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x505 lamp rail

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#1 dan

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Posted 27 May 2019 - 02:21 PM

i accidentally shorted out the X505 lamp rail because i had my multi meter in the incorrect position, i wanted to measure volts but i had forgot it in mA mode. i heard a tiny pop and saw some smoke coming from the main board. i shut it off right away.


the lamp rail still puts out 12v, the cassette deck still seems to work and the MMS gets power. what the heck did i blow?

#2 Transistorized

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Posted 27 May 2019 - 03:16 PM

Any box that releases the mysterious blue smoke and still works....deserves to be put back together and enjoyed :lol:

Why were you measuring voltage for the lamp? Was it burnt out? The first thing I would check would be the regulator for that circuit but if its putting out 12V you were probably spared in that department. A pop and smoke could very well be a capacitor/resistor.

The only part of a circuit that I have ever seen smoke and not inadvertently cause a noticeable change in performance is a resistor and occasionally a capacitor. Make no mistake about it, the tolerances in that circuit have changed despite noticeable performance degradation.

I was really hoping you had reached the stage of enjoyment with this thing. Analogalley.com has an led kit that can enhance the boxes appearance and works well if your lighting isn't working.

I know it will bother you as to what smoked in there but I'm afraid you'll never get to enjoy this thing. If it's working, slap it together and rock that thing :-)

#3 dan

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Posted 27 May 2019 - 04:39 PM

i had my lighting circuit  and my regulator for my BT connected and i wanted to measure if there was any drop in voltage from the regulator. i should have just left it alone. i pulled out the x505, no physical signs of damage, still comes out as a NPN transistor in the component tester. i wonder, does zener d505 have a glass case? i wonder if it popped and that's what made the sound? i remember last time i smelled this smell, i cooked a zener once. but it could also be a ca or resistor getting hot. i tested the caps nearby in the circuit diagram and their ESR was fine.  the only thing i notice, and i'm not sure this was the case before because i never checked,  x505 kinda gets hot while playing a cassette, mms on, and about 70mA from my led/BT circuit. if you hold your finger on the metal back of the x505, it starts to hurt in about 5 seconds. i heard a rule for silicon transistors is that if you can't hold on to the plastic part of the package for at least 3 seconds, it's running too hot.  i don't know. it the heat normal in this case?


P.S, this was supposed to be done last week. i had an issue though. i employed npn switching to control my light control system and when when i hooked up my BT and switched the npn off, the power fed back though the audio ground and cooked my BT module. it took me 2 modules to figure out what was happening. the circuit worked great until i hooked up the audio. lol.


i redesigned my light control circuit and now i am using pnp transistors so the switching is done on the high side so the ground is always connected which should prevent the current from feeding back through the audio ground of the BT module and cooking it.


i rigged up a system where there is only 1 button, the dial lamp button.  initially lights are off. 1st press, dial is illuminated white with vu meters white. 2nd press, vu meters are white, left side of the dial is white, right side of the dial is blue, 5v regulator turns on and bluetooth is on. 3rd press goes back the the default position with lights and bluetooth off.


just waiting for my other BT modules to arrive. the whole circuit i made should use under 90mA off the X505 rail.

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#4 Fatdog

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Posted 27 May 2019 - 06:25 PM

Having the dial light switch to blue when Bluetooth is on is a nice idea.  :cool: :yes:

#5 dan

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Posted 07 June 2019 - 08:40 PM




update, i think it must have been R517 2.2 ohm that got hot and made smoke.  i checked its resistance and it measures about twice the resistance.  it seems like the purpose of R517 is a current limiting resistor. at 15 volts, it would limit the current to a theoretical 6.8 amps and would dissipate a theoretical 102 watts of power! (R517 is rated for 1 watt), that is if the transistor x505 doesn't burn up first or a trace melts, but the M70 has a fuse and the most power that could be dissipated by the resistor if all the power available went through that resistor is around 47 watts.


now the question is, with R517 having twice the value now, does it effect performance in any way? i know x505 isn't rated for 6 amps but i wonder if for short durations it can let such current pass? like how a 5 watt resistor can tolerate hundreds of watts of power for short bursts without damage. with twice the resistance on R517, only half the theoretical current can pass now.  does x505 power the amp?  i wonder if R517 is effecting the sound?


as far as i can tell from the schematic,  x505 only powers the MMS board, autostop board, the lamp and the power led.