Fixing a HP34C

[agarza]

I have a HP34C that I want to bring back from the dead.

The only sign of life is the display, sometimes some segments lit up.

Any suggestions besides cleaning it?


BTW, The HP34C was my first HP calculator.

[agarza]

Forgot to mention, I just bought it.

Also, it seems that there are 2 components missing from the power supply board.

Anyone has a schematic or a photo of the board?

[teenix]

I don't think the components are missing, other power modules, that I have seen, do not have them either. They may have been installed but were considered redundant by the engineers.

Because of the original solderless construction, there could be open circuits causing the failure especially if the spongy layer has collapsed over the years. This provides the pressure to hold the integrated circuits connected to the flexible circuit film. The same applies for the display module.

Upon re-assembly, you would have to check all the connections to see if power and signal paths are still valid. This could only be accomplished with an oscilloscope. Without one you could only apply pressure to the circuit board in the hope that it springs back into life.

Having said all that, there could be failed components causing the issue.

Solderless assembly was probably seen as an innovative design back in the day, but over time maybe not so good, hence latter models were back to using solder again.

I have seen examples where the parts have been soldered to the film layer, but you would have to be careful not to overheat it.

Here is a service file for E series calculators.

cheers

Tony

[Steve Simpkin]

In looking at your power supply photo, it appears very similar to one I found at https://www.sbprojects.net/mylab/hp34c/

Apparently there are no components at those two locations.

[agarza]

(07-12-2023 11:52 PM)teenix Wrote:  Upon re-assembly, you would have to check all the connections to see if power and signal paths are still valid. This could only be accomplished with an oscilloscope. Without one you could only apply pressure to the circuit board in the hope that it springs back into life.

The problem is getting the oscilloscope probes on the circuit board once assembled.

(07-12-2023 11:52 PM)teenix Wrote:  I have seen examples where the parts have been soldered to the film layer, but you would have to be careful not to overheat it.

I was thinking of soldering the components directly "a la surface mount". Do you have any photos of someone that has done that?

(07-12-2023 11:52 PM)teenix Wrote:  Here is a service file for E series calculators.

Thanks!

[agarza]

(07-13-2023 12:00 AM)Steve Simpkin Wrote:  In looking at your power supply photo, it appears very similar to one I found at https://www.sbprojects.net/mylab/hp34c/

Yes, it is the same. So no components were there in the first place.

Thanks

[BobVA]

(07-13-2023 12:24 AM)agarza Wrote:  

(07-12-2023 11:52 PM)teenix Wrote:  I have seen examples where the parts have been soldered to the film layer, but you would have to be careful not to overheat it.

I was thinking of soldering the components directly "a la surface mount". Do you have any photos of someone that has done that?

I had a 34C that went from intermittent to completely dead over the course of a few months. Disassembly and cleaning didn't help.

As a last ditch effort I soldered all the components to the board, and it's been working for a year (with light use).

I worked *very* quickly, using lead-based solder, and still wound up melting a couple of traces through the board (fixed with bodge wires). I tripled checked part identification / orientation and was extremely careful to avoid making any solder bridges. Rework would be very difficult - it's like working on a circuit board made out of chocolate.

[teenix]

You might be able to avoid soldering by using conductive paste or glue, probably messy unless applied carefully. I used conductive glue to reattach a Spice LED chip to the display circuit board after removing the lens assembly.

With either approach, you would need to make sure the silver pads on the circuit film and IC pins are free of any corrosion. The pins can oxidize over time and make proper connection difficult.

cheers

Tony

[John Garza (3665)]

The Spice series was the sure sign HP had 'jumped the shark'. Solder-less construction is not innovation it is regression to save money. Many many many years ago the ancient art of soldering was applied to electronic circuits to both join and eliminate any possible air gap in electrical connections. This avoids corrosion (even the very slight kind that may not be apparently visible) between connections that could develop if it was exposed to the atmosphere - with its oxygen and humidity.

Clean all corrosion before you start, use thin solder with a low temp. Go slow and take your time.

Even if you were to bring it up to factory spec - with solder-less connections again connecting properly - it's only a matter of time until it fails again. Of course, you could opt to keep the refurbed machine in a controlled environment or sealed in an airtight anti-static bag until you wanted to use it. Maybe OK for a collection unit, but not a daily driver. Soldering is the way to go if you intend to use it regularly.

-J

[AndiGer]

Concerning the power supply „missing“ component:
Has to do with decimal point / thousands separator.
Especially see the last post by Katie in this old thread:
https://www.hpmuseum.org/cgi-sys/cgiwrap...read=20346
Andi

[agarza]

(07-13-2023 03:44 AM)BobVA Wrote:  

(07-13-2023 12:24 AM)agarza Wrote:  I was thinking of soldering the components directly "a la surface mount". Do you have any photos of someone that has done that?

I had a 34C that went from intermittent to completely dead over the course of a few months. Disassembly and cleaning didn't help.

As a last ditch effort I soldered all the components to the board, and it's been working for a year (with light use).

I worked *very* quickly, using lead-based solder, and still wound up melting a couple of traces through the board (fixed with bodge wires). I tripled checked part identification / orientation and was extremely careful to avoid making any solder bridges. Rework would be very difficult - it's like working on a circuit board made out of chocolate.

Thanks for the tip.

What are the traces made up, so that they melt so easily?

[agarza]

(07-13-2023 04:04 AM)John Garza (3665) Wrote:  Solder-less construction is not innovation it is regression to save money.

Couldn't agree more.

I can imagine HP engineers going against the solder-less design. But, as always, administration wins.

[agarza]

(07-13-2023 05:31 AM)AndiGer Wrote:  Concerning the power supply „missing“ component:
Has to do with decimal point / thousands separator.
Especially see the last post by Katie in this old thread:
https://www.hpmuseum.org/cgi-sys/cgiwrap...read=20346
Andi

Ohh... I get it.
The jumper selects the radix.
Cut = comma
On = decimal point

[agarza]

Ok, so after cursing HP for the "solder-less" design, I decided take the plunge
and solder the chips & display. Yes... you have to go slow and take your time (as recommended earlier).

I did not solder the power supply. It is still pressure conected.


Sorry for the ugly soldering.

And after the time consuming reassembly..... It still did not work!

As a last option I swapped the power board with a HP33E.

And to my surprise... it worked, well sort of. The continuous memory is not working nor the dot for radix. I'm assuming it's all the power-board's fault.

Anyone have the hp34c power-board schematic?




Thanks for all your help!

[BobVA]

(07-13-2023 02:24 PM)agarza Wrote:  Thanks for the tip.

What are the traces made up, so that they melt so easily?

It's not the traces, it's the board. If you put too much heat into the trace it melts its way down into the board, leaving a hole.

I definitely agree with the "take your time" advice above, but don't do that while you've got the iron on the board :-)

If I recall correctly, my procedure was: heat up the component lead, flow a little solder on that and get off as soon as that flowed smoothly on the pad. Normally I'd touch the iron to both but that risked melting the board. I was using my usual iron temperature of about 375C - a different temperature might have worked better, but I was hesitant to experiment.

[teenix]

There is a circuit for the HP-33C power supply on this web page, created by Tony Duell. I'm pretty sure it will be the same as the 34C

http://www.hpcc.org/cdroms/schematics5.0/index.html

I can't access the site just now, so maybe it is down for some reason.

If you cannot get access, I have a copy of the PDF.

cheers

Tony

[AndiGer]

There is a power-supply of HP-38C at
https://keesvandersanden.nl/calculators/powersupply.php
Andi

[teenix]

Both appear the same, except Kees has R7 as 2K2 and Tony's drawing shows as 110R.

cheers

Tony

[Kees van der Sanden]

I had a second look to this resistor. It measures 2140 Ohm.
The color coding is red - red - red.
A resistor of 110 Ohm has color coding brown - brown - brown.
The difference between red and brown is not always easy to distinguish.

Kind regards,

Kees

[teenix]

(07-15-2023 06:50 PM)Kees van der Sanden Wrote:  The difference between red and brown is not always easy to distinguish.

Especially with my old eyes, these days I have to rely on the ohmmeter a bit more for parts like that :-)

cheers

Tony