expand...

Took some quick measurements at the source follower JFET all referenced to ground.
Drain = 7.9V
Source = 2.8V
Gate = 0.38mV
Common node at R2, R3, R4 = 2.5V

Looks like the FET might have some issues.  Also, found the trimmer R6 was a bit flaky.  Was only 3.4v at input to HV
converter, when I adjusted slightly it jumped to 7.9V.

I also measured the HV at the top pcb with a 10Meg DVM.    Only got a few milivolts negative.  I expect either the HV converter
is bad or doesn't provide enough current to even get a measly 2-3V at the meter.

Regards,
Jeff

In my haste to check this mic out I made a mistake in the measures.    My 10Meg DVM won't provide good measures at
the gate or source pins of the JFET.    I measured 38mV at gate  and 2.8V at source but simulation shows I should get approximately 200mV and  7.6v respectivly (see attached schematic in previous post).    When I open the mic up again I'll try measuring across R3 (680 ohm) at source.    But, it may be quicker just to disconnect the capsule and inject a small signal, if it works then I can  focus back on the HV converter.  I simulated that circuit as well and found the HV converter should provide about 70V at the capsule, if working.

I still suspect its the HV converter and I'm looking at either rebuilding the original if the transformer is still good, or  building a new one based on modern  IC's used for camera flash chargers.  Parts count is about 25 so not sure if I can squeeze into the original housing.  Specs show 100uV maximum output noise.

Here's a possible replacement for the bad HV converter.  Circuit is based on a Maxim APD controller.  In this circuit its set to 70v but can be adjusted with the divider at the FB pin.  Input range is 5-11V.  Its capable of 5mA and  low ripple.  Very small 4mm x 4mm TQFN and low parts count means it could easily be built to fit in the location of the original.  Comments?

Regards,
Jeff

The LT3571 could be a good candidate as well.  Only 11 parts plus IC.  Its a 3mm x 3mm QFN package.  Also an APD bias controller. 

Regards,
Jeff

deveng said:

Here's a possible replacement for the bad HV converter.  Circuit is based on a Maxim APD controller.  In this circuit its set to 70v but can be adjusted with the divider at the FB pin. 

Click to expand...

Looking at the data sheet it gives 76V max (LT3571 you quote later is about the same). The Sony capsule needs 140V.

Best, M

Might have to look at a flyback controller if I can find the right transformer.

I got the HV converter out and it was not as tough as I thought it would be.  The can opened easily as well.  I checked the transformer, no opens.  I'll pull the electrolytics out and check them.  I'm verifying all the other components now and so far it looks fine.    If I get lucky, I won't have to deal with a new HV converter. 

Regards,
Jeff

Upon a more thorough check I found that the 3rd coil may be the issue.  Pulled the yellow, white and blue wires and (with red
being the common to tops of all coils) got 2.3 ohms red-->yellow,  2.2 ohms red-->white and then 90 ohms red-->blue. 

So its back to a new HV converter design.  I'm now looking a boost converter controller that will provide 140v.  I've got
samples on order.

Regards,
Jeff

Well I could slap myself for not staying on task.  I suspected a strong possibility for a non-working HV converter would be
dried out electrolytic caps.  I pulled them from the DC converter an quickly measured the capacitance and found they were
both "normal".  I didn't have any of these values on hand so I moved on.  Distracted by the possibility of a bad coil in the transformer, I measured and found the last coil to be 90 ohm.  Which is turns out is probably correct!  So,  I decided to just put in
whatever close capacitance values I had to see if it would come alive.  It did.  The surprise was that the HV converter is an inverting DC converter, 9v to -140v.   

I verified the converter is working by adding a string of high R resistors as a divider.  Measuring the last in the chain would not be significantly changed by a parallel 10Meg DVM nor current limit the converter.  I also found the switching frequency was very close to the spec at 22Hz.  I'll order new caps for the DC-DC converter as well as the head amp.  If these were bad its a good idea to replace the head amp electrolytics as well.   

I've already put some time into a new DC-DC converter retro-fit design so I'll go ahead and bread board one up.  The IC I have can work with the 9V or 48v input and can be designed to output either +140v or with a couple of changes, -140v. 

The moral of this story is don't trust standard capacitance readers for "good" or "bad" caps.  Even if the cap reads good capacitance value, it may have high ESR and send you on a goose chase.  I knew better so shame on me for falling for this after all these years! 

I should know if the mic is fully working for sure in a few days.

P.S>  Thanks to Stewart for sharing his experiences with his DC converters.  Thanks to him I got back on track with a repair.

Regards,
Jeff

Great that you have found an issue
You can also change tantalum C4 for low esr aluminium electrolytic.
In case like this for the quick test am using multimeter with diode checking mode.
Am checking all the diodes, electrolytic capacitors (especially tantals - common issue in C414EB) then transistors, transformers etc.
HV you should measure before R5 or even disconnect one leg.
After R5, with 10M DMM, you can use 1GOhm in series with meter (+) and change range for mV. Then you multiply the result. It's not so super accurate but it show approximate value much proper than 10M input.

Yep, the tantalum was already on my list for replacement with an electrolytic!

Regards,
Jeff