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2600 Jr. - white noise instead of audio


x=usr(1536)

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Having a problem with my short-rainbow 2600 Jr. that I can't seem to find an exact match for, so am not entirely certain as to what the cause may be.

 

Basically, instead of audio, it only outputs white noise.  Sound had been working both before and after a recap which included replacement of the styrene audio caps with the same type.  This happens on three different TVs, so it doesn't seem to be an issue with the displays I'm using.  RF has been retuned and the audio inductor adjusted; no change in either case and the picture is fine.

 

The one thing I haven't tried yet is swapping out the TIA, largely because my spare one has been organised into temporary oblivion.  Once I can find it, I'll give that a shot.

 

Any ideas?  I've found plenty of no audio troubleshooting documentation (including in the 2600A service manual), but nothing for this situation.

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From what you describe my first thoughts would be that either no audio is being produced (TIA) or it is not reaching the stage where it is eing mixed with the viddeo.

Unfortunately tracing audio and video signals through the circuit without and Oscilloscope which most people don't have would be difficult and even then ideally you would also need a test program that outputs an unchanging video image (i.e. solid Red Screen) and audio (i.e 1KHz tone).

Unless a better idea comes along, if you only have a multimeter the best you could do is try and continutiy trace the singal path as much as you can as even with a fixed audio signal it may be diffficult for a Multi Meter to produce a consistant reading that can be traced through voltage wise, if it can measure frequency you may have a little more chance of tryng to find if the singal is present anywhere starting form where it is mixed with the video and working back to the TIA.   

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4 hours ago, Stephen Moss said:

From what you describe my first thoughts would be that either no audio is being produced (TIA) or it is not reaching the stage where it is eing mixed with the viddeo.

Yep, we're on the same page there ;-)

4 hours ago, Stephen Moss said:

Unfortunately tracing audio and video signals through the circuit without and Oscilloscope which most people don't have would be difficult and even then ideally you would also need a test program that outputs an unchanging video image (i.e. solid Red Screen) and audio (i.e 1KHz tone).

The test program I can supply.  The oscilloscope... Sadly, that's one item I don't have.

4 hours ago, Stephen Moss said:

Unless a better idea comes along, if you only have a multimeter the best you could do is try and continutiy trace the singal path as much as you can as even with a fixed audio signal it may be diffficult for a Multi Meter to produce a consistant reading that can be traced through voltage wise, if it can measure frequency you may have a little more chance of tryng to find if the singal is present anywhere starting form where it is mixed with the video and working back to the TIA.   

I'll go back and re-check.  Continuity was checked both to and from pins 12 & 13 of the TIA and seemed OK, but re-testing can't hurt.

 

My money's on the TIA's audio section being toast.  I'll redouble efforts to find the spare.

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