[volt-nuts] Fluke 5205a Repair

David Garrido d.garrido at me.com
Tue Sep 29 14:23:47 EDT 2015

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.


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

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

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????




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