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First: I've completely reverse engineered both the EW904B as well as the EW904BN programmer cards. Complete circuit diagrams are by now available. Try to find them on this site (or: 1), 2), 3), 3A, 4) ) on the main EPROM page and place them next to this story and in your workshop. So you know what you are doing. The following improvements are also indicated on them. EW904B problems..... The TL497 circuit is susceptible for instability due to unideal ground connections or layout. The circuit as a whole (with power stage) produces by its nature most of the noise that causes instability. It interferes with itself!! If unwanted feedback occurs switching is unstable: i.e. not completely on or off of Q4. The drive at the base of Q4 doesn't get low all the time it should be. Or switching at a too high frequency: a normal on-off cycle is interrupted a few times. This produces a lot of unnecessary heat in 21 or 25 Volt mode. Q4 must be driven in saturation in ON mode (and no glitches) and be completely off in OFF mode. Switching on/off should also be not too slow, that causes even more heat. If you find yourself capable of modifying this circuit place your oscilloscope beside the PC and watch for yourself the peculiarities on this board. Use a test PC with sufficient "open space". If you don't have a scope available to check, don't do anything! Testing takes place at 25V Vpp mode. Tapping an "X" after program start takes you to test mode, you'll see the rest for yourself. We will use 2 scope channels simultaneously. To start set time/division on 50usec. Both probe's are connected to the L's, one on each side. That's on top of the board and the most easiest to reach. It is unsafe to connect ground to the board, it could slip off and ruin your PC!! Safest is to put a solder tag/lug (?) under the board connection screw with a piece of wire soldered to it for ground. Hopefully there is not to much hum/noise between case and ground. One probe gives voltage indication, that's the one connected to the side of the C of Q4: setup 10V per division in DC mode. The second probe is placed between the current limit-R and the L: 1V / div in AC mode. This will give an indication due to voltage drop of the current spikes. If voltage triggering stays unstable, try current triggering, negative going. If the circuit is working properly the current indication gives a triangular current dip downwards, some 100 - 200 usec pause, and again a similar dip. If the dip goes rapidly steeper down near the top: that's ferrite saturation caused by a bit too high current and a rapid reduction of the L value. This getting steeper shouldn't become too high: getting hot coils!! Current level about 0.5 to 1 Vpp; 1Vpp = 1 square. Going down is Q4 on (saturated), voltage at the collector = 0, now loading di/dt. Current curve going up again is Q4 off, discharging coil di/dt via the rectifier in the electrolyt. And charging this capacitor. Voltage pulse on Q4-c is now dependant from the output voltage = 25V. If the charging by di/dt stops, ringing or oscillation may occur, or more serious: a bit bumping up and down before it stops, current could flow through the by-pass diode if already placed. If not you have undesired undershoot. This again is a cause of instability. During the pause between pulses the voltage at Q4-c is equal to the input voltage = 12V. If you measure a nearly continuous square wave at Q4-c the circuit is wrong and Q4 gets hot rapidly. But: nothing has to be defective to be in this mode!!! Only the layout and the component values are causing this error mode!! The circuit could possibly burn out if you leave it in this situation for a longer period of time. See my digital scope printouts (Tektronix 2232 with PC software) for an indication of the problems. What to do about it?? First be very, very careful and very cautious: mind static (put your boots off (NO joke), don't wear static clothing, I've a metal strip at the table edge that is grounded via 1 meg-Ohm. So on purpose grounding is safe for me (too). Don't forget to switch the PC off to insert, or to remove the card. Wait a moment to let voltage drop. Don't spill solder on your SMD components. Cover parts of the inside from your PC with cardboard if you plan to be incautious. And so on......... Making some R's in the 904B equal to the values in the 904BN is a good idea to start with, but wait a moment. First reduce ground related problems and switching noise. Reduce noise on the +12V: increase the value of C5 from 100nF to 4.7 or 10uF tantalum. This gives excellent buffering. You could also solder some electrolytes in the empty places at the card edge, 2 for +5V (C21 and C6) and 1 for +12V (C20). 4.7 or 10uF is OK. Mind plus and minus.... Reduce ground related unstability: solder a short isolated wire (about 3 cm) and not too thin (0.3 to 0.5 Qmm) at the back between the ground of Q4 (emitter) to the ground of (the increased) C5. Measure before that the points you are connecting are already connected and grounded. Indicate these points with a marker pencil! More ground wires could be needed to other points, but this is not tested. Only one appeared sufficient in most cases. (I've tested 3 cards and changed 2 cards EW904B, the other is a 904BN) Test it now already. Watch on the scope for the difference/ improvement for only ONE extra wire from ground to ground .......-.... (and a buffer cap) Going on with soldering..... To protect Q4 from the negative pulses during switch-off and ringing: place a real switch diode in parallel to Q4 to give those currents a by-pass. No undershoot allowed. I took a BYD33G diode, relative fast (250 nsec) and max 400V reverse voltage. BYVxx series or RGPxx series are also OK. I soldered it to test underneath, later I drilled two holes near the emitter and put the diode over there. Connected between e and c of Q4. You can see the small wire ends in the picture above. As I also service TV sets and (PC-)switch supply's for my hobby I have suitable components available. A TV-service shop could supply equivalent devices for a few pennies. To reduce switch-off times it is necessary to reduce the R-be of Q4. R20 = 18K is much too high. A resistor in the range of 330 Ohm to 1K ohm is more appropriate. In the 904BN series 1K is chosen, so I also did here. A standard 50 Hz rectifier is used for D2: a 1N4002 diode. This is by no means a switch rectifier, it's more a slow mains rectifier. Because the power is low and the frequency not very high, it doesn't burn out or even become hot. But if you increase one or both, it could burn. I've tested it. So D2 must be replaced by a better diode, one that is more suited for this task. I've again chosen a TV equivalent: RGP10M. The earlier mentioned types are also OK. The ringing / oscillations are also damped a lot by this improvement. The circuit also gets unstable due to the high frequency of the ringing. It is more stable if the wild oscillations are a bit reduced. Texas Instruments (the builder of the TL497) suggests a 1 Kohm parallel R to the coils. Indeed this helps but consumes too much power. I tested a few R-C series combinations and found a series circuit of a 4.7nF cap. and a 150 Ohm R excellent. No waste of power and a nice (slower) sinusoidal switch off. It is remarkable that the minor above changes have a dramatic stabilising effect on the circuit. Measure it again. If you are not satisfied yet with the result (that ought to be improved a lot already!!!) you could finally give the current limit circuit a bit more "headroom" by reducing the current limit-R. Making it the same as in the 904BN series couldn't be a problem so you could reduce it from 1 Ohm to 0.5 ohm. The same is valid for the drive limit R: reducing this R from 100 to 56 ohm works sometimes OK to reduce false spikes in the Q4 ON period. For testing purposes solder a resistor with the same value on the back in parallel. If you're not satisfied or if it doesn't help you can remove it without any further damage. Those false spikes could also be caused by stray capacitance that disturbs the timing circuit around the timing cap. of 1nF or the circuit traces that are connected to the voltage pot's. I tried to reduce / compensate them by adding a small cap (47pF) from pin 2 to ground or to Vpp, but that didn't work. A second EW904B board didn't need those R reducing, that board worked already perfectly with the other improvements. Probably a TL497 change could help...? Also a TXT file added (here on a web page) with info about the timings and problems (a.o. the A13) related to it. See a separate web page for the possible rebuild of the circuit part for the A13-Vcc switch. (1000H, 2000H, a.s.o. errors) This part is working much too slow. The I/O vector could also be changed: 390Hex is NOT the only address an EW904B could work on. Only after some soldering and patchwork. Also see a page with some remarks related to the newer EW904BN programmer and improvements. Success with the rebuild of your programmer, hopefully nothing gets blown....... Soldering is at your own risk, you know...... ?? Walter Geeraert Vlissingen The Netherlands walterg@cumail.nl P.S. I remember another peculiar thing about the 904B series: the copper conductor trace on the board underneath the top of the PC connection bracket is connected to +5 volt. If the paint cracks you have a short circuit in your PC!!!!!!! PC power supply's are strong.... I noticed. I've seen (in other occasions) more than once fumes coming out of the floppy opening (shit, shit, shit ......-!!!!) So I've placed a small piece of photo film (0.1 mm) between the copper trace and the bracket, that's a lot safer...... { Some fire examples, not always everything goes well: In some cases an electrolyt was blown which was connected reverse: once a Discovery modem card was to blame. Aluminium foil all over the machine..... I once cracked a +5 V wire myself: it was a very thin wire for a LED: plastic isolation molten away everywhere: firework...... Once a tantalum cap exploded, bluegray fumes out of the floppy drive: what a smell........... A switch supply power FET in a PC monitor became "un-well" due to a circuit error: it exploded (300V DC), pieces of it were in the ceiling.... (and not in my face) ow!, ow! } W.G. |
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