Solid State Music V1B question

[Tony Duell]

> 
> All:
> 
>  
> 
>             I'm playing around with the SSM V1B video board in my Altair,
> and I have a question about errata and board modifications, specifically
> with regards to the +12v power supply.
> 
>  
> 
> I have three boards. One is "condition unknown" and two are working. The
> +12v regulated supply consists of a small-value series resistor, a zener
> diode and a cap. One of the boards has a 100 ohm, 1/4w resistor which is
> charred and when operated, continues to smell. The other working board has a
> 220 ohm resistor and the same zener diode and appears to work fine with no
> smell. The manual specifies a 100 ohm resistor and a 1N4742 diode.
> 
>  
> 
> The "condition unknown" board has a 7812 regulator soldered to the pads
> replacing the resistor and the diode. From what I can tell from the
> schematic, the +12v is used only for the character generator chip. When
> turned on, I get a display full of well-formed garbage, which is probably
> just that it's displaying the random data in buffer memory.
> 
>  
> 
> Does anyone have a view on what the best kind of repair might be?


Technically, the best repair is to use the 3-terminal regulator, properly 
heatsinked. I have no idea how much currect the character generator chip 
draws, or indeed what the chip is. But I can't believe it's anything like 
that 1amp rating of the 7812. 

If you want, for historical accuracy, to use the resistor + zener, it's 
worth knowing how to work out the values of those components. 

What you need to realise is 

1) The resistor is what actually drops the voltage. The voltage across it 
is the diffeernce between the supply voltage (nominally 16V on an S100 
machine, but it could be highetr) and the load voltage (here, 12V)

2) In order to drop that voltage, the resistor needs to pass a certain 
current. Part of that current goes through the load (the character 
generator chip). The rest goes through the zener diode. The zener current 
will vary with the supply voltage (obviously, the higher the supply, the 
more voltage needs to be dropped across the resistor, therefore more 
current needs to be flowing through the resistor). 

3) The zenre currnet also varies with the load current. Since, for a 
given supply voltage, the current through the resistor is constant, the 
zener current must increase if the load current decreases.


How to calculatr things .

1) Take the worst case of minimum input voltage and maximum load current. 
Add one a suitable zener current (say 10mA) to get the current through 
the resistor. Calculator the voltage drop accross the resistor (diffeence 
between minimum input voltage and load voltage). Calculator the resistor 
value. Go down to the next lowest prefered value if you have to.

2) Now take the maximum possible input voltage and re-calculate the 
voltage drop across the resistor. Calculate the power dissipated in the 
resistor under those conditions (power = (voltage drop)^2 / R). Use a 
resistor of at least that power rating.

3) For the same (highest) input voltage, calculate the current through 
the resistor. Now assume that all that current goes through the zener, 
that the load is drawing nothing. Calculate the power dissipated in the 
zener (remember the voltage across it is the wanted load voltage, here 
12V). Make sure you use a zener that can dissipate that amount of power.

Let's work through an example. Suppose the character generator draws 
50mA, and that the input voltage can vary between 14V and 20V

1) Minimum voltage drop across the resistor = 14V-12V = 2V
   Current = 50mA + 10mA (zener current) = 60mA = 0.06A

   Resistor = 2/0/06 = 33.33 Ohms. Use a 33 Ohm resistor

2) Maximum voltage drop across the resistor = 20-12 = 8V
  Power = 8*8/33 = 1.94W. Use a 2W resistor.

3) Currnet through the resistor under those conditions = V/R = 8/33 = 0.242A
   Assume all that passes through the zener, which therefore dissipates 
   0.242*12 = 2.91W. That's a big zener!

>From the values you've given, I think I've rather overestimated the load 
current, but you get the idea

-tony