SV: Re: [sdiy] repairing a BBD delay

[Bob Weigel]

SOme scopes have a ground lift switch.  My old HP has a ground lug with 
a metal plate I can swing over to the differential low lug :-).  But 
possibly just using a 2 to 3 pin power adapter (cheater as they are 
often called) is the answer.  That way there is no shield ground 
hopefully unless it's broken or mal-designed and has some grounding of 
the neutral which should never be because 120V could be inverted by a 
hack electrician  putting 120V on the case!  Always watch that.  Never 
assume :-).  So best thing to be safe is take your meter, set it for 
200V scale AC, and get a good ground on one lead.  Take the other and 
verify that you can put it in a socket and get 120V.  Then ground lift 
your scope with the adaptor and measure to make sure no significant AC 
voltage is riding on the case.  Sometimes the 'atmosphere' in the area 
will put 80V even on it..but if it's 120V you know you have something 
bleeding.

Know thy equipment before getting started basically.  Then find a good 
massive capacitor and crobar it with a screwdriver er...hehe..no just 
kidding ...  (Harry and I have an ongoing joke about this..) But 
seriously once you know the case isn't grounded anymore (check impedence 
with the outlet ground still hooked ot one probe, to the case of the 
scope or a bnc connector shield or whatever.   If it's high 
impedence/off scale then the scope isn't grounded)  -Bob

Karl Ekdahl wrote:

>Hmm... i figure, the MN3101 as well as the MN3005 are
>using this weird scheme where VDD goes to GND and GND
>goes to +15. It's as in the example schematic in the
>datasheet for the MN3005 (couldn't they just have
>renamed GND +15 and VDD GND?).
>
>Anyway, how do i lift my scope from ground? do i have
>to get a non-grounded power cable or something, sounds
>like danger.. (?)
>
>Karl
>
>--- Bob Weigel <sounddoctorin at imt.net> skrev:
>
>  
>
>>Yeah the 3101 should have a clock output.  Make sure
>>the ground pin is 
>>good.  Sounds like maybe it's lost the ground and
>>the whole chip is 
>>floating.  Hmm..wait on the diagram..that's right
>>pin 1 is ground and 
>>pin 3 is Vdd (-15V)
>>
>>Sooo.  you are not measuring from that reference
>>frame I assume or it's 
>>in a design where everything is cap isolated and
>>they are using ground 
>>on pin 3 and +15V on ground.  Anyway measure with a
>>ground lifted scope 
>>so you don't short anything and put the ground then
>>on pin 1 and see if 
>>any signal is out of 2 and 4.  First. I have some of
>>those but they're 
>>easy to get compared to the bbd's themselves.  -Bob
>>
>>Karl Ekdahl wrote:
>>
>>    
>>
>>>Hi list, i've got an old delay that has stopped
>>>working. Inside i found a MN3005 driven by a
>>>      
>>>
>>MN3101. 
>>    
>>
>>>I'm thinking that the MN3101 might be busted as i
>>>cannot see any output signals at all on the scope
>>>      
>>>
>>from
>>    
>>
>>>the MN3101, using a DMM all the pins but pin 3 is
>>>      
>>>
>>at
>>    
>>
>>>15V or 14.5V. 
>>>
>>>How do i debug this, should there be a "seeable"
>>>signal at pin 2 & 6 that is fed to the BBD or does
>>>      
>>>
>>the
>>    
>>
>>>MN3005 eat the whole clock signal? Should i be able
>>>      
>>>
>>to
>>    
>>
>>>get a oscillator signal on pins 5,6,7 - i'm
>>>      
>>>
>>thinking
>>    
>>
>>>that perhaps the surrounding components on this end
>>>might be busted?
>>>
>>>There is sound coming into the MN3005 at pin 7 as
>>>should be, but there's nothing else coming in or
>>>      
>>>
>>out
>>    
>>
>>>of that chip.
>>>
>>>Any help greatly appreciated!
>>>
>>>Karl
>>>
>>>
>>>	
>>>	
>>>		
>>>      
>>>
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>>
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>>>
>>>      
>>>
>
>
>
>	
>	
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