Tascam 238S Dolby S Module

[SkywaveTDR]

It seems that after all this time of repairing the 238 decks, I have come across this S model with the Dolby S module. This unit has leaking caps of the surface mount type on a very small and compact double sided board.
My question is- Has anyone seen of or have a 238S service manual as I can not find any proof that a manual for this unit even exists. I am afraid that many of these S units are going to need extensive work on those Dolby S modules and thus make the unit too expensive to repair.

 

[wkrbee]

I have the 238S service manual.PM me.

 

[snclvn]

wkrbee, it would be valuable if you uploaded photos of that here for the future!
Cheers.

 

[SkywaveTDR]

I just got off the phone with another well known Teac Tascam technician in CA and he says there is no diagrams for the Dolby S board.
I have asked many people but there just is no such data I can find.

 

[snclvn]

What a trip. I was saddened to learn when I bought my dbx model that there was a nicer 238S with Dolby...now I’m quite satisfied with what I’ve got.

 

[SkywaveTDR]

This is wise as the dbx works much better than Dolby S and it does not break down like the junk Dolby boards do. Unless I find out otherwise in the future the Dolby S option may be unrepairable.

 

[rjones1]

Hi SkywaveTDR,
Have you made any progress on fixing any of the bad Dolby S boards on the Tascam 238S. I have one and most of the boards are non-functional. From what I have read, there was a problem with the Dolby S chips on the board is that correct? Also I have read it could be leaky caps. Do you have any additional information you could share if these are repairable?

Thanks,
Rick Jones

 

[SkywaveTDR]

I have had no information on these at all or if there was any communication with past technicians or service managers, it was to say that there is no information. It is again the cheaply made SMD caps on the modules and it will take a lot of time to restore the cards times 8 which really does not make the project feasible. Who would pay $1600 or more to fix one of these?
I have one here but I am getting ready to send it back as there is nothing more to be done with it.
Some manufactures dropped the ball on this and they don't care about the customers with them now.

 

[rjones1]

Thanks for the reply Skywave, sad that Tascam won't give more info on this issue. Sounds like they are covering up a known issue or it's been so long that there is nobody left at Tascam that knows or cares.

My unit also have the speed up issue, I plan to recap and clean the capstan motor circuit board. I have some experience in replacing SMD components. Hopefully my techniques will work on this board.

 

[witzendoz]

Here is the story on Dolby S boards, I had the same problem with the boards in a MSR16S, The actual Dolby S board was made under licence by Dolby which is why Tascam can't help you. If they are like the boards on the MSR they are nearly impossible to fix, what you have to do is replace all the caps as they go out of spec and eventually the board stops operating. The only people that can fix them are micro soldering repairers.

I fixed my problem by a complete fluke, I bought a spare parts machine to rob the boards, when I pulled them it must have been a very early build and the boards were different with bigger caps fitted, I put all of them in the good MSR16S and never had a problem, plus they would be easier to fix.

Alan.

 

[rjones1]

Thanks Alan for additional info.

 

[rjones1]

I spent most of the weekend working on my 238S and discovered some interesting findings regarding to the Dolby S module.

I confirmed that I had 3 channels that would playback Dolby S and 5 that either had no output and or distorted output.

I took each channel card out and cleaned both sides of the Dolby S modules (modules are a separate circuit board mounted on each channel board) with rubbing alcohol and tooth brush. On some boards it took some extra effort with a cloth and small tweezers.

There is some type of corrosion on the modules, and is some cases there a jell like substance on the modules. Most of caps looked to be leaky but the corrosion is in addition of the cap issue. The main channel board doesn't have these problems.

The cleaning fixed all but one of the boards. The last board that's still having problems has some popping going on, this board originally had no output. So still more work to be done on this one.

I am now able to playback 7 of the 8 channels, and they all seem to be working fine, but I will be doing more testing.

 

[rjones1]

Here is a follow up to the 9/24 post along with some pictures that I took of the circuit boards. They are not the best because I was using my cell phone camera looking through the lens of my microscope.

This picture show the corrosion / contamination (Left circle) on the Dolby S circuit board along with a jell like substance (Right Circle)


Here is the same area after cleaning.


The corrosion / contamination could have been possibly caused by not removing all of the flux during the manufacturing process. After cleaning the whole board on both sides, the board was operational again. This worked on all but one board, it still has some noise and pops. These boards are 26 years old.


Front side of the Dolby S module, notice the Sony chip with the Dolby Logo.

This is the Dolby S encoder chip, according to the data sheet (attached) for this part, this chip does the encoding and the decode is done by an op amp. Most likely that's what the chip is doing on the left side of the board. It's a dual Op amp P/N 5218s. Also wanted to mention that the data sheet includes an application circuit and best I can tell matches the resistors and capacitors connected to the Dolby S chip. The data sheet doesn't have any application circuit for the decode.



This is the backside of the Dolby S module, most of these caps have corroded solder joints and should be replaced.


This is the Record / Playback channel board with the Dolby S module.


I still have the one board mentioned above that's not 100% working. I plan do more work on this board in the coming days. I will update this post on any new findings.

Rick Jones

 

[SkywaveTDR]

Thanks Rick,
This gives me the idea that I should try and clean the boards that are here to see if I can realize and improvement. Solder flux usually causes no problem as it is not reactive. The corrosion and that you see is usually from leaking electrolyte from those garbage caps. Of course the through hole caps work better as those are what I put on the motor boards. The problem continues on.

 

[AmpAnalog]

I'm a bit late to this thread, but perhaps there is some useful info in this PDF link.

https://238pro.com/tascam238sownersaddendum.pdf

 

[SkywaveTDR]

I read the document and even saved it but it offers very little information for the repair of such boards. The way these are made, they are a product to be avoided if you intend to use the Dolby S part of it.

 

[rjones1]

Want to give an update on getting my 238S working again. As mention in my post in Setp 2018 I had cleaned all of the Dolby S modules and got all but one channel working. When I tried the unit after a few months of setting (Jan 2019) this last channel that was somewhat working before was now was dead again in Dolby mode.

I removed all of the caps from this Dolby S module and clean the board very thoroughly. This board was the worst of the 8 with a lot of leakage form the caps. I replace all of the caps but still the no output in Dolby S mode. It worked when Dolby S was turned off.

After several hours of troubleshooting I discovered that the analog switch (CD4066) on the channel board was not sending any signal to the Dolby S module. Replacing this part resolve the problem. From what I can tell in tracing the circuit, the CD4066 directly connects to the Model with no coupling caps. I think it maybe a possibility that the leaky caps could have damaged the CD4066.

I also had to replace the caps on the motor control board to fix the transport speed problem as seen by so may others.

I know replacing the SMD caps is challenging, but what I have found in making this possible is having a hot air solder station (https://www.amazon.com/Tek-Motion-D...on/dp/B01MR2IWBN?ref_=bl_dp_s_web_12711278011) for removing the caps and a Microscope (https://www.amazon.com/AmScope-SE40...ion/dp/B005C75IVM?ref_=bl_dp_s_web_3016947011) for replacing them and of course a good soldering iron with a small tip. I could not have done it without these items. Since I do build electronics with SMD devices I already had these.

Now I can finally get back to digitizing my old tapes which is what started all of this .

Rick Jones

 

[SkywaveTDR]

Thanks Rick,
This is very helpful where otherwise we are on our own. I don't have hot air but the unit we had at Hughes I used a few times. I will see how it goes with your same methods.
That is once I find the time to get back into them.
Sam,

 

[Laszlo]

Hehe Rick, your last statement is ringing some bells, and I just realised how this works. I actually bought a Teac X-300 on EBay some half a year ago in order to digitize some old tapes I inherited from my dad and grandpa. But now I still dont have the tapes digitized, instead I have accumulated since then four tape recorders, two mixers, three (!) different noise reduction units, a vintage integrated amplifier, and am just in the middle of building my analog testing and repair lab I can only tell I am having fun!
Laszlo