Hi,
I'm working on a Neumann U67 that was sent to repair because it had low end loss.
The Studio has two U67 mics, one of them is fine but this one at some point lost low end.
The Mic works and although the sound is not ultra thin or radio-like there's a noticeable low end loss.
The owner had 2 telefunken EF86 tubes as spares in case needed.
I started the repair by checking the PSU,
The Filter electrolytic capacitors were originals, took them off and there's some bubbles on the underside.
Those were all replaced by new Panasonic Caps.
The PSU power transformer doenst have an 240V input tap so power is hard wired to the 220v tap, a previous engineer revised some resistor values so the voltages would drop to the schematic values while being fed by 230/235 VAC mains power.
I tested all the PSU voltages with the mic plugged in with tube in place and after tweeking the value of one of the resistors the voltages were spot on , 210V and -6,4V.
Next I tested the tubes,
tesla EF806S and two telefunken EF86. The 3 tubes tested fine in the Hickok 6000A tube tester.
Then tested one by one for sound in the U67, all of them sounded the same, low end loss was present regardless of the tube used.
I visually checked the capsule and gave a gentle clean, although it didn't look dirty or dusty at all.
Next with the EF806S tube installed I followed the schematic and measured the voltages at the mic itself.
here is the schematic:
The first voltage measurements are marked on the schematic with a carbon pencil.
I had 140v at the junction of R12 and R13 instead of 59V, also at the capsule Pin 1 I had 30v instead of 59V.
Even with those wrong voltages the mic was working, and was just noticeable because it lacks a little bit of low end.
Started measuring resistors and lifting legs.
Found that R12 was open, resistor gave no reading on the DMM.
Replaced R12 and measured again, those new voltages with R12 replaced are marked on the schematic with a Red marker pen.
Now HT rail droped from 210v to 206/207V (this doesnt seem problematic as Neumann states +-2% tolerance on the HT rail)
Heater voltage drop from -6.4 to -6.22, seems to be good enough
At the R12 and R13 junction the voltage is now 48V , better than the 140V reading with the dead R12 but still 11 volts less than the schematic value of 59v.
Strangely at capsule pin1 there's only 8V now and at Pin 5 only 3V instead of 120V.
Even with these strange voltages the Mic still works, and to be honest it seems to sound the same as before R12 replacement.
I'm really tired now and need some help before moving on.
Do you know by looking to the schematic which are the components I should suspect next?
Thanks
I'm working on a Neumann U67 that was sent to repair because it had low end loss.
The Studio has two U67 mics, one of them is fine but this one at some point lost low end.
The Mic works and although the sound is not ultra thin or radio-like there's a noticeable low end loss.
The owner had 2 telefunken EF86 tubes as spares in case needed.
I started the repair by checking the PSU,
The Filter electrolytic capacitors were originals, took them off and there's some bubbles on the underside.
Those were all replaced by new Panasonic Caps.
The PSU power transformer doenst have an 240V input tap so power is hard wired to the 220v tap, a previous engineer revised some resistor values so the voltages would drop to the schematic values while being fed by 230/235 VAC mains power.
I tested all the PSU voltages with the mic plugged in with tube in place and after tweeking the value of one of the resistors the voltages were spot on , 210V and -6,4V.
Next I tested the tubes,
tesla EF806S and two telefunken EF86. The 3 tubes tested fine in the Hickok 6000A tube tester.
Then tested one by one for sound in the U67, all of them sounded the same, low end loss was present regardless of the tube used.
I visually checked the capsule and gave a gentle clean, although it didn't look dirty or dusty at all.
Next with the EF806S tube installed I followed the schematic and measured the voltages at the mic itself.
here is the schematic:
The first voltage measurements are marked on the schematic with a carbon pencil.
I had 140v at the junction of R12 and R13 instead of 59V, also at the capsule Pin 1 I had 30v instead of 59V.
Even with those wrong voltages the mic was working, and was just noticeable because it lacks a little bit of low end.
Started measuring resistors and lifting legs.
Found that R12 was open, resistor gave no reading on the DMM.
Replaced R12 and measured again, those new voltages with R12 replaced are marked on the schematic with a Red marker pen.
Now HT rail droped from 210v to 206/207V (this doesnt seem problematic as Neumann states +-2% tolerance on the HT rail)
Heater voltage drop from -6.4 to -6.22, seems to be good enough
At the R12 and R13 junction the voltage is now 48V , better than the 140V reading with the dead R12 but still 11 volts less than the schematic value of 59v.
Strangely at capsule pin1 there's only 8V now and at Pin 5 only 3V instead of 120V.
Even with these strange voltages the Mic still works, and to be honest it seems to sound the same as before R12 replacement.
I'm really tired now and need some help before moving on.
Do you know by looking to the schematic which are the components I should suspect next?
Thanks
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