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Nagra IV-S - input chain issue


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Hi,  new to this forum, but have read a number of helpful threads here before and thus when i found myself in a pickle and in need of some pointers and advice I though i would ask this here. 


some background:  I have been using Nagra for some time, mostly a 4.2  for sound effects and tape loops.  I  have some experience in problem solving these, internal fuses  and so on, many moons ago I got a very thorough guide at Pro Sound (nyc) all recorded to tape on the machine we inspected.

I recently borrowed a rather well used 4 IV-S for digitisation purposes, this machine works well for playback, but off-course i want to also try the nice preamps and do some recording on it, give it a good clean, motor functions is good, rollers, heads seem ok, then I check the recording functions, thats when i realized that the left mic input was almost inaudible compared to the right, both in monitoring and on tape,  i tested with a multimeter the voltage and reading is good on all the steps of the pre amp. when measured of the xlr,  the right channel is perfect both with line and mic on all voltage setting, but the left i have to almost max out to hear and you can hear i distorts when you turn the gain right up, but its still not loud very out, this volume and distortion is reflected in line output, headphone and on tape . iIt plays back a stereo tape true. i wonder at what stage the fault could be, feels like its after the pre amp ...  has anyone had similar issues.  any information welcome.  thanks for looking.  ( its is a old machine  approx 1974 )  

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Do you have line input cables for it? If you record using the line input and there is no fault, it is easier to locate the fault.


A perhaps trivial suggestion: The IV-S is a great recorder, but the rotary switches used in it are not as durable as the Elma switches in the mono machines. Have you tried giving the switches a bit of exercising? Both in the mic power selectors, the filter selector and all other switches?


on www.nagraaudio.com you can download the service manual, and in it are some good overviews of the signal paths.

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If one channel works well then one way to rule out a preamp fault is to interchange them. It does not require too much practice but you must follow a good guide (unscrew the correct screws for instance.) 

The way you describe it could be the line amp. But that’s just guess. 
I am familiar with the inside of the 4.2 not much the one of the 4S but you should find Nagra schematics easily. Most electronic modules can be changed. 

What does it record on the tape? If I recall correctly on the 4.2 the signal goes from preamp to line amp for monitoring and line output AND simultaneously to the amp for the magnetic head (I forgot the name of the module) so you could hear nothing but still record correctly on the tape. 

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Unfortunately the IV-S does not have individual mic preamps, it has a stereo preamp assembly, so it is not possible to switch modules between channels...


Normally the module connectors does not fail, but you could start with reseating the signal processing modules (no need to involve the A22 motor board behind the battery compartment. I have attached a block diagramme of the IV-S, so that you can see the signal flow. As you can see, in REC mode (and MIKE In enabled) the output of the mic preamp is on the middle pin of each level control, which is quite easy to find. So if you follow the signal path from the input, it should be possible to find out where it disappears.


And as always: Look out for contact problems, intermittent switch problems etc.



IV-S synoptic.jpg

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Thanks for your replies !  I have ordered some 7 pin binder connectors and will make up new line in and out’s for it. there has been a leak for sure at some point. should mention It also has a mod: a switch to turn on and of the speaker in playback and a added 2.5 kΩ   ±2% (G) resistor  (found inside back electrical tape) on a white and purple cable that looks added in the region of the speaker, have to remove speaker to see where it terminates, its not on the speaker, the added switch is on the speaker circuit. @ Dela thanks for the synoptic diagram ! thats a good route to follow. It does disappear somewhere must be possible to find with the diagram. I will post back with a possible fix : ) 

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