Fluke 8808A not measure

[eduaqa]

I bought this multimeter as part, and I'm trying to fix
Initially when powering on, it was beeping, beeping and display was come on and off fast
Doing a initial investigation I notice that the 7805 marked was warming ... I removed it and test the rail that it supply voltage and was short circuited
I discovered to IC (latches) marked on the picture in short ... I replace the 2 IC and the rail now is having around 24ohms resistance, which still seems having some type of short circuit, because is cause a consumption of 370mAH, what seems too much, because when compared with the others rail which read very high resistance

When I turn it on now, the display works, the push button on the front change as should, however none of the measures works
I noticed that the 7805 is warming, as it should because it's providing 370mAH, also I notice that doesn't matter what function I choose I don't hear relays "clicking" and relays are also warming ... also measuring the voltage on the relays S and R coil that have around 4V, on both ... seems strange, I understood that should not have voltage on both at the same time, and maybe even not continuous

Any help is welcome

[stj]

a number of the 8pin chips look a bit strange in the picture

[eduaqa]

sorry, what you mean? the quantity of 8pin chips? or those 2 that were replaced (16 pins)? Those that were replace are onsemi HC573A, exactly as was before, and just in case ... I tested pin by pin on those replaced to detect maybe jump solder between pins or not soldered pin and they are all good (not short between pins, no desoldered pin)

[stj]

i mean in the picture atleast 2 8pin chips look like the top is discoloured or damaged.

[eduaqa]

Understood … those 2 (16 pins)cycled on the picture are new from DigiKey , the 2 that was there before was short circuited (0 ohms between gnd and vcc)
on that situation it was not even turn on because of the short circuit on the rail and the rail was measuring 0ohms
after I removed the 2 old one the rail become around 24 ohms , and here is my problem it’s still too low , should measure near open link
so something more on the rail is still almost shorting the rail
After soldering those new rail keep reading around 24omhs , so the 2 new should not be the ones causing that low measure
im thinking that I will need desolder all components in that rail until the short goes, but try to avoid because will be a very hard work

[CapLeaker]

A 24 Ohm short sound like shorted MLCC to me but could be something else as well.. Inject rated voltage for that rail using a bench power supply and crank it to one amp. Use 99% IPA and put some on the SMD's. If a cap is shorted it will get hot and evaporate the alcohol quickly. Saves you from fiddling around taken unnecessary parts off.

[eduaqa]

Thanks
I used that method to detect the 2 ICs i discovered in short (in the first round when rail was 0ohms) ... those 2 in the first picture (see the green wires connected to the 7805)
I have a fluke thermal camera so I used it to detect what was warming
But now the only thing warming was the relays after a few minutes,
So I focused on that ... note on the new picture i desoldered 3 of the 5 relays ... after that the rail went to 140ohms (what is still high, should be Open Link)

Now look the pictures ... relays a connected direct to the 5V rail and if I'm understand correct, that 3 pin device marked as 2A and a E or 3 on side is a NCHANNEL MOSFET and D is connect to the 0V rail ... when a tested it, between the D and S its shorted (conducting) ... i thought that maybe some residual voltage on the G was putting it on conducting state, but even if I short G and D, it still conducting between D and S

Am I assuming correct that those device are NCHANNEL MOSFET? and should not conduct when I short D and G?

I still have other 2 relays to desolder ... it seems that 20ohms was the coils from relays as all NCHANNEL MOSFET are conducting they are "clamping" the 0V to the coils.

[stj]

does the base have a resistor inline?
my databook says 2A is a PNP 200mW transistor.
FMMT3906

[redwire]

The MCU might have outputs stuck high and turning on all relays - but no. Low power Ohms should not read anything then until the transistors get turned on. So it doesn't make sense unless all the relay driving transistors have shorted.
DS2E-SL2-5VDC-C-H245 latching relay has two coils 69.4Ω each. I think two transistor high-side to drive them SET/RESET.

[eduaqa]

GreatSage is right, that was a PNP transistor, and was shorting the C and E ... what put energy on the coils, and i didn't read this before, so first i though was N Channel, as I replace had the same problem (N Channel act as a diode and keep coils energized)
below i'm posting all step that could help somebody if need:

#1 meter was turning on, but all reading was showing OL in all functions
#2 I open it and I detected that transformer was just generating like 4-5v where it should be generating around 10V
#3 Disconnected the transformer from the board and than it was doing 10.5v, so some short circuit was in somewhere
#4 I detect that one of the 7805 was warming.
#5 I removed it and then the transformer starting generating then 10V as supposed.
#6 Testing the ground and +5v out on the board (with the 7805 removed), it showed 0ohms, so really a short after 7805
#7 here one mistake ... I looked around the 7805 and saw a few components around, so i decide to remove them ... mistake,, the board is multilayer (obvious), but i didn't noticed that on in a first look ... so after removing the components still showing 0ohms
#8 so then I discovered that there was a lot of components, so removing then was not an option
#9 so, with the 7805 removed, used an external power supply and injected 5v on that rail, looking for heating and was easy, those 2 chips I marked initially was heating (the picture is after replacing, not sure why seems discolored), removing them remove the 0ohms reading.t
#10 however, that reading was still 22ohms, what seemed to be low ...
#11 easy, use the same process with the power supply... not that easy ... this time no obvious heating and the AMP usage on the power supply was not too high ... so what to do
#12 leave it on for some minutes and noticed that the relays mainly those lite orange was warming a little
#13 so I a get a point to focus
#14 measuring the relays saw that all coils was energized at around 5v
#15 another mistake, and here is where you need be patient and understanding before starting soldering/desoldering ... i decide to remove the relays (this was a error, and that was a mistake
#16 ohms went high to 800ohms, now appears good
#17 but, why the relays was energized? I discovered that those sot23 that was connect to 0v and to the coil, was shorting the 0v rail and as the 5v rail was connect direct to the other side of the coil relay it was causing the relay been energized
#18 another big mistake ... looking on the sot23 it had a marking code 2A and a letter E turned 90 degrees ... some google and a found that was a N Channel MOS ... big, big ... big mistake
#19 replaced those sot23 and the relays continued getting 5v on all coils ... so something wrong
#20 here again think before do ... course that would happens N Channel with 0V connect to the gate and relay connect the source and the other end of relay on 5V will act as diode and will energize the coils
#21 so, what now?
#22 remove the N Cnannel and soldered the PNP transistor, the multimeter worked
#23 Also a notice and this is very important the relays that are lite orange are what is called SRCRS relay with 2 coils (Set and Reset), what that means? means that they should not be energized, but, just send a pulse to switch the Set or Reset