Back to main EPROM page|
First: circuit part round Q3 / 7805 modifications Put in readiness in front of you: All the schematics to understand what's going on, especially the A13 mod circuit diagram, if possible some images taken from the link on the main page. 270 Ohm 1/3 Watt 100 Ohm 1/4 watt 1N4148 small signal diode 1N4002 medium signal diode 100 nF miniature ceramic capacitor Watch the solder side of the PCB. Remark: The extra ground wire to the GND of C5 (now a tantalum C) for the TL497 adaptation could be in the way. Loosen if required and fix again later. To the same GND point of C5 (on the solder side) the 1N4148 will be connected (the diode side with the "bar"). The other end is to the ground plane of Q3 (=7805). In parallel the 100 Ohm R is soldered. If possible use some shrink tubing to isolate the R. Leave the ground wire to the emitter of Q4 loose for now. Turn board upside down. Component side is facing to you now. In the same line as the pins of Q2 and Q3 an extra diode ( 1N4002) will be placed on the component side. Drill extra holes. On the solder side: connect it in the output trace of Q3, cut trace between the two connection points. Component side: In the open space between Q2 and Q3 the R of 270 Ohm will be placed: drill extra holes. One side will be connected to the output trace of Q3, the other end somewhere to the ground plane underneath Q3. Connect ground wire to Q4 to common ground point again, the two extra components (R + D) should stay in position. Component side: small ground trace between ground of Q3 (centre point) and GND of C5 must be cut (very small) just above the printed character G on the PCB (= near ground of Q3). Scratch off the paint on the piece near (and to) C5. Place the mini ceramic 100nF C between this trace and the output pin of Q3. The removal of the old A13 control components. A heated solder suction or solder removal device is advised. If not you could ruin your PCB! To be removed: Q7 - NPN transistor D1 - small signal diode R11 - 1Kohm R12 - 330 Ohm R13 - 1Kohm Also remove the solder in the feed through hole in the +5 V line to R12. Place ready in front of you: 100 Ohm for position R11 470 Ohm for position R13 180 Ohm placed on solder side (later on) 22 Ohm BAV21 22 R and BAV21 in series on position D1 180 nF (or 150 nF + 33 nF) both encapsulated versions - totally isolated capacitors - mounted on the solder side in parallel to the 180 Ohm. Solder them : R11, (100 Ohm), R13 (470 Ohm), position D1 BAV21 and 22 Ohm in series (lift them a little in the middle, let them form an angle, R going up, D going down). Diode arrow (bar) towards the +5 point. DO NOT solder the 180 nF and 180 Ohm yet. This will be done later at the solder side. Preparations to the MAX626 circuit. Please see the images, if available. The IC will be mounted on the component side, but upside down in "dead beetle" method, with shortened pins UP. This uses less space. Put thin isolated transformer (winding) wire (0.3 mm) to pins 2 and 4 with a small bend around pin 3. Scratch the paint beforehand and tin where it will be soldered. Put a piece of isolated assembly wire (telephone quality - 0.4 to 0.5 mm) to pin 2, going up (= to the top side of MAX626) , about 2 cm long and cut. Put thin winding wire to pins 5 and 7, then going up for a few centimetres and cut. Only tinned where it will be soldered. Place, between the pins 3 and 6, a tantalum capacitor of 4.7 uF and a 100nF ceramic capacitor in parallel. Solder to pin 3 (minus) and pin 6 (plus). If fixed, solder to both pin 3 and pin 6 a 2 cm isolated wire, going to the top side again. From the isolated wire pieces: partially remove the isolation, leave 8mm to 1 cm isolated. Tin if needed beforehand. Winding wire: leave about 1 cm isolated, tin the rest. Adjust all 4 wires and put the circuit upside down on the component side of the board. IC marker indication to board-bracket. Put the wires in the following holes: Pin 2 - input - wire to old base pad Pin 3 - ground - wire to old emitter pad Pin 6 - +5 - wire to +5 feed through hole or +5 hole a bit further away Pin 7 - output - winding wire with a small bend to the old collector hole or old R12 hole near collector. Press everything very cautious in place, bend/rotate a little to adjust, and fix with solder. If upside down pins of the MAX626 look to long, cut them a bit. Only now: solder over the old D1 connection points, on the solder side on the back the 180 Ohm R and the 180 nF (set) of capacitors in parallel with each other. This again is in parallel with the series circuit of the 22 Ohm and BAV21. Open the ZIF socket unit and look for an open space where a resistor was planned. This is from the pin 26 line to ground. Check with an Ohm meter! An undershoot protection diode preferably a schottky type: BAT41 must be soldered there. Bar mark to pin 26. Check everything thoroughly with the schematic, and be very sure it's OK. That's all to improve the A13 circuit. If not done all ready: change the Vpp pull-down R's : make them a little lower to speed-up the circuit. See my schematics above for where and how. To correct the maximum voltage again: add some pull up R's to compensate. It is an obligation that the TL497 changes have been added beforehand to make this part more stable, or else you come into serious trouble with your Vpp voltage supply. Due to more power drain in 21 and 25V mode. Originally this circuit part is unstable due to a wrong ground loop. Walter Geeraert Vlissingen The Netherlands |
Copyright© 1990 - 2024 Walter Geeraert - All Rights Reserved
|
Back to main EPROM page |