Sony C37 FET repair

[deveng]

For those interested, here's some pics of the HV converter after repair.  Note, there is a 200Meg resistor
in the shrink tube on the HV output line.  That messed me up at first as I didn't notice it. 

 

[deveng]

Another one, back in the container with some of the foam to keep the loose transformer from bouncing around.  Another piece of foam is placed on the opposite side of the transformer and then the final thin piece over the top of it.

 

[deveng]

.....just need to put a couple dabs of adhesive on the edges of the pcb board to attach to the container and
then bend the tabs back over.  Ready to re-install. 

I've got the new head amp caps but haven't installed yet.  Hopefully today.  When I get those caps in I'll test the mic before I re-mount the DC-DC converter.    I'll probably replace the trimmer inline with 9V to adjust DC to the input of the converter.  The original is a bit flaky.    The new one is bigger but I think I can squeeze it in. 

Regards
Jeff 

 

[deveng]

This mic doesn't have the external 9V supply and I hate replacing 9V batteries.    Not to mention the cost.  If the mic works, I'll build an external supply for it.    Down the road it would be nice to have a new HV converter that runs off phantom.    If the new converter is a required it would only take a couple of mods on the mic to make it work at 48v.

Regards,
Jeff

 

[deveng]

The mic is alive. 

Since I was confident it would work, I went ahead and re-installed the DC -DC converter.  Replaced the trimmer and both the 30uF and 10uF caps on the head amp pcb.    Actual original value of the "10uF" tantanlum was 1.5uF.  Not sure where the trimmer should be set though. and there's nothing in the manual describing where that should be set.  I removed the flaky one, measured it and set the new trimmer to the same value.  At the original value it set the input to the DC converter to 6.5v.  That seems low.    Does anyone have information on how to set this trimmer?

Next step is to do some listening tests as thus far I only know the mic passes signal.  Damned if I can't get that silly spring for the internal 9v battery back on!

I'll post some pictures of the head amp pcb as soon as I can.

Regards,
Jeff

 

[deveng]

Head amplifier photo before cap replacement.  The 2 replaced are the Elna 30uF on the left and the small 1.5uF tantalum hiding behind the 103k greenie to the right. 

Regards,
Jeff

 

[deveng]

Does anyone here have a working C37FET that can measure the voltage just past the trimmer going into the DC converter.    Also,
as long as you're there measure the trimmer value?

Regards,
Jeff

 

[deveng]

Recorded some scratch vocals with this mic and was pleasantly surprised at the sound.  Smooth with no sibilance.  Output is low though.  I used one of my custom API 312 mic pre's with an EA2503 and an original Huntington NY 2520.  Nice.

Regards,
Jeff

 

[charlesaudio]

Sorry for old thread revival. Would you have cues for disassembling this mic ? It's not an easy task. I'm working on a c38b but it's close enough for the construction.
Thanks in advance.

 

[deveng]

Sorry for old thread revival. Would you have cues for disassembling this mic ? It's not an easy task. I'm working on a c38b but it's close enough for the construction.
Thanks in advance.

Document everything with pictures. The disassembly is not trivial for some of the mic's internals. If you are attempting to remove the HV converter it's buried up under the capsule and will require some delicate work to get it out. One particularly difficult item was reassembly of the battery holder spring mechanism. I had to rig up a custom tool to get that back together successfully. If the work you are doing does not absolutely require removing that, don't do it. It's been a long time now so I don't remember all the details of the disassembly.