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Thread: Tascam 312B Project

  1. #71
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    Powering and diagnosing the board outside the mixer may be a problem. . I don't have a power supply,or a oscilloscope (although those may end up on a Christmas list......)
    You don’t need to get a separate power supply. You can run jumpers from the +15V, 0V, and -15V rails on the buss PCB to those terminals on the input PCB. Use something like these:

    Philmore 500 Mini Hook Grabber IC Test Leads,Set of 5 Colors,16" Long,New | eBay

    I started thinking last night about shotgunning some parts and had some general questions about that:
    Try and resist the urge to just start blindly replacing a bunch of stuff.

    1. Looking through some threads last night, I noticed you use TI op amps and not less expensive ones (.....honest..... I'm not stalking you Cory..... you just happen to mention 5532 op amps more than others.... ). Is there a spec you follow or is this just a brand (or parts house ) preference?
    Often times you might just be getting what you pay for. Google “counterfeit opamp”. Generally you don’t save much (or you might even pay *more*) for opamps from some eBay vendor. I’d rather have assurance what I am getting is coming from an authorized dealer and is a genuine part. I generally get my stuff from Mouser. I know it’s a genuine chip. Most of the opamps I use are TI parts...or available from TI. So I just do that. Maybe not necessary, but I’d rather not wonder.

    2. Should I socket all op amps?
    It’s generally a good idea on these phenolic boards (not as robust as glass fiber), if you’re removing a chip anyway, replace with a socket. It’ll pretty much avoid having to torch those lands again, and also avoid having to apply heat to the chip. But avoid getting the cheapest sockets. Get “machine pin” sockets. Better contact with the contacts on the opamp. Like these:

    https://www.mouser.com/Search/Produc...lkey575-199308

    3. Who is the best small quantity vendor to buy electrical components from in terms of customer service.? The reason I ask this is I noticed some of the amps on my 312b are obsolete (like in the monitor section). If I need some of those I would like to talk to someone about replacements.
    What amps are obsolete on the M-312? I don’t think so.

    I use Mouser. Digikey is also another top-notch option.

    One other thing.......

    I was looking at putting my replacement input card in and was looking at the mods that were done. Here's the replacement card:

    Attachment 101106

    and here's the one I took out of my 312

    Attachment 101105

    I looked in the schematics and couldn't find those resistors (?) anywhere. I assume, I'll need to replace those blue ones with ones like were originally like the mixers right?
    Those resistors are not stock. And the parts they replaced I don’t think they were resistors.

    I forgot about those. I’m away from the M-312 for a few days. I don’t recall what those resistors were doing there from memory. Can you tell me what the labeling says on the board for those parts?

    I need to see what the parts are and what they are doing in the circuit.

    Cory

  2. #72
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    Wait...I’ll answer both our questions...

    Those resistors go between the Phantom power connector and terminals 8 and 10 on the card right? Where the hot and cold conductors from the mic jack connect (pins 2 & 3 of the mic jack) right?

    You can’t find those resistors on the schematic because (I’m guessing) you aren’t looking at the M-300B supplement...you’re looking at the schematic out of the original M-300 (not B) manual...no phantom power on the non-B M-300 mixer, so no phantom resistors. And you won’t find them labeled on your spare card because it’s a non-B card, and somebody added them. But the *only* difference between the non-B and B version input PCB is the phantom connector and phantom resistors.

    Can you put up a pic of the solder side of those blue resistors?

    They used decent quality parts.

    Measure their resistance. 6.8Kohms right?

  3. #73
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    Measure their resistance. 6.8Kohms right?
    On the replacement board, each resistor measured 2.97 Kohms

    Can you put up a pic of the solder side of those blue resistors?
    It should be a high enough resolution that you can zoom in.

    file-nov-15-1-00-40-pm-jpeg

    Just for comparison sake, I measured the resistance on the original board and they were 3.9 Kohms each, and here is the solder side of the original board I took out of the mixer.

    file-nov-15-1-01-37-pm-jpeg

    I guess the shape of the components is throwing me a bit. The blue ones look like resistors, but the brown ones off the original board look like film capacitors? Again, its probably my lack of familiarity with the components



    Another thing.... I inspected the original board more carefully and noticed a blackened area right where the equalizer pot that is squawking is. I don't know if its coincidence or not.....Here's a pic:

    file-nov-15-1-04-17-pm-jpeg

    In the upper part of the blackened area, I scraped away some of it to see if the board was damaged underneath.... it doesn't appear to be.......
    Brad Bachelor
    "Marrying an old bachelor is like buying second-hand furniture." - Helen Rowland

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    Can you put up a better picture or two of the phantom power resistors on the stock input PCB?

    They are resistors...you wouldn't be measuring 3.9Kohms if they were caps.

    Looks like the replacement board from the M-300 is the same as the M-300B.

    You should be fine with those replacement resistors...no need to replace those.

    That blackened area, I'm pretty sure that's just from soldering...blackened varnish. There's nothing that would be flowing through that potentiometer that would heat things up like that, so I'm pretty sure its just scorched varnish. If you want, gently take a small flat-blade jeweler's screwdriver and scrape the stuff away and put up another pic or two.

  5. #75
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    Can you put up a better picture or two of the phantom power resistors on the stock input PCB?
    Here is a picture off the original board:

    file-nov-16-10-27-46-am-jpeg

    You should be fine with those replacement resistors...no need to replace those.
    Good Deal!! I think I'll try to get that board in this afternoon then.

    Payday comes next week, so I think I'll get those leads ordered to power the board that needs fixing. If this replacement board works, I can spend a little more time diagnosing the other one.
    Brad Bachelor
    "Marrying an old bachelor is like buying second-hand furniture." - Helen Rowland

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    Those resistors just look different because they are older metal film 1/2W parts. I think those are decent quality parts, but so are the replacements on your replacement board.

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  8. #77
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    Success!!
    file-nov-16-9-07-10-pm-jpeg

    Did I tell y'all my nickname was Shakey McShakeson? That was some close quarter soldering there..... I decided to use the spare board while I start to gather the leads and tools I need to do the troubleshooting. Everything on Channel 9 now seems to work.... Now on to the monitor section....
    Brad Bachelor
    "Marrying an old bachelor is like buying second-hand furniture." - Helen Rowland

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