[volt-nuts] Improving Solartron 7071 and 7081 (m k)

David C. Partridge david.partridge at perdrix.co.uk
Sat Feb 2 06:18:49 EST 2013

I suspect probe ground lead inductance in retrospect.  

As for the RF burst it matches the clock and I can pick it up with a probe with the earth lead connected to the probe tip. Pass it near the ribbon cable connecting the earthy processor board to the lower earthy logic board, or near the 74LS TTL chips on the erathy processor board and you will see it fine. 

David Partridge
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of Bill Ezell
Sent: 01 February 2013 21:49
To: volt-nuts at febo.com
Subject: Re: [volt-nuts] Improving Solartron 7071 and 7081 (m k)

>   As far as I can determine this appears to be caused by a big spike as IC403 (74LS02 on the earthy processor board) switches.  You can see the spike at TP 3 on the earthy processor board.  This spike goes  below ground by about 1.4V every 0.66uS.

This is interesting. I don't see this at all. I just probed the tp and it's a very clean TTL signal, no undershoot at all. I'm using a 300 Mhz scope (Tek 2465A). Not only is the tp clean, the +5 is also clean. I'm using the ground point very near the tp.

Silly question - have you checked your probe compensation?

There is a very small rf burst every 0.66 uS I can see on the floating board, but it's not on the digital power line, I checked that on one of the ICs on the floating board. I'm seeing about 20 mv bursts at the AC buffer test point you mentioned before, the burst is quite short, and the burst frequency is much higher than any of the clocks in the meter, so something is ringing, but it's not getting coupled out very far. I'll probe around some more.

Bill Ezell
They said 'Windows or better'
so I used Linux.

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