The Great Cobb
New member
ok
Sorry, double post.
Sorry, double post.
Tascam 388 problems Part 2
- Thread starter The Great Cobb
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evm1024
New member
Close enough to be the same. Odd.
Does it appear the the series pass transistors get hotter than the heatsink (much hotter that is) - Be aware that the transistor cases are likely at unregulated voltage levels so don't touch them with the power on and don't touch them with a pad of your finger least you want to get burnt. (Use the top of your finger at the first joint).
It does not appear that we have an overload based ont he current readings. So now I'm thinking about a lack of heatsink grease on the series pass transistors that leads to thermal runaway.
--Ethan
Does it appear the the series pass transistors get hotter than the heatsink (much hotter that is) - Be aware that the transistor cases are likely at unregulated voltage levels so don't touch them with the power on and don't touch them with a pad of your finger least you want to get burnt. (Use the top of your finger at the first joint).
It does not appear that we have an overload based ont he current readings. So now I'm thinking about a lack of heatsink grease on the series pass transistors that leads to thermal runaway.
--Ethan
The Great Cobb
New member
So you want me to heat it up then turn it off and feel the series pass transistors? Where are they located on the machine?
Is it not strange that the values slowly drop over time?
sweetbeats
Reel deep thoughts...
I'm not thinking that the current drop over time is a concern...Ethan?
The transistors Ethan is talking about are the ones on the internal surface of the heatsink...pull the bias cards and the reel servo pcb and you'll see them screw-mounted to the internal side of the heatsink all in a nice row...
Do this...the heatsink mounts with just two screws to the chassis. You can take that off and there is enough slack (IIRC) to turn it and see the underside of the pcb to which the transistors mount. Does it look scorched at all? browned? How do the transistors look? Try Ethan's temp test.
Ethan, quite a stumper. I was starting to wonder about measuring resistance across the transistors to try and capture maybe a bad solder joint or something. Haven't thought deeply about that yet...worth pursuing?
Cobb, also look closely at the wiring that goes to and from the transistor pcb...does the wiring look at all like it has been overheated? How about the connectors from the transistors on the mother pcb? Look at everything. Look for heat damage...
The transistors Ethan is talking about are the ones on the internal surface of the heatsink...pull the bias cards and the reel servo pcb and you'll see them screw-mounted to the internal side of the heatsink all in a nice row...
Do this...the heatsink mounts with just two screws to the chassis. You can take that off and there is enough slack (IIRC) to turn it and see the underside of the pcb to which the transistors mount. Does it look scorched at all? browned? How do the transistors look? Try Ethan's temp test.
Ethan, quite a stumper. I was starting to wonder about measuring resistance across the transistors to try and capture maybe a bad solder joint or something. Haven't thought deeply about that yet...worth pursuing?
Cobb, also look closely at the wiring that goes to and from the transistor pcb...does the wiring look at all like it has been overheated? How about the connectors from the transistors on the mother pcb? Look at everything. Look for heat damage...
The Great Cobb
New member
heat
OK well i took out the heat sink and looked at it as well as both sides of the little board its attached to. Nothing looked damaged or burnt or even slightly discolored.
I also put everything back in and let the heatsink get hot, then touched the resistors as you said (power off etc). They werent any hotter than the sink itself. If they were, it wasnt enough that my hand could tell the difference.
OK well i took out the heat sink and looked at it as well as both sides of the little board its attached to. Nothing looked damaged or burnt or even slightly discolored.
I also put everything back in and let the heatsink get hot, then touched the resistors as you said (power off etc). They werent any hotter than the sink itself. If they were, it wasnt enough that my hand could tell the difference.
evm1024
New member
OK if the transistors and the heatsink are the same temp then we (should) have good thermal conductivity.
You might try this where you don't let the heatsink get fully hot - just let it start to get hot and then test the transistors to see if they are much hotter.
Thermal runaway does involve changes in values as the part(s) heat up.
When you measured the currents did you let it run for a while? We are looking for the thing that takes more than its normal share.
Quite a stumper.
--Ethan
You might try this where you don't let the heatsink get fully hot - just let it start to get hot and then test the transistors to see if they are much hotter.
Thermal runaway does involve changes in values as the part(s) heat up.
When you measured the currents did you let it run for a while? We are looking for the thing that takes more than its normal share.
Quite a stumper.
--Ethan
The Great Cobb
New member
well
I didnt leave it on for a really long time during the testing. All of the tests resulted in a slowly declining number. Mostly i shut it off when putting in or pulling out parts, because i thought it might be safer etc.
I didnt leave it on for a really long time during the testing. All of the tests resulted in a slowly declining number. Mostly i shut it off when putting in or pulling out parts, because i thought it might be safer etc.
sweetbeats
Reel deep thoughts...
Cobb,
I used my 388 for the first time in a real-world setting so it got to do more than idle...mixer busses working, transport motors working and all that, and my heatsink was to the point in that lower left corner (i.e. the side closest to the mixer...the corner we've talked about that's hot on yours) that it was hot. I could still nest my fingers in the heatsink vanes but it was not that cozy warm feel. It was what I would classify as "hot" and if it was any hotter I'm not sure I would have wanted to keep my fingers in there for any duration of time. I looked for a thermometer around here that would measure that temp range but couldn't find one.
What do you think?
Maybe your 388 is running normal?
I used my 388 for the first time in a real-world setting so it got to do more than idle...mixer busses working, transport motors working and all that, and my heatsink was to the point in that lower left corner (i.e. the side closest to the mixer...the corner we've talked about that's hot on yours) that it was hot. I could still nest my fingers in the heatsink vanes but it was not that cozy warm feel. It was what I would classify as "hot" and if it was any hotter I'm not sure I would have wanted to keep my fingers in there for any duration of time. I looked for a thermometer around here that would measure that temp range but couldn't find one.
What do you think?
Maybe your 388 is running normal?
The Great Cobb
New member
388
I have considered that, but mine gets hot pretty fast even just sitting idle, and also the fuse in F3 position blew before, and is already yellowing again from just maybe 45 minutes total operation time on it. When i say "yellowing", i mean, its in there among other brand new fuses of the same kind. They are bright and shiny silver and F3 looks a touch aged already.
It is something to consider though, but i just wonder why the fuse is taking damage even though the measurements on it were all normal.
Do you think it could be something in the mixer section causing it? The only reason i ask is because when that fuse blew the first time, everything on the machine still functioned fine except for no audio would pass through the mixer channels. You could still fast forward and rewind etc, and all the VU meter and other lights worked. Or, is the fuse 3 problem being caused by something else, and it just coincidentally affects the mixer section once it blows?
BTW, i think it great that you are finally able to run yours. Other than the interesting tone you get from the recordings, i like how smoothly it handles the tape.
I have considered that, but mine gets hot pretty fast even just sitting idle, and also the fuse in F3 position blew before, and is already yellowing again from just maybe 45 minutes total operation time on it. When i say "yellowing", i mean, its in there among other brand new fuses of the same kind. They are bright and shiny silver and F3 looks a touch aged already.
It is something to consider though, but i just wonder why the fuse is taking damage even though the measurements on it were all normal.
Do you think it could be something in the mixer section causing it? The only reason i ask is because when that fuse blew the first time, everything on the machine still functioned fine except for no audio would pass through the mixer channels. You could still fast forward and rewind etc, and all the VU meter and other lights worked. Or, is the fuse 3 problem being caused by something else, and it just coincidentally affects the mixer section once it blows?
BTW, i think it great that you are finally able to run yours. Other than the interesting tone you get from the recordings, i like how smoothly it handles the tape.
sweetbeats
Reel deep thoughts...
Sheesh, Cobb...go to bed. 
Okay...right-right-right.
I was totally not thinking about the fuse and how quickly it heated up, etc.
So I was reading up on thermal runaway...how good are you with a soldering iron? Could you touch up the solder joints for those transistors that are mounted to the heatsink? Remove the heatsink, rotate it upside down to access those joints? Can't recall for certain if that is possible.
And on your question, the transport motors and such run on 24V and the logic runs on 5V...meter lamps...I'll have to look what those run on again, but the point is that, for the most part, the 15V rails power the mixer and audio stuff. So if F3 is blown other stuff will still be powered and can run. So one would think that it is something in the mixer causing trouble;but we measured how much current the mixer is drawing and it is normal. I'm wondering about the TR PCB or the regulator itself...
Okay...right-right-right.
I was totally not thinking about the fuse and how quickly it heated up, etc.
So I was reading up on thermal runaway...how good are you with a soldering iron? Could you touch up the solder joints for those transistors that are mounted to the heatsink? Remove the heatsink, rotate it upside down to access those joints? Can't recall for certain if that is possible.
And on your question, the transport motors and such run on 24V and the logic runs on 5V...meter lamps...I'll have to look what those run on again, but the point is that, for the most part, the 15V rails power the mixer and audio stuff. So if F3 is blown other stuff will still be powered and can run. So one would think that it is something in the mixer causing trouble;but we measured how much current the mixer is drawing and it is normal. I'm wondering about the TR PCB or the regulator itself...
The Great Cobb
New member
solder
I am not a master solderererer of any kind. Ive done it, but im not great.
Let me ask you this, how costly and difficult would it be just to buy replacements for the TR PCB or the regulator if they are in fact the problem?
I am not a master solderererer of any kind. Ive done it, but im not great.
Let me ask you this, how costly and difficult would it be just to buy replacements for the TR PCB or the regulator if they are in fact the problem?
sweetbeats
Reel deep thoughts...
I have no idea. I would be pretty surprised if Tascam had replacements in stock or available.
The transistors can be found at places like Mouser or Digikey or the like.
Tell you what...I've got a spare loaded heatsink assembly. I'll send it to you. You swap it out and see if that fixes the problem.
What do you think? Anybody else think that's a good or dumb idea?
The transistors can be found at places like Mouser or Digikey or the like.
Tell you what...I've got a spare loaded heatsink assembly. I'll send it to you. You swap it out and see if that fixes the problem.
What do you think? Anybody else think that's a good or dumb idea?
The Great Cobb
New member
parts
So, what do the parts look like that need to be swapped out? Also, would there have to be any kind of soldering etc? Im just trying to grasp what all we are talking about part-wise that has to come out.
And thank you immensely for your continuing help.
So, what do the parts look like that need to be swapped out? Also, would there have to be any kind of soldering etc? Im just trying to grasp what all we are talking about part-wise that has to come out.
And thank you immensely for your continuing help.
sweetbeats
Reel deep thoughts...
Two screws and the heatsink/TR PCB assembly is free from the chassis. and then there are three (IIRC) connections to unplug on the mother PCB and the one other one. I can detail it when I pull the one here but it is really easy after removing the PSU, Rell Servo and Bias PCB's.
The Great Cobb
New member
parts
Just let me know what all you need pulled and ill do it! If you can take a pic or two it might be a big help, but if its not necessary then im sure i can still do it.
Just let me know what all you need pulled and ill do it! If you can take a pic or two it might be a big help, but if its not necessary then im sure i can still do it.
sweetbeats
Reel deep thoughts...
Okey-doke. Give me couple days and I'll take pictures of the removal process.
sweetbeats
Reel deep thoughts...
TR PCB / Heatsink Assembly...how to get that bad boy offa there...
1. Turn the dern thing off and unplug the mains power.
2. Turn the unit on (yes with it unplugged) for a few minutes to drain the PSU caps.
3. Remove the cardbay cover, fuse PCB shield and cross-brace (the one for the bias, reel servo and PSU cards)
4. Remove the bias, reel servo and PSU PCB's
5. There are three plugs to remove from the Mother (2) PCB at P7, P8 and P14. they are obvious as they come straight from the TR PCB Assembly to the Mother (2) PCB...one white (P7), one red (P8) and one black (P14). Unplug 'em.
6. There's one more plug to remove and it is J801 on the Capstan Servo PCB, but first you'll need to clip a couple wire ties off from the bundle of wires that go up to the Capstan Servo PCB. The Capstan Servo PCB is the one mounted to the inside back panel of the 388 directly behind the main transformer and the Fuse PCB. This is the bundle of wires and I'm pointing to where the clips used to be...may be different on yours.
7. Now you can unplug the white 3-pin plug from J801 on the Capstan Servo PCB. This one...
8. Now there are just two screws to remove that mount the whole assembly to the back panel. These ones...
9. And there you go.
Installation is the reverse of removal.
1. Turn the dern thing off and unplug the mains power.
2. Turn the unit on (yes with it unplugged) for a few minutes to drain the PSU caps.
3. Remove the cardbay cover, fuse PCB shield and cross-brace (the one for the bias, reel servo and PSU cards)
4. Remove the bias, reel servo and PSU PCB's
5. There are three plugs to remove from the Mother (2) PCB at P7, P8 and P14. they are obvious as they come straight from the TR PCB Assembly to the Mother (2) PCB...one white (P7), one red (P8) and one black (P14). Unplug 'em.
6. There's one more plug to remove and it is J801 on the Capstan Servo PCB, but first you'll need to clip a couple wire ties off from the bundle of wires that go up to the Capstan Servo PCB. The Capstan Servo PCB is the one mounted to the inside back panel of the 388 directly behind the main transformer and the Fuse PCB. This is the bundle of wires and I'm pointing to where the clips used to be...may be different on yours.
7. Now you can unplug the white 3-pin plug from J801 on the Capstan Servo PCB. This one...
8. Now there are just two screws to remove that mount the whole assembly to the back panel. These ones...
9. And there you go.
Installation is the reverse of removal.
The Great Cobb
New member
Thanks
Thats awesome! Thank you thank you thank you! I really hope this works...
Thats awesome! Thank you thank you thank you! I really hope this works...
The Great Cobb
New member
ok
Alright, i did everything, it is out and ready to go.
Alright, i did everything, it is out and ready to go.
The Great Cobb
New member
Well
Well i tried Sweetbeats heatsink just now in my 388. It still got hot as hell.
(siiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiigh)
Im not sure what to do. The machine functions etc but the sink is getting very hot. Hot like what youd expect from an old tube bass amp head from the 70s. Arent there like 500 people here on this forum who have multiple flawless condition 388s that they are sitting on that could possibly test the heat on their own heat sink so i can compare their results to mine? I would greatly appreciate it. If i put my fingers into the grooves of the sink for longer than 10 seconds it feels like im about to burn my hand. Thats with the machine being on for about 10 minutes, sitting idle.
Thanks again to Sweetbeats of course for his generosity and help.
Im feeling a bit lost on where to go with this thing, i hate to say.
Well i tried Sweetbeats heatsink just now in my 388. It still got hot as hell.
(siiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiigh)
Im not sure what to do. The machine functions etc but the sink is getting very hot. Hot like what youd expect from an old tube bass amp head from the 70s. Arent there like 500 people here on this forum who have multiple flawless condition 388s that they are sitting on that could possibly test the heat on their own heat sink so i can compare their results to mine? I would greatly appreciate it. If i put my fingers into the grooves of the sink for longer than 10 seconds it feels like im about to burn my hand. Thats with the machine being on for about 10 minutes, sitting idle.
Thanks again to Sweetbeats of course for his generosity and help.
Im feeling a bit lost on where to go with this thing, i hate to say.