Tascam 388 - New issue?

TAPESOURCE

TAPESOURCE

New member
Hi everyone, It's been a few years since I own a Tascam 388, I bought it for a low price at the time cause it was a big project to make it work again. With lots of patience and reading through the internet, I’m almost at the point when everything works correctly. By the way, I first want to thank a lot all the Home Recording, tape machine community and sweetbeats for your amazing work and knowledge.

Not far from last week, I wanted to use my 388, turning it on, no sound in the L/R bus. I searched on forums to see if anyone already had this problem, and I came across the thread "New 388 problem". Turns out that the culprit was the MC14069 chip. I shotgun the chip, and the problem was solved. Amazing.
So now that this issue is fixed, last night I have planned to record a bit but I experienced a new issue:

The issue is : monitor 5 and monitor 8 are only fed by tape out 5 and tape out 8 respectively, even if nothing is assign to them.


1 - When MON pgm/cue is depressed (up) and no assign button is pressed: everything works correctly, I have sound via the Monitor section if I turn the mon gains up.

2 - When I press the assign LR button: Works fine, no sound as long as the faders are down. Channels 1 - 8 = works properly

3 - When MON pgm/cue is pressed (down): if nothing is routed to any MON bus, with faders all the way down: no sound through channel 1 to 4 and 6 and 7, but channel 5 and 8 are fed by tape out 5 and tape out 8. Beside this, everything works fine for the other channels.


I listened to the PGM OUTs at the back of the deck, when I assign something through PGM 5 and 8, everything works as it should.
When I try to send sound through PGM IN : I got nothing on PGM 5 and 8 as when I do it via the mixer.
I tried swiping the REC/PLAY PCB cards but the issue is fixed to channel 5 and 8.
Same with the DBX cards.


Maybe someone already experienced this, I really don’t know where the problem is, maybe it comes from the MC14069 chip again or somewhere there?
I don't know if i have good explanations skills so tell me if something is not clear...

For information : I have good soldering skills, I got an Oscilloscope and barely know how to use it, I’m learning how to troubleshoot etc…


Thanks a lot for your concern,
Pardon my english writing,
Greetings from France,


R.001
 
Hello sweetbeats,
Alright, they're on their way, I'll do it tomorrow or later this weekend and report back.
Thank you!
 
Hi,
I just replaced those chips today, but the issue remain the same...

I did not mention the fact that tape sound still passes in monitors 5 and 8 when PGM Masters ODD and EVEN faders are at their lowest.
I'm gonna look at the schematics again to try figuring out where to look first.
 
Ahhh…interesting. Okay.

Try this first…swap the channel 5/7 R/P amp card with channel 1/3. Tell me if the problem follows the card (i.e. if now monitor mixer channels 1 & 8 pass tape signal regardless of the position of the PGM MASTERS), or if the problem is still with monitor mixer channels 5 & 8.

The above is just to test/verify what I think is going on and that’s that the logic chips on the BUSS A PCB are starting to go also…U109~U112. These are also 4000 series parts (4966), and handle source switching of the monitor mixer channels, either post PGM MASTERS fader from the BUSS B PCB, or from the R/P amp cards.

Sorry for misdirecting you to the 4011 parts in the MONITOR PCB…if I’d spent more time chewing on your first post I might have pointed you to the BUSS A PCB, but I was in a hurry, so, sorry for that. Your latest post helped me understand better what’s going on. Regardless, if my hunch is correct your issue is with the 4966 parts U109~U112 on the BUSS A PCB, then it was probably good preventative maintenance to shotgun the 4011 parts on the MONITOR PCB. For years now I’ve seen this trend with issues related to the L/R main buss auto-switching functionality controlled by U104 on the BUSS B PCB, okay? But we also have similar functionality on the BUSS A and MONITOR PCB assemblies also handled by 4000 series logic and I’m starting to see issues cropping up with those parts too.

Are the parts Sanyo or JRC? I don’t know if the parts are not happy on the +/-15V power…it shouldn’t be a problem, at least according to the Sanyo datasheet…the parts are good for up to a 40V supply differential…but they also can operate on a very low voltage differential so maybe 35+ years of power at the higher end of the spectrum is causing the parts to reach the end of their service life. But it’s definitely a trend impacting all the 4000 series parts on the 388, and I’ve even run into this on one of the mixing consoles from that era too…maybe it was an M-300 series console? I can’t remember. The good news is through-hole 4000 series parts are still readily available and relatively cheap. This will change over time though as there is an industry trend away from through-hole parts and as they are phased out prices will go up. It’s already happening.

I recommend getting TI branded parts from a reputable and authorized vendor like Mouser or Digikey.

[EDIT] My memory was correct…the M-300 series consoles use a 4966 part, U405 on the MONITOR PCB, for source control of the MONITOR OUT jacks and headphone amp…either the SOLO buss or the output of the MONitor select switchrack. I helped somebody suss out an issue that ended up being a failed U405 chip.
 
Last edited:
Hi,
Thank you so much for your last post and sorry for the late reply, i was busy these last days…

Sorry for misdirecting you to the 4011 parts in the MONITOR PCB…
Click to expand...
No problem with the 4011 parts really, what is done is no longer to be done! ;)

Try this first…swap the channel 5/7 R/P amp card with channel 1/3. Tell me if the problem follows the card (i.e. if now monitor mixer channels 1 & 8 pass tape signal regardless of the position of the PGM MASTERS), or if the problem is still with monitor mixer channels 5 & 8.
Click to expand...
So, like you told me to, I swaped the channel 5/7 amp card with 1/3, and the problem was still on monitor mixer channels 5 & 8.

Are the parts Sanyo or JRC?
Click to expand...
For information, the stock chips U109 U110 U111 and U112 on BUSS A PCB are LC4966 from Sanyo.
I looked for TI equivalent by comparing the data sheets and end up buying the CD4066BE from Texas Instruments.

I just picked up the package today and changed those parts and I’m happy to say that MON 5 & MON 8 are now assignable on the mixer and work as they should.
BUT, I need to mention that it’s a 50% problem solved cause MON 5 & 8 are still fed by tape from their channels even if nothing is assign to them…
I mean, I can add audio source in monitor 5 and 8 from any channel on the mixer, but even if send nothing to those bus, the tape still pass audio to MON 5&8 like before changing U109 to U112.

If anything is un-understandable in my post, do not hesitate to tell me.

Again, thank you so much, you were right on those 4000 series chips!
 
Huh! Okay. I’m not sure how easy this will be for you to do, but the REC arming switches, among other things, pass control signal to the electronic source switching of the monitor mixer. So what we want to do now is see if the control signal passed by the track 5 and 8 arming switches is the same as other track arming switches. These 8 channels of control signal propagate from the MONITOR PCB at P121 (should be 8 pins and red in color) to the BUSS A PCB at P104 (should be 8 pins and white in color) via an 8-conductor ribbon cable. Pins 1~8 correspond to track arming switches 1~8 respectively. If I’m reading the schematic correctly, switching those arming switches should turn on and off +15V under certain conditions.

Set the CUE/PGM switch in the CUE (up) or unlatched position.

Set the INSERT switch in the ON (down) or latched position.

Load some tape and put the transport in PLAY mode.

Set your DMM to DC volts and check the voltage on each pin of P104 on the BUSS A PCB as you arm and disarm the corresponding track. With the arming switch in the armed (down) or latched position, the corresponding pin at P104 should measure +15V. When the switch is in the disarmed (up) or unlatched position the voltage should be at or close to 0V. Check each track/pin this way. We are looking to see if pins 5 & 8, which correspond to tracks 5 & 8 behave differently than pins 1, 2, 3, 4, 6 & 7…like +15V is present regardless of the position of the corresponding track arming switch.
 
Hello Sweetbeats,

Thank you for your answer.

Just passed the whole night on the console and the schematics and solved my issue by changing some burned diodes on the MONITOR PCB.
(Sorry I was too tired and could not put a report after it…)


So there was 3 dead diodes on the MONITOR PCB that was D116 / D115 & D113.

If i understood the schematics correctly these diodes are just after the CUE/PGM switch on the PGM circuit. In my case with those burned diodes i suppose the Channel 5 & 8 were stuck in the CUE position, that is why the PGMs master faders were not effective at all.

I replaced them and everything was back to normal. That’s a problem solved!


On the other hand, I found out that the LC4966 replacement chip : CD4066BE was heating a bit too much. I measured 115/120 degree Celsius 4/5 minutes after turning on the machine. Is it a normal situation for an IC?

I don’t remember feeling the LC4966 this hot through the back of the mixer… I mean, I can put my hand at the back of the console without touching and feel the hot comming out of the BUSS A PCB.

I changed U109 until U112, as I understand in your previous post, do that move was correct?
(We are ok that this symbol « ~ » means « until »?)

I fear that I made a mistake chosing CD4066BE even tho the specs were quite the same with LC4966 unless the 4966 can go with +40v on DC supply voltage while the 4066 can only take +20V. I can’t understand how i missed that…

Do you think it is not the right replacement chip?
Or, maybe something before the chip is dead and cause overheating?
Could you correct me on this ?

Being a noob, I learn by doing all this and make easy mistakes so thank you for your understanding…


I’m happy cause the console is 100% usable, but now i’m scared to let the power on cause these chips are heating quite a bit and I don’t want them to burn everything inside the deck… :facepalm:
I was too excited about resolving the issue, but I should have wait and take the time to ask if it was the right replacement part… :D

Again, thank you for your time and your answers!
 
Apparently the 4066 maximum temperature is 125 degrees C, at least for one manufacturer, but that kind of temperature doesn't sound at all right to me, not for a signal switching chip.
 
Okay…nice job on discovering the failed diodes, and yes I think what you would have found with the test I suggested is +15V always present on pins 5 & 8 of P104 on the BUSS A PCB…and then I would have had you check those diodes…beat me to it.

And on the CD4066 yeah wow…you’re right. Max input voltage at Vdd pin 14 referenced to Vss pin 7 is 20V…the LC4966 is max 40V. U109~U112 (and yes I use “~” to mean “through”) are power by a differential of 30V (+15V at pin 14 and -15V at pin 7), so we’re cooking those CD4066 parts. I’m not finding a current equivalent for the LC4966 in my usual searches. I’m going to reach out to Mouser tech support and see if they know of anything. You also have the option to buy NOS LC4966 parts, you would just want to do that from a reputable seller and scrutinize the parts for signs of counterfeit activity. I’ll let you know what I find out. You can also step down the power at the CD4066 power input pins but that wouldn’t be my preference if it was my 388.
 
Well, Mouser couldn’t help. They are working up a quote for a non-stock part. I’ll have info in 24-48 hours, but the reality is there will be some huge minimum quantity requirement…it’s a dead-end.

And you could try to kludge a pair of dual bilateral parts in there but you likely would have a crazy mess of 64 flying leads…no way. SO…

Honestly if it was me I’d be sourcing original LC4966 parts because I did some more searching and there is just nothing in this package and configuration that handles more than 20V. A place called utsource has them. I’ve ordered from these folks before. I believe they are based in Asia with US distribution…they deal in surplus electronic parts but at volume…lots of different parts and don’t have big minimum quantities, very reasonable prices…the parts I got were genuine…I consider them a reputable vendor. A friend of mine who purchases a lot of parts doesn’t think twice about ordering from them. You’ll pay shipping, but IIRC it was a nominal amount…they have over 49,000 NOS LC4966 chips in stock, minimum quantity is 2, and you pay $1.33 each, which is not bad at all for NOS genuine parts like this, but there is a big price break even at a quantity of 10 or more. You’ll pay $2.65 for two of them plus shipping…get 10 and you pay $5.97 plus shipping. If it was me I’d be getting at least 10, but I’m a small-time parts hoarder because I know through-hole parts are going to be increasing hard to find and more expensive. Here they are:

https://www.utsource.net/itm/p/11714733.html

It is reasonable to assume that these NOS genuine parts will last a good long while…many, many years if not a decade or two or more.
 
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