Hey guys,

I was wondering if someone could help me troubleshoot this mic mod I'm working on, swapping out the circuit board on a Sterling ST-55 with a microphoneparts 84-55 circuit.

The build went pretty well - I accidentally singed a cap pretty bad my first go around, so originally only the HPF channel was working, but I swapped it out. I plugged it into a 48v preamp and the mic was sounding great - low noise, great signal, obvious improvement from the original. But then after 30 seconds or so the signal started waning, and then became intermittent. (The same had happened my first time around, except only with the HPF signal.) DC voltages across the board are within range of what they should be.

My first thought was that there could be some problem with the capsule. I carefully re-flowed the connections to the capsule at the very least, to try to rule out any weakness there. It didn't help.

Does this ring any bells for anyone?

Thank you!


Khron

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #1 on: June 08, 2020, 04:56:31 PM »
Uploaded image doesn't work.

Glossing over that, i encountered a potentially similar issue in some JFET gain stages i played with some time ago, where i forgot to add a resistor between the JFET gate and ground.
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Gus

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #2 on: June 09, 2020, 07:34:20 AM »
What side of the capsule leads did you reflow?

Did you clean the flux off the PCB?
Did you do a search at this forum for how to test what could be wrong?

Is this a KM84 like circuit?

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #3 on: June 09, 2020, 10:25:02 AM »
Thanks, Khron, I wonder if it's the JFET - but I did install all recommended resistors. I tried again to attach a photo.

Thanks, Gus. Yes it is a KM84-style circuit. I have not located any troubleshooting threads specific to this kind of build, but I would be glad if you could point me in a direction. Re: the capsule: I reflowed the connection to I suppose it was the rear side of the capsule, as I did not trust myself to mess with the front-side lead which enters the center disk of the ST-55 "disk resonator" capsule.  I was careful not to touch any of the brass parts of the capsule.

Yes, I did clean the PCB with flux cleaner.

Khron

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #4 on: June 09, 2020, 10:34:02 AM »
Any chance you could upload those photos as .jpg or .png, as opposed to the current .jpeg?

Something doesn't compute with the forum backend(?), because they won't show up (tried in two different browsers).
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #5 on: June 09, 2020, 10:56:06 AM »
Sure, here's a no 'e' jpg!

RuudNL

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #6 on: June 09, 2020, 03:12:05 PM »
Still nothing...
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #7 on: June 09, 2020, 03:58:36 PM »
On the back page of the build guide is a photo of a populated board which indicates what the measured voltages should be at specific points in the circuit. Have you tried checking those measurements to see if your mic matches what Matt says they should be?

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #8 on: June 10, 2020, 01:45:49 PM »
Yes, all the voltages are within the correct ranges as per Matt's guide.

analogguru

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #9 on: June 10, 2020, 02:56:50 PM »
Still nothing...
No wonder, the pictures are neither .jpeg or .jpg, they are uploaded in the Apple-only iOs11 HEIC-Format....

Here is a converter for it, (I won't do the job, because IMHO people should better put their incompatible i-xxxx's to the trash):
https://imazing.com/guides/how-to-convert-heic-to-jpeg

« Last Edit: June 10, 2020, 03:00:03 PM by analogguru »


Khron

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #10 on: June 10, 2020, 02:58:22 PM »
Oh for f**k's sake, they even made their own "custom" JPEG format? Jeeeeeeez... :o

No wonder, the pictures are neither .jpeg or .jpg, they are uploaded in the Apple-only iOs11 HEIC-Format....


Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Sredna

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #11 on: June 10, 2020, 05:30:14 PM »
Maybe this works?
Hmm... in which direction do the electrons actually go?

cyrano

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #12 on: June 10, 2020, 08:32:01 PM »
No wonder, the pictures are neither .jpeg or .jpg, they are uploaded in the Apple-only iOs11 HEIC-Format....

That's not Apple's format. It's an ISO format. It's just new and Apple is one of the early adopters.

It should have .heic as extension, though.
Why is it people love to believe and hate to know?

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #13 on: June 11, 2020, 07:53:03 AM »
Thanks, Sredna. And heres a converted jpeg of the other side.

RuudNL

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #14 on: June 14, 2020, 07:48:12 AM »
Is this a KM84 circuit with some (crazy) big capacitors?
There is a solution for every problem!

http://www.vansteenisaudio.nl

Khron

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #15 on: June 14, 2020, 08:19:49 AM »
Can't have "big tone" without big caps now, can we? ::)

Is this a KM84 circuit with some (crazy) big capacitors?
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Gus

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #16 on: June 14, 2020, 04:17:44 PM »
Still not enough information for a good guess

I have not found a schematic of what if anything was changed from a  KM8x gain stage circuit
found this does this match?http://www.audioimprov.com/AudioImprov/Mics/Entries/2015/12/31_Microphone-Parts_T-47_Kit.html

Need the drain and source voltage after the microphone is on for some time.
What voltage do you measure at the Zener?
Need the voltage at the other side of cap from the drain at the cap transformer node.

What is the capsule charge voltage at the 2.2Ks? node to the first 10meg resistor? You will not be able to measure the charge voltage after unless you know how your meter loads the circuit you could calculate the charge voltage with some math

I don't understand how you can tell it sounds good if it only works for 30 sec

I am still not sure what side of the capsule connections you re soldered at the PCB or at the capsule? If at the capsule you could have damaged it

As Khron posted check the 1gig? gate to ground connections.

Does the source voltage drop tor go up when the microphone is powered up for some time?

Do you have any test gear?

When you posted intermittent does it work OK stop then work again are you tested with voice or a speaker? If you use a speaker you remove moisture from you breath and possible dirt on the capsule causing the charge to dissipate and the signal to go away until the capsule charges back up.
« Last Edit: June 15, 2020, 06:48:31 AM by Gus »

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #17 on: June 20, 2020, 10:35:39 AM »
Is this a KM84 circuit with some (crazy) big capacitors?

It is indeed :)

Re: microphone parts ST-55 circuit mod - intermittent signal problem
« Reply #18 on: June 20, 2020, 10:40:34 AM »
Gus,

Thanks so much for the detailed reply. I was in fact testing with my voice.

I swapped out the capsule with a new microphone parts CK12 capsule, and it did the trick. The old, original capsule was quite dirty, I wonder if the situation you were describing was indeed at play. Looks like the circuit was all good. Perhaps one day if I want to salvage that capsule, I'll have to look into ways of cleaning it.

Thanks, all.

Still not enough information for a good guess

I have not found a schematic of what if anything was changed from a  KM8x gain stage circuit
found this does this match?http://www.audioimprov.com/AudioImprov/Mics/Entries/2015/12/31_Microphone-Parts_T-47_Kit.html

Need the drain and source voltage after the microphone is on for some time.
What voltage do you measure at the Zener?
Need the voltage at the other side of cap from the drain at the cap transformer node.

What is the capsule charge voltage at the 2.2Ks? node to the first 10meg resistor? You will not be able to measure the charge voltage after unless you know how your meter loads the circuit you could calculate the charge voltage with some math

I don't understand how you can tell it sounds good if it only works for 30 sec

I am still not sure what side of the capsule connections you re soldered at the PCB or at the capsule? If at the capsule you could have damaged it

As Khron posted check the 1gig? gate to ground connections.

Does the source voltage drop tor go up when the microphone is powered up for some time?

Do you have any test gear?

When you posted intermittent does it work OK stop then work again are you tested with voice or a speaker? If you use a speaker you remove moisture from you breath and possible dirt on the capsule causing the charge to dissipate and the signal to go away until the capsule charges back up.


 

Related Topics

  Subject / Started by Replies Last post
7 Replies
2383 Views
Last post December 17, 2004, 12:54:33 PM
by Gus
8 Replies
2935 Views
Last post January 29, 2007, 06:30:56 AM
by HiString
4 Replies
1472 Views
Last post April 20, 2007, 03:02:30 PM
by SSLtech
7 Replies
619 Views
Last post May 15, 2019, 03:46:41 AM
by maxoche