Post Reply 
ACT available
04-09-2019, 04:39 PM (This post was last modified: 04-09-2019 04:40 PM by Dreato.)
Post: #421
RE: ACT available
ACT 25E/IR printing problems:

Recently I updated two HP25 calculatiors to the 25E ACT. No problems here - everything worked great! After this, I added the IR circuit to both ACT boards, and installed the IR LEDs in these 2 calculators. One prints perfectly (see photo), and one gives occasional errors. I swapped the ACTs between these 2 calculators, and the problem followed the ACTs.

   

I though that I may have made a mistake soldering the SMD parts on the ACT, possible shorting the 33 ohm resistor? So I removed them and soldered new parts on again, but still have the problem. I also reflashed the 25E v 1.14 firmware, to see of that helped. It did not - same problem. Any suggestions from the helpful readers of this forum?

Thanks and Best Regards,

Dave
Visit this user's website Find all posts by this user
Quote this message in a reply
04-09-2019, 09:29 PM (This post was last modified: 04-10-2019 11:48 PM by PANAMATIK.)
Post: #422
RE: ACT available
Did you try to replace the NPN transistor? Perhaps you have chosen a unsuitable type? I recommend BC847 or BC817 with high current gain. The firmware follows the HP82240 timing very accurately from the early versions and is not responsible for the problem.

Bernhard

That's one small step for a man - one giant leap for mankind.
Find all posts by this user
Quote this message in a reply
04-09-2019, 09:44 PM
Post: #423
RE: ACT available
Yes - I used the BC847 transistor, the Vishay TSAL4400 LED, and the 2.2k ohm and 33 ohm SMD resistors. It's very puzzling! Identical components in both working and non working IR circuits. All other functions are fine. I flashed the ACT to HP29 firmware, which also worked perfectly, but which still showed the random printing error. Scope traces looked OK.

Scratching my head!?
Visit this user's website Find all posts by this user
Quote this message in a reply
04-10-2019, 01:40 AM
Post: #424
RE: ACT available
I've got a follow up on my IR printing problem. I hooked my scope up to the IR LED, and compared the waveforms on the working and non working calculators. The waveforms were very similar. Since the printing errors were reduced when the calculator was farther from the printer, or when the calc was off axis, I thought maybe the IR receiver in the printer was set to a very high sensitivity. I opened up the printer, but there was no sensitivity adjustment. However, when I covered the IR receiver photo transistor with my finger, allowing almost no IR light in, the printing was perfect.

The problem is in my printer. The IR receiver is VERY sensitive, and there's no adjustment that I can see. I added some attenuating material behind the printer receiver window, and all is well now.

Regards,

Dave
Visit this user's website Find all posts by this user
Quote this message in a reply
04-10-2019, 04:20 AM
Post: #425
RE: ACT available
Final update on my printer issues. I went down the rabbit hole: After narrowing my printing issues to the printer itself, I decided to look at the schematic (attached - from Kees Vandersanden's site). I traced the IR signal through the U5A flip flop. From there, it goes to the M80C50 processor where decoding occurs. This processor pinout shows a crystal oscillator at pins 2 and 3. However, this printer uses a cheaper LC oscillator. I decided to measure this frequency with my old HP frequency counter. When I attached the probe, the printer would print the output from the suspect ACT, error free. The frequency counter presents a load of 1 Meg ohm and 10 pf. I soldered a 10 pf capacitor across L3, and the frequency changed enough to assure error free operation with both my 25E ACT calculators.


.pdf  HP82240B_Schematic.pdf (Size: 40.15 KB / Downloads: 17)
Visit this user's website Find all posts by this user
Quote this message in a reply
04-10-2019, 11:55 PM
Post: #426
RE: ACT available
Bravo for tracking down the problem to the high sensitivity of the printer receiver. You could replace the 33 Ohm resistor of the ACT circuit by 100 Ohm, this would attentuate the IR stream and would also solve the issue.

Bernhard

That's one small step for a man - one giant leap for mankind.
Find all posts by this user
Quote this message in a reply
04-11-2019, 05:10 PM
Post: #427
RE: ACT available
RE: 100 ohm resistor. I actually tried that, with no effect! This is what led me to open up the printer. Looking at the signals at flip flop U5A- they all looked the same from both my HP25E ACTs - attenuation made no difference in the appearance of the signals. I was actually attenuating the signals by placing my finger over the IR receiver diode. This led me to look for bad solder joints, and the M80C50 chip which decodes the digital signals to driver the printer module. The spec for the printer specifies the pulses at 32,768 Hz rate (you know this from the HP82240 interface guide), but that decoder chip had a simple LC oscillator. My experience working on the elusive HP45 hidden stopwatch feature (see my website https://davidreaton.com/hp-calculators/hp-45/ ) told me that LC oscillator frequency can vary widely depending on component values. All the 'Classic" series use a simple LC oscillator to multiplex the display LEDs. I played with swapping capacitors into this circuit to adjust the frequency, with limited success, until I found the MEMS oscillator, which gave the required 784 kHz frequency with only 20ppm drift.

Therefore I measured the frequency at the M80C50 oscillator pins. When I clipped the frequency meter on, the printer began printing flawlessly. I assumed that the RC load from the frequency counter input changed the LC oscillator resonant frequency enough to bring it into spec. I added a 10 pF ceramic cap to this circuit, and now it works well. BTW, the schematics shown for the HP82240A and B models (from Kees Vandersanden's site) show C values of 47pF and 390pF, respectively. I couldn't see the caps values in my printer, so I don't know which one is correct.

To be perfectly honest, I don't know if the LC oscillator modification is the true, definitive answer or not. I know the IR attenuation observation I made, DID WORK, but I don't know why!?? Maybe putting my finger over the IR receiver diode added some stray capacitance? I'm a chemist, not an electrical engineer!

Clipping the frequency meter on made the difference. I guess it's an example of the Heisenberg uncertainty principle. The act of measurement changes the very thing you're trying to measure!

Sorry for the long winded post. It was a long, strange trip, indeed! Thanks for all feedback, help and suggestions.

Regards,

Dave
Visit this user's website Find all posts by this user
Quote this message in a reply
Post Reply 




User(s) browsing this thread: 1 Guest(s)