Tascam 38 issue blowing fuses 503 +504

Discussion in 'TASCAM Analog Forum' started by leitrim_lad, Feb 24, 2021.

  1. leitrim_lad

    leitrim_lad New Member

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    Hi folks, i just purchased a tascam 38 secondhand on ebay, its my first tape machine and the seller told me it had been brought to a tech and everything was ok with it. However when the machine arrived there was loads of issues. I have gotten through most of the mechanical issues (belt change, washer at the back of the flywheel gone to mush etc)

    however im stuck with as issue where it keeps blowing fuses 503 and 504.

    It was playing back audio on the repro head but there was an issue with it not playing back audio on 3 channels when in sync so i took out one of the cards with an issue (channel 2) and tried resoldering where the connectors meet the card. When i put the card back in no channels would play audio from repro/sync or input. When i removed that card and left it out the rest of the cards would play audio on repro again.

    Tried this a second time and now the fuses seem to keep blowing and no audio will play or UV meters dont show any activity on any channels

    Any help would be greatly appreciated

    Thanks
  2. SkywaveTDR

    SkywaveTDR Well-Known Member

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    I hate to say it but you better get the deck to a known Technician that has worked on these before as it is clear you are going in a negative direction with regard to fixing the deck. You are going from a simple one channel problem to major blowing of fuses. This should never happen.

    I had a Tascam 44OB on the bench the other day and the sync led would not go out when I switches modes. Again the channel 1 card was pulled out and an examination of the work done on the card showed me connector soldering that was horrible and two joints were joined together that should not be. Just because a person owns a solder iron does not make them a technician. There are a lot of years past buying equipment in college that then prepare you for the work on electronic products that you must go through as well as a number of years of experience in doing the work. I get questions from a lot of people and some of them are clearly no where near the level that they must be to work on this product.
    If fuses are blowing then you have some power supply lines shorted probably on the card you were soldering on. The joints on the 38 are small enough to be shorted with little effort and so using the correct solder iron station such as a Weller with a fine point tip and at my age I use head magnifiers t see my work. Even I solder some joints together once in a while as I convert poor consumer grade soldering to Aerospace grade but when I accidentally solder I immediately unsolder the two joints with a Edsyn DS017 and then solder them again with Kester 44 and .031" solder. The reason some of these solder together is the solder they used for wave soldering is not so good and then the flux may cause the solder to travel between two closely positioned pins. When doing small connectors like that I clean the pins with Denatured alcohol and a old tooth brush to make clean the solder joints and then look at them with magnification. Then when inserting cards on a deck there is an experienced feel to knowing how to install them. Some just push them in and that bends pins- this is very bad. The professional way I teach is to position the card at the connectors and slowly jockey the card down and in pressing on alternate sides. There is a feel you get when after doing a lot of machines that you know it is positioned properly before applying any pressure. Some cards on machines have to be taken out a dozen times depending on what was done to them. I have worked on several hundred machines when at Teac since 1982 to now and I have fixed them all 100%. This is why people send me unit left and right. I will have 186 units at shop by today's end.
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  3. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Claims on E bay about a Technician worked on a unit are almost as good as saying the unit does not work. Most people that claim to be Technicians are NOT. Unless the name and shop name of the Technician is given and a length of warranty is stated then what they tell you is as good as the rest of the fertilizer they pass on E bay. I would have filed a claim with E bay as with that many issues it is clearly not as described. I work on E bay purchased units all the time and I can tell you that what they say is 99% of the time fraud and deception. The only good machines are ones that I service and my service records are presented but most people that have me work on their decks do not give them up so easy. If you Get a unit that was serviced by Tom Brucker, Curt Palme, Russ Bachmann, Gerhard in FL or some of the other well know technicians, then at least you have a chance to get a working unit- a no name technician that works on Cell phones is a card changer and he is completely out of his element to work on these kinds of machines.
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  4. leitrim_lad

    leitrim_lad New Member

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    Hi SkywaveTDR and thanks a million for the lengthy reply. I have removed the board i was working on (all boards infact) and the fuses are still blowing. I went through the card I was working on and cant seem to find a short anywhere on it. I desoldered completely anything i had worked on with an fr301 and resoldered from fresh.

    Im still trying to figure out what fuse 503 and 504 are for. Would you know off hand what they are for by any chance?

    Yes ive a feeling I got slightly done over on the ebay sale.. Ill just have to take it on the chin and put it down as a lesson learned i suppose this time. :(








  5. SkywaveTDR

    SkywaveTDR Well-Known Member

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    The first thing I say is why are you working on a deck without the service manual? The schematic in the service manual has the fuses and show what they power. Next the reason a deck will blow fuses when cards are out is because the short is on the motherboard and the cause is probably bent over pins on one of the connections due to bad insertion of cards. look for bent over pins or one touching each other. The fuses are being blown because there must be a short on pins 9 and 10 of P171 of the mother board and like I said this is the plus and Minus 15Vdc to cards- The only way this can happen is pins shorted together.
    Stereomanuals.com sells the most correct and readable manuals- the others are crap.
    To help further I would put meter ground in a chassis hole then in diode test- that beeps when you have a short- you could also use Ohms, test pins 9 and 10 and if either one conducts to the chassis which is ground then you need to go find out why- they should read a very high resistance with cards pulled out. If you do not know where P171 is on the motherboard then that is why you need a service manual for this unit. The pins on audio card for +15V are-
    106Pin 4, 107 pin 3, 105 Pin 5 on audio cards which also means motherboard where they plug into.
    The Negative 15 is on 106 pin 3, 107 pin 4 and 105 pin 6.
    The connector number make sense when you look at the audio card connector schematic.

    This kind of machine is far too complicated to work on without technical documentation- I will not do it like that and I have worked in electronics for 47 years. This is what you need from
    Stereomanuals.com. TAS38-USM
    http://www.stereomanuals.com/man/rep/tascam/index.htm
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  6. SkywaveTDR

    SkywaveTDR Well-Known Member

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    One more tip to help localize problem- unplug P171 and I bet the fuses will stop blowing. If you resoldered any feed through pins make sure some are not soldered together shorting the +15 to -15 supplies. As I have told persons over forums, do not keep blowing fuses but use the DVM Diode test or Ohm setting to tell you if the conditions exist that would cause fuses to blow- you do not need power in machine to use a meter this way. Once the DVM reads high readings after you found the problem the fuses will no longer blow- that is a rather serious condition.
    IF the DC output remains shorted after P171 is unplugged then the short is somewhere else.

    The 38 deserves a complete recap of the power supply as the caps are old but the main reason would be to remove that yellow glue that eats through component leads due to acidic condition. I remove all signs of that glue from boards. Since it makes no sense to put back old electrolytics I put new ones on and using 105*C types as I do, you will be upgrading the part type. I also clean the solder side of board with denatured alcohol and old tooth brush to give a real professional clean condition.
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  7. leitrim_lad

    leitrim_lad New Member

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    Thank you so much for getting back to me on this, i really appreciate it. Ive done more investigation, checked pins 9 and 10 on P171.. No shorts or grounds.. When i plug out P171 from the board the fuses still blow..

    Im suspecting i must be in trouble on the power supply?

    I noticed the red/dark yellow kinda glue under all the caps.. I havent tried to remove the power supply yet. It seems to look like it will take a little time to get into..

    Thanks again
    Leo


  8. leitrim_lad

    leitrim_lad New Member

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    From further investigation I think i might possibly have some bad diodes.. D813 and D814 when i take them out of circuit and test using a multimeter im getting nothing from either of them.. Diode 812 and 815 seem to be ok.. I dont seem to be getting anything from 817 either out of circuit.. I havent taken 816 out of circuit yet but when i put the multimeter across it in circuit im getting nothing.. Any help would be greatly appreciated..
    Thanks
  9. SkywaveTDR

    SkywaveTDR Well-Known Member

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    The problem with assuming these were the cause of the problem not be all that correct- they might have been damaged by the problem that was the result of where the problem was in the first place. It is best to determine where the short is and them make sure that is gone before fixing power supply parts. Power supply parts are usually pretty robust and the last items to be failing. Just from experience of all these years- I don't remember replacing bridge rectifier diodes in these decks in all my 47 years. I have the trait to upgrade parts but even if you put 2.5 amp or 3 amp diodes in if the short is still there then they can be damage too- fuses take a long time to burn out and all that time those diodes are conduction more current that they were designed for and probably why they are out now. In other words fix problem, do not blow more fuses and let the DVM find the short not the fuses.
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  10. leitrim_lad

    leitrim_lad New Member

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    Thanks a million again Skywave TDR.. Is my theory here right?... The 15V + and - rails only feed the main board/cards.. If i plug out P171 from the main board where the cards are seated the short is still there.. this leads me to believe the short has to be in the power supply?

    I mixed up the diodes that are gone, its 812 and 813.. When i take them out of circuit and test the short is still there on one side so you're right I think my short is still there and probably damaged these diodes. I don't have a clue where to even start looking for the short however.. Nothing looks out of place on the board (blown cap/resist etc)

    If you had any direction it would be greatly appreciated

    Thanks
  11. SkywaveTDR

    SkywaveTDR Well-Known Member

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    A short can be anywhere. But the thing is what caused it? Power supply parts just do not short under regular conditions. Some capacitors (Electrolytics) can short on occasion but it is rare in these decks. The DVM (Digital Volt Meter) will be able to tell you when the short is gone. If you are at Diodes on the power supply then that must mean you have it out of the deck- kind of hanging out the back. The Plus/Minus 15 Vdc lines only feeds P171 and the other lines that go off for muting reasons are through large resistors of 100K or 82K and then can not cause a short from off the negative side. The D812 and D813 are on the positive side of the split supply so the problem is on that side so measure with P171 unplugged from pins 1(ground) to Pin 2 (+15). There are parts capacitors on both sides of the voltage regulator but they are all after the diodes that burned out. The regulator transistor Q807 is heat sinked and these can have insulator problems or if it was taken off and remounted. When you measure the collector- tab of the transistor to the aluminum heat sink there is not to be any conduction- If this is shorted to the aluminum then that is sending the input voltage to ground as the heat sink is grounded by mounting to the chassis. I use new Silpads for TO-220 transistors I got off E bay. Also note if the insulation washer is put on the transistor- some people work on these and leave parts off such as
    screw the transistor down without an Insulator washer and of course an instant short is the result. The power regulator transistor should be sitting on a Mica or Silpad, then a plastic washer is put on the mounting hole of the transistor then a screw and the nut or threaded aluminum allows the mounting of the screws so that when ohms tests are done the transistor tab and aluminum heat sunk never conducts- if it ever does you have something wrong. You don't have to blow fuses to find this just use the ohm meter or DVM on Diode test.
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  12. leitrim_lad

    leitrim_lad New Member

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    Thanks for all the help.. So I sourced 2 diodes and changed them d812/d813 and my issue with the blowing fuses has now stopped. However everything seems to be going well until the capstan motor suddenly stopped.. It spun for a second and then completely stopped.. I looked at the power supply and now R829 seems to have blown.. Please see attached picture. Any help would be greatly appreciated.
    Thanks

    https://drive.google.com/file/d/14qtvBLq-V89YfETI1GCIT3NsPLK3VQIC/view?usp=sharing
  13. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Well your picture tells me that whoever put that in there does not understand electronics very well and that the large size allowed for the part would suggest that the part is a 3 or 5 watt resistor- Does that look like a 3 or 5 watt resistor? The fact that it burned out is not a surprise to me in that it looks like a 1/4 watt part which is 20 times smaller than it should be. The resistor is suppose to be a 22 ohm probably 5 watt sandohm resistor. If a 22 ohm resistor gets hot or the voltage collapses at the load end, then I have to think the Capstan motor that this feeds is shorted inside or other things like the thrust clearance is gone on the flywheel or it could be a tight or wrong belt. As I tell my trainees, you need to know what you are doing in these and the guy that put the small resistor in does NOT.
  14. Evgeny

    Evgeny New Member

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    I should begin from the start message of this topic:
    It's almost impossible to make advices "by phone", but first of all I should check new belt, better to say a tension of new belt. SkywaveTDR (a great respect to him!) has repeatedly said on different forums how to replace this belt, specified its dimensions and all the replacement procedure including lubrication and clearance setting. So, please, take the trouble to ask him to specify the correct belt tension. Note that R829 restricts the current via capstan motor at a safe level about 1A, and remember that current is just proportional to the mechanic moment on a motor shaft. So if you replace R829 by 3-5W-resistor, your mechanic problem (if it exists) may remain and next time you will get overheating the motor, detonation or low speed stability and so on.
    And best of all, try find a good specialist, which can do this job without help!
  15. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Amateurs contact me a lot after they go in and make a mess of their deck and then they want me to fix it when it is impossible to know what they did to it. Some units I have seen with vacuum cleaner belts on them another I have seen two round belts where a flat belt should be. It seems the brainless types are surfacing with much more regularity now that we have the same in the Presidency. It is wiser to just take the unit to a good technician in the first place otherwise like the deck I work on this time which as yellow paint on the screws and many screws left out of the unit from SC is another example. People tell you all the time that they know what they are doing and this is the results. This was an X1000RBL. Why does a person who knows what they are doing separate the 120Vac wires from the cables that come from the heads that carry mV levels? This guy had them all collected together. Why does my deck hum?
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  16. explodingPSYCH

    explodingPSYCH New Member

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    Did you ever find the source of the problem? I'm also blowing the same fuses, so I must be shorting something. I'm going to check those specific diodes, but it sounds like the problem is further down the line.

    I'm also curious if there's a reference for where the fuses point to, as I couldn't find clear direction in the service manual. (Maybe I overlooked something?) I have a post here:

    https://www.tascamforums.com/threads/tascam-38-fuses.9122/
  17. explodingPSYCH

    explodingPSYCH New Member

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    The resistor in question is smaller than the outline of what was originally designed for the PSU board, but this seems to be a modification made by Tascam (much like unpopulated components on the channel cards, etc). I say this because both of my 38's have the same smaller size/wattage resistor (also with the heat shrink on the legs).

    What is strange to me is that @leitrium_lad reports that D812 and D813 are his bad diodes, where the diodes that measure wrong for me are D814 and D815 (the outer diodes in that row of four, where his are the inner diodes).
  18. SkywaveTDR

    SkywaveTDR Well-Known Member

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    First people try and work on this complicated piece with manuals from Hi Fi Engine which one I have you can barely read some of the connector numbers. Most the manuals out there in Pdf are missing the schematic of the power supply which was with the original manual as a fold out.
    Tetrakan Supermonobloc web site has a manual with schematics. I am looking at it at after page 93.

    NOW, why does 504 and 503 blow out- it is easy really. That is the supply that make the plus or minus 15 Vdc supply. I have told you already where the problem is but like a lot of amateurs that are not real technicians with a DVM that would find this quickly, you fail to go looking. In power supply diagnosis you use a method called divide and conquer. Unplug connector P171 as this is what goes to the motherboard. If diodes are bad in a deck it is not because they fail all the time but that they were blown out by the work on the unit. When people go in resoldering motherboards with their large tips they joint two pins together and then start blowing fuses. The plus and minus supplies are next to each other in this board design and if you would use a DVM in diode test mode you would already find the problem- look for pin that have 15 and -15 on them that are joined incorrectly together- this is the most often found problem when amateurs work on stuff as the Pro's don't have this problem or if they do it is from previous work. If the short that is on the two polarities is gone when you unplug P171 then that tells you the short is on that board. Now go to the schematics and find which pin is 15 and -15 then on the motherboard which pins of those go to traces that have a feed through pins next to each other. If you put a DVM in diode test to find the short from 15 to -15 then when you make corrections to stop the beep then the fuses will not blow but then if the diodes have gone bad put 1.5Amp ones in there and elevate them off the board for heat dissipation. In my experience these deck do not fail in this regard except where worked on incorrectly. Yes the motherboards do need resoldering but with the right tools and I use hood magnifiers but then I am no longer 20 years old.
  19. SkywaveTDR

    SkywaveTDR Well-Known Member

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    This may help- https://tetrakansupermonobloc.home.blog/2019/04/26/tascam-38-operations-and-service-manual/

    That resistor put in is not factory as they never do stuff like that- it is obviously an amateur knowing very little. Most these kinds of resistors are either wire wound 5 watt or a fuse resistor 2-3 watts. You need to see what was in the schematic and a Triangle sign means that variance from what they specify is dangerous. From further review this resistor is R829 which is a 22 ohm probably 3 watt resistor. That 1/4 or 1/2 watt resistor will burn out right away which is why the guy that put that in did not look at the size that was in there- that is NOT factory.
    Last edited: Jan 8, 2022
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  20. explodingPSYCH

    explodingPSYCH New Member

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    I appreciate your help, but you didn't "tell me" anything before. I'm new to this thread.

    I'm just pointing out my findings and correcting the issue about the resistor.

    Thanks for the additional info. I have a feeling something just arched while I was testing for connectivity related to another issue. After cleaning the unit with compressed air and replacing the diodes the machine is working again, save for the initial issue I'm trying to fix (no output in repro on one channel).

    It seems very unlikely that his machine, and BOTH of my machines would have been modified by the same "amateur". And this person also carefully put in heat-shrink on the legs of the resistor to match other stock components inside the machine?!

    It seems more likely that Tascam made an adjustment to their design as shown by other deviations to the originally printed circuit boards. I would be curious to see pictures of other's PSU to confirm if this is a wide spread hack someone is doing.