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Author Topic: Different NU67 - different output levels?  (Read 5000 times)

klaus

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Re: Different NU67 - different output levels?
« Reply #15 on: October 13, 2015, 12:30:31 pm »

Because of the dangerously high voltages present inside the power supply, none of the experimenting described or recommended in these posts should be done by a lay person. Take the supply to a qualified technician instead!
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Kai

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Re: Different NU67 - different output levels?
« Reply #16 on: October 13, 2015, 07:29:04 pm »

Yes, I forgot to mention:
NEVER MANIPULATE OR TOUCH ANYTHING INSIDE OF SUCH A PSU WHILE THE POWER PLUG IS CONNECTED.
EVEN AFTER DISCONNECT LETHAL VOLTAGES ARE STILL PRESENT IN THE CHARGED CAPACITORS FOR A LONGER TIME.
SPECIALLY IF THE THING IS BROKEN THESE VOLTAGES MAY NOT BE DISCHARGED FOR HOURS.
IF YOU ARE NOT SKILLED GET SOMEONE DOING IT FOR YOU!!!

All the best
Kai
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klaus

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Re: Different NU67 - different output levels?
« Reply #17 on: October 19, 2015, 01:52:46 pm »

Hello KOEN,
Please update the forum: how have you solved the issue?
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Klaus Heyne
German Masterworks®
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Koen

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Re: Different NU67 - different output levels?
« Reply #18 on: October 20, 2015, 04:27:23 pm »

Hello to everyone here.
Although we didn't have the right schedule at hand (in the meantime, Martin from Neumann has send me one), we think we managed to get the PSU to work as it should.
We first measured all the stages in the circuit and found out that the transformer put out a hefty 227V. With a load in place, the polarisation voltage was a staggering 299V! Firstly we tried to attach the 240V tap of the transformer (it is an older model, so 220V only on the circuit board, but there actually is a 240V tap located on the transformer. This brought down the voltage to about 280V, but the filament went down to about 4,8V and we were unable to bring that back up to the required 6,15V. Eventually we went back to the 220V tap and replaced the diode bridge. We subsequently augmented the dropping resistors (R1 and R2 on the newer schematic, but R2 and R3 on our circuit board) to 18 Kohm and 10 Kohm respectively. We also saw that there was no 160 Kohm resistor around R7 (R8 on our circuit) and put a 100 Kohm in there (as per an other, slightly different schematic we found).
In the end, we managed to get a polarisation voltage of 209Vdc. We also discovered the variable resistor on the filament rail was 'stuck' at 26 ohm and there we strapped a fixed value in parallel (actually 3 x 8 ohm resistors) untill we reached 6,18V under load. The microphone has been switched on and in use since three days and it performs flawlessly.
In the meantime I received a schematic for the NU67 with the 03 suffix on the circuit board. It seems no one had tampered with the PSU as I suspected at first but surely the transformer must have been way out of spec. Anyway, it now works as it should. Many thanks for your detailed input Klaus, Kai, JJ Blair and Uwe!
I'll try to attach the early schematic here, it's interesting, with the extra small filter caps etc. compared to the later ones. (I hope I don't break Neumann's forum rules here). Hope it works...
Greetings from Belgium,

Koen.
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Koen

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Re: Different NU67 - different output levels?
« Reply #19 on: October 20, 2015, 04:29:27 pm »

Schematic
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Kai

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Re: Different NU67 - different output levels?
« Reply #20 on: October 20, 2015, 09:15:44 pm »

Congratulations!
You have successfully redesigned the PSU  :)

Jokes aside - I suppose Neumann service or more likely anybody else has replaced the transformer with the wrong type once during the long time this PSU has lived.
They must have used what they could get and didn't care about the result, ending up with a mix of different version's parts and a wrong voltage.

E.g. other versions of the NU67 used a single diode rectifier in the high voltage branch, compared to the full wave bridge rectifier in your schematic. This, besides different values of R7 aka R4 (the HV shunt resistor) results in different voltages.
Now this all doesn't matter too much any more - within a reasonable range the resistor values can be adapted to what transformer is there to achieve proper voltages. That's what I proposed and what you did.

The only thing that makes me think "what ..." is your mentioning of the variable resistor being "stuck".
In that case it should either be replaced or unsoldered and cleaned (that's what I usually do if possible) to make it work again.
Cancelling the option to adjust heater voltage might turn out unhandy in the future, e.g if you switch cables. An exact heater voltage is essential to optimize tubelife.

All the best
Kai

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klaus

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Re: Different NU67 - different output levels?
« Reply #21 on: October 20, 2015, 09:38:55 pm »

Most of the trim potentiometers in the NU67 heater voltage rail are stuck because some of the sealing lacquer used to lock the setting in place after factory calibration seeped into the shaft of the switch that turns it.

A simple solution: dab a few drops of acetone into the cup formed by the switch shaft and lock nut. This will usually free the mechanism again.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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