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Thread: Old School LED Help Needed

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    Old School LED Help Needed

    Good Day All-
    If someone could point me in the right direction, I would appreciate it. I have an old A&H Syncon A that has been highly modified. There was an LED meter bridge added in the 1980s that I have very scant documentation for. One of the PSUs went bad and took out the logic. We replaced all the ICs, but still no control.

    I need someone that is adept in Old School LED Tech - preferably theory & design to help. If you know somebody please let me know. Many thanks in advance.

    PS. If this belongs in a more appropriate forum, please let me know which one. Thanks!

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    Hi and welcome to the forum. There are plenty of people versed in LED technology and application here, but more information is needed as there's too little to base any constructive reply around. Is there a brand or manufacturer for the meter bridge, or at least what are its origins? Can you post the documentation you have?

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    Old Timer J M Fahey's Avatar
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    Welcome
    Agree and add: you do not have a "LED" problem, those are just "lamps" which can be replaced if failing, no big deal.

    Your problem lies with the Electronics driving them, a very different problem.

    Is that meter bridge an A&H product?
    In that case, you might get a schematic for it, either from A&H, any legacy company which replaced it (some 30 years have gone by) or most probably some A&H fans and aficionados Forum.
    If it was made by some relatively small and obscure Company ... itīs almost impossible.
    Again, search for some A&H Forum.

    IF you get the schematic we might try to help; otherwise itīs practically impossible.

    Absolute worst case, I think a meter bridge is basically an add-on Vu Meter, so get another which should be functionally similar an use it or, if you dare, search for a schematic or project, wouldnīt be surprised if an old Elektor (or similar) magazine from the 80īs carried such a project, and build your own.

    Hey, you might even house the "new" one (built with the old recipe of course) inside the old case and keep everything visually the same.

    Hey, look at this:
    https://www.gearslutz.com/board/geek...terbridge.html

    they claim:
    Meterbridge???
    I have a Thatched cottage Audio S2/ Allen heath GS3 desk here that's great except its desperate need for a meter bridge. Anyone know where I can find one. A&H never made one is there a universal option?
    which was answered by the suggestion of building something like this, for which they mention a project or kit:


    thereīs many such projects around.

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    Juan Manuel Fahey

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    More Details

    Photos in the next day or so . . .

    This meter bridge is either a "one -off" or a prototype. There is no manufacturer's name, no way of ID nothing. Originally the Syncon A came with VU meters and had a large PSU to power them. The original PSU was used to power the bridge. I got hand drawn diagrams from the previous owner on how to hook it up. That's it. No schematics. No parts list.

    It was working fine until the PSU failed and my tech believes a surge was sent to the logic board and fried a number of components. Again, this is on a breadboard - custom.

    Please hold comments until we test the repairs we've done on Wednesday. The meters themselves are okay. It's the logic control that's the problem.

    Thanks for your help and interest thus far. I will update in a few days. Please sit tight.

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    Click image for larger version. 

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    This is the logic board that is giving us the problems. My tech is swearing a blue streak because the traces are so tiny and fragile. After the PSU spiked the voltage, we replaced all the ICs, checked and verified the transistors, caps and resistors. He found that a couple of traces had been separated by the voltage spike and repaired them. There was a short that was also repaired. The biggest problem with it is, that theres no documentation. We spent 4 hours on it today and it still doesn't work.

    What this board does (I think) is control how the meters act to the engineer; peak, rms, etc. There are 4 buttons on the console that are marked: Analyze; Analyze in PFL; VU / Auto & Peak Hold. There are 56 channels of input meters - 40 LEDs each. Main L & R also 40 LEDs. 6 Auxes - 20 LEDs each. All these have their own PSU at +5V.

    The logic board wants to see +15, common and -15. When the PSU failed, it took out a couple ICs which were replaced. We replaced the PSU with two of these:

    https://www.jameco.com/z/RT-65C-MEAN...tt_323548.html

    At this point, I think it might make sense to look for a different logic board and try it make it work with our system. I'm going to start researching this today. Hopefully, this is pretty standard stuff that might make a retrofit a little easier than trying to fix a circuit without a map. Input always welcome.

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    Old Timer J M Fahey's Avatar
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    Ouch!!!!
    Itīs very home made and the PCB is a nightmare.
    Made by ironing (literally, using a clothes iron) a paper print on a bare copperclad board, then acid etched, is a system valid only for protoyypes and nothing else, way too unreliable.
    Itīs a miracle it survived all these years, I guess because it was not moved around.
    Typical of noob designers, traces are unneccessarily thin (since thereīs tons of unused space) and poor ink protection plus irregular home etching means some tracks are broken or very easy to break just before assembly.

    A lot of words to say itīs unrepairable. *even* if you had the schematic.

    But not all is lost; I *think* (not my area) that a Meter bridge is just a Meter **add on** , generic and able to be somewhat attached to Mixers, maybe just by plugging them into some XLR out?

    So itīs *function* may be replaced by "something which does the same".

    Please answer:
    * do you have any more of these? (Picture)
    * how many?
    * connection details?

    as a side curiosity: what do you need these for? ... your mixer seems to have a Ton of Vu Meters of different kinds and very sophisticated.

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Please answer:
    * do you have any more of these? (Picture)
    * how many?
    * connection details?

    as a side curiosity: what do you need these for? ... your mixer seems to have a Ton of Vu Meters of different kinds and very sophisticated.
    I can shoot more pics if you like. I only have the one control board.

    Power Input: +15V, Com.(0) and -15V
    There are 4 ribbon connectors that connect the logic board to the meters. One each to channels 1-14; 15 - 28; 29- 42 and 43- 56. Power for the actual meters is supplied at +5VDC by an ancillary PSU. The meters are supplied audio by each channel.

    Forgot to mention two additional ribbon connectors coming from the console switches. they're the ones between the caps & resistors in the lower right.

    The original meter bridge was abandoned in about 1984. Long gone.

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    Last edited by dorkjackson; 05-23-2018 at 10:36 PM. Reason: Additional Info

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    In extreme circumstances I've been pushed to drawn up a schematic and determine what voltages or logic states I should expect with this type of job. It's a relatively simple board and it's much easier to visualize if you then mark up the schematic with some baseline voltages and truth-tables for the switching. I don't often have need for one, but one of those old-school logic clips is handy in these situations, at least to see what effect the switching has on the IC pin states. At least you know what the ICs are so you'll know if any pins aren't responding.

    In changing the logic board, you may need to change a lot more - this is a system after all. It's unlikely that you'll find a drop-in replacement and you could be faced with an even greater task in getting it all working and importantly - calibrated. Are those presets on the board for calibration?

    The plus-side of drawing up a schematic means that you could have an improved board made with correctly-sized tracks and pads.

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    Thanks for the reply. I get your point. To answer your question, I'm sure at least one of the 4 presets is for calibration. There are also 2 pots on the board itself. It's an old analog desk which we are restoring, so I haven't much experience with the meters at all. However, sometimes to solve a problem, it must be torn down and rebuilt, and I think you're right about that in this case.

    Th fact that the control board is so "prototyped", is grounds for me to believe that the meters are going to be finicky about what they want to operate them. I think we have to suss out the problem with this board, repair it and build a new one on a more robust chassis.

    Any and all comments / advice welcomed. Thanks again!

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