NV73 Input Transformer Strange issue

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qmp audio

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I finally had some free time,  and built the NV73 i had sitting in a box for quite some time.  I'm having a strange issue, and can't seem to figure out why this is happening.  Perhaps someone has some ideas to help me figure this out.

The symptoms and issues are identical to 2x NV73 i built at the same time.  So whatever it is,  its happening to both.

Issue:  Signal is not getting past the input transformer.  With the input transformer in place,  it is a VERY low output.  So,  i thought something was wrong w/ the transformer or connection.

However,  i can't find any issues.  everything is connected where it should be.  I'm seeing continuity between the terminals and pins.  The resistance measure appropriately across the primary and secondary (~25ohm across each primary,  ~125ohm across each secondary).  So, both new carnhill's (VTB9045) seem to have no issues.

I traced the signal with a scope,  and saw the sine wave is not coming out of the secondary (pin7).  So something is happening where it isn't making it though.

If i bypass the input transformer,  and simply connect card edge pin 10 to the NV73 input (pin 7 on the terminal),  the amp functions at it should!  So,  i can confirm this issue is in the xfmr.

nv73_xfmrBypass.jpg


However,  with the transformer installed,  i can't see any issue.  everything is connected where it should.  Transformer measure correctly.  And nothing is shorting out.

If i scope the input pins,  i see the sine wave:

nv73_input-xfmr.jpg


However,  if i scope the secondary pins (pin 7 on the terminal),  i just see a little offset,  no sine wave:

nv73_input-pin7.jpg



Any ideas?  I spend hours looking at it and measuring the xfmr connections.  I can't find any issues with it.  Also,  the xlr connection going in functions with any other modules in the slot ( tried multiple slots and cables,  no issues with those )
 

Bonnie1

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Jan 24, 2016
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Not being too familiar with Carnhill's VTB9045...
You say there are 2 pri. and 2 sec. windings?
Are the 2 pri. windings connected in series/parallel?
Secondary?
 

qmp audio

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First,  Thanks for the input!

radardoug said:
Transformer windings are wired out of phase on one side or the other.

This is what i was thinking too.  But the transformer pins connect right to a pcb,  and tracing the connections they are matching the schematic.    But it would make sense there is a phase issue,  i just can't find it.  will try probing both inputs simultaneously on the scope to see if i can see clearer.

Bonnie1 said:
Not being too familiar with Carnhill's VTB9045...
You say there are 2 pri. and 2 sec. windings?
Are the 2 pri. windings connected in series/parallel?
Secondary?

There is a hi/low z switch which changes the connection from series to parallel.  I put the schematic below,  along with images of the pcb and transformer closeup.  i did trace out w/ a meter and the connections match the schematic.  There are no shorts or malfunctions with the switch.

nv73_inputschem.jpg


nv73_inputclose.jpg


Im going to inspect the rack further just to make sure there is nothing strange there.  But all of my other modules work as expected, so i do not think i will find anything out of the ordinary. 
 

qmp audio

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radardoug said:
Transformer fitted upside down on the sub board?

Yes,  this was my first thought.  i inspected and confirmed it is oriented and soldered correctly on the sub board.

im getting 25ohm between terminal screws 2-4
3 - 5 = 25ohm
2 - 5 = 50ohm
7 -10 = 268 ohm
10 - card edge 5 = 10ohm

tf pin 6 - chassis = ~0 ohms

According to this data sheet i found,  that is what i should see:
http://www.vintageaudioprojects.be/new/index.php?main_page=product_info&products_id=49
 

Youngwhisk

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Oct 13, 2012
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Is it possible there is a reversed connection inside the transformer? It would still measure resistance correctly, wouldn't it?  Maybe sub a known good transformer...
 

qmp audio

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Mar 4, 2011
Messages
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Location
Chicago
Thanks everyone for the input!  helped point me in the right direction.  The issue is solved (or at least the issue with the nv73s... there never was one)

Once i worked backwards and tested the transformer on my bench w/ xlrs jumped directly to the transformer,  i realized there couldn’t be an issue with the nv73

The issue was the rack slot.  The module i pulled out & slot had no issue,  so something must have gotten knocked loose when i put the card extender in the slot.  Time to pull the 51x rack apart and find the (likely) cold solder joint on that slot.    I swear the first thing i did was move the slots around. 
 

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