Tascam 238 - changed caps still jet speed motor - help appreciated

Discussion in 'TASCAM DIY Repairs and Mods' started by Joshb_, Dec 10, 2021.

  1. Joshb_

    Joshb_ New Member

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    Hi.
    I really hope I can get some help with this.

    I changed the usual electrolytic caps, C2 and the zener. Of the old caps I removed x2 10uf caps were bad and the 1uf cap was bad.

    I damaged a pad on C13 ( my fault) but all other pads are still ok

    I tried through hole and then individually checked smd's in case i used a duff through hole cap initially. but its just the same as it was then i started, with the motor going super fast.

    I am getting 9v at the new zener and power to IC1 and continuity seem ok around the caps.

    are there any other components i need to check? I don't yet fully understand the circuit.

    I am in the UK so getting e replacement board is hard.

    I would really appreciate some tips and thoughts. cheers..

    Josh
  2. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Usually the reason for servo run away is due to the FG signal being lost. There is a test point to the left of IC1 and this must have a good Sine wave on it. It is between two resistors and near the mount screw. Foils get damaged by leaked electrolyte on the foil pattern going into the IC one input. Do not leave any SMD caps on the board as they are not long term parts. C2 should be replaced with a Mylar or Polystyrene cap as on here. C2 is the clear plastic one that some think is C5 but C5 is above. C2 is immediately to the left of Zener diode symbol. I adjust the thrust bearing end play for some play but also maximum sine wave amplitude but not too tight.
    upload_2021-12-10_22-22-12.png
  3. Joshb_

    Joshb_ New Member

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    Hi thanks for the quick reply, I have replaced all caps with through hole versions, still no luck. all traces seem ok for continuity. I put a scope on TP1 and the waveform was a mess, not at all a smooth sine. does this mean its IC1?

    Is C2 supposed to be 1n or 47n? i have read both.
  4. SkywaveTDR

    SkywaveTDR Well-Known Member

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    The distance from the pickup coil make a lot of difference. That is why I adjust the back screw while keeping the flywheel turning but also to get the maximum sine wave amplitude. If the wave is a mess the C2 is there to filter that I am told. Another guy uses .01 Mylar caps there and I have for many years used .001uFd Polystyrene which is what I determined was on there. ANT - Alex backwards wrote a schematic but it may have different references as I think there were a number of different boards. I can not upload files here so if you want the schematic send me a E mail at skywavebe@sbcglobal.net. Out of about 136 motors I do have a couple that still have problems- they can be worked on out of the deck. I have a piece of wood to support the motor with a hole that it sits in and then a 54K resistor is used at the Pitch control terminals- if the pitch control terminals are not closed with some resistance the motor will not run. Values of higher than 54K have been measured in decks so it might be as high as 82K.
    I have all the chips on the board here in stock but other that a couple of BA6411 power IC's that burned up and cracked there have been no IC failures that I have had to change the SMD IC's. Foils can be compromised under chips and the ohming out of components to the IC pins is required as the break can be where you do not see it. I have also had some coils leads break due to tension on them and maybe the flux they used. One coil I had the end lead was lost so I had to find it after taking the coil off the board and then patching it then gluing it back on. This was a touchy job but it can be done. [​IMG]
    Last edited: Dec 11, 2021
  5. Joshb_

    Joshb_ New Member

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    Hi. the fun of buying an item that someone has already 'tried to repair' I finally have the machine working!

    All the 10uf smd caps were bad, the 1uf was bad and the 4.7uf was bad.

    The FG wire had broken off where it attaches to the FG board

    I also replaced IC1 but i dont think i needed to as is was still playing up, thats when i noticed the grub screw was completely backed off which took me a while to work out as being an issue.

    How much end float is their supposed to be or is it just a case of getting the biggest sine wave before it starts to square off?

    Other issues I found were D3 and D4 on the power PCB were dead and a track was broken on the channel 1 input RCA which meant removing the mother board etc.

    It runs pretty well now, I possibly have a bit of flutter though. I read if its not mechanical it could be C16 or the traces around it?

    Thanks for all your help SkywaveTDR, i really appreciate it.


    Denis at https://238pro.com/ has also been amazing in helping me narrow down faults. If I lived in the USA i would have definitely sent my machine to him for a full service and calibration.

    cheers


    Josh

    [​IMG]
    Last edited: Jan 10, 2022
  6. SkywaveTDR

    SkywaveTDR Well-Known Member

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    The spacing is determined with a scope set to watch the sine wave on the test point. The distance is not known as a measurement. The wow and flutter is most often found related to the oil used in the capstan bearing. At the higher speed this should be pretty low and I have had decks go down with AMS oil and good Idler tire to .03%. If the pinch roller is not fully engaging or the wrong pressure or size then that can cause issues. I have had to replace only one roller I think.
    D3, D4 are for power mute. Anything broken or otherwise damaged would indicate a bull in a china shop person working on the deck. Track one might be a bad solder joint. The connectors if aggravated enough will crack joints as the soldering is not a high grade type.
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  7. SkywaveTDR

    SkywaveTDR Well-Known Member

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    I did one the other day and the sine wave was very small when I started and as I turned down the thrust screw as I watch the scope the sine wave increased very much more and at the end started to clip. At that point I back off from the clip point to a sine wave and leave it there. The wow and flutter also went down. Initially it was around .07% and then after oiling again with AMS oil and the adjustment it is near .03%.
    Joshb_ likes this.