Solid State Music V1B question - test-drb@ccmp.vtda.org

15 Jul 2005

Dwight:

	Thanks. I will add the back-biased diode. Should I also add a small
filter at the inout of the regulator or is that not needed since the
distance between the source point (buss connector) is fairly close to the
regulator?

Rich

-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Dwight K. Elvey
Sent: Friday, July 15, 2005 1:07 PM
To: cctalk at classiccmp.org
Subject: Re: Solid State Music V1B question

Hi
 The typical rail voltage is around 18 volts some place. This
should be on the order of 1/3 watt someplace for the resistor.
The 1/4W resistor is way too small. I'd tend to think that
you might be better with the 220 ohm resistor. The diode of
1W is a maximum of 83 ma. I'd suspect that you only need about
1/10 that much to get reasonable regulation. Even at 15V
input, a 1.8K would be enough. One would need to figure
how much current the video chip needed, before selecting the
right value resistor.
 Since you have one board with a three terminal regulator,
you might add a 10 ohm resistor to the input lead of the
regulator to work as a temporary current sensor. The regulator
draws some current but it is usually small. Add about 50%
to this value and then add 8ma for the diode. From this
you should be able to calculate a reasonable dropping
resistor. Figure a minimum supply of about 15 volts and then
figure the wattage with the 22 volt level.
 Or, as was suggested, go to a 3 terminal regulator for all.
I've seen a number of negative regulators fail, when used
under minimum load. I suspect this is actually caused
by back current when the supply is turned off. Any filtering
capacitors after the regulator may hold longer than the
input filter capacitors. This causes significant back current.
When using a 3 terminal regulators under a minimum load, as
you'll be doing, I suggest that you add a 1N4001, back biased,
...
 from the input to the output lead ( for a positive
regulator, band on the input lead ).
 When working for a company doing embedded machines, we saw
this problem and we had no more returns after adding the
diode.
Dwight

...
 From: "Cini, Richard" <Richard.Cini at
wachovia.com>

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