(Topic ID: 261887)

explain MPU flashing


By oldschoolbob

3 months ago



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  • 24 posts
  • 6 Pinsiders participating
  • Latest reply 3 months ago by oldschoolbob
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    #1 3 months ago

    I'm working on a Bally -35. I get the flicker - then the LED goes on then off (first flash) - then the LED goes on then off (second flash) - then the LED goes on then off (third flash) - then on and off (forth flash) - then the LED stays on. Wouldn't that mean the PIA at U11 has a problem? Or does it mean the 555 timer is not turning off the LED?

    I replaced all the sockets including the U11 socket. The board was DOA when I got it.

    Thanks

    Bob

    #3 3 months ago

    Thanks ForceFlow, I checked the PIA in my Neoloch tester and it tested good. But I changed it anyway. Still no fifth flash. I'll check the socket for continuity tonight. I'm sure that's where the problem is.

    Bob

    #5 3 months ago

    Thanks Quench, I think your diagnosis is spot on. I lost a solder pad on pin 4. I tried to repair it by using a machined pin socket and soldering on the top side. I probably caused a short to the adjacent trace. I didn’t have any shop time today but I’ll check it as soon as I get back to the shop. (The dark spots in the photos are sharpie marks.)

    IMG_4204 (resized).JPG

    IMG_4219 (resized).JPG

    On a brighter note – I got my ROM programmer and I “THINK” I burned my first EPROM tonight. It’s Leon’s test ROM. I haven’t used it yet so I don’t know for sure if it works. As always I had a devil of a time loading the driver for the programmer but it seems to work fine now.

    Thanks

    Bob

    #8 3 months ago

    Thanks Andrew, I guess I should call that a half-flash. I've changed the PIA chips three times. (all three tested good on the tester) I think the problem is with the new IC socket I installed.

    Quench, this time I followed your example. I removed all the IC sockets. I pried off all the plastic covers then heated each pin and removed the pin as the solder melted. (but I didn't pull them out with my fingers) This is the soldering station I have (set at 550* F.) :

    https://www.circuitspecialists.com/csi-premier-75w-Soldering-Station.html

    But this still leaves solder in the holes. To remove the remaining solder I use a desoldering station similar to this (mine is no longer available).

    https://www.circuitspecialists.com/Compact_Desoldering_System.html

    IMG_4011 (resized).JPG

    This is where I lose the solder pads. The only pads I lose are the pads without a trace connected.

    This is a Bally board and I seem to have more problems with Bally boards. There is no temperature reading on the desoldering tool - only a potentiometer without any markings. I guess I had it set at about 80 percent. Maybe I have it set too hot.

    When I removed the header pins I used the same method (one pin at a time). Then removed the solder left in the holes with the desoldering tool. This time I set the heat to about 60 percent. That worked much better.

    Once I get this board fixed I have another Bally board to repair. I'll try less heat next time.

    Thanks

    Bob

    #11 3 months ago

    Always learning. Dave is right - tips are important. I usually use the larger pointed tip for most work but when I worked on that SMD board I switched to a small chisel point. That worked really well there. I also used it to remove the sockets and headers. But I switched back to the pointed tip for installing parts.

    I also learned from the SMD board that flux helps. Before I removed anything I hit it with a little flux and melt the solder before turning the board over to remove the parts. The down side of the flux is it leaves a sticky residue that difficult to remove. I seldom add solder because it seems so counterproductive - but I've seen where it does help.

    I think my biggest mistake is using too much pressure on the pads with the desoldering gun. That's one thing I know I'm doing.

    I don't mind the time expensive method. I use to use the other method of desoldering first but this way seems much safer to me. Besides I have nothing better to do.

    By the way Quench, how's the weather where you are? It's been in the single digits (*F) here lately. (another reason I'm not getting much shop time - too cold down there). If you have better weather maybe I'll grab that other Bally board and stop by and you can show me your technique on repairing boards.

    Thanks guys.

    #17 3 months ago

    This has been a very interesting thread. I know I picked up a bunch of tips and tricks. I really enjoy rebuilding these old boards except when I lose a solder pad. I'm getting better but not 100 percent yet. Learning these tips and practice maybe I'll make it.

    Another thing I'd like to hear about is what you guys do to repair a lost pad. What is your method? How do you stitch the connection?

    Bob

    #18 3 months ago

    I had a little shop time today (a little warmer today). I was certain I had a short or open circuit somewhere so first I traced out all the pins in the missing pad areas. Everything checked out fine. Then I removed U11 and looked close under the socket - I even looked with my USB magnifier and held the board up to a bright light to view through it. I still found nothing. When I traced pin 2 (PA0) I noticed it was connected to U20 and U20 has been replaced before. I checked my parts box and found a new U20. Replacing it made no difference.

    I didn't have time (or patience) to do any other tests. Tomorrow I do the resistance check that Quench suggested back in post 4.

    Thanks

    Bob

    IMG_4210 (resized).JPG
    #20 3 months ago
    Quoted from Quench:

    Do a resistance check on those U11 pins with respect to ground looking for any with much lower resistance. If you find any with lower resistance than the others then unsolder the respective bypass capacitor on that signal to see if it's resistive (shorted). You can boot the board without that capacitor.

    I checked the resistance on pin 2 through pin 9. All showed about 7.4 K except pin 8 showed 4.9 K. Pin 8 is connected to C65.
    I checked resistance of C64 = 7.3 K. Then C65 = 4.9 K. Then C66 = 7.4 K. I pulled out the ground side of C65 and tried to boot - it still locks on at five flashes. With one leg of C65 disconnected I checked its resistance - it shows open (O.L). I checked the capacitance of C65 and it shows 0.49 nF.

    I'll check my inventory for a new C65 and replace it.

    But why didn't it boot with C65 disconnected?

    Thanks

    Bob

    #22 3 months ago
    Quoted from barakandl:

    Here is the kicker... when I short two digit drive PIA ports together i then get about 4.7K resistance across ground.

    Thanks Andrew, After you said that I looked at the photo I posted before. Looks like there is something (solder?) between pin 33 and the trace from pin 8. Pin 33 is D0. And that's exactly what your talking about.

    I'm not in the shop right now but maybe after dinner I'll check there for a short. If that is a short it would explain why it still won't boot with C65 disconnected.

    Thanks

    Bob

    IMG_4204a (resized).jpg
    #24 3 months ago

    I gobbled up dinner and when down to the shop. Yep, I had a short between 8 and 33. First I tried to clean it up with solder wick. That didn't work so I tried to scrape it off with a small pointed scribe. I finally got it cleared. Then I connected the power supply and got 6 flashes. I reconnected C65 and tested several more times. It works great. We brought another one back to life. Thanks guys - you're the greatest.

    All I need to do now is install the headers.

    Before

    S20200216_0003 (resized).jpg

    After

    S20200216_0004 (resized).jpg

    Thank you all.

    Bob

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