TASCAM 688 MIdiStudio problem with metering and switching on track 3

[altruistica]

Had a chance to use the unit today and found it was 'behaving' and some of the problems I had seemed to disappear as I used it....very strange as I have re-capped the power supply PCB and checked the new voltages and apart from the negative rail measuring -11.94V (which I thought was fractionally low for new caps....the + rail was +12.11V) everything else seemed fine. I had a chance to hook it up to a Technics SX-WSA1 that I used as a sequencer between 1997-9. I still have floppies that I could load in. The 688 is a midi-beast as it generates its own FSK tone which it lays to TK8, then spits out midi clocks to sync your sequencer.....very clever for the time.
I'll make a few videos once I'm totally au fait with its foibles.

 

[sweetbeats]

Late to the party…yes…RF shielding…Coated on one side. The stuck clip LED may be the opamp driver latched up. I’d need to look at the schematic. As you are experiencing it wouldn’t effect the audio because the metering and clip circuitry are buffered from the audio path. You’ve done a LOT of work. I’m impressed. What at this point poses as a “must fix’ task to you, Al?

 

[altruistica]

Hey Sweets,
I guess I'm just an inquisitive type....I like to understand things. I realise that I've probably jumped in the deep end with the most complicated cassette portastudio that was ever built, but you learn a ton of stuff from doing it. I'd like to get the unit to a point where it can produce recordings free of any distortion or hum that is there because of broken components. The wires coming from the recording head are interested because they're just lying on top of the components of the P/B PCB, which have the steel plate covering the. I wonder if they should be tidied up if possible. The same with the wires that go the monitor boards. I may be getting rf humfrom those. I've printed off on A3 some of the schematics just so I can have a go at trying to understand them more. There is a circuit called 'POWER MUTE' which I'm not sure is something that operates to make the routing switching silent, or is something else?

I have been able to sync some stuff from the Technics workstation and it has been interesting to hear the DBX cassette flavour, so much so, that I actually pulled the trigger on a really good looking 238 unit (the rackmount version) yesterday along with a new old stock RC88 remote. I paid top dollar (well EURO) for the units but I'm hoping these are hardly used units and the seller seems to sell Studer and other top equipment and has had a new capstan motor installed before selling it....so I'm hoping it will be as good as new. The limitation I see in the 688 is, for someone like me, in the mixing stage. I think I'd be hard pushed to get a 'professional' mix out of the 688 itself. The best thing would be to take the 8 outs (or 7 if using synced stuff) and put it through my Soundcraft Spirit. I did try the 688 and 238 about ten years ago (when at one point I had two 238 synced up using a Tascam ATS500). I then moved 'up' to MSR16, then MS16, then MSR24, then the Fostex G24S. The reason why I've ended up back at the 688 and 238 is because of the medium itself. Cassettes are still cheap. They still work (I've been using cassettes which are thirty years old, and they still sound great) and they're nice and small. It's a part of recording history that I skipped....I went from an Arion double speed four track with a dodgy mixer, through to getting a job that had a Fostex R8. I used that until I could afford a Soundtracs desk and my first ADAT. To think I paid £1500 for the Alesis BRC remote for ADAT machines, when I could have had the whole 688 for the same price in 1993 makes me smirk.

So, I want to try recording stuff to the 688. This will allow stuff to be done free of a computer. Maybe try syncing that to Studio One (I don't know if Studio One will sync with just midi clocks) and off-load takes into Studio One. I have Studio One outputting 24 tracks via the Cymatic into the Soundcraft Spirit desk and mixing from there. The 238 could be used to bounce stuff from say the 688 if I decide to stay out of the box completely. There are endless possibilities with these pieces of equipment. I also want to try recording my wife's voice to the cassette and see what gives. This was done all in Studio One the other day.......an antidote to the madness we're seeing all around today.

 

[sweetbeats]

I don’t doubt that you’d be able to sort out the remaining issues that are encroaching on the audio path…but i suspect it will test your patience. The hum on mic inputs 4 & 7 is likely a common issue as is the issue effecting track monitor 1, 3, 5 & 7.

I totally understand the drive to sort it out. That resonates entirely with me. The 688 is a tough subject indeed because of the complexity, density, and some fragility. Prices on the 238 have gone bonkers in the last few years, but they are way easier to work on, are a much more robust build, and if the seller replaced the capstan motor assembly, which should include the servo assembly as well (the servo board is mounted directly to the back of the capstan motor assembly), that is the main Achilles heel of the 238. So if that’s been addressed, huzzah. If it is a dbx model and not the Dolby S model, then double huzzah. The 238 has better sonic potential than the 688 because you aren’t limited by the 688 mixer. As far as Porta/MIDIStudios go the 688 and 644 are going to fare a little better as far as sonics go in that generation because of the electronic assignment switching. And overall the sonics are fine. But you can do better with a 238 and a separate higher quality mixing console.

The R/P head wiring laying all about I wouldn’t worry about that. Nothing wrong with tidying it up, but it is high quality cable, and is properly shielded, and honestly there’s not a lot going inside the 688 that’s going to cause interference anyway…it’s really more about what’s *outside* the 688, and there’s a decent enough amount of shielding in the chassis design there.

The POWER MUTE circuit is just that…a muting circuit to avoid popping in the outputs when the device is powered…would be heard as popping or thumping in the speakers.

I’m not sure if I addressed everything I wanted to address from your post. I do know I wanted to say WWWWOWWWW regarding the song you linked. Wow. Some talent going on there in the performance, arrangement and production. Thanks for sharing.

 

[altruistica]

Yes, the 238 is the DBX version and was described by the very good professional seller as:
'Tascam 238 8-track Cassette Recorder, has hardly ever been used and is like new. PCB Capstan Motor Replacement has been replaced by a technical Dutch company. Incl original pakking.'

The setting of the Lord's Prayer I did in 2019 (just at the piano)....I'm not sure why. I've been an atheist all my life, but the last ten years I started listening to a guy called Rupert Sheldrake and another guy called Marty Leeds. Both have led me to see creation in a totally different way, along with consciousness and the idea of a soul. I've always thought the Lord's Prayer has some beautiful words and sentiments, but in churches in England, both catholic and protestant congregations seem to turn these sacred words into nothing but mumbled dirges. I wanted to try and elevate the words to their rightful place. Also singing the words involves ones soul much more than just speaking them. I orchestrated the setting using Native Instruments Symphony Series Sample Library within Studio One 5 last week and Toni, my wife, sang it in a couple of takes.

I've just started watching this on YouTube, but I can only manage short bursts as it really needs re=watching until it sinks in, but seems like a good place to further understand schematics at an elementary level.



If anybody has other sources which might help understanding this stuff better, I'm all ears.

Thanks for your thoughts on the problems. I've just uploaded a video on the hiss problem with the CUE MONITOR path. If the problem becomes less after a certain time the unit is on, does this point more to a faulty capacitor that maybe reaches its operational spec? And because the problem is not apparent on the 'MIX' or track armed state, does this look more likely that I will find the problem on the input stage of the R/P PCB ....yet it's only on the Left channel groups? Thanks too for the kind words on the music (nearly forgot the music with all this fixing going on).



Cheers,
Al

 

[altruistica]

I had the machine on for a couple of hours today, recording some test stuff and erasing some tapes. After this length of time, the white noise on the CUE MONITOR BUSS was significantly reduced to the point where you had to have the monitor knob up full to hear it. There was still slight hum on inputs 4,7,8 and 10.

 

[Papanate]

Now I don't know if that problem is the same one I've got, but I made a short video here to demonstrate it.

Any help offered, gratefully received. Thanks, Al

Seems like you have a grounding problem.

 

[altruistica]

Cheers Papanate, yes that seems to be the consensus. I'll have to open it again and see if there's anything obvious......what's strange is that it affects 4, 7, 8 and 10. It seems a little abitrary, not like they're channels next to one another on the BUSS PCB. When I got the unit I checked the input channels through and they didn't have the hum, so either the things I've fixed have created another problem, or I have created it trying to fix the unit. Fun and games....

 

[altruistica]

YAY!
I had the input channels off the mixer again today to try and cure:
1. The overload led on channel 5. It is fully lit
2. The overload led on channel 8. It slowly creeps on, though not as much as channel 5
3. The hum that has appeared on channels 4,7,8 and 10
4. The noise on the CUE MONITOR BUSS when then unit is in any mode other than 'MIX'.

I started by having another good visual look at the BUSS PCB with my super goggles. I couldn't see anything obvious, so I elected to run solder over all the joints. After inspecting the input boards, it was more likely that if there was a dry joint between the input boards and the buss board, it would be on the buss board, as the ones on the input board are doubled up (they are like the female socket which the male buss pcb inserts into). I then set about removing a couple of transistors from around the led to check them. I compared the ones on channel five with the ones on channel four, and couldn't see any difference.....so these are not the culprits. To make sure I swopped them over with one another. It was only at looking at channel 4, that I noticed the ground pin on the XLR was loose. They are soldered straight onto the board. I couldn't imagine when it came loose, but then wondered if the same thing had happened to the other boards on removal and re-fitting. Sure enough, channels 7,8 and 10 all had loose ground pins. I then elected to remove all the input boards and run new solder over all four joints.

I put the 688 back together and was disappointed that the re-soldering of the buss board hasn't made a difference to the CUE MONITOR problem. Also, I hadn't cured the LED on channel 5 either.....though now led 8 illuminates straight away....I wonder if in time, they're all going to light up?
BUT....and it is a big but......the unit sounds tons better. The hum has all disappeared on all channels. I really wanted this sorting, because I want to use the pres on a drumkit and I also want to insert a Presonus ACP8 that I have on up to 8 channels. The CUE MONITOR buss fumctions as it should if you keep the 'MIX' button enabled......which really isn't a problem. I just did a test recording and the matrix switching is silent and the unit sounds tons better.

So, for now, I'm going to start recording with the unit and see how far I get. I'm pretty sure the 'noise' problem is component related....because it gets less and less the longer the unit is on.

Onwards and upwards....

 

[altruistica]

So I woke up thinking.......it can't be that hard to find out what's causing the overload LEDs on CH5 and CH8 to illuminate all the time. Sweetbeats had said, 'The stuck clip LED may be the opamp driver latched up'. I'm not sure which bit this would be, but I have found a reference to an overload LED design which was an interesting read. It mentioned that noise could be introduced on the power lines. Could the two problems I have, namely noise on the CUE MONITOR BUSS and the LED constantly on be the same problem? I've put it all in the attached PDF (Overload preamp design) to save looking for stuff, with the Tascam 688 Input Board schematics on Pg5 and Pg6. From the 688 manual the parts I think it concerns are:
Q1 - Transistor 2SC1844
Q2 - Transistor 2SC2603F
Q3 - Transistor 2SC2603F
U1 - Op amp uPC4570HA
U5 - Digi Transistor DTA124ES

I've also added the data sheets for these parts.

If anybody that understands this stuff like I understand looking at a full score and has the odd half an hour to peruse, I'd really appreciate your input. I've got the 688 to a state now where I can use it, so basically I don't want to mess it up looking for the last 2%....On the other hand I don't want to use the unit knowing I'm making the situation worse.
Cheers,
Al