FSBUS Not working.

[Guest danowar]

I have been searching the forum for the last couple of days looking for an answer to why my newly built FSBUS isn't working.One problem I did find and correct, is that I had all the transistors backwards, except the large one with the heat sink. I am not an electroncs expert and I guess I made the wrong assumption when looking at the component layout drawings. Anyways, that is fixed now.I am getting the "Read after Write" error (big surprise huh :(). When I run the troubleshooter, I am only getting 8.47 volts to the MCLR when the checkbox is checked, and I am getting 8.07 when it is unchecked. Pin 12 and 13 have 5.04 at all times, whether or not the box is checked.Hopefully someone here has a quick answer for me, otherwise if you need more information, let me know.Danowar

[Guest Erups]

Recheck the cable connecting the Com3 card to the serial port.Recheck the solderings, you must not have any shorts.Be sure you are powering the card with 15v or more.Read this post http://forums.avsim.net/dcboard.php?az=sho..._id=13786&page=it is full of "check here, check there".

[Guest lacko]

Hi Danowar,when I see the schematic of FSCOM and thinking about your problem, i quess then your Tr2 & Tr3 are broken. Probably Tr1 also. Try to exchange them with new ones, but take care of the direction of them.May be not this is a reason, but you can try.Regards, Lacko

[Guest GusB]

It's unlikely TR1 TR2 are bad from powering up the FSCOMM card with them backwards. Base to emmiter and base to collector both would look like a p-n junction in the same relative direction, so at worst case you clamped R5 & R6 to ground. With values of 22K for base resistors you can't pull enough current through them to damage anything (although, I personally feel those values are to high if you used low Hfe transitors). The MCLR line sitting at 5V when in "flash mode" is a sure sign that TR1 is either "gone", just not connected or the flash switch isn't wired correctly. Do you know what the voltage on C11 is?? It would indicate if the 555 voltage doubler is still working, even though the 7812 might be blown. Also check the C11 votage without the 7812 in circuit as it "could" load down the output from the doubler.I don't understand why I see recomendations for having the input above the specified 9V. The voltage at C11 should be the input voltage, less a diode drop, plus that same voltage times the duty cycle (DC).V at C11=(Vin-Vdio)+ ((Vin-Vdio)*DC) (Duty cycle is always less than 1)At most, you'll need a minimum of 1-2 volts of overhead for the 7812 to regulate, so I'd expect something around the 15-18 volt range on C11.Hope some of this helps!!!Gus

[Guest Erups]

Every LVR on the market (even low dropout versions) necessitate more voltage to provide the voltage written in their datasheet.So for a 7812 to actually provide 12V, 15V are necessary.Why don't you see such recommendations fitting?

[Guest GusB]

Hi ErupsAll that I'm saying is, that the recommended 9V input should be more than enough to provide the needed voltage for the "voltage doubler", "bootstrap supply" or whatever you want to call it, to supply the 7812 with enough voltage to provide the regulated ~13.4V for flash mode. Circuits similar to this one are very commonly used for driving the high side FET in synchronous buck DC-DC converters, so I don't understand recommendations to use higher input voltages. The one thing that will happen for sure if you increase the specified 9Vin to 12 or more, and that is increased power dissipation in the 7805 & 7812. All that "extra" overhead voltage multiplied by the current being drawn through those regulators will be dissipated as heat.You seem to be very knowledgeable in electronics, but in this very thread you recommend at least a 15V input to the card. That's what I don't understand, as it shouldn't be nessessary, that's the whole purpose of the 555 circuit.RegardsGus