mata_haze

Neumann 476B problem
« on: August 17, 2009, 05:07:08 AM »
Hello everybody,
I have just bought a couple of Neumann 476B.
the owner claimed there was discrepancy between hot and cold pole at the output, in fact the cold output is roughly 6 dB lower than the hot.
both units are connected via their own 13 pin connector, all connections are made by me so I don't think there are are problem on that.

the signal seems to be pretty healthy until the output transformer (pin 1 between C37 and R66) but then it becomes uneven if I put my scope directly at the transformer output( pins 5 and 6).

I also know the modules were not really looked after and the previous owner had smoke coming out of them.
unfortunately I cannot see anything obvious, I have recapped most of the unit, went trough connections and so on, but nothing has improved.

Both units behave the same although one was working fine for a while (when I just bought it) and then went wrong.


Now the question is:
by looking at the output transistor configuration, would it be possible for the output to be uneven if one of the two is not working?
IE if the output pair is not actually paired or it is faulty would that cause any difference at the level in the output trafo?

any (really any!) suggestion will be much appreciated.




Best,
Mattia.


[silent:arts]

Re: Neumann 476B problem
« Reply #1 on: August 17, 2009, 05:39:54 AM »
mhm, -6 dB?

think about it ...
any change when using the phase switch?

mata_haze

Re: Neumann 476B problem
« Reply #2 on: August 17, 2009, 07:10:19 AM »
mhm, -6 dB?

think about it ...
any change when using the phase switch?

Hello,
it's actually 10dBs...
I am measuring the signal just after the output trafo therefore BEFORE the phase switch  and there is a 10dB difference between pin 5 and pin 6.

Best,
Mattia.

[silent:arts]

Re: Neumann 476B problem
« Reply #3 on: August 17, 2009, 08:06:28 AM »
measure the module connector, pin 11 & 12.
any difference "in phase" and "reverse phase"?

my bet is: a short in the phase switch / wiring.
worst case is a broken output transformer, but I doubt this.

btw, transistors in front of the transformer can't be the reason.

mata_haze

Re: Neumann 476B problem
« Reply #4 on: August 17, 2009, 07:45:41 PM »
I am measuring the signal right after the transformer with phase switch disconnected.
I am probing on pin5 and pin 6 of the output transformer.

I have even tried to disconnect the output transformer and connected a general OEP instead.
shows same problem.
I am puzzled...

 ??? ??? ??? ??? ??? ??? ???

Best,
Mattia.

[silent:arts]

Re: Neumann 476B problem
« Reply #5 on: August 18, 2009, 03:00:02 AM »
I am measuring the signal right after the transformer with phase switch disconnected.
I am probing on pin5 and pin 6 of the output transformer...
to which reference?
is your signal fine at output 2 ?

mata_haze

Re: Neumann 476B problem
« Reply #6 on: August 18, 2009, 05:39:06 AM »
I am measuring the signal right after the transformer with phase switch disconnected.
I am probing on pin5 and pin 6 of the output transformer...
to which reference?
is your signal fine at output 2 ?
I am measuring between Pin5 and Pin6 of the output transformer referenced to ground ( PIN 5 at the output connector).

signal is fine between pins 4,9 (references to pin 5) of the output connector.
is this what you meant by output 2?

thank you for helping me out.

Best,
Mattia

abbey road d enfer

Re: Neumann 476B problem
« Reply #7 on: August 18, 2009, 06:20:07 AM »
I am measuring between Pin5 and Pin6 of the output transformer referenced to ground ( PIN 5 at the output connector).
Mattia
This is absolutely normal behaviour for a transformer. The ouput is floating, which means it is not referenced to ground. The only valid measurements you can make are either using a balanced input audio test set, or shorting one pin to ground and measuring the other, and vice-versa. Both measurements should be identical.
If the transformer was perfect, you would measure ZERO, nothing, but the transformer has some leakage capacitance between its secondary and ground. The combination of this capacitance and the impedance of your measurement set generates SOME reading. Another transformer will give different measurements.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

Related Topics

  Subject / Started by Replies Last post
4 Replies
3560 Views
Last post February 16, 2005, 03:44:02 PM
by Leslie West
5 Replies
2367 Views
Last post November 17, 2005, 10:27:03 AM
by rodabod
13 Replies
6561 Views
Last post September 24, 2006, 06:35:19 PM
by pucho812
16 Replies
4959 Views
Last post February 20, 2007, 05:05:47 PM
by Jazz