A strange behaviour when my classic calculator is connected to a wall adapter

[aurelio]

The machine (a 67 with a battery pack)) does not turn on if the wall adapter is plugged in ...
if I unplug it the calculator instead turns on and works correctly, what do you think, something something wrong with the charger?
If I swap it, the calculator works normally (I mean turns on)

the same problem with another classic calculator: the same behaviour.

I tested the voltage on the terminals of both the adapters following what i read on this previous post, I mean:

for the first adapter the "outside pins" is 4,9 V (supply for the calculator's logic chips). on the other outside pin (a constant current source to charge the NiCd.) the open-circuit voltage is around 16,2V.

for the second one respectively 5,3 and 16,1V

It seems these values are both in the range, where must I check forward ?

thank-you for any advice
Aurelio

[AndiGer]

If you have the plug connected to the calculator but the charger not to the mains the calculator will not turn on. You didn't mention if you have connected the charger to the mains.

[BobVA]

It does sound like the 5 volt supply has failed on that charger. It's probably going a lot lower than 4.9v under load.

[Geoff Quickfall]

I am with Bob on this. The adapters contain a large electrolytic if I am not mistaken. That electrolytic cap is around 45 years old and those adapters do get warm when used. It is probably failing open.

Geoff

[aurelio]

(04-21-2020 03:04 PM)AndiGer Wrote:  If you have the plug connected to the calculator but the charger not to the mains the calculator will not turn on. You didn't mention if you have connected the charger to the mains.

Andi, the charger is connected to the main, I couldn't measure any voltage output as described otherwise

[aurelio]

(04-21-2020 04:59 PM)Geoff Quickfall Wrote:  I am with Bob on this. The adapters contain a large electrolytic if I am not mistaken. That electrolytic cap is around 45 years old and those adapters do get warm when used. It is probably failing open.

Geoff

I did not consider it, it means that usually the drop of voltage when the plug is connected to the calculator is lower than 300mV, 'cause the second adapter supplies with the open-circuit 5,3V.
I'll open the adapter searching the electrolytic, so.
I'll report.
Thanks to all

[Geoff Quickfall]

Yes, i am sure there is and electrolytic cap and as it fails to open it becomes a resistor limiting the voltage and current passed through it resulting in the lower voltage measured at the plug end.

Do let us know. And take some quick photos.

Ah, something to ad to the book, which I am working on during COVID, an appendix on adapters, I must have around 30 including some defunct ones to sacrifice to photos

Geoff

[Massimo Gnerucci]

(04-21-2020 05:25 PM)Geoff Quickfall Wrote:  Ah, something to ad to the book, which I am working on during COVID, an appendix on adapters, I must have around 30 including some defunct ones to sacrifice to photos

Waiting... :) :D

[Geoff Quickfall]

How did I know!

More like this:

Stewie and brian

[Massimo Gnerucci]

(04-21-2020 09:20 PM)Geoff Quickfall Wrote:  How did I know!

More like this:

Stewie and brian

Geoff, please go on at your own pace (unfortunately I still have many years before retiring, and I know you have so many interests to follow) but, please, keep on accepting a little teasing, tongue in cheek, here and there. ;)

Great gag from Family Guy!

[aurelio]

the 400 uF capacitor is OK, anyway I swapped it, I'll post the pics later.
The voltage is even a little decreased (4,8V)
I have to investigate forward

[teenix]

(04-22-2020 10:21 AM)aurelio Wrote:  the 400 uF capacitor is OK, anyway I swapped it, I'll post the pics later.
The voltage is even a little decreased (4,8V)
I have to investigate forward

What is the voltage when plugged into the calculator?

There are only 3 components for the charger logic supply, maybe one has failed or has a poor solder connection.

cheers

Tony

[aurelio]

(04-22-2020 11:18 AM)teenix Wrote:  

(04-22-2020 10:21 AM)aurelio Wrote:  the 400 uF capacitor is OK, anyway I swapped it, I'll post the pics later.
The voltage is even a little decreased (4,8V)
I have to investigate forward

What is the voltage when plugged into the calculator?

There are only 3 components for the charger logic supply, maybe one has failed or has a poor solder connection.

cheers

Tony

the voltage is 0,3V on both the sides, when plugged into the calculator
I've retouched all the soldering points, they seems good
the voltage now on the "outside pins" is 5,2 V (supply for the calculator's logic chips), but still not enough

[teenix]

(04-22-2020 02:25 PM)aurelio Wrote:  the voltage is 0,3V on both the sides, when plugged into the calculator
I've retouched all the soldering points, they seems good
the voltage now on the "outside pins" is 5,2 V (supply for the calculator's logic chips), but still not enough

I don't have one of these chargers, so I can't give a comparison.

When plugged into the calculator....

The voltage for the logic side of the regulator should be around 4 volts. This mimics the battery supply.

What voltage do you get across the electrolytic capacitor? I would imagine about 6-8 volts.
If you can check the zener diode it should have about 4.7 volts across it.
The resistor should measure about 470 ohm.
The transistor might have failed also.

Edit: Just thinking, if the voltage is a bit low, it might be worth checking the diodes to see if any have failed. You might be getting half wave instead of full wave rectification.

cheers

Tony

[aurelio]

(04-22-2020 05:24 PM)teenix Wrote:  

(04-22-2020 02:25 PM)aurelio Wrote:  the voltage is 0,3V on both the sides, when plugged into the calculator
I've retouched all the soldering points, they seems good
the voltage now on the "outside pins" is 5,2 V (supply for the calculator's logic chips), but still not enough

I don't have one of these chargers, so I can't give a comparison.

When plugged into the calculator....

The voltage for the logic side of the regulator should be around 4 volts. This mimics the battery supply.

What voltage do you get across the electrolytic capacitor? I would imagine about 6-8 volts.
If you can check the zener diode it should have about 4.7 volts across it.
The resistor should measure about 470 ohm.
The transistor might have failed also.

Edit: Just thinking, if the voltage is a bit low, it might be worth checking the diodes to see if any have failed. You might be getting half wave instead of full wave rectification.

cheers

Tony

Thank-you Tony I'll check asap,, but if the last voltage test yesterday gave me (un plugged) 5,2V for one side and 16,3 on the other, today the situation is worst, 0,3V a 0, 2V, hope to have not created more damages...
I have a couple of questions
1) this charger should have a fuse but I don't see any, is it normal ??!!
2) It is a model with dual-mains-voltage selector, when I opened it the switch fell apart and I'm not sure if I put back in the slider the two contact springs correctly, do you have any pics which shows that selector, I haven't?

[teenix]

(04-23-2020 04:27 PM)aurelio Wrote:  Thank-you Tony I'll check asap,, but if the last voltage test yesterday gave me (un plugged) 5,2V for one side and 16,3 on the other, today the situation is worst, 0,3V a 0, 2V, hope to have not created more damages...
I have a couple of questions
1) this charger should have a fuse but I don't see any, is it normal ??!!
2) It is a model with dual-mains-voltage selector, when I opened it the switch fell apart and I'm not sure if I put back in the slider the two contact springs correctly, do you have any pics which shows that selector, I haven't?

As I don't have one to verify, the fuse might be built into the transformer. I don't know if the switch is repairable.

It looks like the switch became faulty. The higher voltage might have resulted from accidently moving the switch to the lower mains input setting, causing the transformer to output a much higher voltage. This can be damaging to the calculator and the charger.

Please be careful when fiddling with mains equipment. It is unforgiving to the unwary.

cheers

Tony

[aurelio]

(04-24-2020 06:29 AM)teenix Wrote:  

(04-23-2020 04:27 PM)aurelio Wrote:  Thank-you Tony I'll check asap,, but if the last voltage test yesterday gave me (un plugged) 5,2V for one side and 16,3 on the other, today the situation is worst, 0,3V a 0, 2V, hope to have not created more damages...
I have a couple of questions
1) this charger should have a fuse but I don't see any, is it normal ??!!
2) It is a model with dual-mains-voltage selector, when I opened it the switch fell apart and I'm not sure if I put back in the slider the two contact springs correctly, do you have any pics which shows that selector, I haven't?

As I don't have one to verify, the fuse might be built into the transformer. I don't know if the switch is repairable.

It looks like the switch became faulty. The higher voltage might have resulted from accidently moving the switch to the lower mains input setting, causing the transformer to output a much higher voltage. This can be damaging to the calculator and the charger.

Please be careful when fiddling with mains equipment. It is unforgiving to the unwary.

cheers

Tony

to avoid misunderstandings, the switch is not faulty at all, it is only taken apart and I'm not sure how to insert the springs into putting it back, I guess the flat part towards the slider . The selector was positioned correctly, so I don't think the transformer gave on the output a much higher voltage, but I will take the time to carefully disassemble and test everything.
I found this interesting site where David proposes an alternative schema
The problem is my local electronics shop doesn't have the NTE ones suggested by .

I have found in many other posts regarding the topic also an interesting conversion table provided here by Katie, but unfortunately in our case they have been not enough to find them
Somebody may help in that?


Edit: I found the equivalents, not without emotion, here on the Jaques Laporte site, thank-you Jaques

[Paul Berger (Canada)]

(04-24-2020 11:03 AM)aurelio Wrote:  to avoid misunderstandings, the switch is not faulty at all, it is only taken apart and I'm not sure how to insert the springs into putting it back, I guess the flat part towards the slider . The selector was positioned correctly, so I don't think the transformer gave on the output a much higher voltage, but I will take the time to carefully disassemble and test everything.
I found this interesting site where David proposes an alternative schema
The problem is my local electronics shop doesn't have the NTE ones suggested by .

I have found in many other posts regarding the topic also an interesting conversion table provided here by Katie, but unfortunately those indications did not help me and the seller to find them
Somebody may help in that?


Edit: I found the equivalents, not without emotion, here on the Jaques Laporte site, thank-you Jaques

The flat side of the contacts in the switch go against the circuit board so that they bridge the pads on the board.

I have fixed a few of these chargers and the most common fault I have found is breaks in the cord which are often deceptively intermittent. The most common place for them to break is near the calculator connector. Try flexing the cable a bit while measureing the output.

I have replaced the transistor in the logic supply, a MJE800 in at least one but I don't recall if the symptom was low +5 or none at all.

Paul.

[aurelio]

(04-24-2020 12:52 PM)Paul Berger (Canada) Wrote:  

(04-24-2020 11:03 AM)aurelio Wrote:  to avoid misunderstandings, the switch is not faulty at all, it is only taken apart and I'm not sure how to insert the springs into putting it back, I guess the flat part towards the slider . The selector was positioned correctly, so I don't think the transformer gave on the output a much higher voltage, but I will take the time to carefully disassemble and test everything.
I found this interesting site where David proposes an alternative schema
The problem is my local electronics shop doesn't have the NTE ones suggested by .

I have found in many other posts regarding the topic also an interesting conversion table provided here by Katie, but unfortunately those indications did not help me and the seller to find them
Somebody may help in that?


Edit: I found the equivalents, not without emotion, here on the Jaques Laporte site, thank-you Jaques

The flat side of the contacts in the switch go against the circuit board so that they bridge the pads on the board.

I have fixed a few of these chargers and the most common fault I have found is breaks in the cord which are often deceptively intermittent. The most common place for them to break is near the calculator connector. Try flexing the cable a bit while measureing the output.

I have replaced the transistor in the logic supply, a MJE800 in at least one but I don't recall if the symptom was low +5 or none at all.

Paul.

Paul, I've checked the integrity of the flexible cable at first and again after each attempt, it's OK, my suspicious is to have damaged the components while testing the voltage in different points of the circuit using unproperly the probes .
Infact at the beginning I had the constant current supply (16,4 V) and the logic supply
4,9V (unplugged the calculator), maybe this second value too low, but now I get just ,03/0,2 V) instead...aargh

waiting for the replacement parts...

[Paul Berger (Canada)]

Since you have one of the version that is switchable and I am guessing it is a later version that does not have screws that fasten the circuit board onto the bottom, however I believe that the holes in the board and the bottom of the shell are still there, even if the screws where not present. having the board fastened to the bottom makes it a lot easier to measure things inside the charger. I would wonder what the voltage is across the capacitor, also note these transformers are not capable of providing a lot of current so if the capacitor fails short it will drag the whole thing down.

Paul.