TRX2 Microphonics

General discussion

TRX2 Microphonics

Postby G8GEV » 01 Jan 2012 10:06

Any advice on microphonics on the TX2 main PCB? Turning the audio gain
right up with the loudspeaker attached to the case causes the set
to oscillate. The most sensitive components to tapping seem to be C4-7 and
C12 - 15 imediately after the mixer

Thanks all

John
G8GEV
 
Posts: 3
Joined: 17 Dec 2011 16:40
Location: England

Re: TRX2 Microphonics

Postby OH7SV » 03 Jan 2012 08:26

John,

The AF gain is huge but typically there has not been problems with acoustic feedback.

I guess you have tested with an external speaker. Is it ok?

Please check that the main board capacitors C4...C7 and C12...C15 are right G0G type. The G0G color is grey compared to the X7R brown color. X7R type capacitor is "a very good" condenser type microphone, C0G type capacitor is much more immune to mechanical change.

Have you already adjusted the AGC? When it is adjusted properly the AF gain is little lower.
If the caps are right type there can be some unbalance in the AF differential path. That can be checked by measuring the operating point DC voltages according to the voltage table http://www.nikkemedia.fi/juma-trx2/rev- ... -table.pdf

73 Matti OH7SV
User avatar
OH7SV
Moderator
 
Posts: 72
Joined: 11 Nov 2011 07:00
Location: Finland

Re: TRX2 Microphonics

Postby G8GEV » 03 Jan 2012 15:49

Thanks Matti -
The capacitors _are_ brown, but so are the replacement ones I got from Farnell. I also got a set
of film capacitors Farnell 969-5087, PANASONIC - ECHU1C473JX5 at 42p each, and these have
almost completely cured the problem.
The PCB no longer is sensitive, but the individual capacitors are - tapping C4-C7 produces about
one quarter of the noise of tapping C12-15, wich is what you would expect as the latter have 4x
the voltage across them, and a change in capacitance of 1 part per million will give a voltage of
about 4 microvolts.

In due course I intend to try the following mod which should eliminate it completely -
Return C4-C7 ground to the 1.2 volt reference C1 (nil dc across them)
Delete R9-R12, replace C12-15 with short circuit
Change R5 R6 to two 100R 0.1% in series, with ~470R 0.1% to ground (this sets ouput
dc to ~5v to maintain headroom)

It also saves quite a few components.

Regards

John
G8GEV
 
Posts: 3
Joined: 17 Dec 2011 16:40
Location: England

Re: TRX2 Microphonics

Postby OH7SV » 03 Jan 2012 16:32

John,

During the design phase I tested several cap types for C4...C7 and C12...C15. Murata GRM31M5C1H473JA01D (grey in color) caps performed very well. I wonder if the brown caps you got with the kit are Murata type.

Your modification idea is good. I have also sketched quite similar circuits. You are right that the DC voltage across a cap is bad because of condenser mic effect. But you might not get rid of all the DC because the LO oscillator is slightly detected and it can generate some DC which is not exactly the same as the muxer input bias voltage 1.2V. Anyway your mod should remove most of the DC. Additionally to condenser mic effect there is the piezo effect in capacitors which takes action without DC.

Let me know the results of your modification.

73 Matti OH7SV
User avatar
OH7SV
Moderator
 
Posts: 72
Joined: 11 Nov 2011 07:00
Location: Finland

Re: TRX2 Microphonics

Postby G8GEV » 03 Jan 2012 20:48

Will do - thanks - now receiver working well so it might take a bit of time to get round to it.....

Happy new year

John
G8GEV
 
Posts: 3
Joined: 17 Dec 2011 16:40
Location: England

Re: TRX2 Microphonics

Postby OH2G » 07 Feb 2012 04:08

Hi'ya all!
I've built two of these fine rigs. As I experienced in earlier projects with same types of ceramic caps and internal speakers I found that, it's a far better way to place the speaker on the underside cover of the radio. That of course doesn't compeletely eliminate the phenomenom, but "tames" it considerably. 8-)

Enjoy Your JUMAs!
73 &C
Penna OH2G
OH2G
 
Posts: 1
Joined: 24 Nov 2011 12:56
Location: Finland


Return to Juma Discussion

Who is online

Users browsing this forum: Bing [Bot] and 0 guests

cron