Tascam 688 EQ repair


I've been having some issues with my tascam 688, more specifically with two channels (1 and 2). There's a problem with the EQ section of these two channels, it seems that whenever I play back audio that was recorded on channels 1 and 2, and I play around with the EQ knobs of channel 1 and 2, especially the low end/high end, the audio sort of just loses its treble, or even the volume just drops down. It's like if I tinker around with those knobs, there's a loose connection somewhere thats causes the audio's treble/volume to drastically drop.

I've tried cleaning that EQ section with deoxit, but the problem still persists. I was thinking, if it's not a problem that can be fixed with deoxit, maybe it has to do with the wiring, maybe a loose connection? I opened up the tascam to see if I could maybe get a good look at that EQ section, but alas it didn't seem to be that easily accessible and I didn't want to wander too far into the depths of this machine...

any thoughts on how to fix this problem? any help is appreciated, thank you!
I wouldn’t use that on carbon element potentiometers…that’s a general electronic parts cleaner.

I would get some DeoxIT F5 Faderlube for cleaning the pots and faders, but my gut-hunch is that’s not your problem. It sounds more mechanical than just dirty pots. If it was me I think my first step would be to get at the components and inspect for damage.

But, yes, back to cleaning, to do it right, clean them and drain them and inspect them you really need to disassemble and pull the channel PCBs out. I’ve never heard of a problem like you describe…

Any way you can post a link to a video so we can see what you’re doing and hear what you’re hearing?
Good to know, I'll get some proper DeoxIT F5 just to be safe.
But yeah I didn't have the courage to venture too much in the machine and pull out the channel's PBCs, in fear of messing something up...
And yes I will be posting a video as soon as I can!
I understand. Yeah just start with the video. The 688 and 644 are nicer than other period cassette multitrackers as far as the mixer construction since the channel cards are vertical…easier to isolate when troubleshooting, and generally take a hit between than a horizontal monolithic PCB. BUT the pots are plastic shaft smaller bodies units with little reinforcement as to their mounting…it’s just the way it is with this echelon of gear. I think the video will help me understand better what’s happening and maybe offer suggestions.

I hope this shows a bit better what I was trying to explain... Basically the problem is on channel 1's low EQ fader and channel 2's high EQ fader.
It's like the low fader on channel 1 doesn't even affect the bass at all, playing with it only seems to affect the track's treble and highs.
And on channel 2, the high EQ fader only works when a certain pressure is applied on the fader.
But basically both of these faders have to have some kind of pressure applied at a certain angle on them to achieve some sort of EQ'ing.
To me, I would've assumed this was simply just a problem that could be fixed by cleaning the pots/faders, but it might be something else, I don't know.

The video is helpful.

No, cleaning is not going to help that. That is a mechanical issue. You either have a damaged wiper, which is like a set of tiny metal fingers or contacts the rest on the element. If the pot experienced some trauma that could tweak the shaft and bend the wiper so it no longer makes contact with the element reliably. I don’t think this is the case here because the plastic shafts on these pots would likely snap before internal damage could occur. The other thing that can happen is if enough dirt or debris gets into a potentiometer, it can get in between the wiper and the element interrupting contact, but that doesn’t really occur with closed bodies sorry potentiometers…happens more with linear slide potentiometers (“faders”) with a horizontal element. So the other thing that can occur is some trauma and compromised solder joints. That’s the first thing I would check…the solder joints associated with the faulty pots.
Thanks for helping me troubleshoot this sweetbeats! (Also, my bad I kept saying ''faders'' when referring to the pots)
I will have to open it up once again when I got time.
I'm guessing I'll probably have to remove the channel to inspect the pots' solder joints...

I actually used one of your videos on opening up the 688 as a guide, very useful info!
will update soon with the repairing process
Alright, so I opened up the Tascam and removed channels 1 and 2 and lo and behold I spotted that there were indeed some loose solder joints on the problematic pots...

Soldered them and then closed the machine back up again. Channel 1 somewhat works a bit better, but there still is a bit of the same problem... same thing for channel 2.
My guess is what you said about the damaged wipers...
I probably would have to just replace those pots entirely maybe?