U67 troubleshooting

[madreza]

Hi Guys,
I have a probleme with a U67
it works fine but after a while ..... volume goes down ( 15dB approx. )
in cardio and omni the volume is low ..... when switching in fig 8 everything is normal
if you switch the mic off and wait a few seconds and then switch on ..... everything is fine again
seems a bad cap to me but how to troubleshoot it ?
the capsule is new so ....

 

[3nity]

Is this a D-U67??

 

[madreza]

nope ! it's a real one

 

[madreza]

nobody has any idea ????

 

[poctop]

Was the issue there before capsule replacement, can you monitor you polarisation voltage somehow, Capsule connection,  switching clean.. take no prisonner
Start with all basic psu voltage up to mic as a start.  can you tell more about the issue, use the schematic and try to identify wich node you think might be the issue check the reference voltage,  check polarisation voltage scheme , last resort cheap chinese capsule we never know.

Hope this helps,

D

 

[madreza]

Hi Dan ,
thanks for helping ....

the issue was there before replacing capsule
I tried another capsule .... the issue is still there

how can I monitor the polarisation voltage ? ( I guess a multimeter is of no use here .... can an oscillo help ? )
I know I should compare reference voltage but don't know how to measure it. psu voltage is fine .
there should be a bad cap somewhere , causing a voltage drop after a while

here is the issue :
when I turn on the psu , the mic perfectly works in cardio omni and Fig 8
after a few minutes the level drops. I assume it's a linear drop ....
this drop affects the cardio and omni ..... but when I switch to Fig 8 the level is back to normal


thanks again for your help

 

[MagnetoSound]

here is the issue :
when I turn on the psu , the mic perfectly works in cardio omni and Fig 8
after a few minutes the level drops. I assume it's a linear drop ....
this drop affects the cardio and omni ..... but when I switch to Fig 8 the level is back to normal

Hi madreza,

If the backplate voltage fails, there will be no capsule bias until fig. 8 is selected, at which point the +120v at the rear diaphragm will be divided across the capsule and sound will be restored (although arguably the bias will drift somewhat from front to back as the charges on either side of the capsule attempt to equalize as the other components around the backplate exert their influence).

My guess (for now) is that there is a bad resistor in the 60v path to the backplate - this could be R13 (the top of the divider), or R2 (the feed to the backplate).

R13 is much more likely, as it sees the load of the divider bottom-half (R12) and the grid bias network.

There is the alternate possibility of a capacitor going shorty, in which case this is probably C4.

 

[madreza]

Man, you're awesome
it was not R13 but R2 that was faulty ....
now it perfectly works

thanks a lot
Cheers

 

[MagnetoSound]

Glad to help.