Tascam 688 with no audio signals in or out

John Mackenzie

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Tascam 688 midistudio
Hello again. Following on from previous posts about tape transport problems, which are now fixed, I now have a more major problem. The unit switches on , all scenes are usable, all transport functions work.

i have no audio or signals , either from tape or inputs. All I can hear on monitoring, is a low level 50Hz hum.
I think, due to a bit of carelessness, I had somehow wired up outputs to inputs when trying to use it in parallel with a computer's audio output. ( I can't remember what happened exactly). All the meters went into full RED, and there was a nasty smell. Is this likely to be an audio amp issue or a power supply issue? Everything else , tape transport, scenes, selector buttons etc etc are all working as they should. No funny smells when switched on. Something must have fried, I think, but don't know what. I would love ANY input to this problem. I have a service manual getting delivered from USA, which might help me diagnose the issue. Parts may be a problem to source ?
 
Dear Sir John, Looks like your output IC's & or preamps are fried. If you open the unit you can visually check the items just below the tape transport mechanism with a heat sink on top. Check your other posts I have replied to some with detail of power supply. Wish it will help.In 644 amp IC 's are BA6209.
 
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Thankyou rohmal. This is what i suspected. I do hope I can get it repaired ! I will let you know of progress, once I get the service manual .
 
My one plays back the pre-recorded tapes on all 4 channels. When put to record mode I can hear signal on all 4 channels. Record light comes on blinking and play also happens and I hear no erasing as my pre-recorded tape plays with the input signal from monitoring out and nothing is recorded also.

I read Your Problem also.
Did Sir John, If you check your power supply. It should have on pin 1 -0V, pin 2-+10V, pin 3-negative 10V and on pin 4-+7V. If all is OK and check when a pre-recorded tape is played you have LCD meter level display on it when OUTPUT button pressed for display. If all is OK your sound output is blown. In my unit 644 the out put IC's are BA6209 ( seperate for each channel). There may be a pre-amp also. If you open the unit You can visually see any burnt part as if it did smell before stopping. I have included a sketch of power supply socket on unit seeing from behind in to the unit. So when you check your power supply chord it should be mirrored (flip along horizontal axis). Contact me if You need any help more with the findings.Good Day to You.
 

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Hello again rohmal. A small update on repair to my 688. I took the unit apart today, removing the top which has all the faders etc, then removed metal plates underneath to expose the circuit boards. On close examination I found one IC LC4066B to be damaged. It had a burn hole in the centre. I don't actually know what this chip does, but seems to be quite a common device in machines like this. I do have a data sheet for it which I downloaded. One thing I can't find however : The number on the chip is LC4066B 9M8. There are 4 chips in there, two with that number, and two with LC4066B 9M9. i don't know what the difference is between them, if any, and can't find any reference to 9M8 or 9M9 in my internet searches. My worry is more that something has caused this chip to blow out, so simply replacing it might not be the answer. It is also surface mounted on the circuit board, not in a socket, so replacing it will be very difficult. At least I found something wrong in there !
 
Hello rohmal. I hope you are still able to discuss things. I have taken a few photos of 688 , to let you see damage I found. There are Four LC4966B IC's in here. Only one has blown, a you will hopefully be able to see in my poor photos. (if sending photos works on here? ). I have included pics of IC's of the same No. . 4 IC chips, only one damaged. Hope you find this of interest. It has been nice speaking to you, and your offer of help.Inside 688.jpg blown IC.jpgIntact IC 9M9.jpg Inside 688.jpg
 
Dear John, IC LC4066 looks to be a switching circuit unit. Most probably it may be used to select inputs and outputs to the unit. It looks like it has got cracked due to over heating but there's no evidence of resistors discoloring due to heating around it. It is good to start from your power supply.unit socket. Check it for voltages between it's 4 female pins. The unit is supplying DC to 688, There should be a common ZERO Volts. Check other three pins with it. If it is OK. and your unit is working fine with other functions, check if you can change the inputs to channels using LCD display and related buttons. May be no sound is due to this chips not switching audio paths now. 688 has the same setup as I know my 644. I have 4 channels in 2 input boards and you have 8 channels in 4 input boards. I am away from home at the moment. I'll be at home tomorrow evening 6 with good internet. I'll send you a photo of my unit inside then. I;ll also have look for this IC LC4066 in my unit boards. To replace the burnt chip just cut the pins from top, one by one. Then get rid of it. De-solder the remaining pin parts one by one. Put a socket (14 pin Dual In Line) for the IC and any future IC replacement will be an easy task.
Better to check transistors around that area also(or for the channel the IC is in) with a multi-meter diode check function. If the unit can be plugged in safely in open I;ll write to you tomorrow.
 
Thank you again rohmal for your interest in this repair. I hope you got a chance to read my other message about being offered another 688 to use for spare parts. I am very excited about this !!. I hope we can keep in touch and that we can both get our Tascams working again. i agree with all of your comments above. I have been doing a lot of research on the Internet on this.
 
Hello Rohmal. We seem to be communicating in different threads on here. It might be better to do this on some sort of private message type thing, on this forum ( I have never used that option), or perhaps on Facebook, or even email, if we have the need for a more private discussion. Let me know your preference for this. My email is jmacke2174@aol.com, and on Facebook, I am just John Mackenzie , with added search items, Peebles, Scotland to find me. I have a pic of a 12 string guitar as my avatar pic, so should be easy to find , out of the hundreds of people with my name. ( It is very common, world wide ). If you use Facebook, then let me know how to contact you on that. I did find a couple of rohmal names, but did not know if any of them were you? I examined my circuit board with my magnifier and found 8 REC LEVEL pots , also REC PLAY LEVEL pots. They are labeled as such on the circuit board. I am not sure if your low recorded signal is possibly to do with the recording heads, rather than a fault on a board, but you probably know better than me about such things. It is a difficult thing to diagnose, I admit.
 
Dear John I have no face book account. My email is rohanamallawa@gmail.com I also searched in my boards and found the REC level pots but want to be sure about how they are on the circuit before I meddle with them. As your unit has 8 channels there are 8 and mine is only 4 for 4 channels. As you said demagnetizing the heads is not really a matter to my problem. It may be a weak signal processing on record path as prerecorded tapes are playing back fine on it.
 
Hello rohmal. I would presume that if pre-recorded tapes play as expected, then the heads should be fine. I wonder what could reduce signal on recording? There are so many possibilities, given the complexity of these machines. I am still astounded by what has been built in my 688. The functions that these machines can perform are outstanding, for analogue technology. I am not surprised that they cost so much , when new. I was lucky to get mine , used, (about 6 months old) for about 2/3rds of retail price. it had never been used for more than a few hours.
 
Dear John, so we'll stick to this thread here after. If you need send any to my email. I have attached some photos with this. I used 2 wood pieces to keep the unit apart as I can power it up and work easily from back side accessing the PCB's for tuning and going in front for play/rec/set functions. I have only one 4066 marked as BU4066B in my unit. The prefix letters before 4066 gives you the maker of the IC so LC4066B is the same by another maker. If you want you can find it on net. It is a TAPE INPUT QUAD ANALOG SWITCH. So if only one is blown in your unit check to find the path that the IC is driven. Four switches. If there are transistors they can be faulty. Otherwise the IC itself became faulty and blew 20171102_220758.jpg IC BU4066B 14 PIN DIL is here1.jpg 20171102_223030.jpg BU4066B on PCB1.jpg 20171102_221607.jpg 644 Trim Pots on REC-PLAYBACK  PCB1.jpg up. Check when you receive the cct logically any reason for it to blew other than catastrophic failure. I have marked my pots in my drawings but we'll wait until you receive the schematics to see how it goes. If nothing else is bad on your unit just replace it with a socket underneath and see. Its only a switching unit for tape inputs.
 
Dear John here are the data sheets for 4066B attached. Check and compare both sheets.
 

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Hello Rohmal. Thank you again. You give me hope !.. I now have to be patient and wait for news of the possible " for spares" 688, and the service manual. I had a message from USA, saying the manual had been posted, but delivery is not due until around 10th Nov.
I have not heard any more from the man who has the spare one. It would make things a lot simpler if I can just change one of the boards, but I will need to check power supply lines before fitting a new board. I hope you have success with your 644 before I get this fixed.
 
I have sent an email to you. See it and write to my email. I'll help you to check other things until the schematics arrive.
 
Dear John here are the data sheets for 4066B attached. Check and compare both sheets.


I have already downloaded data sheets for the IC. The one I can buy readily here in UK is NEC4066B, but I will check the specs before ordering them.
 
Dear John,
As You said above wait until the schematics arrive. Start checking from your power supply. It looks like you should check all the power to IC's on the PCB's. It is good to check all transistors in place generally for bad ones. Before doing all this get the IC numbers from the PCB's and down load the data sheets for them. There are quad analog switches, Noise reduction Dolby part ones, motor drivers, head selection switches, REC/PLAYBACK IC's. Operational amplifiers with single and dual voltages are a few to describe. So by the number, you at least know for what it is therefore. It's not an easy task but patience and time will carry you ashore. Most probably service schematics do have voltages that should be present at various points. So when they are wrong you can suspect some thing wrong there. Lets wait until the schematics arrive. Mean while you can search and down load the IC data what you can see on your PCB's. May be You'll need an O'scope to trace the input signal path if things did not come out easily. But by logical and tactical trouble shooting you will be able to repair it.
 
Hello rohmal. i have successfully removed the LC4066B chip, after a bit of struggle getting access to both sides of the PCB, nice and neat. I now need to source an equivalent chip, but not sure what I need to compare ? There are quite a lot to choose from locally in electronic suppliers stores, but I want to be certain I buy the correct thing, and also a 14 pin DIL socket to make things a bit tidier. I hope you can suggest the correct one ? Still waiting for the Schematics to arrive, so not in any rush. I just wanted to make a start. I used a solder sucker with my iron, and it all worked nicely. I had watched a video about other ways to do it, using copper wire around all the pins at once, then heat the wire with solder. It looked a bit messy, so i just did one pin at a time, and it all popped out. Keep in touch !! Thank you for being a patient ,helpful man, I do appreciate it.
 
Another quick update. My Service manual for 688 has arrived ! The manual itself is a normal A4 size of booklet, but the schematics are on A3 . I find this a bit overwhelming to be honest, not having a background in Electronics ,or reading this kind of stuff. I can follow some of it, but so far I not been able to find any simple information on voltages expected at different places. There are several test points on each PCB, but I don't know what to expect to be measuring there? If any of this can be of use to you, please let me know and I can scan whatever you need. i am going to order CD4066B IC's ( you have to buy a pack of 5), and have crossed checked their compatibility with original one. They are available locally and are cheap price. My next step is to start checking power supply lines etc. There are some references on the diagrams to voltages at various points. Most of the diagrams are very clearly printed, so should scan OK. The only problem is with the A3 sheets, as I can only scan A4 at home. I may be able to find a place to get them reduced . It is a pity that I am no longer at previous work, as I had access to wonderful photocopiers. Anyway, if you need anything , let me know.
 
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Dear John, It's nice to hear that you got the service manual. CD40066B is a CMOS QUAD BILATERAL SWITCH. It has four electronically controlled switches each stable at one of two states. The test points marked should have test voltage either in place or separate with a chart or on a page given in another place. Normally there should be some explanation to mark that point on the cct. as it helps the fault testing/finding. Take your time if you can scan the A4 to PDF format using a bit high 300dpi resolution. If the file is big you can go downwards. A3 pages scan it in multi-parts. You can take all the time in the world. I am in no hurry. Keep the scanned documents ready and let me know. By studying them I can tell you what to look step by step. I'll send you an email also. Start from the power supply side. See my unit pictures and open your unit in the same way and keep two wood strips to keep the unit in a power-able setup where you can check and operate also. When any voltage readings are weird you can check that area thoroughly. Hope may be you cct.'s are similar to mine so I can follow them also. Let me know the progress.
 

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