Solid State Music V1B question

[Cini, Richard]

The 1N4742 is a 1w part which at 12 volts is well within the amperage
ratings of a 7812.

That's the way I'm going to go. Thanks.


-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Allison
Sent: Friday, July 15, 2005 8:55 AM
To: cctalk at classiccmp.org
Subject: Re: Solid State Music V1B question

>
>Subject: Solid State Music V1B question
>   From: "Cini, Richard" <Richard.Cini at wachovia.com>
>   Date: Fri, 15 Jul 2005 08:39:42 -0400
>     To: "'General Discussion: On-Topic and Off-Topic  Posts'"
> <cctalk at classiccmp.org>
>
>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?
>
> 
>
>            Thanks.
>

Rich,

The best fix is the three terminal regulator.  The resistor and zener 
diode works well but the correct value of resistor is dependent on the 
raw 12+ volt power on the S100 buss.  Since I've seen that bus run from 
barely 15V (marginal for three terminal regulator) to as high as 22V
the right resistor varies greatly. I'd speculate (I don't know which 
character rom) on the required 12V power needs that the 100ohm is 
plainly too low and maybe 220 is right.  If the resistor is too low 
the Zener diode will overheat and if too high the regulation is poor.
Zener diodes have power ratings like many parts and if exceeded they
fail.  Using a three terminal regulator eliminates a lot of guessing.


Allison