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Project Specific Discussions => Microphones => Topic started by: skeptic12 on October 16, 2020, 08:30:00 AM

Title: SE Gemini hum woes- any schematics?
Post by: skeptic12 on October 16, 2020, 08:30:00 AM
Hi

Has anyone had to deal with a Gemini (mk i)?
This one has a hum- I am reading around 100-135mv DC on each side of the output.  They tubes have been swapped with known good and quiet ones

Any schematics or suggestions?  I'm not up to speed on transformerless tube mic circuits like this
Title: Re: SE Gemini hum woes- any schematics?
Post by: kingkorg on October 16, 2020, 11:38:38 AM
Could be helpful.

https://groupdiy.com/index.php?topic=37698.0
Title: Re: SE Gemini hum woes- any schematics?
Post by: RuudNL on October 16, 2020, 03:02:24 PM
Interesting information kingkorg! (Didn't know about this schematic.)
Title: Re: SE Gemini hum woes- any schematics?
Post by: skeptic12 on October 17, 2020, 07:30:47 AM
thanks! I hadn't seen that either
I will take a look at that and see if it resembles the SE circuit                               
Title: Re: SE Gemini hum woes- any schematics?
Post by: Gus on October 17, 2020, 08:41:50 AM
How many tubes have you tried?
Sometimes with MICROPHONES you need to try a few.
There have been times I tried a number of tubes and could not get low noise until I used a certain build of a tube.

Did it hum with the set of tubes before?

Was the microphone dropped at any time The microphone looks to be big and heavy from reading some reviews. I would check if a solder connection could have been broken.

Have done a PCB visual inspection?
Does it smell like a burned part?
Do any parts look bad?things like an expanded or leaking cap

Things to check the grill to body to shield from microphone to power supply. It is possible you don't have a good connection. The microphone was opened and tubes changed. Sometimes paint can cause a bad connection.

Is the cable good check connections end to end with a meter set to ohm check for continuity and them shorts between pins that should not be there.

More information will help people to give you things to check.
Title: Re: SE Gemini hum woes- any schematics?
Post by: skeptic12 on October 17, 2020, 01:02:33 PM


I replaced the capsule as the last one had moisture related problems.  When that noise had gone away, I heard the low hum and asked the owner about it.  He said he had noticed a hum before the capsule had gone whooshy

I tried a few tubes that I know to be low noise/microphonics

I notice two trim pots on the board in the mic, probably some sort of bias adjustment?  They don't affect the hum level

It doesn't look to have been dropped and I have checked solder joints.  I replaced electrolytic caps in PSU and mic to see if that helped.  But was left with the same low hum on the output- 80-100 mv of DC being read

The other odd thing is I read around 80 mv DC between the mic body and earth on the PSU.  mic body attached to the mic connector via the shield in the cable but I am not sure how the ground scheme is supposed to go as I can't find a schematic for this