[volt-nuts] Fluke 5205a Repair

[Chuck Harris]

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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