Hum problem HELP! SOLVED.

[erik_DK]

Hello.

Sometime ago I started this thread about a Danish broadcast mic-limiter and how to modify it to act as a normal mic pre without the limiter.

Since then I have modified six of them for myself - they all work flawlessly.

Then lately a guy contacted me to ask if I could fix eight of them for him, which I have now - almost - finished. They are all serviced, recapped and modified as per the description in my former thread. Seven of them work super, but the last one has some hum left. Not that much, but definitely enough to annoy me.

All eight are tested with the same microphone a Shure SM57
They all have completely identical PCBs
I used the same power supply and the same connector on all of them.
They all have the exact same grounding scheme.
They are in fact all 100% identical.

But number eight makes this hum, when I hold the microphone in my hand. With the mic on a stand, there is no problem. Once I touch the chassis of the case with my other hand, the hum disappears. Making a connection with a test lead from the housing of the mic to the case also solves the problem.

I have inspected all soldering points with a magnifying glass.
I resoldered all ground connections.
Nothing helps.

I changed one transistor T8, which was defective. A 2N696 for a BC140, which I had in my drawer. Both general purpose transistors, and the amp works fine.
Only thing I have not tried yet is changing the 23 pin Tuchel connector, which shows a little corrosion (but nothing more than the seven working modules).

Is it possible that a corroded pin in the connector, eg. the pin that takes the screen of the mic cable, causes the problem.
If yes, what would you recommend for cleaning the connector?
Or are there other possible reasons?

Kind regards Erik

 

[Ike Zimbel]

Sounds like a Pin-1 problem to me. I would make sure that Pin-1 of the XLR is making good contact to the chassis.

 

[micolas]

Hello.

Sometime ago I started this thread about a Danish broadcast mic-limiter and how to modify it to act as a normal mic pre without the limiter.

Since then I have modified six of them for myself - they all work flawlessly.

Then lately a guy contacted me to ask if I could fix eight of them for him, which I have now - almost - finished. They are all serviced, recapped and modified as per the description in my former thread. Seven of them work super, but the last one has some hum left. Not that much, but definitely enough to annoy me.

All eight are tested with the same microphone a Shure SM57
They all have completely identical PCBs
I used the same power supply and the same connector on all of them.
They all have the exact same grounding scheme.
They are in fact all 100% identical.

But number eight makes this hum, when I hold the microphone in my hand. With the mic on a stand, there is no problem. Once I touch the chassis of the case with my other hand, the hum disappears. Making a connection with a test lead from the housing of the mic to the case also solves the problem.

I have inspected all soldering points with a magnifying glass.
I resoldered all ground connections.
Nothing helps.

I changed one transistor T8, which was defective. A 2N696 for a BC140, which I had in my drawer. Both general purpose transistors, and the amp works fine.
Only thing I have not tried yet is changing the 23 pin Tuchel connector, which shows a little corrosion (but nothing more than the seven working modules).

Is it possible that a corroded pin in the connector, eg. the pin that takes the screen of the mic cable, causes the problem.
If yes, what would you recommend for cleaning the connector?
Or are there other possible reasons?

Kind regards Erik

Before anything have you tried with a DMM set on the electrical continuity scale (min ohms) to measure the Tuchel connectors? And the connection of the connector to the GND of the PCB?

 

[erik_DK]

Sounds like a Pin-1 problem to me. I would make sure that Pin-1 of the XLR is making good contact to the chassis.

Before anything have you tried with a DMM set on the electrical continuity scale (min ohms) to measure the Tuchel connectors? And the connection of the connector to the GND of the PCB?

Thank you both of you!
Problem is solved. Missing connection from the connector of the case to the GND. So no screen connection from the mic to the case. So they were in fact only 99.9% identical. I would have expected a lot more hum under these conditions.

This is what I love about groupDIY. Input from others, that makes you look carefully one more time. I could have polished that pin in the connector for hours.

/Erik

 

[JohnRoberts]

Sounds like a Pin-1 problem to me. I would make sure that Pin-1 of the XLR is making good contact to the chassis.

"Pin 1 problem" generally refers to a design issue where chassis ground current flowing into pin 1 generates a voltage (typically hum) that the audio path does not adequately reject.

Indeed this sounds like an open pin 1, that is "a" pin 1 problem but not "the" typical pin 1 problem often discussed. This may be fixable by reflowing a few solder joints or perhaps replacing a faulty connector.

JR