M
mem0ry
New member
I recently inherited a Tascam 424 MK1 that had been sitting unused for years in a house with heavy smoke exposure. I've been slowly cleaning and restoring it. I should say up front - I'm pretty new to electronics troubleshooting and repair in general, so I've been learning the basics (multimeter use, soldering, reading a schematic) as I go through this, and I could definitely be missing something obvious.
For my first real test, I set up two Shure SM57s (dynamic, so no phantom power needed) as an XY stereo pair into channels 1 and 2, to record acoustic guitar. Right away I ran into a loud buzz/hum.
Initial symptoms:
The buzz changed noticeably when I touched the mic cables
The hum was present even with both mics completely unplugged, just listening through the headphone output with the input channels empty
I moved the unit around to different outlets and got buzz in different rooms - my guitar amp and stereo speakers also buzz a bit, so I was worried this might be some general interference issue rather than something specific to the deck
The hum changed when I touched the any metal on the unit, or other metal objects in the room
That last one pointed toward a floating ground reference - the deck runs off a simple 2-prong DC wall adapter with no earth ground anywhere in the chain, which seemed consistent with the hum shifting when I touched metal.
I ran a ground wire from an exposed bit of the chassis to a real ground (an outlet's ground screw). It helped, but only partially - didn't fully clear the hum, which told me there was more than just a floating chassis going on.
From there I got a soldering iron, a multimeter, and the service manual/schematic, and started tracing the ground path by hand. Found that Input 1's jack had a completely dead ground connection - no continuity at all, where every other point on that board tested fine. Photos showed the jack housing itself had been knocked out of its proper position at some point, and I also found the switch that's supposed to mute the input when nothing's plugged in seems to be stuck permanently open. Between the physical damage and two separate electrical faults, I'm planning to just replace that jack.
Separately from that, I noticed Channels 2-4 buzz whenever something's plugged in and the fader's raised, and go dead silent otherwise, while Channels 5-8 stay completely clean under the same test. So there seems to be some general noise floor that the channels with a mic preamp are picking up and amplifying, that the simpler line inputs aren't.
I went hunting for that with the multimeter and tested the DC power jack's ground path all the way through - that came back fully healthy, so I've ruled it out.
My current theory is something's not right in how the chassis/metal frame grounds back to the actual boards inside, but I'm honestly not sure this model even has a single dedicated wire for that - there's not a ton of metal in this thing to begin with, so it's possible the ground is made through a mounting screw contact rather than an actual wire.
Given I'm still fairly new to this, I'd really appreciate input from anyone who's worked on one of these (or similar old Tascam gear) and knows how the internal grounding is actually set up, or has ideas on where else to check next. Happy to answer questions about what I've already tried if that helps narrow it down.
For my first real test, I set up two Shure SM57s (dynamic, so no phantom power needed) as an XY stereo pair into channels 1 and 2, to record acoustic guitar. Right away I ran into a loud buzz/hum.
Initial symptoms:
The buzz changed noticeably when I touched the mic cables
The hum was present even with both mics completely unplugged, just listening through the headphone output with the input channels empty
I moved the unit around to different outlets and got buzz in different rooms - my guitar amp and stereo speakers also buzz a bit, so I was worried this might be some general interference issue rather than something specific to the deck
The hum changed when I touched the any metal on the unit, or other metal objects in the room
That last one pointed toward a floating ground reference - the deck runs off a simple 2-prong DC wall adapter with no earth ground anywhere in the chain, which seemed consistent with the hum shifting when I touched metal.
I ran a ground wire from an exposed bit of the chassis to a real ground (an outlet's ground screw). It helped, but only partially - didn't fully clear the hum, which told me there was more than just a floating chassis going on.
From there I got a soldering iron, a multimeter, and the service manual/schematic, and started tracing the ground path by hand. Found that Input 1's jack had a completely dead ground connection - no continuity at all, where every other point on that board tested fine. Photos showed the jack housing itself had been knocked out of its proper position at some point, and I also found the switch that's supposed to mute the input when nothing's plugged in seems to be stuck permanently open. Between the physical damage and two separate electrical faults, I'm planning to just replace that jack.
Separately from that, I noticed Channels 2-4 buzz whenever something's plugged in and the fader's raised, and go dead silent otherwise, while Channels 5-8 stay completely clean under the same test. So there seems to be some general noise floor that the channels with a mic preamp are picking up and amplifying, that the simpler line inputs aren't.
I went hunting for that with the multimeter and tested the DC power jack's ground path all the way through - that came back fully healthy, so I've ruled it out.
My current theory is something's not right in how the chassis/metal frame grounds back to the actual boards inside, but I'm honestly not sure this model even has a single dedicated wire for that - there's not a ton of metal in this thing to begin with, so it's possible the ground is made through a mounting screw contact rather than an actual wire.
Given I'm still fairly new to this, I'd really appreciate input from anyone who's worked on one of these (or similar old Tascam gear) and knows how the internal grounding is actually set up, or has ideas on where else to check next. Happy to answer questions about what I've already tried if that helps narrow it down.
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