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Author Topic: Flea 12 hum  (Read 1129 times)

Oleh Malyy

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Flea 12 hum
« on: January 12, 2019, 07:54:50 am »

I bought used Flea 12.
Looking to serial, maybe early model.
As I can see it fully copying schematics of AKG C12, except power supply, of course.

I think i have problem with this Flea12.

Please donwload archive with "test" files:
http://www.mediafire.com/file/jvx63m2r33uxsm7/Flea12_test.zip/file
in archive:
Rec1: Flea12 and Flea249 at one time, preamp Millennia Media HV-3C. Gain for Flea12 - 48.5.
Rec2: Flea12 and my "copy" of Telefunken Ela M 251 (CT12 capsule, Haufe T14/1 transformer, AC701 tube).

As you can hear (and see with spectrum analiser) Flea12 have a little bit hum at 100 Hz (I am in Ukraine and here 220v/50Hz).
Not a lot, but it is.
Maybe that is normal for C12 type microphone - I don’t know .
Please tell me.

I has changed capsule for new CT12 - the same result.
Tried with different tubes (GE 6072 5star too), for all tubes voltages (anode and hitter) was adjusted - the same result.
Checked cable,  look like it good.

Maybe, such small hum is normal, please tell me.
If not, please tell me what can be the reason(s) and how to fix that - I would appreciate any ideas.

Thanks,
Best regards

PS. sorry for my bad English.
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AusTex64

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Re: Flea 12 hum
« Reply #1 on: January 12, 2019, 01:49:05 pm »

Confirm the mesh headbasket is well grounded to the body. I suspect a ground or dirt problem somewhere.
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klaus

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Re: Flea 12 hum
« Reply #2 on: January 12, 2019, 03:01:14 pm »

No, not even "small" hum is normal.

Please confirm that you have compete shield and ground coverage, from head basket all the way to mic pre-amp input: with headphones on, hold the head basket against the XLR that plugs into your mic pre amp, and  report back whether the ground hum has disappeared. If it has: you need to look at all possible intersections where ground and shield may have been interrupted.

First and primary suspect would be mic and XLR-3 cable connections: shield and ground MUST be connected on both ends of both cables to ground and shield pins (where applicable) and to the housing lugs or clampdowns of all four connectors.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Oleh Malyy

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Re: Flea 12 hum
« Reply #3 on: January 13, 2019, 01:52:16 am »

Yes, headbasket is well grounded to the body.

A lot of thanks for all answers.
Will try and check all connection.
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klaus

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Re: Flea 12 hum
« Reply #4 on: January 14, 2019, 01:32:42 pm »

Before I move on to more troubleshooting tips: Did you perform the test I recommended?
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Klaus Heyne
German Masterworks®
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Oleh Malyy

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Re: Flea 12 hum
« Reply #5 on: January 15, 2019, 01:36:05 am »

Yes. Thanks.
Has found that at the microphone side in the cable shield and ground was not connected. Conneted these. But without effect - hum still is.
Has checked all cables from microphone to preamp(s). Shield and ground connected at both sides (also has tried without this connetion at one side of cable - without effect too)

>>>> ... from head basket all the way to mic pre-amp input: with headphones on, hold the head basket against the XLR that plugs into your mic pre amp, and  report back whether the ground hum has disappeared. ...

No, hum not disappeared.

I thinking to buy and change transformer to Haufe T14/1.
Now in microphone is this tranformer
https://flic.kr/p/S5UyNW

But I don’t think that hum can be connected with transfrmer.

Please tell me more ideas/tips.
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Kai

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Re: Flea 12 hum
« Reply #6 on: January 15, 2019, 07:52:55 am »

100Hz usually means a problem with the power supply, stray field induced hum has components at 50Hz, 150Hz, 250Hz...
These are not present in your file.

Checklist:
What about the room the mic is located- is it dead quiet?
No flourescent lamp active? They can have accoustic hum.
All connection balanced or shortcut to ground on a signal wire - check with ohm-meter (not likely, but...).
Did you check the DC supply voltages? They need to be perfectly ripple-free too (Oscilloscope or sensitive AC Milli-Voltmeter).
What if you switch off the PSU - what happens in the few seconds the mic is still active, what afterwards?
Hum with PSU off still there -> next step unplug the mains connector on the PSU, ... hum?
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Oleh Malyy

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Re: Flea 12 hum
« Reply #7 on: January 15, 2019, 12:09:14 pm »

Thanks for your reply!

>>> What about the room the mic is located- is it dead quiet?
Yes. As possibly.

>>>No flourescent lamp active? They can have accoustic hum.
Tried without any lamps. Hum is.

>>>All connection balanced or shortcut to ground on a signal wire - check with ohm-meter (not likely, but...).
All connections are balanced.

>>> Did you check the DC supply voltages? They need to be perfectly ripple-free too (Oscilloscope or sensitive AC Milli-Voltmeter).
With sensitive Milli-Voltmeter, voltages looks like stable.

>>> What if you switch off the PSU - what happens in the few seconds the mic is still active, what afterwards?
After turning of PSU - hum not present.
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Kai

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Re: Flea 12 hum
« Reply #8 on: January 15, 2019, 02:34:10 pm »

I bet the supply voltages are not ripple-free.
How much AC voltage do you see on the anode and heating voltage?
Is your AC voltmeter sensitive enough to detect that - what is the lowest your meter is capable to measure.
Should be well below 1mV AC.
Measurement needs to be done on the connector, either on the microphone or PSU side, with the microphone connected.
If there is a separate PSU ground line in the cable this needs to be reference for the measurement, not the shield.
Be careful, there are dangerous high voltages.
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Oleh Malyy

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Re: Flea 12 hum
« Reply #9 on: January 15, 2019, 02:44:29 pm »

I have instruction from Flea how and where in PSU to adjust hitter (5.92v)
and anode voltages (55.1v at R17 400kOhm resistor - I think 64.9v at anode).
And all is done with this instruction.
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Kai

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Re: Flea 12 hum
« Reply #10 on: January 15, 2019, 03:58:24 pm »

DC adjusting does not change AC ripple on the DC power lines.
You need to measure AC, not DC on the power lines.
Maybe something is broken in the PSU, maybe there is a bad construction (I've seen those with other manufacturers).
AC should be way below 0.001V, or it can creep into the audio.
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klaus

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Re: Flea 12 hum
« Reply #11 on: January 15, 2019, 07:59:57 pm »

Thanks, Kai, super helpful hints.

The owner needs to troubleshoot filter caps as next step, but I recommend that a trained technician is doing the testing, because of dangerous voltage conditions inside the power supply:

Turn off power supply. with alligator clip wires, parallel the last of the heater filter caps with one of the same value/voltage rating. Turn on the supply. Hum gone? Make the reverse test: now disconnect one of the alligator leads: hum back? It's a bad filter cap.

If there was no change in the hum in the heater circuit, repeat the test for the B+ filter caps.

NEVER CONNECT a capacitor while the supply is switched on! ONLY REMOVE one while it's on.
Not following this will severely shorten tube life.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Oleh Malyy

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Re: Flea 12 hum
« Reply #12 on: January 16, 2019, 12:39:31 am »

A lot of thanks for all replies, tips.
But as I see, I must to send it to my friend that have more skills/knowledges/instruments,  as I am.

Best regards
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klaus

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Re: Flea 12 hum
« Reply #13 on: January 16, 2019, 02:02:30 am »

Pass these tips on to your friend. They are all useful and methodical.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Oleh Malyy

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Re: Flea 12 hum
« Reply #14 on: January 16, 2019, 07:21:54 am »

Yes. Of course. Will tell.
A lot of thanks!
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