Fluke continuity beeper on 787 processmeter

[softwarejedimaster]

Beeper in continuity mode is not working and this is a difficult one to trouble shoot without the correct schematic.  That's why I'm posting here.  I was working of off a schematic for a fluke 87 but seems to have some major differences.  I have checked the piezoelectric element and it generates a tone from my frequency generator.  Connectios to the element are good.  I scoped the output to the element pads and it shows vbatt DC, not a square wave as I was expecting.  I replaced the hex inverter ic and the dual n channel and p channel ic.  I suspect a transistor which is supposed to switch in this signal to the piezo is at fault. But i dont want to replace all of them.  All other functions on this meter work perfectly.  Just no beeping in any mode.  Does anyone have an idea how this beeper is driven in the 787 or what the problem might be?  Thanks in advance to anyone with some words of wisdom.

[retiredcaps]

The 87 and 787 are completely different despite their similar numbering.

Can you post clear focused pictures of the pcb area where you are looking?  I don't have this meter.

I know modemhead, a member here, has access to a 787.  He may have some ideas?

[retiredcaps]

According to the 787 manual, the meter should beep each time it gets a new min/max value.  Does the beeper beep in this mode?

From page 18 ...

"The beeper sounds when a new maximum or minimum is recorded. Auto power-off is disabled and auto ranging is turned off during MIN MAX recording."

[softwarejedimaster]

There are no beeps in any mode.

[ModemHead]

Yeah, the 787 is a different animal altogether from the 87. The piezo driver circuit is probably similar though, as I recall it's driven from some paralleled CMOS inverters.  That 4069 inverter package is connected directly across the battery for max power to the piezo.

I just got in from a long drive, but when I get a minute I'll take a look at the one here and see if I can deduce the path, which probably includes one or more BJTs if the 87 is any indication.

[softwarejedimaster]

Your insight is much appreciated.  Yea I should have posted these pics when I created the thread.  The 4069 inverter was the first thing i though of - so I replaced it and same behavior.  Next was Q100 on the opp side of the board, now I was grasping.  So before I end up replacing a lot of stuff, its better for me to try and understand what is going on here.  Thanks for the assistance.

[softwarejedimaster]

Not sure why this thread was placed here,  I distinctly recall creating this in "Repair"  in any event, can the moderator/admin move this thread to "Repairs"?  where this makes more sense.  Thanks!

[Muttley Snickers]

I had a similar problem on an 83 that I rebuilt whereby the continuity button was making permanent contact with the PCB on power up which disables the beeper in all modes, it turned out that the foam packer that I installed to put pressure on the LCD because the holding tabs were broken was too thick and causing the button to be held in, may or may not be related, anyway you already have the best guys on the job so I will leave it to them.

[softwarejedimaster]

Excellent statement, I didn't even think of this as a possibility, and I remember using this feature on the 87V a few times.  Probably has this on the 787 as well.  I have powered this up numerous times though with a power supply on the 9V adapter without the membrane keys in  position,  so I guess I unknowingly ruled this out a while back.  But thanks for the input - I would have never guessed this without your post.

[Excavatoree]

Mr Modemhead,  is the back "shield" with the speaker common between the 787 and the 87?   

[ModemHead]

Here's the beeper path. I don't think Q100 is involved.  Signal originates from U12P49, through 100K base resistor R25 to a PNP BJT level shifter to U1P1, with 100K pulldown resistor Rx.  My unit has a partial silk screen but the BJT and pulldown are not marked so I just called them Rx and Qx.

[ModemHead]

Mr Modemhead,  is the back "shield" with the speaker common between the 787 and the 87?

Looks exactly the same to me.  Even the front piece is the same, it has the holes for R21, R34, C2 and C3 trimmers even though those components do not exist on this board.

[retiredcaps]

OT:

@Excavatoree, your 787 post is asking about the Fluke 787! Spooky!! 

Inside joke among the long time Fluke DIY people here.

[softwarejedimaster]

Thanks for the circuit trace and schematic.  That helped a great deal.  My circuit did not trace the same way so I knew there was a problem, it turns out it is not even component related.  I have included some pics of where you said the sig originates and ends up going to the piezo. 

I wasn't getting continuity from pin 49 to the first 100k resistor - bingo

The signal path take 3 vias and crosses both sides of the board before getting to the 100k resistor.  There seemed to be a break at the last via next to the 100k resistor.  Top side of the via wasn't wasn't connected to the bottom side, what the heck?  The rest of the ckt rung out just fine.  I connected a bodge wire from the first via to the last and reassembled.  Switched the unit on and put the thing in continuity mode and shorted the input leads and a beautiful tone emerged.  I will do a proper repair using solder to permanently fix.

Thanks to Modemhead and retiredcaps for their posts - you guys are awesome.  Without a schem or a working version of this board - this would have been 10 times harder.

[retiredcaps]

There seemed to be a break at the last via next to the 100k resistor.  Top side of the via wasn't wasn't connected to the bottom side, what the heck?

Modemhead did this via repair for Fluke 179.  It isn't publicly documented yet, but I can share the pictures.  The filename describes what is happening.

Modemhead's website

http://mrmodemhead.com/blog/all-posts/

also has other cases of corroded traces, paths on Fluke meters.

[retiredcaps]

PS. I will have the administrator moved this to the repair subsection.