Solid State Music V1B question

[Dwight K. Elvey]

Hi Richard
 You most likely won't have any troubles but it is
recommended to have a 1uF to 10uF cap at the input lead.
These things can oscillate in the 100KHz to 1MHz range.
I remember trouble shooting a burn-in board that
was running about 20 amperes and going on and off
at about 900 KHz. I was surprised that they'd not
gotten a visit from the FCC. I guess the antenna was
not that effective. I didn't have a radio handy to
try but I suspect it would have been noticeable
at close range. ( the voltage was 15Volts with about
100 regulators )
Dwight

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