Tascam 244 | electronics troubleshooting in playback mode - tracing a sine wave

If you have no signal from the headphone or line out sockets of a Tascam 244 multitrack cassette recorder in playback mode, this video demonstrates how to figure out where you are losing the signal and narrow down where the faulty component etc lies, minimising the amount of deconstruction, resoldering etc that needs to be done.

The signal flow is quite complex, as follows;-
Magnetic heads
Record Ampl (signal routing only)
Playback Ampl (Opamps and trim pots)
DBX decode (Expander, op amps, trim pots, )
Cue amplifier (Routing only - tape out & cue monitor branch off here)
Power & Output Ampl(routing to mixer)
Mixer channel (Fader; EQ; Pan pot, after which mono becomes stereo)
Power & Output Ampl (L&R buss opamps 1)
Master Fader
Power & Output Ampl (L&R buss opamps 2; line out & aux out branch off here)
Playback Ampl (Headphone amplifier section; opamps)
Headphone Jacks

I’m testing at just a few easily accessible points to establish how far through this signal flow the audio is getting, few enough to locate which of the several printed circuit boards involved has an issue. Further signal tracing of the problem board would them be required to circle in on a small number of suspect components so visual inspection, testing, solder reflow etc could be attempted.

I’m using a low-tech and unconventional audio probe here -
A crocodile to crocodile cable and cheap computer audio monitors.As it turned out though I could hear the sine wave in the room okay, its barely picked up in the video audio...not great for demonstration purposes but at least it shows you this sort of testing can be done with cheap gear you already have?

Also I’m introducing a sine wave from a function generator instead of using a test tape because the unit under test has a faulty motor...if the motor worked, I’d use a cassette with loud recordings of test tones as my source signal - if you don’t have such a tape or the means to make one, any loud cassette recording would be adequate for testing purposes.

Please not that in a previous video on a 424MK III where I didn't have a schematic for the unit under test so there was a chance I might touch DC by accidentwhich might do somethign nasty to the amplifiuer I was using to hear the signal, therefore I had a 0.1uF capacitor wired between the probe handle and the positive wire going to a guitar practice amp to allow the AC signal into the amp but block any DC (capacitors block DC): in this video I'm not using a capacitor at all, because I am familiar with this circuit so I know I'm only going to touch the AC signal path.

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