[volt-nuts] Fluke 5205a Repair

Chuck Harris cfharris at erols.com
Tue Sep 29 16:06:16 EDT 2015

Hi David,

My eye keeps wandering over to U40, U43, and that abortion of
tantalum capacitors, and snubber networks hung on the inputs to
U40, U1 and U2.

Whenever I see stuff like that, I know the engineer was getting
frantic over being toasted by glitches.  FF clocks just aren't
at all amused when they see a glitch come on their clock, or
preset/reset lines.

Also, old tantalums have a habit of becoming noise sources as
they short, and unshort bits and pieces of their oxide layer.

I have seen a few that tested ok, except that their leakage current
kept spiking and returning to normal. You might not notice that
sort of behavior on a power supply bypass, but it is bound to
raise an eyebrow when it happens on a logic input line, or
an integrator (not this circuit, I know).

Capacitors hung on logic lines are always a bandaid fix.

-Chuck Harris

David Garrido wrote:
> Hello All,
> I am making a little more headway on this issue of the OVERLOAD & FAULT led’s
> being triggered and shutting down the system.
> In paragraph 3-34 on page 3-4 of the manual it states:
> Once U2-3 has been pulled low and a 3-9ms delay, the Operate FF J Input, U41-4 is
> supposed to go high.  And it does, BUT the next thing in the line is a short delay
> caused by R121 & C43 and that signal showing up at U41-1 should be low.  It is NOT
> and neither is the Q Output (U41-14).  So I thought ok GREAT, only pin 4 should be
> high and both of the others must be low.  Easy fix, plop in another SN74106N and I
> should be GTG.
> Not so much………..still the same issue after warm-up, the OVERLOAD led comes on,
> followed very shortly by the FAULT led.  Now I checked U3-9, it is low and U3-8
> high.  Next was U4-9 and it was high and pin 8 was low.  On to U40-9, it is low
> and pin 8 is high which is what was pushing U41-1 high.
> So I went and checked R121, she measured 389 ohms and C43, a 4.7uf TA, measured
> 5.1 with 1.2 ESR.  That all seems good.
> BUT……………….
> So I decided to try putting the amp into the OPERATE mode and see if that changed
> the input to U41-1as it is supposed to follow the low from U3-8, it did.  Yes,
> U3-8 is now 100mV along with U3-9.  U4-9 is low and U4-8 is high.  U40-9 is high
> and 8 is low.  Now the signal at U41-4 is high as before, The signal at U41-1 is
> low as expected, BUT………..the signal at U41-14 did not go low following the low at
> U41-1.  Hmmmmmmmmmmm
> Scratching my head………..
> In summary in the standby mode:
> U3-9 LOW U3-8 HIGH
> U4-9 HIGH U4-8 LOW
> U40-9 LOW U40-8 HIGH
> U41-1 HIGH U41-4 HIGH U41-14 HIGH
> In summary in the OPERATE mode:
> U3-9 LOW U3-8 LOW
> U4-9 LOW U4-8 HIGH
> U40-9 HIGH U40-8 LOW
> U41-1 LOW U41-4 HIGH U41-14 HIGH
> I could use a little more brain power as this too seems to be a bad U41-FF, but I
> changed that.  So unless my other U41 FF was bad in the very same way……………what am
> I missing????
> Thoughts?
> Cheers,
> David
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