Acorn Products Z80 SECOND PROCESSSOR 409 用户手册

address lines are shorted together and that all data lines are 
operating and not tied together.

6.3.9 Power Supply

Check the 250 mA. type T mains fuse, accessible at the rear of the 
unit (see section 2).

Check for any loose, disconnected or broken leads.

After making sure that the second processor is disconnected from the 

mains supply, check the mains switch at the rear of the unit.

Overload protection of the second processor is provided on the second 

processor Z80 board itself. Fuse FS1 protects against overcurrent and 

thyristor Till protects against overvoltage by blowing the fuse.

The overvoltage protection circuit functions as follows:

D3  is  a  5.1V  Zener  diode;  rail  voltages  greater  than  5.1V  appear 

across  resistor  R11  whilst  spikes  are  absorbed  by  capacitor  Cll.  If 

the rail voltage across R11 exceeds approximately 1V, thyristor TH1, 

which  is  capable  of  drawing  8  amps.,  conducts  and  blows  the  1  amp. 

fast-blow fuse FS1. Supply rail cut-off is therefore achieved if the 

voltage reaches approximately 6.1V.

If  fuse  FS1  is  blown,  it  could  be  due  to  either  overvoltage  or 

overcurrent  and  there  is  likely  to  be  either  a  short  circuit 

somewhere  or  the  power  supply  board  is  faulty  (it  is  supplying  too 

high a voltage).

Disconnect  the  two  power  supply  leads,  brown  and  black,  from  the 

second processor PCB and connect a 10 ohm 2.5W resistor between them. 

Measure the voltage across the brown lead (+5V) and the black lead (

ground) which should be in the range 4.95 to 5.25V with a maximum of 

50mV noise (peak-to-peak, 0-50MPz bandwidth). If the voltage is out 

of  spec.  then  set  it  to  5V  exactly  using  the  trimmer  which  is 

accessible through a hole in the power supply board. See Fig. 9.

If 5v cannot be obtained and/or the noise level is out of spec., 

replace the power supply unit.

Now remove the resistor connected across the power supply leads and 

reconnect them to the second processor PCB, ensuring correct polarity. 

Test  the  current  drawn  by  the  second  processor  PCB  from  the  +5V 

supply. The board should draw 600 - 800mA from the power supply.