CAD GXL3000 Intermittently Loosing Capule Voltage

[Potato Cakes]

Hello, everyone,

Years ago I modded two CAD GXL3000 with a Linear 87 mod from Microphone-Parts.com. I changed out the capsule and the kit changes out of the two stock boards, which has the FET, capsule connections, and the DC blocking cap on the FET drain. It worked wonderfully until about 6 months ago when audio would die when you started talking into it. When you stopped talking, you could hear the audio come back and any low level ambient noise that was in the room. You resume talking into it and it would work for about 5 seconds then die again. Then the ambient noise. Then repeat. I started with changing out the FET but that did not solve the problem. I did notice that the voltage on the source of the FET (2N3819) was about 2VDC when it was "working" with just low level ambient noise. When you talk into it and the audio dies, this measure 4V and stays there till the audio comes back. On the stock board that was left, there is a 2N5551 in place of the 2SA1015 which I don't believe I changed.

To me it sounds like the voltage on the capsule it self is being drained completely when you speak into it and then it recharges when not in use. The electrolytic capacitors in that would be connected to phantom power test fine, so I'm suspicious of the capsule even though I've never seen this happen. The 2N5551 could have something to do with it but doesn't explain the voltage change on the source connection of the 2N3819 that corresponds to when the microphone does and doesn't pass audio. Curious if anyone else has seen this and if I'm way off in my current assessment.


Thanks!

Paul

 

[Khron]

C8 electrically leaky? Could also measure the voltage where R11 connects in-between the two source resistors.

Photos of the FET area?

 

[Potato Cakes]

I'll get some photos of the FET area when I get back to the studio. C8 on that schematic is not going to be the same as that was on the board that was replaced with the mod, for which I do not have a schematic. However, your inquiry regarding C8 has led me to my answer I think. I will report back soon.

Thanks!

Paul

 

[Potato Cakes]

Here is the FET board. Unfortunately the issue was not what I thought. I think I will replace the electrolytics first and see what happens.

Thanks!

Paul

 

[pucho812]

Are you sure there isn’t a capsule issue?

 

[Potato Cakes]

I am not sure. But I don't have a spare capsule. It will be easier to change out the electrolytics first.

Thanks!

Paul

 

[mjmajilton]

Hello, everyone,

Years ago I modded two CAD GXL3000 with a Linear 87 mod from Microphone-Parts.com. I changed out the capsule and the kit changes out of the two stock boards, which has the FET, capsule connections, and the DC blocking cap on the FET drain. It worked wonderfully until about 6 months ago when audio would die when you started talking into it. When you stopped talking, you could hear the audio come back and any low level ambient noise that was in the room. You resume talking into it and it would work for about 5 seconds then die again. Then the ambient noise. Then repeat. I started with changing out the FET but that did not solve the problem. I did notice that the voltage on the source of the FET (2N3819) was about 2VDC when it was "working" with just low level ambient noise. When you talk into it and the audio dies, this measure 4V and stays there till the audio comes back. On the stock board that was left, there is a 2N5551 in place of the 2SA1015 which I don't believe I changed.

To me it sounds like the voltage on the capsule it self is being drained completely when you speak into it and then it recharges when not in use. The electrolytic capacitors in that would be connected to phantom power test fine, so I'm suspicious of the capsule even though I've never seen this happen. The 2N5551 could have something to do with it but doesn't explain the voltage change on the source connection of the 2N3819 that corresponds to when the microphone does and doesn't pass audio. Curious if anyone else has seen this and if I'm way off in my current assessment.


Thanks!

Paul

Sounds like a bad capsule. I’ve seen that symptom many times. The 35mm capsule in that mic sounds great. You can get a new one from Advanced Audio.

 

[Voyager10]

Can you disconnect the lead to the capsule front diaphragm, then try the mic in omni or figure-8 mode (talking into the back of the mic)?

If it stops misbehaving it's probably the front capsule that's bad. If it does the same, it's probably one of the capacitors starting to break down.

 

[Potato Cakes]

I did inject signal into the FET with the capsule disconnected and it the audio was consistent, so my initial thought about the capsule being bad is probably the case. I have just never had a capsule go bad so I thought that something else like a bad cap was more likely.

Thanks!

Paul

 

[Khron]

I did inject signal into the FET with the capsule disconnected and it the audio was consistent, so my initial thought about the capsule being bad is probably the case. I have just never had a capsule go bad so I thought that something else like a bad cap was more likely.

Thanks!

Paul

Is the capsule particularly dirty? Or can you measure any (meaningful) resistance between the backplate and the diaphragm wires?

 

[Potato Cakes]

Have not gotten that far yet. I'm doing three things at once and I was hoping to find the issue without taking the head basket apart. Looks like that is now not an option. I'll see if I can get back to this in the next couple of days.

Thanks!

Paul

 

[Voyager10]

Looking at the photo, the yellow wires appear to be the capsule's front and rear diaphragm connections, and can be unsoldered without opening up the headbasket. The front and rear are likely to develop faults independently (through dirt or damage) so you can test each one in isolation by disconnecting the wires one at a time (use omni mode). If the fault disappears when one side is disconnected, that side of the capsule is faulty.

 

[radardoug]

Classic symptoms of a dirty capsule.

 

[Potato Cakes]

So the front capsule looks like it got dust on it and somehow exposed to moisture, creating circular patterns which do not come off with wiping with a dry cotton swab. The back side has the typical dust particles which are easily brushed off. Never seen this before but I guess you learn something new every day. I'll have to set aside some time to do the whole cleaning procedure.

Thanks!

Paul

 

[pucho812]

Capsules don’t go bad per se, they do get dirty and sometimes get moisture trapped under the membrane. This causes the capsule to short which is heard as audio dropouts.
You can clean it and that can address a lot of the issues. You can reskin it but that requires special tooling. A clean capsule will address your drop outs.

 

[Khron]

So the front capsule looks like it got dust on it and somehow exposed to moisture, creating circular patterns which do not come off with wiping with a dry cotton swab. The back side has the typical dust particles which are easily brushed off. Never seen this before but I guess you learn something new every day. I'll have to set aside some time to do the whole cleaning procedure.

Thanks!

Paul

A stopgap solution would be to turn the capsule around (so what was the rear becomes the new front).

And i would argue that, with center-terminated diaphragms, it's more important that the clear ring area of the diaphragm is clean, rather than the metallized area. Dust and moisture there will be a far greater issue (diaphragm shorting out to the backplate) than the added weight of the dirt. Slightly depends on the amount though, of course...

 

[Potato Cakes]

I may just replace the capsule all together. I don't remember which one I put in there. This will be on the back burner for a couple of weeks. I'll figure it out later. Thanks for everyone's help!

Thanks!

Paul

 

[orenj]

Hey sorry to revive this conversation but I also got an GXL3000 from CAD microphones. It started recently to having issues. When I use it in cardioid, omni or eight figure polar systems. It works. Then I come near It and start talking It cuts out for some seconds and the start again to work. Sometimes It make an ambient sound in recordings like an electrical rumble. I have opened It unscrew the capsule and I dont find any issues except a part black and greasy underground the capsule like a gummy cylinder (I dunno if its normally that black and greasy.) I need help

 

[Khron]

I have opened It unscrew the capsule and I dont find any issues except a part black and greasy underground the capsule like a gummy cylinder (I dunno if its normally that black and greasy.) I need help

You could consider posting some pictures? One is worth a thousand words, as the saying goes...

 

[orenj]

Yes totally, here somes pictures. I have buy this mic from it's original owner last year and was working until last week. I doesn't owned any other CAD GXL3000 so I doesn't really know how it is supposed to look alike.

Thanks for your help ! And your fast answer.