Tascam 788 Portastudio disassembly

[iamsetsetiam]

It looks like I am going to have to try taking my 788 completely apart to get to some no longer functioning buttons(primarly the rewind and stop buttons). I have tried the contact cleaner spray from the outside around the problem buttons as described in another thread, to no avail. If anybody has taken apart their 788 I would much appreciate any tips etc.

Thanks

 

[sweetbeats]

I’ll PM you a link to some service instructions including disassembly instructions. I don’t know what your experience is with this kind of stuff. Depending on the type of switches, it may require good soldering skills if replacement is needed. But you’ll need to tear it apart to see what you have and to be able to test and assess what needs to be done. I’d have responded sooner but you didn’t tag me like I suggested in my Tascam 388 thread in the analog forum, and I don’t regularly look at this forum…just the analog forum.

 

[Slouching Raymond]

I know nothing about 788s, but have taken a lot of things apart.
I would just say take photos while you dismantle, so that you have a record of what bits went where, and what colour the wires were.
I always lay out any removed screws in the same pattern as they were on the device.
Using a camera set for close-ups would help.
Good Luck.

 

[iamsetsetiam]

I’ll PM you a link to some service instructions including disassembly instructions. I don’t know what your experience is with this kind of stuff. Depending on the type of switches, it may require good soldering skills if replacement is needed. But you’ll need to tear it apart to see what you have and to be able to test and assess what needs to be done. I’d have responded sooner but you didn’t tag me like I suggested in my Tascam 388 thread in the analog forum, and I don’t regularly look at this forum…just the analog forum.

I’ll PM you a link to some service instructions including disassembly instructions. I don’t know what your experience is with this kind of stuff. Depending on the type of switches, it may require good soldering skills if replacement is needed. But you’ll need to tear it apart to see what you have and to be able to test and assess what needs to be done. I’d have responded sooner but you didn’t tag me like I suggested in my Tascam 388 thread in the analog forum, and I don’t regularly look at this forum…just the analog forum.

Thanks again Sweebeats for that link. I can solder a bit, tho hardly have "good soldering skills" I bought a cheap as-is 788 off ebay to with the thought to use it to practice the disassembly process and maybe I can also get some parts that work off it, especially those switches. It doesn't come with the 788 power supply which are extremely unique, hard to find, and pricey. I do have the power supply for mine, tho of course.

 

[iamsetsetiam]

I know nothing about 788s, but have taken a lot of things apart.
I would just say take photos while you dismantle, so that you have a record of what bits went where, and what colour the wires were.
I always lay out any removed screws in the same pattern as they were on the device.
Using a camera set for close-ups would help.
Good Luck.

Thanks for the suggestions about the photos and screws - this will be an adventure for sure. As a mentioned in my post to sweetbeats, I have another 788 coming that I bought "for parts" and I will get some practice with tearing apart that one hopefully and also maybe get the switches from that one to replace the faulty ones in mine.

 

[sweetbeats]

I find it is always beneficial, if I have a particular device or machine I really like, to find a backup “parts machine” or spares donor. You seem to be committed to your 788, so good move I think getting the second unit. It also does indeed afford the opportunity to practice disassembly/repair with lesser consequences than learning in your primary device. And yes, maybe the keyboard assembly will be fully functional it it is possible to trade them out. How easy the swap is will depend on whether or not any connections to/from the keyboard assembly are hard-wired or connectorized. Feel free to post pics as you get things opened up. I’ll also take a close look at the Service Manual. I only just acquired it myself.

 

[sweetbeats]

Okay it looks like the transport control switches are on a combined monolithic “gather” PCB with the mixing section controls/encoders and A/D/A circuitry. So there is no smaller “key” or “keyboard” PCB assembly to swap. You’ll be swapping the entire upper PCB.

Also, FYI, the external “power supply” is just a step-down AC transformer to a 6-pin mini DIN connector. It feeds AC to the regulated supplies that are onboard the 788…+5V and +6V for logic and digital circuits, +/-12V for analog audio.

 

[sweetbeats]

All of the push switches on the gather PCB are “tactile” push switches. These are readily available for a low cost through a number of online electronics parts suppliers.

 

[iamsetsetiam]

Lots of great info already, thanks Sweetbeats! I will get that 788 from ebay soon and go from there. I have attached a pdf from the Tascam site that shows some of the disassembly stuff for purposes of replacing the eeprom.

 

[iamsetsetiam]

Also, regarding that massive 788 power supply, I found the following info from someone who apparently "built" one successfully.:

Putting together a Pwr Supply for a 788 and need to know the pinout for the mainboard jack. Shield = Ground, Pins 3-4 = Ground. Are pins 1-2-5-6 all hot? (12VAC). Came without the P-788 and have a few nice supplies that can be used. Thanks in advance----Jim p.s. jack is the PS2 jack.

788 Solved

Couldn't get anyone to respond so I dig in and figured it out! Tascam info very vague--turns out the supply is 24vac split into two 12vac lines. If phased corectly two separate 12vac supplies can be used. (works like a charm). Pin out-----pins 1-2 12vac---pins 5-6 12vac across 1-2 & 5-6 reads 24vac. Pins 3-4 and shield are common and should read 12vac to 1-2 & 5-6. So this was the solution I used--up and running----------Jim

 

[iamsetsetiam]

@sweetbeats Ok here are some pictures of this disassembly in process. The first (leftmost) picture is a closeup of the area on the 'back' of pcb corresponding to where the problem switches (closeup of those switches in the 4th picture) are on the other side of the pcb. I am stuck at what to do at this point. I am also wondering if it's possible that the those plastic 'buttons (part 15 page 23 in the 788 document you sent me, and second picture here) could be part of the problem. Also, as I mentioned initially, I did try spraying contact cleaner from the outside around the problem buttons/switches and while it didn't fix them and actually seems to have made them worse, there is one weird thing: Right after I sprayed them (again this is all from the outside before any disassembly) they worked great for a little while - at least 5-10 minutes I think. I went through this pattern of spraying to see what would happen a number of times always with this same result. I thought this might perhaps be a "clue". Thanks - I await any thoughts/ideas etc.

 

[Slouching Raymond]

iamsetsetiam, those switches are very similar to the ones on my Korg DX16XD, and I have loads of pictures like these, from when I was investigating mine.
The first thing you need is a buzzing digital multi-meter, to check out the switches you think are dodgy.
Each switch looks to have a connection at each or its 4 corners. It will have 'make' contacts, and 'break' contacts, but they may not all be required for correct operation.
A slight variation on mine was that some of the switches had a surface mount LED next to them, which illuminated the mechanical keytops, shown in your second picture.
You need to buzz out the connections of a good switch, while pressing it, and not, to find out what 'makes' and 'breaks' occur. All the switches will be the same.
If you identify a mal-functioning switch, you'd be better off replacing it, or at least try to spray cleaner into it, through whatever gap you can find.
Perhaps there is a button somewhere that you never use, and you could swap over a good switch from there?
It is possible that the actual switches are all perfectly good, and that your problem is the mechanical connection between the switch and the key cap. I did find this once.
That would be easier to fix.

 

[sweetbeats]

I agree with much of this ^^^^^.

I don’t see contacts at all four corners. I see a joint to ground at the top of each switch that goes to the metal body, and then two contacts. If there were four contacts this would be a double-pole single-throw switch (DPST), but they look like single-pole single-throw to me (SPST), and the schematic confirms this. These are called “tactile” switches. You can search for them at Mouser or Digikey…other places as well. They are very common and you should be able to find replacements, but because there are literally thousands of different kinds, you have to have your “ducks in a row” as far as the specifics of yours. We know they are “through-hole” parts (they mount to the PCB through the board rather than mounting to the surface), they are SPST OFF-ON momentary switches, they are horizontally mounted, they are not lighted…but you also need to know the width and depth of the body of the switch, how high the top of the switch body is from the PCB, how far out the actuator protrudes, the shape of tip of actuator (concave, flat, etc…), and also the distance between the contact pins that are soldered to the PCB. Do you have a dial caliper? This is the best way to get accurate measurements of all these things. If you don’t have a dial caliper they are not expensive…you can get one from someplace like Harbor Freight online. Once you have all this information it might be easiest, rather than sifting through the thousands online, to just call Mouser or Digikey and ask for help identifying a compatible switch…Or open a chat with a sales rep. You would just start the conversation by saying you need help finding replacement SPST horizontal-mount through-hole tactile switches. If this seems complicated this truly is just how it goes when trying to keep older electronics gear going. The good news is you can still get these parts.

As far as spraying contact cleaner, if I’m understanding correctly, your efforts so far have been limited to jetting cleaner through the gap around the plastic buttons on exterior of the 788? If so, that is a useless effort. Like @Slouching Raymond is saying, the cleaner has to be jetted directly into the switch by any means you can…probably where the actuator goes into the switch body. You jet, exercise the switch (press it rapidly 25 or 50 times) and jet again to flush. Let it dry overnight, exercise again and try it out. It looks like what you’ve done so far just made a mess all over the PCB. Very important question: what contact cleaner are you using? Brand and type please.

Also, what is going on with capacitor C5? There is a group of four axial capacitors in a row, C4~C7 near a couple of the switches…does C5 appear damaged or is there just crud on it? The picture is too blurry when I zoom in but something doesn’t look right there.

So you can try properly treating the switches with an appropriate contact cleaner first as described above, but before you do anything I’d like to know what cleaner you are using. My hunch is that in the end the solution is going to be new switches.

 

[iamsetsetiam]

iamsetsetiam, those switches are very similar to the ones on my Korg DX16XD, and I have loads of pictures like these, from when I was investigating mine.
The first thing you need is a buzzing digital multi-meter, to check out the switches you think are dodgy.
Each switch looks to have a connection at each or its 4 corners. It will have 'make' contacts, and 'break' contacts, but they may not all be required for correct operation.
A slight variation on mine was that some of the switches had a surface mount LED next to them, which illuminated the mechanical keytops, shown in your second picture.
You need to buzz out the connections of a good switch, while pressing it, and not, to find out what 'makes' and 'breaks' occur. All the switches will be the same.
If you identify a mal-functioning switch, you'd be better off replacing it, or at least try to spray cleaner into it, through whatever gap you can find.
Perhaps there is a button somewhere that you never use, and you could swap over a good switch from there?
It is possible that the actual switches are all perfectly good, and that your problem is the mechanical connection between the switch and the key cap. I did find this once.
That would be easier to fix.

Thanks much for responding. I have a multimeter that buzzes when there is continuity, but I am not sure where I would place the multimeter probes, which also might be too big for this as well. Ya, I was wondering about that key cap to switch issue as well - what is the "easier" fix for that. The 788 in these pictures is the one I bought for parts and not the one I need to fix - on this for parts 788 those switches worked fine before I disassembled it.

 

[TalismanRich]

There are several videos on Youtube that show how corrosion under the metal snap causes the button to fail. This is one such video:


I was looking around last night and ran across this in a Google search. Unfortunately Full Compass isn't showing anything like it in stock. Its the closest thing that I can find that has the ground lug. I don't see any 4 point switches with the ground lug.

 

[iamsetsetiam]

I agree with much of this ^^^^^.

I don’t see contacts at all four corners. I see a joint to ground at the top of each switch that goes to the metal body, and then two contacts. If there were four contacts this would be a double-pole single-throw switch (DPST), but they look like single-pole single-throw to me (SPST), and the schematic confirms this. These are called “tactile” switches. You can search for them at Mouser or Digikey…other places as well. They are very common and you should be able to find replacements, but because there are literally thousands of different kinds, you have to have your “ducks in a row” as far as the specifics of yours. We know they are “through-hole” parts (they mount to the PCB through the board rather than mounting to the surface), they are SPST OFF-ON momentary switches, they are horizontally mounted, they are not lighted…but you also need to know the width and depth of the body of the switch, how high the top of the switch body is from the PCB, how far out the actuator protrudes, the shape of tip of actuator (concave, flat, etc…), and also the distance between the contact pins that are soldered to the PCB. Do you have a dial caliper? This is the best way to get accurate measurements of all these things. If you don’t have a dial caliper they are not expensive…you can get one from someplace like Harbor Freight online. Once you have all this information it might be easiest, rather than sifting through the thousands online, to just call Mouser or Digikey and ask for help identifying a compatible switch…Or open a chat with a sales rep. You would just start the conversation by saying you need help finding replacement SPST horizontal-mount through-hole tactile switches. If this seems complicated this truly is just how it goes when trying to keep older electronics gear going. The good news is you can still get these parts.

As far as spraying contact cleaner, if I’m understanding correctly, your efforts so far have been limited to jetting cleaner through the gap around the plastic buttons on exterior of the 788? If so, that is a useless effort. Like @Slouching Raymond is saying, the cleaner has to be jetted directly into the switch by any means you can…probably where the actuator goes into the switch body. You jet, exercise the switch (press it rapidly 25 or 50 times) and jet again to flush. Let it dry overnight, exercise again and try it out. It looks like what you’ve done so far just made a mess all over the PCB. Very important question: what contact cleaner are you using? Brand and type please.

Also, what is going on with capacitor C5? There is a group of four axial capacitors in a row, C4~C7 near a couple of the switches…does C5 appear damaged or is there just crud on it? The picture is too blurry when I zoom in but something doesn’t look right there.

So you can try properly treating the switches with an appropriate contact cleaner first as described above, but before you do anything I’d like to know what cleaner you are using. My hunch is that in the end the solution is going to be new switches.

Thanks for your thoughs and suggetions. A reminder that these pictures are from the 'for parts' 788 that I just disassembled - didn't spray this one. The contact cleaner that I sprayed on my 788 is LPS CFC free Nu contact cleaner - this one: https://www.emisupply.com/catalog/05416-free-contact-cleaner-11oz-aerosol-p-52630.html
That was the one recommended in some other 788 thread by someone who has successfully fixed some 788 switches/buttons by that spraying around the buttons from the outside etc. No I don't have a dial caliper. I'm not sure my soldering skills are adequate, but if the spraying method you describe doesn't work on my 788 I could try swapping out those switches from this for parts 788 I guess.

 

[iamsetsetiam]

A better picture showing the connections of two of those switches - I really am not knowledgeable about this stuff, but it kinda looks like 5 connection points per switch. The switches are at the bottom of the pcb - I put arrows pointing to them. Thanks.

 

[sweetbeats]

You can use what you want. That “contact cleaner”, while I’m sure a fine product, is not a dedicated metal-to-metal contact cleaner. It’s a more general “electronic parts cleaner” for cleaning boards after soldering, cleaning dirt and other residue or foreign contaminants from electronic parts, etc. as such you can also use it to clean metal-to-metal contacts of those same things. In this case I’d be using something that is specific for cleaning and deoxidizing contacts. My go-to is DeoxIT D5. I believe it does more than what you are using. Again, use what you want, and if it worked for somebody else maybe it will work for you too. I still say flooding it from outside the device and crossing your fingers in hopes it will somehow leach into the tactile switches is not a good use of time and resources. You’ll have much better luck applying it directly to the switch.

Thanks for the clarification the pics are of the parts unit “gather” PCB.

Well, the parts unit is non-functional right? Or do you know that? Since all of the control functions for mixer and transport are all on that one PCB, there’s a high chance if something was dead with the parts unit it is a problem on that PCB. But there’s no harm in trying I suppose. I’m still curious about what’s going on with C5, but I’d need a better picture.

As far as switch replacement, you can find any local electronics repair shop, bring in the PCB and new switches and they can put them in for you for a fee. I’d also be open to doing it, PM me if you want to discuss that. Either way you need to do the leg-work to find the parts.

 

[sweetbeats]

A better picture showing the connections of two of those switches - I really am not knowledgeable about this stuff, but it kinda looks like 5 connection points per switch. The switches are at the bottom of the pcb - I put arrows pointing to them. Thanks.

No. Two of those solder joints go to a diode. The three contacts for the switch are in a row, the big one is the ground lug for the switch shell, the other two small ones are the switch contacts. Just put your test probes on the two small solder lands and check for continuity with the switch pressed.

 

[sweetbeats]

There are several videos on Youtube that show how corrosion under the metal snap causes the button to fail. This is one such video:


I was looking around last night and ran across this in a Google search. Unfortunately Full Compass isn't showing anything like it in stock. Its the closest thing that I can find that has the ground lug. I don't see any 4 point switches with the ground lug.

View attachment 128589

Can you share the link to that product? I searched on full compass but couldn’t find it. It looks like it might work, I’d like to see if the webpage for the product has any details about the part that could be shared with a parts retailer to help in the search.