Confusing resistor package size choices in Fluke 289

[gamalot]

I was repairing a Fluke 289 with a faulty LCD, and the problem was actually easy to locate, the +20V rail was only a little over 15V. I disconnected the 0 ohm resistor R157 and used an external power supply to provide +20V, and now everything works fine.

I measured pin 3 of the boost chip U21 (LT1615), and the voltage there was only about 0.85V instead of the reference voltage of 1.23V I expected. A new LT1615 would save this Fluke 289, but the prices from Digi-Key and Mouser are too expensive, so I might go to LCSC.

It's just a simple 30 minute repair and nothing worth to talk about, what really bothers me is the choice of package size of the two voltage divider resistors R145 and R146 by the Fluke designer, R145 is 121K in package 0805, and R146 is 2M in package 0603.

Why? 

[robert.rozee]

likely Fluke were using component values and packages that are already used elsewhere on the PCB; the more different components (size or value) used in an assembly, the more machinery is required on a production line and hence a higher production cost. as an example, if a design uses many 10k and a few 20k resistors, production cost may be less if each 20k resistor is replaced with two 10k resistors in series. similarly, if a design uses some number of 10k resistors but in a mixture of 0805 and 0402 package sizes, it may be cheaper to just use 0805 sized throughout.


cheers,
rob   :-)

[gamalot]

likely Fluke were using component values and packages that are already used elsewhere on the PCB; the more different components (size or value) used in an assembly, the more machinery is required on a production line and hence a higher production cost. as an example, if a design uses many 10k and a few 20k resistors, production cost may be less if each 20k resistor is replaced with two 10k resistors in series. similarly, if a design uses some number of 10k resistors but in a mixture of 0805 and 0402 package sizes, it may be cheaper to just use 0805 sized throughout.


cheers,
rob   :-)

I had similar thoughts, but I checked the schematics and found that there was only one resistor in the 121K 0805 package and two in the 2M 0603 package (the other one was R293, unpopulated). If it was to reduce the BOM list, I can't explain why they used the 0805 size. Maybe they just had the device in stock.

[floobydust]

I think it's to make room for C93.

[coromonadalix]

ok  re - found schematics  loll   

https://www.eevblog.com/forum/testgear/fluke-189iian-(289)-service-manual/msg4745819/#msg4745819

[gamalot]

I was about to reply to your PM and saw this. 

[Kean]

I had similar thoughts, but I checked the schematics and found that there was only one resistor in the 121K 0805 package and two in the 2M 0603 package (the other one was R293, unpopulated). If it was to reduce the BOM list, I can't explain why they used the 0805 size. Maybe they just had the device in stock.

This is likely BOM optimisation across products.

While they may not appear elsewhere on this PCB, they probably use those resistor values on other older designs in different package sizes. The PnP lines will be used to assemble various products, and those particular resistor value/size combinations are already loaded in reels on the machines.

If the 121k parts were used multiple times on a new design then they might have chosen to move to 0603 and have to load up a reel of them taking up a new slot or requiring changeover.  It often just doesn't make as much sense for a single part placement to do that if there is PCB space to place the existing size.

[mpeters3]

Hello,
Can you tell me where you accessed the wiring diagram for the Fluke 289 motherboard. I am attempting to purchase a display and was wanting to know the dimensions of the LCD screen. I am going to attempt to replace the screen on one I have along with other missing parts, fuses, leads, etc. Where is the best place to shop for parts for one of these models. Is $100.00 bucks about the going rate for a new/used LCD screen for one of these? Let me know what you can about these items. Thank you for your time. I liked your post about the repair you did on the display issue you had on yours. Have a great day.
mpeters3 out. 

[gamalot]

For the circuit diagram, see the link in coromonadalix's reply.

I don't know about buying a replacement display, maybe someone else can give advice.

[mpeters3]

Hello gamalot and coromonadalix,

Thank you for your replies. I will check out the diagrams. I take it that the fluke 189 schematics are the same for the PCB or similar to the schematics on the fluke 289? They did not change much or add to much between the two models.? Where are these schematics obtained from on the web or are they just available on others forms pages here? I am a newbie to this type of repairs. I mainly fix cars and trucks, but I have some scope training on cars/trucks, and some electrical desires I dabble in with little projects here of fixing DMM's that I want to attempt to fix that I have sitting on my work bench along with life events. I'm sure no one but me has these issues of life getting in the way of fixing projects. ha ha.
  Truely thanks for the help.
mpeters3

[gamalot]

I believe the 189 II is the exact same thing as the 289, except for the model number. I don't know where people get it from, but it showed up on the forums and matches the circuit of my 289.

[TimFox]

I do not know the exact circuit of the Fluke 289.
However, for critical applications a larger package is preferred either for self-heating or voltage co-efficient of resistance (important in a voltmeter).
(During my career, I had to beg the PCB designer to allow a 1206 package for critical resistors, when the usual company default was 0603.)

[mpeters3]

Hello gamalot and all,

Does anyone know where I can find a replacement piece, new for this LCD connection point "J3 LCD" on this PCB that has been damaged that I am going to attempt to fix. see pic. As you can see its mangled pretty well from someone. Not me. I am just the attempting fixer. A part number and source supplier would be helpful if anyone knows whom to purchase one from. Does fluke sell parts such as this to the public? I can hear it now, you're going to attempt to fix this.... . Well, I figure I fix cars, and I have worked prior as a solder tech for Pegatron fixing modems. Let me know, I appreciate any feedback. If I can barter car questions for my green newbie motherboard repair questions I'll do so. Best I can offer via forum pages. Thanks in advance.
Mpeters shalom, shalom.

[gamalot]

Hello gamalot and all,

Does anyone know where I can find a replacement piece, new for this LCD connection point "J3 LCD" on this PCB that has been damaged that I am going to attempt to fix. see pic. As you can see its mangled pretty well from someone. Not me. I am just the attempting fixer. A part number and source supplier would be helpful if anyone knows whom to purchase one from. Does fluke sell parts such as this to the public? I can hear it now, you're going to attempt to fix this.... . Well, I figure I fix cars, and I have worked prior as a solder tech for Pegatron fixing modems. Let me know, I appreciate any feedback. If I can barter car questions for my green newbie motherboard repair questions I'll do so. Best I can offer via forum pages. Thanks in advance.
Mpeters shalom, shalom.

It's hard to tell what model Fluke used, but I guess most straight mounted 1mm pitch FPC connectors should be suitable as a replacement.

[coromonadalix]

the best would be to find some 20 pins connector with the right pitch on a small pcb  and grossly  add wires to it,  because on the main pcb  you dont have side tabs  to help / hold the connector,   pcb traces are badly damaged

you would need to work with thin wires, use uv glue, scrape the top wires if they have varnish,  and try to solder another connector on top of everything ...  IT IS repairable but ...  tedious job ahead

picture attached,    but minus the 0.1" idc  connector,  you have many on ALI ......  with different pitch

[Kean]

It's hard to tell what model Fluke used, but I guess most straight mounted 1mm pitch FPC connectors should be suitable as a replacement.

Measuring the pitch should be easy - almost certainly 1.0mm, but 0.5mm is common too.
Then find a 20-pin vertical SMD FFC/FPC connector that fits in the space.  Check the FPC contact orientation (top/bottom) and the SMD pins alternate sides.

A starting point
https://www.digikey.com/en/products/filter/ffc-fpc-flat-flexible-connectors/ffc-fpc-flat-flexible-connector-assemblies/399
Then narrow down by pitch, mounting type, number of positions, etc, and look at some of the pics and datasheets.

Based on a quick glance, Molex 0528082070 or 0528082071 look like possible options.

[mpeters3]

    Hey Thank You!!! Gamalot, Coromonadalix, and Kean, for the assistance and education. Wow, you guys are genius status in my book. Thanks for the help. Gives me hope of the repairs being able if I can get the right microscope and tools to do it justice.

Thanks again. I'll be back. And it's not an Arnold reference.
Mpeters3