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Radio problems



  • Really, thanks a lot! Good info in your post. I was thinking about the voltage regs, particularly the LM318, and a pass transistor. I think, counting all the 194 bulbs behind the instrument panel (2), and all the microbulbs (at 60 mA a piece), the regulator should be able to handle 2 amps or so. That means, the bare 318 will be toast in no time (only 1.5 amps). Also, the problem of going this way is that you get a 1.5 volt drop with any of these regulators, so you usually need 15v to get a nice 12v...

    In any case, I think that I can control the duty cycle of a 3055 (which handles 90 watts...) with a 555 timmer. If I etch (spelling?) a board that fits the board on the dimmer switch, I could easily fit everything on the current switch housing, and even use it's nice little pot to control the triggering of the 555 (varying the RC constant...).

    In any case, what you said about the illumination lead of the radio being able to handle anything from 0 to +BATT gives me more confidence on what I'm doing. Personally, I think that the guy who owned the car before had it installed and they botched the installation (thus the burnt dimmer in the first place).

    Well, it will be cheaper (10 bucks for all parts), but it'll take a while. In any case, all this talk brings back good memories. I will have to find where the heck I stored that protoboard I had...

  • swschradswschrad Posts: 2,171
    yeah, by all means... what's the dash fuse, something like 10 or 15 amps? might want to use a pair of 3055s with some half-ohm balance resistors series on the emitters. I whipped up a little pass-transistor train speed control in HS, and with an amp draw, that tranny (I think it was a TR-02, universal 2n554 replacement) lasted three minutes. so you will want to find a place to stash a remote heat sink if you have appreciable drain, and I would expect at least 5 amps now that you caught me in a contradiction.

    as for the botched install... truth be told, there are a lot of "professional installers" that secretly hope to be hired as the new "rally monkey" if the present one falls off the scoreboard. I had a dealer guy futz around ordering the wrong radio for my car by using acid flux and eighth-inch solder to tack-solder harness wires directly to a different one in the mid-70s, and had an installer sorta hack up the radio cutout expanding it one millimeter to replace a wimpy factory radio with an "exact fit" sony in 96.

    have fun breadboarding! -- I'm almost done restoring an old Philco 65 chassis in a 155 (?) table case for something different.
  • I got all the stuff and made a little protoboard. It works fine with a 0.25 amp load (a single 194 bulb...). Now I have to bring all this mess down to the car and try it out (this weekend's project).

    As for multiple transistor pass gates (that's how I called them), oh boy, I rather stay away from them. One of the instruments I maintained was an old NMR (the research version or the MRI used in medicine). It had a controlled 100v, 100 amp power supply (with a ripple of less than 1 ppm), and it had a pass gate with 16 big'ol transistors (those that bolted to the heatsink, which was watercooled in my case). I remember one blew, and we had to replace ALL of them because we could not get them balanced - One always drew more current and invariably burnt. In any case, a single 3055 (or equivalent) should handle 3 to 5 amps at 12 volts. That 'should' be plenty...

    As for the botched install, I actually took the radio out and there was nothing wrong with the harnesses (that I could tell). So my guess is that this Mazda radio is like some aftermarket radios I've seen which don't dimm the lights with the dimmer switch. Some even have their own dimming mechanism. The only way to find out would be to move my car's radio to the wife's car, but she's not thrilled about that.

    Thanks for all the help and ideas. I will give an update when I test it on the car...

  • swschradswschrad Posts: 2,171
    you ever see the power supply for one of the old Varian medical accelerators, the ones that occasionally "went wild" according to 60 minutes? looked like the high-power section of a 5 KW radio transmitter, including two 3cx2500 tubes (think 5-gallon water jugs with plate caps and glass chimneys.)

    I agree, balancing transistors is the pits, you should shotgun all the emitter resistors as well when you get after a heat sink full. it does matter more if you are putting out big-[non-permissible content removed] pulses than if you are not running several hundred audio watts most of the time.
  • Wow - The NMR I was talking about was also Varian! The power supply, which powered a big magnet, consisted of a transformer that weighed in at ~ 200 pounds, and a big rectification circuit. This was three SCRs (the big ones, bolted to the same watercooled heatsink), each of which was itself triggered by another, smaller, SCR. The three phases rectified were combined, then passed through a bank of capacitors (each one was a 'quart' in size, and had screws to hold them to the boards), and this went to the transistor pass gate, which was controlled by the feedback from the magnet (to keep the magnetic field steady).

    I remember one time one of the phases was not triggering properly and I started mucking around with a scope. I accidentally shorted one of the large, 50 AMP, SCRs, and the thing blew up with a huge bang and sparks. I had little bits of SCR casing impaled in my arms (it was like glass shreds - no fun...). It was then than I started understanding what people meant when they said that someone 'was so scared that he had sh*t his pants' (you make your own conclusions...).

    Anyway, too much nostalgia. The circuit works fine with the load from the dashboard lights. The 3055 does not even get warm, so I think the actually load is much less than what I imagimed (anyway, I learned to design with a worst-case scenario approach). I tried today with the car. I don't think I can drive around with the cables coming off the dashboard and the protoboard floating there, so this week I will have to remember the old skill of etching PCB board. More later.

  • swschradswschrad Posts: 2,171
    varian did a lot of medical equipment in the 1980s, the whole scanning tomography field was wide open and everybody dove in. the x-ray giants were well positioned, of course, and varian built some of the best high voltage, high-frequency, high current transmitting tubes and components, so it was to be expected they would buy a few smaller outfits and go there. I should also say I screwed up in describing that power supply, 3cx series tubes are ceramic... this used the 3-2500Z series, big honking glass bottles about halfway between the size of washer fluid gallon jugs and 5-gallon water jugs. two of those with sockets and chimneys cost what a chevy nova did.

    back to dashboard controls... yeah, there is a lot less power loss (and heat) to take care of when you use a switched power supply to cut the voltage down into a load, so that transistor will do fine.

    if you can fix those money-suckers, you can probably even figure out how to light up a bad front panel on the current crop of car radios :-D -- that's a real load of one-off displays from Hell ;)
  • I have blown the stock subwoofer under the drivers seat of my 1997 Subaru GT wagon. Does anyone know where to get a new speaker, or know what size the stock woofer is? I contacted Crutchfield, but they were unable to help. Thanks.
  • Could you just leave it like it is? (:o]
  • swschradswschrad Posts: 2,171
    you can always go to dealer parts. as an alternative, get to the speaker, measure its depth from the back of the front panel, its width across the mounting plate, and start looking for replacements with that data.
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