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Anyone have Tech Tip #8, 820 Printer Reset Problem?

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Michael Current's Atari Library lists a collection of service bulletins, including the following:

 

Quote

Tech Tip #8
  820 Printer, 11/17/82
  Reset Problem

 

Does anyone have this tech tip? I have long suspected that my 820 doesn't work because the processor isn't coming out of reset properly. Even the paper advance doesn't work if the printer isn't working after I turn it on, though I can see that the paper advance signal is making it to the IC. However, the chips seem to be good because if I power cycle the printer, once every 100 or so cycles it comes up okay and will both advance paper and print. It gets tedious to switch on, press paper advance, switch off, switch on, press paper advance... :)

 

I would be interested to see what the tech tip says, or whether anyone has solved the issue with 820s that only seem to work every once in a while.

 

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I suggest a complete clean and lube check any sockets, clean all sensor switches etc.

then check PIA, transistor array, power switch pcb- clean that switch check voltages over time, solid state switch, print ok switch, mpu....

there is a strobe fix for those with double character etc. and a reset fix.

 

the tech tip might be in the administrative faults manual..

 

I can't imagine it's not on AtariAge somewhere.

 

Curt do you have the technical service tips all in one place?

Edited by _The Doctor__

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Man, I'd like to have this anomaly (and Fix) recorded.  I have had an 820 for over 30 years, never a problem, solid as a rock.  However, I only use it a couple of times a year (print out calendars, listings, etc.).

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What is the processor and other socketed IC's in the 820? Anything that could be swapped into another device to verify function? Ie. 6507, 6532, 6810, 6820?

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I miss my old ker-chunk ker-chunk.

:)

 

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So according to the manual, there's a 6507 CPU, a 6532 PIA/RIOT and a ROM.

 

The 6507 and 6532 can be tested in an 810, 850, 1050, or even a 2600. :)

 

The ROM has 12 address lines in the schematic, so will be a 4KB 2332 type like the 850 and 1050, and can easily be dumped by inserting it into a socketed brown shell Atari cart and using the DOS 2 "K. Binary Save" menu option. Then it can be compared with a known good ROM. (Which I don't have, may need someone else to do the same unless someone knows a location online...)

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Still no luck finding the tech tip. Here's the reset circuit:

 

820_reset.thumb.jpg.bbe37250c1b237628871ea773e6254be.jpg

 

If I understand this correctly, C109 and R106 form an RC time delay circuit of around 1 second. The charging current flowing through C109 and R106 causes Q102 to initially turn on, pulling pin 1 of the 6507 and pin 34 of the PIA low (grounded through R134) to hold them in reset. After C109 is charged, Q102 turns off and pins 1 and 34 go high (+5V through R108, 4700ohm resistor). This also causes Q103 to turn on, and a current flows through R110, Q103, and R134 to ground. This raises the voltage on the emitter of Q102, causing it to go further into cutoff and ensuring it is no longer trying to pull the reset pins low.

 

The next thing I'd like to do is look at the +5V supply and reset pins on an oscilloscope; the +5 supply should be stable before the reset pins go high. If it isn't, I imagine that using a larger capacitor (maybe 2.2 or 4.7uF?) to increase the time delay would give the supply enough time to stabilize.

 

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Thanks hunmanik!

 

”We are not aware of any service problem...”

 

I was hoping there was a service problem with a suggested fix. My printer won’t work even when it’s been off for a long time before I turn it on. I’ll still try increasing the capacitance in the reset circuit to see if that makes a difference.

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you might try going the other way if that fails... I really think you have another issue and that is why I outlined a grocery list to check out and uploaded the fsm....

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Finally got a chance to test my theory. First I hooked up an analog voltmeter to the reset line and turned on the printer. It does stay low for a little less than a second before going high, which seems like expected behavior. However, the pointer wiggles slightly as if there is some noise before rising to 5V. Then I clipped a 10uF capacitor in parallel with the existing 1uF cap (10uF was the smallest value I had on hand that would make a difference). This time it took 4-5 seconds for the reset line to go high, but when it did, the printer worked! I was able to repeat this behavior over multiple power cycles, so my confidence that I’ve identified an issue is rising. My new theory is that there is some noise on the power line that couples to the reset line when the power is ramping on the printer, and if the chips aren’t held in reset long enough, this noise causes the chips to come up in an unknown state.

 

I went to the parts store and picked up some smaller values to try (1, 2.2, 3.3, and 4.7). Smaller is better since larger values will mean a longer wait time after turning on the printer before I can start using it. Once I find the smallest value that seems to work reliably, I’ll solder it in and use the printer for a while to see if the issue has gone away.

 

Doctor, to your point about other issues, I was fairly certain those weren’t the case because I had already reseated/replaced ICs. Also, I figured those parts were good because every once in a while, the printer would work fine when I turned it on. But always good to reseat ICs as a first step :) You mentioned a reset fix, were you referring to the tech tip?

Edited by electronizer
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