Author Topic: Repair Fluke 77 series I -> U1 really dead?  (Read 339 times)

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Offline ElecSeb

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Repair Fluke 77 series I -> U1 really dead?
« on: February 21, 2019, 08:54:03 am »
It has been a while and this DMM has been on my desk ever since (collecting dust)...
Now the time has come to pick the device back up and hopefully revive it, but I have a gut feeling U1 is really dead...

I have cleaned the PCB and replaced 2 broken C, furthermore I have placed back the C mentioned in this topic of mine:
https://www.eevblog.com/forum/repair/repair-fluke-77-i-starnge-c-not-in-schematics!/

But after rebuild the DMM is still dead. So started to measure and according to the SM:

  • Put the UUT function switch in the VDC position -> Done
  • Using a DVM, check +VDD (TP-1) or the positive battery post for 3.1V dc +-.1V -> TP-1 (red probe) with P15 (U1) = 0.03V
  • Using a DVM, check +VDD (TP-1) or the positive battery post for 3.1V dc +-.1V -> +bat & -bat = 9V
  • Using a DVM, check +VDD (TP-1) or the positive battery post for 3.1V dc +-.1V -> TP-1 (red probe) with -bat = 9V

Do i read the SM manual wrong or is the U1 just terminated/defect. Do I have some other options to test?
Read on an other topic, on eevblog, about 2 minuted 400 degrees heating on U1 (reflow internal bonds)? Could this be a solution?
 

Offline ModemHead

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Re: Repair Fluke 77 series I -> U1 really dead?
« Reply #1 on: February 21, 2019, 01:48:50 pm »
If U1P8 (Vdd, TP1) is at 9V battery voltage with respect to U1P48 (Vss), then U1P15 (analog ground) should be 3.1V below Vdd.  U1 has an internal rail-splitter to create a bipolar supply this way. So if it's not happening, there isn't much else to blame.

Try making the measurements with sharp probes right on the chip. There could be some value in reflowing the (external) leads on U1 in case there's a dry/cracked joint, which is definitely not unheard of with U1 on the 7x meters.

No experience with trying to reflow internal bond wires, but I do vaguely recall someone talking about that here.
 

Offline ElecSeb

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Re: Repair Fluke 77 series I -> U1 really dead?
« Reply #2 on: February 21, 2019, 02:08:40 pm »
If U1P8 (Vdd, TP1) is at 9V battery voltage with respect to U1P48 (Vss), then U1P15 (analog ground) should be 3.1V below Vdd.  U1 has an internal rail-splitter to create a bipolar supply this way. So if it's not happening, there isn't much else to blame.

Try making the measurements with sharp probes right on the chip. There could be some value in reflowing the (external) leads on U1 in case there's a dry/cracked joint, which is definitely not unheard of with U1 on the 7x meters.

No experience with trying to reflow internal bond wires, but I do vaguely recall someone talking about that here.

I already soldered the chip with additional new solder (to test), no result.
Might reflow this weekend just to test. If it is not doing a thing, then sold as parts.
 


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