Deutsches CPC-Forum > Off-topic

Good forums to help fix electronics?

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Bryce:
An MP3 player with a 10kHz sinewave recording can be used instead of a signal generator.

Bryce.

tjohnson:
Thanks, I had that thought after I wrote my response.  Hopefully I'll be able update tomorrow on the results.

tjohnson:

* Right so I made a little cable up and hooked it up to CN041 on the board for deck B.
* I set the test tone to 1khz I used spotify to play audio test tones 1khz at 0db, starting with vol turned down.
* I hooked the scope up to c130 which comes off L104 (I check both C130 and C230 and the same end result)
* I used a set of headphones to listen.
* Without pressing play I heard nothing.
* Pressing play I heard the test tone ok without the same level of distortion however as I turned the volume up distortion was heard although not quite the same and I started to pickup noise on the scope.
* When playing the test tone at normal audible volume there was no additional noise at c130 than what I observed without playing.
* I then hooked up the head back onto the board at CN041 and played the tape and observed noise at c130 (see picture) and the horrible playback sound through the headphones.
* I also recorded, obviously this didn't record to the tape as the head wasn't connected, and observed what I assume is the bias signal at 107khz (see picture).
* So I'm a bit perplexed as to what the issue could be and the source of the noise.Sorry about the bullet point, the forum put everything into one big Paragraph.

tjohnson:
 I have spent ages trying to work out what is happening and have now have an interesting update.
 
I then fed in the signal from deck A on one channel into the B connector CN041 on the board again and played from deck A.  I heard nothing but upon connecting deck B back up it now plays perfectly fine without any distortion but recording is now not working where it was before. 
 
 So the issue is now reversed, Deck B is now playing without distortion where it wasn't before but now doesn't records silence where it recorded before.
 
 The board has alot of ICs and a main processor, I can see in the block diagram something called record mute, I'm wondering if this is some sort of record muting issue.  I'm not quite sure how that's suppose to work but maybe it should mute on playback but not on record and wasn't muting before on playback so putting a high voltage signal in during the playback hence the massive distortion but is now stuck in mute.....atleast now I have more of a chance to tracking the issue down I think as I can monitor for the record signal....the mystery continues.

Bryce:
If this is an expensive piece of kit and you seriously want it back working (and both of those seem to be the case), then the first thing I'd do is replace every electrolytic capacitor. When the more complicated parts such as multiplexers, Op-AMP's, CPU's etc have issues, you usually loose an entire channel or the entire device is dead. Distortion means it's working, but something is outside the specs. On really old valve radios, this could also be a resistor drift issue, but not here, the only thing that goes bad are the electrolytics.

Bryce.

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