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Thread: Tascam 388 extremely hot control pcb resistor...

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    Tascam 388 extremely hot control pcb resistor...

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    Hello, I was just wondering if anyone can give some tech advice...

    I have recently been fixing up a Tascam 388, the machine now seems to be all working as far as I can tell, all the transport functions, and it passes audio on all channels, Though I have not yet fully tested every single audio routing.

    My problem is a thermal resistor on the control pcb (2w 47k) R405 which is running extremely hot and heats straight away after turning the machine on. Somethings not right there and is drawing too much current I guess?

    When I first got the machine there were burned flux/heated marks from this resistor on PCB and the inside casing of the machine it's self, the old resistor seemed fine when I tested it, but I replaced it anyway. There is a cap close to it that looks to of had some heat now also, after having it on a short time to test. Visually everything else looks ok - I did recapped all the board and hoped this would get rid of the problem, but has not. I'm thinking it must be a short somewhere on the board? or a power supply problem?...

    When testing voltages on the mother board (2) at power supply pcb pins, I'm getting two high readings: P6 pin 1 should be +25v (reading +40v) and P6 pin 3 should be +15v (reading +20v)
    Would this be related to my control board resistor problem??
    Is something drawing too much power? or not enough power to get the higher volt readings? or is this a different problem altogether?

    Has anyone had this problem before? or If anyone has any ideas of what I should look at and check next regarding this it would be a great help and greatly appreciated.

    Thank you in advance,
    Robin

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    P6 pin 1 is the 35V NST rail, not 25V. P6 pin 3 is the 15V NST rail. These are both unregulated power rails. 40V and 20V respectively are fine IMO.

    Power resistors get hot. It is normal. You titled this "extremely hot". How hot is that? They can easily get too hot to touch. That's why they are often flameproof resistors. BUT...R405 on the Control PCB Assy is not a power resistor. It looks like its associated with the power-on mute circuit...it is a ground resistor on the "reset" signal associated with one side of the 6VAC rail. It doesn't appear to be associated with P6 of the Mother 2 PCB Assy/Power Supply PCB Assy.

    I've had issues in the 388 with certain components benefiting from more robust mounting to abate long-term heat damage. Again, components get hot...this is normal...even to the point of the PCB discoloring from the heat. When it ends up being problematic is if the heat generated by normal operation degrades the integrity of the solder joints and solder and traces on the PCB associated with the component. As those degrade the resistance associated with those traces/solder joints increases. With increased resistance you get increased heat and now you have a downward spiral...more heat, more damage, more heat...so the way to address this is when mounting a new component, don't just stuff the tails of the resistor (in this case) through the holes straight and solder in. Instead scrape off the green coating on the traces a little ways back from each solder land, exposing the bare copper trace. Tin the exposed copper trace with solder, and then when you stuff the new component, bend the tails to lay on the trace so as to increase the contact area between the component tails and the traces on the board. Even better, before you solder the component, use something to clamp the tail to the trace using a clothes pin or something like that so when you solder it together there is a better chance the component tail is actually in direct contact with the copper trace when you solder it up. And I don't have a 388 Control PCB on-hand, but likely Teac installed R405 spaced away from the board, and then you likely installed the replacement part the same way. That is good. If they DIDN'T do that, that is another step you could do to allow more air-space around the component for better heat dissapation.

    I'm leaning in the direction of thermal management and proper installation of R405 vs there being a problem because you say everything seems to be functioning normally. Typically if there is really an acute problem, either something doesn't work, or components go *poof* up in smoke instantly rather than just getting hot.

    SO...maybe clarify how hot R405 is getting (do you actually smell that hot burning electrical smell? Is there smoke?), and maybe clarify how you mounted it and if there is room for improvement there per my advice above, and when you say the transport seems to be operating normally, are we talking about you can power it up and leave it powered on for a day and using it throughout a day?

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    Hi sweetbeats, and thank you.

    Regarding voltages: ok great, that make more sense if p6 pin 1 is +35v. I think I confused myself with that one, my mistake.
    I guess if they both read within a 10% tolerance or something then that's ok?

    Regarding the resistor R405: I replaced it with a flameproof resistor same spec and yes, it is mounted about 1\2 an inch off the board. I tested the temperature with a heat probe on a multimeter, with the machine being on for 10 mins the resistor is too hot to touch and had an average reading of 85 degrees. I'm not sure how accurate that thing is as its only a cheap meter. (Just for reference I tested the rear heatsink with it, was around 30 degrees.)

    Maybe it normal? I can't imagine it running that long or all day at that temp? I was only aware of it because of the state that area was in when I first got the machine. There's no smoke, or electrical smell but I have only run it for 30 minutes tops when first test things again, I notice that again and then turned it off as I didn't want to fry anything, not knowing if normal or not etc.

    On your advice I will re-fit R405 to the board as you described above with the tails through and on to the copper traces to see if that helps and will report back.

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    All sounds good.

    On the power rail variance, they are unregulated power rails, so the tolerance is much less of a concern as compared to a regulated rail. With a regulated rail the variance concern is that it is within the rated spec + or -. Like if it is a +15VDC regulated rail I think I like to see that within +/-5% of that. Of course this is application dependent. With an unregulated rail, generally its more about ensuring that the voltage is above a minimum threshold, and less about how far above that minimum threshold the voltage is. My hunch is that with the 35V NST rail that means it needs to be at least 35V, but I don't have any concern with a reading of 40V there, or likewise 20V on your 15V NST rail...and that's a 30% variance on the 15V NST rail...but I think that's fine. To be completely sure you have to look at all the components that are powered by that rail or passive components in that circuit that may see that full voltage if there is a failure in the circuit, but I don't think that is necessary here. I'm blanking on what the "NST" means...I think @skywaveTDR or @wkrbee clarified this in some thread in the last couple months. But, anyway, I think you're fine there and as we discussed those power rails don't have anything directly to do with R405 on the Control PCB Assy anyway.

    As to your temperature measurement on R405 of 85 degrees (I'm assuming celsius, yes?), a quick look on Mouser.com at a FP 2W 47k carbon film resistor reveals a normal operating temperature range of....

    [drum roll please...]

    -55C to +200C.

    R405 in your 388 is operating WELL within the designed operating spec. In fact it isn't even breathing heavy. Again, on rare occasions I have found minor design issues in this era of Teac equipment with some heat-stressing. Teac did a relatively excellent job with their designs...you can generally see evidence of thoughtful component selection and physical design of board layouts and nice decisions to minimize cost but maintain performance quality relative to the market at which a particular device was targeted...I don't see wholesale corner-cutting as with some other brands, and they generally over-spec'ed their power supplies, etc. But nobody is perfect and after 3 to 4 decades I've occasionally found an area of heat scorching on a board, often times around rectifier diodes. This is usually more due to the board material being phenolic resin vs glass fiber, and usually the solder joints and traces are still good, but the board itself is discolored. So I'll install new parts, make sure they are spaced off the board, make sure there is as much room between the components themselves and from surrounding components, and install them as I described earlier where the contact between the component tails and the board traces are enhanced and go on my merry way without a concern. Stuff gets hot inside these devices. Look at the rear top panel of your 388. You've got cooling slats punched into the panel, right? They're right over the main transformer, Control PCB, Power Supply PCB, Reel Servo PCB, Bias PCBs...all those assemblies have components that get hot...this is normal...smoke? burning smell? Not normal. My Ampex MM-1000 2" tape machine has these hot dog seized wire-wound resistors for setting the holdback and takeup tension. They...get...hot. Like you can smell it hot. Like I could cook something on them. But its perfectly normal. I appreciate you are asking these questions though and wondering. Its not always okay to just go "meh no worries". Asking questions is good. But I hope you are reasonably assured at this point that its okay...I think its fine.

    Reinstall R405 with some enhanced contact with the board.

    Run your 388 for some hours. I am going to assume it will run normally. Check the temp of R405 again after some hours. I bet it is still well within the component operating temp range.

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    Ok... I fitted the R405 resistor again as sweetbeats suggested, it seems all ok. I ran the 388 for 3 or so hours and the temp stayed around the 85c mark. (the resistor is a metal oxide flameproof type 2w 47k) So think that's all ok now!

    I was aware of the heat damage that was there when I received it, so that's why I questioned it I guess. Having only worked on cassette type machines before, I wasn't sure what was normal running on the 388.

    So I'm good on the voltages too, really pleased - thank you for the clarification on this Sweetbeats. NST is that none stabilised.

    There was just a couple of extra things I noticed that could be linked with the voltage thing:

    On the TR PCB, U2 I have a L78N20 (20v), now... I've seen this come up before on the forum that it should be a L78N24 (24V)?
    Maybe it was a change in a revision two of the machine or something? I do have a L78N24 so might swap it out.

    Also from the coming from the transformer, the two orange wire coupling (one which goes through fuse F1 to the power board) and one goes direct to the power board, does it matter if the are the wrong way around? I guess not as it's working, and as long as there's a fuse on that coupling it's ok? but is another thing I'm unsure about.
    At the moment they are the fused orange ends up at P2 pin 4, the direct orange to P2 pin 5. On the schematic I have it's the other way around.

    Thanks.

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    All sounds good!

    I remember something about that transistor on the TR PCB. The tech docs don’t always match the device...subject to change, etc. What is indicated in the service manual? If you swap it out you want to see if there is a complimentary component in the circuit that should be swapped out. I can’t look at the moment.

    And you are fine regarding the orange leads and the fuse from the main transformer. Those two leads form an AC circuit. You fuse-protect the circuit by placing a fuse in one side of the AC circuit. It doesn’t matter which side.

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    Ok that s great, thanks for the clarification on the AC circuit.

    On the schematics I have, which is probably a first revision copy of the machine it's a L78N24 in U2 of the TR pcb. I not sure what it would compliment in the circuit. It doesn't seem to be causing a problem at the moment though.

    Sweetbeats: Thanks for all your input, with the questions I had. I hope it also helps out others. I will carry on testing etc and hopefully get this thing back in use.

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    I don't think I've ever seen revisions of the 388 schematics...by this point in Teac history they weren't that good as far as documenting revisions or at least making them available externally...other companies issues TSBs and the like when there were changes/revisions.

    Looking at the schematics I see now that transistor is the regulator for the +24V transport supply. There is no compliment to it. It should be the L78N24. That's what I would install.

    And you're welcome. I'm glad I could help, and I appreciate we could work through your issues on a public thread because it *will* help others in the future and you are a part of that help now.

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