Replaced capsule in 3 pattern mic but only figure 8 is working

[DFischer]

I grabbed a couple of these while they were on sale, unfortunately they are back up to $200 at the moment. Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet. I seem to remember a thread about replacing the capsule with a capacitor and doing a frequency sweep with REW, but I need to look that up before I can get started.

 

[157audio]

I grabbed a couple of these while they were on sale, unfortunately they are back up to $200 at the moment. Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet. I seem to remember a thread about replacing the capsule with a capacitor and doing a frequency sweep with REW, but I need to look that up before I can get started.
View attachment 142822

Oh cool, I’ll look at that diagram and hopefully learn something! And wow, too bad the sale price is over. I picked up 2 tac700’s as well for $40 each, and another that I gave to my assistant as a gift. Glad I got that in under the wire or I’d be a little sadder about messing up that first one!

 

[micolas]

I grabbed a couple of these while they were on sale, unfortunately they are back up to $200 at the moment. Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet. I seem to remember a thread about replacing the capsule with a capacitor and doing a frequency sweep with REW, but I need to look that up before I can get started.
View attachment 142822

An interesting scheme

 

[micolas]

Oh cool, I’ll look at that diagram and hopefully learn something! And wow, too bad the sale price is over. I picked up 2 tac700’s as well for $40 each, and another that I gave to my assistant as a gift. Glad I got that in under the wire or I’d be a little sadder about messing up that first one!

Too bad it got expensive.
It's a good platform for mods.

 

[DFischer]

I think Khron nailed the polarization scheme in post #16. I'm just a hobbyist, so I have no idea how the DC/DC voltage section works and I can't tell if the circuit is flat or de-emphasized by just looking at it.

 

[Khron]

Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet.

Yes it does - very, very close to the U87A, albeit i'm not 100000% sure you reverse-engineered the capsule connections quite right. Pretty sure "pin 4" should go to ground (too), though.

But the rest is indeed very U87A-like - C16 should govern the high-frequency attenuation.

 

[DFischer]

The metalwork is excellent. I love the die cast frame and the 87 style angled grille. There's also no ringing in spite of the steel body. It seems well damped by the construction.

 

[DFischer]

Pretty sure "pin 4" should go to ground (too), though.

It sure does! There are a few very small plated thru holes on these boards that can be easy to miss. Schematic edited.

 

[micolas]

Yes it does - very, very close to the U87A, albeit i'm not 100000% sure you reverse-engineered the capsule connections quite right. Pretty sure "pin 4" should go to ground (too), though.

But the rest is indeed very U87A-like - C16 should govern the high-frequency attenuation.

But the sequence C14 15 16 does not change the phase of NFB?
Shouldn't C15 be connected to Drain jFET?
C16 of 220nF seems huge.
R16 20Mohm?

***
Are you sure the values are written down correctly?

 

[DFischer]

C16 should govern the high-frequency attenuation.

Would removing it result in a flat circuit then? And I assume that increasing it's capacitance would result in more roll-off, like on the AKG Perception mod?

 

[DFischer]

Shouldn't C15 be connected to Drain jFET?
C16 of 220nF seems huge.
R16 20Mohm?

I checked C15 again and I think it's location is correctly shown.

C16 is a Wima marked" 220/100-". I think that R16 in parallel is keeping me from getting a good measurement in situ.

R16 is marked "206" and measures about 20Mohm.

 

[micolas]

I checked C15 again and I think it's location is correctly shown.

C16 is a Wima marked" 220/100-". I think that R16 in parallel is keeping me from getting a good measurement in situ.

R16 is marked "206" and measures about 20Mohm.

Wima 220/100 means 220pF/100vcc

 

[soliloqueen]

this is one of 797's stock mics, which are kinda good. this is the cr86
View attachment 20230911_38c180ac4f7abf55_427364498796_mp4_264_hd_unlimit_aliyun.mp4

 

[micolas]

this is one of 797's stock mics, which are kinda good. this is the cr86
View attachment 142825

I noticed it too. It is more difficult to obtain here in the EU
I also received an invitation to the 797 presentation on their Facebook group