80-Bus News

Tape Motor switching

For some while there has been a lack of urgency in the switching on and off of the tape recorder under firmware control. Having tried various commercial and homemade interfaces, it was realised that under the R and W command (also CLOAD and CSAVE), LED2 was illuminated (quick, this fellow – eh?). A stirring of the grey matter brought to mind the opto-isolator, which after all requires no more driving than a normal LED. See circuit 1. The internal transistor is effectively switched on by the illumination of the internal LED It was decided that transistor output was not on as large currents occur in the motor drive, particularly at switch on and off; proven by the number of fatalities of BFY51’s having meant bulk purchases in the past. Thus the final motor control was chosen to be electromechanical, under the drive of the BFY51. The whole exercise is heavily over engineered, but this is to ensure a long and healthy life.

PSU Droop

One of the problems of the Nascom 3A power supply, or at least mine, is that the no load output of the +5 volt rail is a nominal +5 volts, but under load drops to +4.8 volts. The first steps in attempting to raise the output voltage, and also providing a 5 amp output, was to replace the LM323 (IC1 on the PSU circuit diagram) with a 78H05 5 volt 5 amp regulator. This was attached to a very large heat sink remote from the board, and connected with very thick cables. To further improve the current handling capacity of the supply, the 9 volt winding on the mains transformer was dispensed with and a new 12 volt 50 VA transformer used for the 5 volt rail instead. The current capacity was now there, but the supply resolutely remained at 4.8 volts.

A resistor, in this case 56R was introduced into the earth lead of the regulator, and had the desired affect of raising the voltage to + 5 volts under load. See circuit 2. (Note: that inserting a resistor this way is quite legit, but as the case of the regulator is the earthy connection it means that the regulator must be isolated from the heat sink, or the heat sink must be isolated from system ground (0 volts). –DRH.) A word of caution though. The exact effect on the voltage of inserting a resistor is difficult to predict, even if you are good at sums, as the final value is device dependant. It is best to disconnect the computer and replace it with a dummy load whilst selecting the appropriate resistor, as life can become expensive if the voltage is allowed to rise above 5.25 volts (7 volts is absolute max. according to the book, but I agree with the precaution. –DRH). Certainly there has been an improvement in the video quality, probably caused by the rumoured increase in speed of TTL at the correct operating voltage (not of course the computer itself, as this is crystal controlled). I thoroughly recommend the use of silicon based heat sink compound between the heatsink and the regulator.

Backplane mod.

Finally, there appears to be some mileage in ‘flow-soldering’ the power tracks of the Vero backplane. Not a new trick, but it does make the whole thing more solid. Mind you it does take some time to avoid indvertently bridging tracks.

No claims for originality are made for these modifications, particularly the last two, but they might help someone out of a hole!!