Small Blue Fella in HP11C calculator

[NivagSwerdna]

I am trying to repair a HP11C calculator that I bought on EBAY as Parts/Not Working. 

I'm working through its various problems but one I came across is finding two traces on the PCB that are shorted together.  Given that these traces make the effort to cross the board I figured out that they were probably meant to not be shorted.  After removing any components that got in the way I have traced these to this small blue chap.



He measures pretty much as a dead short when my DMM is set to continuity.

Is this a capacitor?  It should measure open?

[jemotrain]

I think, it's a capacitor with 180pF.
Or it should be a capacitor...

[NivagSwerdna]

Would it be possible to replace it (I presume it is mica) with a 0603 180pF ceramic which I probably have?

Something special about mica?  (or tantulum) or whatever it is?

[pqass]

Would it be possible to replace it (I presume it is mica) with a 0603 180pF ceramic which I probably have?

Something special about mica?  (or tantulum) or whatever it is?

Tantalums are larger with a "+" on one terminal (anything on the reverse?). They tend to fail short.
MLCC (ceramic) caps if stressed will crack and short (and explode if across power rails).

Replacing it with a 180pf (J=5% tolerance) ceramic cap should be fine.

[artag]

A capacitor is possible, but it might also be a ceramic resonator. I can't find any HP11C documents that describe the oscillator and whether it's RC, crystal or resonator. However none of them should be a short, and I wouldn't expect the tracks to an oscillator component to go right across the board. So the 180pF is fairly likely.

You might get a definitive answer from https://www.hpmuseum.org/forum/forum-4.html
 

[wasedadoc]

I only see 1 similar shaped blue component in the photos https://www.hpmuseum.org/10cint.jpg and https://www.hpmuseum.org/10cmbb.jpg.  Looks like it is across the power rails.  Capacitor code 181J is 180pf 6.3Volt.  If the capacitor itself is not the short then probably one of the chips.  Possibly killed by someone applying excessive or reverse voltage. If so you have effectively reached a dead end.

Edit. If a capacitor the 'J' would mean 5%. (Not 6.3V)

[TimFox]

"181J" is the code for 180 pF/5% tolerance, probably a C0G ceramic capacitor.
THT mica capacitors are usually flatter and redder.
You could replace it with a SMT C0G capacitor by appropriate soldering.
I've never seen a ceramic capacitor with a voltage rating as low as 6.3 V.
If all connections to that capacitor (or at least all to one side of the capacitor) have been temporarily removed, it should definitely measure "open".

[floobydust]

I doubt that capacitor has actually failed. It looks like a Murata or TDK part and I've never seen a dipped ceramic cap fail short.
Something else has shorted, I'd go after the tantalum and electrolytic capacitors first as they have a higher failure rate and do not age well.

[NivagSwerdna]

Likewise I doubted... and a closer look at the board makes more sense...



There is an associated inductor and looking at the traces shows them in || so this is obviously the LC Oscillator.

I'll leave it alone and repair the trace over near the processor.

It's interesting that the LC is geographically so far away from the processor.

[NivagSwerdna]

Just FYI... shows how far away the LC parts are from the processor!

[TimFox]

If the inductor is in parallel with that capacitor, then it would read "short".

[NivagSwerdna]

If the inductor is in parallel with that capacitor, then it would read "short".

Indeed

[vk6zgo]

Although this question seems to have been resolved, it is worth knowing that, although rare, there are resistors out there which are packaged in what looks to be a dipped mica capacitor package.

[WattsThat]

Have you verified you have continuity between the processor pins and the LC tank components? It is a common failure for one of the traces to fail open near the positive battery terminal due to unseen battery corrosion on the hidden (front) side of the board.

If the tank is okay, check for fractured soldering on the IC pins. It requires magnification.

It's interesting that the LC is geographically so far away from the processor.

It’s the only place that has the required clearance. They eventually did a redesign with smd parts and one chip. Those have the later LCDs with few connections and different multiplexing schemes.

[NivagSwerdna]

Thanks.  Indeed a broken trace, but actually under the processor which I find pretty weird.  Repaired (along with some other traces that had corrosion damage) and all working now. 

[WattsThat]

I never lift chips or pull the board if I can avoid it. I have an inventory of the different version boards from 12C lcd donor units - which are a huge help in finding open traces as you experienced, looks like you have perhaps done the same.

With the help of those stripped boards, I created lcd pin outs for both versions of the LCDs, been meaning to draw them (properly) and post them on the HP Museum. Those come in handy when you encounter a Voyager with missing lcd segment(s). With some luck, I might get that done before the spring of ‘23 arrives 😉

[NivagSwerdna]

I never lift chips or pull the board if I can avoid it.

Agreed.  I did the exposed side first and then when that failed I did lift the top side chips.  Unfortunately there were also problems on the underside of the board which meant I had to decapitate all the heat stakes.  It is possible I destroyed the trace under the processor when removing it... but we will never know.

All fixed now though...

[NivagSwerdna]

Here we see the clearly obliterated clock trace...

It's quite strange... one clock trace is completely missing... the other has a bit of damage.

I would blame myself for overheating things but the surrounding traces look in great condition.

It wasn't lifted... it was gone!  The device had previously had leaky batteries but it's a long way from the battery compartment to under the processor.

[Runco990]

I love HP and have a few of their calculators including this one.  I also love a challenge....  But I have to ask, WHY are we rewarding and incentivizing ebay sellers to shovel more and more landfill junk for money?  Every time we buy stuff like this, it just empowers them to find MORE busted crap and ask even MORE money. 

I may be out of line here, but being a tech for 45 years, there is just stuff I stay away from when I can usually find a working example for mostly the same price.  Repairing single chip devices almost never ends well....

Hope this was actually super cheap.  But it likely should have been FREE at your local landfill.  Probably WAS.... for the seller.

Nonetheless.... interesting and I hope you actually fix it.

[NivagSwerdna]

Very good point.  I paid £23.48 for this one; that's around $28

It was a gamble; quite happy with the result although it wasn't an easy fix.  Would have prefered $0 

As a general rule I try and avoid non-working consumer electricals on EBAY, they are normally completely knackered, being repair failures of the current owner or their predecessors.

I just couldn't resist this one

It is FIXED. See above

PS
Actually... I guess the busted trace could have been a previous failed repair attempt... !

[Runco990]

You got lucky.   

The Irony is that I picked up a mint 11C a few years ago for $35.  No soldering required.

My argument really is that WE are driving up prices on clapped out junk by BUYING it.  I once put in "used toilet paper" as joke.
Try it.... see what comes up.  I was speechless.  This is where we are......