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A quick scope view to a EW904BN board that I got in hands showed that it is working properly, no ground loop related problems in the TL497 circuit like in the EW904B version. Thanks to the redesign of the ground traces no instability. See the EW904B web page for more explanation and info for this part of the programmer. Advised reading... But........ The safety by-pass diode of the power transistor TIP31C is a small signal one: 1N4148. That is not the way it should be: I replaced it by a safer one: the BYD33G. Replacing C16 - 100nF with a tantalum cap of 4.7 or 10uF is another good idea: more local buffering. One more hint: replacing the 1N4002 (ugh) with a RGP10M : again reduced the ringing pulses during switch-off remarkably. You could finalise the TL497 part with a tiny ringing damping network across the L's: 4.7 nF in series with 150 Ohm is splendid. See for the final result a picture taken from a memory scope. Other problems found in the 904BN card: Diode D5, one of the protection diodes on the PIO (8255) databus (bit-d2) to the EPROM socket is NOT connected to +5V like the other diodes, but to EPROM-Vcc (5 - 6.5V). This diode should protect the PIO against too high Vcc (>5V) and should be connected to +5V to limit spikes. During test mode you could measure on all but one data channel 0V or +5V, only one (databit d2) is only 2.5 - 2.8V high. It looks like this PIO channel is blown, but it is NOT. The current is only leaking away because the circuit is wrong. This is corrected. I disconnected the + side of the diode and lifted it. I soldered a piece (1.5 x 3 mm) of double clad board on top of the original connection point. The diode is soldered on top, it is now fixed in position and isolated from the former connection point. A piece of PTFE wirewrap to normal Vcc: problem solved. Replacing all the diodes by schottky BAT41 types (0.3V drop) gives even better protection. After rebuilding the A13 circuit (among others) in the former 904B programmer (solving the 1000H, 2000H errors in a 486DX-2 PC, A13 timing now being much less than 1 usec), I checked the performance of the A13 circuit in the 904BN version. I did this using my ZIF socket adapter from the 904B version (I only have the board, it was sold second hand as it was for $5,-- A must have for me!!). This socket module is not suited to program EPROM's, some /CE and /OE pins are different. But I presume that the Vcc2 -A13 capacitors are almost identical on this line to pin 26. Going down/ fall-time is OK (< 5usec) but a very wrong dU/dt curve going up / risetime about 300 usec. That looks incredible bad. Are the capacitors really the same (4x 10nF)? Are the 1000H, 2000H errors therefore also in this programmer ..-...??? This part doesn't look full of promise. Due to the already improved circuit design only an extra pull-up resistor (470 to 220 Ohm) on the right spot solved this risetime problem for a great deal. Risetime is now about 25 to 15 usec, which is acceptable for a slow machine only. The R is connected between pin 10 and pin 14 of U3 = 7407. In the 386DX40 the A13 rise time is only stabilised AFTER the /OE (not /CE) read pulse, so this is still NOT OK. Modified with a Maxim driver MAX 627 or MAX 628 (non inverting part) in place of the last 7407 buffer (glued on top of it), would make risetimes << 1 usec. I haven't tested this yet. Due to a lack of the proper (different) MAXIM buffers. Will be continued. I also changed the Vcc to Vpp clamp diode by a schottky version: BAT41. Normal Vcc without Vpp went up from 4.3 to 4.6 volts, looks much better. Vpp to Vcc timings were tricky but OK, smaller than 10 usec. In some programming algorithms Vpp stayed on! The Vpp pots were hard to adjust. They appeared to have the wrong value for proper smooth adjustment: 2Kohm. Calculated setting for all 4 pots is about 500 ohms, so a value of 1K is more appropriate. I drilled an extra hole between the top and bottom hole and placed 1K multiturn trimpots. Smooth adjustment as a result ....-.... Also changed the setting R's by metalfilm types: more stable and less temperature dependant voltage. The changed setting R's are: R3 - 10K, R34 - 100, R35 - 470, R36 - 22, R37 - 470 Ohms. Success with the rebuild of your EW904BN programmer. Here a quick view on the board with all the above corrections. Walter Geeraert Vlissingen The Netherlands walterg@cumail.nl |
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