Dead HP 35

[krazyk]

Just tested the current use by putting a multimeter across the left and right external charger positive pins yields only 0.25mA.

Basically zero power is flowing or being used with the 3.78V battery DC input.

Its reproducable and is always the same at 0.25 milli Amps.

What could cause no current to flow in the circuit?

Is there a better place to measure?

Do I need to start to desolder and test the transistors or would this indicate an open circuit somewhere?

[teenix]

Reading only 0.25mA when the voltage readings were as high as mentioned, seems a bit odd. The voltages suggest the power supply is somewhat functional. Was the meter on the correct setting and was the gold bar removed from the charger port for this test, otherwise it will short the current meter.

In the circuit board download link in previous post there is a diagram of the HP-35 circuit and PCB overlay which might guide you tracing the circuit board for dry joints. If you have a transistor/capacitor checker, you could check those components, otherwise you might have to start replacing them.

cheers

Tony

[krazyk]

Tony

Here is a picture of how I measured current

   

I am puzzled by this too. Is there a better place to measure the current?

[Steve Simpkin]

(10-10-2024 08:08 AM)krazyk Wrote:  Tony

Here is a picture of how I measured current



I am puzzled by this too. Is there a better place to measure the current?

Correct me if I am wrong, but shouldn't the red lead on the meter be in the jack marked "uA mA" when measuring current (in this range)?

[teenix]

The measurement method is ok, but as Steve pointed out, the red lead is not in the mA current socket on the multimeter.

[krazyk]

I hadnt even noticed the mAmp neg multimeter spot.

   
0.50mA though is still indicating no real activity on the logic board.

I double checked those voltages though and they were slightly up at
Vcc 7.6v and the test point voltages of 5.6volts and -11.2 volts.

I changed one of the batteries upping the total input to 3.89volts DC to get to 90% of the correct values.

But only 1/360 of the normal 180mA current appears to be flowing through the circuit.

Its not looking good really.

[AndiGer]

I think you should use uA/mA and COM
Andi

[krazyk]

   

[Steve Simpkin]

Still not quite right. The black lead should be in the "COM" socket and the red lead should be in the "uA mA" socket.

[teenix]

With the multimeter, place the red lead into the mA socket and the black lead into the COM socket. Switch it to 600m position.

That will have the battery current from batt +ve to meter +ve, then meter COM to calculator circuitry.

It should then read a normal current flow.

cheers

Tony

[krazyk]

I need to check for clock signals next because it occured to me that without a clock signal there will be no life to this digital circuit. No life translates to no current I am guessing.

This appears to be the case here really. I collect computers of the late 70s and 1980s so I will check the other compnents for open circuit. The 4 transistors seem to check out using the diode test method and the voltages are 90% of what they should be.

Will see if I can post some DSO images of any clock waveforms. I am betting there will be ZERO.

Kris

[krazyk]

Oh dear

Showing my ignorance here and thank you for correcting my multimeter usage.

   

Now showing around 37mA of current flowing which makes me feel somewhat better but still only consuming about 20% of its normal 0.00 current usage.

Something is not firing up properly. I am guessing that the LED display probably uses up at least 50% of the overall power and it is staying pitch black.

[teenix]

You might like to connect the multimeter ready for voltage measurement and connect black to batt -ve and the red to pin 5 of the CTC IC - the large white one. Pin 5 is on the left side, 5th pin down from top. If there is no voltage there, the calculator is held in reset. You have an early model, and the reset circuit is controlled by the 2 transistors on the right side of the circuit board.

[Hiwi]

Hi Kris,

have you cleaned the powerswitch and removed the old grease?

Can you short the 2 pins with a cable and measure the input voltage directly on the battery contacts?

Don't forget to setup up your meter correct :-)

Can you measure the battery voltage on the pin without any drop? (Agianst minus)

Sometimes the red/black cable are totally oxidized, it is a good advice to change them.

These battery holders have sometimes bad contacts or springs and very thin cables. Do you have a better powersupply?

Ralf

[HPing]

(10-10-2024 12:28 PM)Hiwi Wrote:  Sometimes the red/black cable are totally oxidized, it is a good advice to change them.

These battery holders have sometimes bad contacts or springs and very thin cables. Do you have a better powersupply?

We have some cars with excess resistance from corroded connectors. We see this visually and measure the resistance (e.g. voltage drop).

Cleaning and reterminating might be the next stop.

We also see corrosion migrate from connectors into the wire, under the colored insulation. You can only see that corrosion by removing the wire insulation near the connector. I don't know if this would be a red herring for a low voltage/low current calculator but where your calculator had so much corrosion, anything is possible I suppose.

Getting a good connection with corroded wires can be tricky. Sometimes you can clean the wires and apply a lot of flux. Sometimes we cut the corroded wire away and reterminate / splice in clean wire. Or replace the wire. YYMV with different situations.

[AnnoyedOne]

Some years ago a number of young men (brothers) were crowded around the bonnet (aka hood) of an old car trying to get it going.

So a "dinosaur" walks up a takes a look. The cabin interior light is on but the car won't "turn over".

Said dinosaur comes back with a mulitmeter, checks a few things, and says "replace that" pointing to the battery ground cable.

They do and bingo! the vehicle starts.

"Internal resistance" can be a real bummer

A1

[HPing]

(10-10-2024 03:30 PM)AnnoyedOne Wrote:  "Internal resistance" can be a real bummer

In high current applications, we can see the corrosion and maybe feel (or see) the resistance via hot wires and parts. They are easy to test too.

In low current situations like the calculator, everything is a bit more subtle. Bad soldier joints and lifted pads can be tough to see and intermittent...

[krazyk]

I have inspected both sides of the main keyboard pcb and measured the resistance between the battery terminals and the + and - entry points and the resistance was very low and the continuity beeper went off on the power connections from the battery terminals al the way to the main pcb.

It is a good point though re internal copper going BLACK as I see that a LOT with my retro computer collection. It will not hurt replacing the link wires just to see if it reduces the voltage drop of particularly Vcc. I need to get it to 8.2V.

I have cleaned the power switch already so I dont think thats an issue now

   

I DO own a laboratory power supply and can use IT to supply 3.9 volts at 500mA max just to eliminate those AAA 3 pack batteries from the equation. I am waiting for a delivery of some Enerloop NiMH batteries as thats what I am using in my HP25 and 33C and they work very well.

I have been using 3 semi depleted AAA alkaline batteries that are showing around 1.3V each DC but I am aware that their current delivery capability can also be DEPLETED too. If I find out its THAT I will be very annoyed with myself.

Will update with more info soon.

Kris

[krazyk]

Some views of the MAGNIFICENT Logic PCB of the early 1970s.

   


   


I NEVER get to see such wonderfully assembled PCBs in my retro computer collection.

Ahh good ol American Can Do back in the day. Or was this Singapore like my HP 33C.

Everything LOOKS perfect.

[krazyk]

Sorry stupid software

heres the back side