Deoxit on a dusty 688 midistudio?

[AmpAnalog]

Found a 688 with a capstan belt that needed replacing. Did that, no problem. So now that I have the transport running, every single pot and fader crackles and/or cuts in/out with movement. This unit comes to me from Las Vegas. The layer of dust on the outside was thick. Opening up the case, I blew air out of the chassis and had one heck of a cloud of dust.

Thankfully I found a genuine service manual on eBay, which has been helpful. I'd like to run the 688 through all the tests and alignments I can. But I probably need to remove the corrosion on the pots, faders, and jacks so I have a solid path to put signal through.

Any advice on what I need to do to clean up and restore this 688 would be greatly appreciated. The service manual is helpful and quite deep, but obviously doesn't cover the maintenance basics on how to clean and get this old electronic gear ready for advanced maintenance.

I've got an oscilloscope on the way and am trying to hunt down a Teac MTT or MXT test tape (thanks for the tip, Skywave!). I figure DeoxIT should restore the contacts, but I'm open to hear others' experiences.

Is there a standard checklist for assessing an old cassette multitrack's fitness?

 

[SkywaveTDR]

The check list is going through all the channels of the mixer section and when they are complete then the switch over to Rmx or Tape will go pretty smoothly. The jacks are damaged by abuse and if they reside near the ocean. Any exposure to a salt type atmosphere will be bad for and metallic non-protected device as I have seen decks come from Florida where they were on the back porch. They had a good amount of corrosion started on chassis parts. Of course with the right amount of effort and some repainting will fix it up. It is best to use Rustoleum for such painting.
The Deoxit will go a long way in fixing up most of the problems. If you want to go the full route which takes a lot more time, the mixer can be removed from the plastic case and then you can spray the pots more effectively. The correct way to do it is clean with Deoxit D5 and then treat with Fader Lub after that- you will find out how expensive this can get but the same cost a Technician has to deal with as well. In very bad cases where a jack does not clean up if you have any copper type riffle barrel cleaning brushes a couple of insertions of the right size could help the oxidation be wiped off- The copper ones are softer than the stainless steel ones. Remember you are still working with aged plastic jacks here and they are going to crack if you get too aggressive. The Deoxit and exercising the controls usually gets the noise out and the same with switches. Now knowing how much trouble it is to do all this one is best advised to cover the unit
with a towel or plastic sheet when the unit is not in use. You should see some of the units I get in as they look like they were in a Tractor barn. They go out looking pretty good.
Anyway a 1KHz input signal at -10dBV will be the sine wave you will be using the most. I also clean and polish the heads with Nu Finish that leaves a surfactant that acts as a short term lubrication improving tape to head contact and signal integrity. The MXT111 and MXT116 are really all you need besides the frequency counter or Wow and Flutter meter and the AC millivolt meter- I use the LMV181A type meter all the time from Leader. Bench 2 has 2 of them on it.

 

[AmpAnalog]

Skywave, I deeply appreciate your input; I am partway through opening up the mixer section of the 688 to clean and lube the faders/pots. The copper barrel brush is a terrific idea for the jacks; I think my dad has one I can borrow. I'm looking forward to using the Nu Finish tip.... that's a genius idea. I'll have to scour the 'net for the MXT111 and MXT116 as I assume Teac/Tascam won't sell such an item to an individual (if they're even produced for sale anymore). I have a Leader function generator (which I love), so I will check into the LMV181A meter.

I will say that overall (and with my limited experience) I find the 688 to be elegantly thought out and sensibly assembled. I have an old Tascam 112B I'm concurrently rehabbing and find it easy enough to approach. Must be something about the Japanese engineering from the mid to late 80s. Makes me wonder if manufacturing/engineering today could even build such a machine.

 

[SkywaveTDR]

Probably not. The last Tascam 424 Mk III was labeled made in China and it sure shows.

 

[AmpAnalog]

Amazed at how well built the 688 is; the quality shows at even connector and solder joint. It seems as though many of the components were custom made for this unit. Even the plastics feel quite substantial.

After opening up the right and left echelons I was able to blow out the dust. I used alcohol to wipe down the bare transport assembly and capstan flywheel. I removed the bottom plate underneath the channel/buss section, blew out the dust there too. Discovered that a switch for dbx affecting tracks 5-8 has a broken toggle leaving the noise reduction on tracks 5-8 stuck at "in" which means track 8 with MIDI tone won't be possible until that's fixed.

Anyhow: relieved the screws on the underside of faders 1-10 and master buss faders to gently squeeze deoxit in the fader mech, then worked them back and forth. They move and sound like new.

I've run 1kHz into each input channel and for each bussed the signal through to all 8 tracks. The cue section and Group 1-2 section test out okay. Adjusting the pan pots on various channels yields slightly "unbalanced" signal strength when bussed to the master/buss section as displayed on the meter bridge. The center detent on many of the pan pots seems to be inaccurate when doling out the signal to both sides of the buss in the grouping pair.

I'm taking notes and charting results to know where I stand in this process of rehabbing the 688. I'm even starting to understand the schematics in the service manual to a basic degree. There seems to be some grounding issues/hum, especially in the headphone jacks, but first things first: gotta make sure the signal path is good to go, then calibrate the transport/repo.

 

[SkywaveTDR]

Keep in mind these are not finished project types devices. It was made only for the recording of the concept of a music selection. Once the composer has all the parts defined how he likes them that is when either a Deck like a Tascam 48 or 58 can be used to produce a finished product. There are TV commercials in Chicago that had the audio recorded with a Tascam 38 1/2" 8 channel deck- I know who the studio is because I serviced it. While working in Radio for ESPN, WPNT, Sporting News and finally Moody WMBI, there is not a lot of difference of equipment in order to make commercials or cuts as they call them. You could probably make commercials on the 688 when in good shape and they would never know. Some production studios might use $21,000 Yamaha mixers but then some have been found to use $500 Mackie mixers as well.
The depth of the pockets of the owners defines what they buy. The largest issue I have with some of these Porta Studio from working on them is lack of cable length to open them up enough to make adjustments. They all have a level of aggravation of this type. The 488 has me making adjustments with a small Shure Brothers stylus mount driver that is about 2 " long. One could do research and make some cables but that all takes time to do and I have 200 units to get out of the shop. If you come up with some extension cables for a particular model, I will buy them. It is just a matter of getting the male and female connectors that match and then connecting the wire to them. I suspect the connectors might be Hirose type. It could be Molex too.

When Techs calibrate these decks, they only check if the pan works or not but most the time for a channel strip the pan is set hard left or right. These are most likely Alps pots not a plastic conductor grade linear part. To use that kind of part brings you up into the world of much more expensive units. They were made to get the basic work done. Deoxit is fine for cleaning pots but the final spray should be from Fader Lub. Let me know how you get on.

 

[wkrbee]

If you are not using an external meter to measure the group outputs,the problem could be the meters are not calibrated. Get the service manual and a good meter and do the cal. in the sequence specified in the manual

 

[SkywaveTDR]

That could be correct in that they are not that far off when I do calibration. Of course this is not a Rupert Neve Console