250469 Rev3 Blowing Fuses

Es gibt 22 Antworten in diesem Thema, welches 4.026 mal aufgerufen wurde. Der letzte Beitrag (22. Februar 2024 um 20:56) ist von niallquinn1971.

  • Hello all,

    I need some hints/help with the repair of a C64 shortboard.

    the board seems to work, but there is no audio as the fuse seems to go up in smoke...

    Punkt 1:

    Die Platinenversion ist sehr Wichtig für korrekte Hilfe. Bei Commodore Platinen steht meist ASSY gefolgt von einer Nummer drauf, die wollen wir wissen.

    - 250469 Rev3

    Punkt 2:

    Ist das Gerät modifiziert oder noch Orginal? eventuell andere ROM´s drin?

    - original

    Punkt 3:

    Ist das Netzteil Orginal oder schon mal durch ein anderes ersetzt worden?

    - my own powersupply (can deliver 2a at the 9ac line and 4A on the 5v line) power supply works ok (used to repair 8 other C64 and 2 C128 (with adapter) and 2 Vic20)

    Punkt 4:

    Tritt der Fehler nur auf wenn Zusatzhardware angeschlossen ist?

    -fuse seems to "burn" faster when tape dongle of harness is connected, but burns also with no hardware, seems that when drawing current will make it go faster.

    Punkt 5:

    Tritt das Problem sporadisch auf oder ständig?

    -always, can take 1 or 2 minutes, then fuse will go (sometimes get sound in diagnostic 586220).

    Punkt 6:

    Werden irgendwelche Chips besonders heiss? so das man sich fast die Finger dran verbrennt?

    -no chips get very hot, system works like a charm without 9 volt (only no sound/tod/tape)

    Punkt 7:

    Hat der Fragesteller ein Multimeter und weiss wie das benutzt wird?

    (Die billigen 5-Euro Multimeter reichen für diesen Job aus)

    -yes, I have a multimeter, oscilloscoop, several diagnostic cardridges (ufd,systest64,deadtest,...) and know how to use it

    Punkt 8:

    Hat der Fragesteller irgendwelche Erfahrungen im Löten? wenn nicht gleich sagen.

    -no problem.

    voltages of the 5 DC volt line and 9 DC volt line are correct.

    what I did so far...

    replaced so far:

    Q1,

    Q2,

    CR1,

    C65,

    C63,

    C66,

    CR7,

    Q5,

    CR2 (ripping some traces :cursing:).

    used other (8580) SID (known good) to check if SID is not the issue.

    oh, replaced F1 now 7 or 8 times... to sometimes see it acting as a lightbulb for short time. :(.

    I'm running out of spare fuses... help!

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    Regards,

    Jeroen.

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    2 Mal editiert, zuletzt von war64burnout (7. Oktober 2022 um 20:05) aus folgendem Grund: Bild ins Forum geladen und Link gelöscht!

  • Blowing fuses mean excess current. Measure resistance between +9V and GND. Best place would be Vcc from the SID (Pin 25) to GND (anywhere, Expansion Port shield or Pin 14 SID, ...).

    Most probably a short circuit somewhere. Maybe the rectifier (CR2) is blown, maybe a cap is shorted, maybe a stray wire ...

    EDIT

    Machine turned off, SID pulled, no power supply attached

  • I would like to emphasize one detail that advotax mentioned in his afterthought: always pull the precious SID first. The rest of the C64 must work without it. Put the SID back in only when everything else is fixed.

  • I measured the resistance over pin 28 and pin 14 (9 volt DC and GND) and it's about 8 MΩ, so definitive no short

    CR2 (and CR3 and CR4) measures a 0.6 voltage drop on each diode.

    there is also no short between the pins of transistors.

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  • Now measure resistance between

    - cathode of CR3 (should be the upper wire, easy to reach) and GND

    - cathode of CR4 (right next to CR3) and GND

    - C of Q1 and GND.

    - Tape Port Pin 3 and GND.

    - 9VAC 1 and GND, 9VAC2 and GND, preferably at the Userport (Pins 10, 11)

    - 9VAC1 and 9VAC2 (not to GND)

    Again with the machine turned off, no PSU attached.

    If there are no shorts at these locations, it starts to get interesting.

    What did you replace CR2 with? A salvaged one from another C64?

    And are you absolutely sure you got the right values for F1? Its not an unusual mistake. (don't ask :D)

  • I'll check resistance of the above points tonight, when home again.

    Fuses an CR2 were salvaged from other C64 boards.

    I only had a rectifier bridge rated for 2A in my spare parts, but the one in the C64 is rated 4A, so I took one from another C64 (parts) Board.

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  • So you had ... 8 ... boards to salvage fuses from? :schreck!: Well then the majority of them should have the right rating. :) (but check it anyway)

    If you measured the forward voltage of the diodes in CR2 (all four?) you probably checked all possible combinations and orientations and there are no leaks?

    The 2A bridge rectifier would be sufficient. It's all rated for around 1A. A common reason for a blown fuse - and the reason there is a fuse in the first place - would be an excess draw an the Userport (short circuit and/or too demanding accessories) or a faulty Datasette (e.g. stuck motor) on the Tapeport. VIC and SID alone only draw around 0.3A each at max, so 1A should never be exceeded. On a 250469 they even need less, because their 5V supply comes directly from the PSU instead of the 9VAC-to-12VDC-and-5VDC-supply like the older boards have.

  • I have around 15 boards for parts, most were already missing the fuse when I got them.

    Yes I did check forward voltage and if diode is blocking on CR2, CR3 and CR4. (and between the transistor pins)

    I repair this board for the Club, not knowing what the next owner will do with it, or what he/she is connecting to it.

    So I want to stick on what Commodore used (Not sure if the did this to handle any peak currents, the fuse is a slow version so should also be able to handle peaks above it's rating for short times)

    for 250469 boards indeed only SID, sound amplifier and TOD circuit is using the 9v DC (now realizing I did not check TOD circuit)

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  • most were already missing the fuse when I got them.

    Exactly my experience. :) Question then is, why they are missing. Probably because they were blown. If say CR2 was the reason und you salvage that CR2 ...

    only SID, sound amplifier and TOD circuit is using the 9v DC

    The Tapeport is supplied, too, with "9V unreg" from CR2/C63 which is then regulated through CR1/Q1.

    The TOD cicuitry could be faulty but there should be a resistor in series (560 Ohms as per schematic; no number given; should be the one next to C44) which if not shorted itself but CR6 and/or C44 is shorted to ground should only allow ~20mA of current to flow. Could be checked if you measure resistance between Pin 1 of U23 to GND.

    Not sure if the did this to handle any peak currents

    Most certainly not, at least not to handle 4A peaks. It was probably the one they had available. I am not even sure they always put the same rectifiers in there.

    It's a spec from the manufacturer. It only means the forward voltage is "guaranteed" at 4A for around some milliseconds. This rectifier could never ever withstand 4A without serious cooling. Modern rectifiers for example the KBU8 series are rated for a forward voltage drop of less than 1V at 8A ... for 300µs (microseconds!) ... at a junction temp of 25°C (which itself is virtually only achievable without any current flow at all).

    Anyway let's check everything else first before heating up the soldering iron. :)

  • - cathode of CR3 (should be the upper wire, easy to reach) and GND

    open

    - cathode of CR4 (right next to CR3) and GND

    open

    - C of Q1 and GND.

    open (same point as cathode CR3 and GND ?), to B = 3.1MΩ

    - Tape Port Pin 3 and GND.

    Open

    - 9VAC 1 and GND, 9VAC2 and GND, preferably at the Userport (Pins 10, 11)

    5MΩ,open

    - 9VAC1 and 9VAC2 (not to GND)

    open

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  • same point as cathode CR3 and GND ?

    No. That should be the anode of CR4 - "9V unreg" in the schematic. The same point would be C of Q5. (if board markings equal the schematic)

    So there are no (DC) connections to ground anywhere on the 9VAC rails. Interesting. Did you check the blown fuses for their markings? Its not very likely but still a possibility.

    What resistance is from Pin1 of U23 to GND, and for good measure Pin 1 of U23 to Pin 10 and Pin 11 (Userport) respectively? Thats the TOD circuit.

  • Interisting thread! Adrian Black had a Board from a subscriper, which fuse blows anytime. Turns out the fuse je used was to weak.

    But 8 fuses... 🧐🤔

    Commodore C64, C64 II, Atari 1040 ST, Atari 1040 STF, Atari Falcon 030, Atari Portfolio, Ender V3 Neo V2, Amiga 500 & Gotek

    Bitte melde dich an, um diesen Link zu sehen. Status: unterwegs

    "NUR" das Diagnose Set Harness + 8in1 Diag Modul zum Verleih... Status: ... verfügbar

    Wer das Projekt unterstützen möchte, kann sich gerne per PM melden :-)

  • pin 1 -> gnd = 2M

    Pin 1 -> pin 11 = 8M

    pin 1 -> pin 10 = 500K

    last fuse was rated 1A, fuses before were 1.5A

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  • Hm. Pin 1 -> Pin 10 (which is 9VAC1) should be 560 ohms (no k, no M, just ohms). It's this part in the schematic:

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    You could measure the current. The probes to either side of the fuse holder, no fuse inserted. Leave the SID out and dont forget to switch your DMM to AC and current. Then power on the computer. Now watch how (fast) and to what extent the current rises. Switch off at 1.5A max. Thats the current the fuse would still allow so everything else has to withstand that also. If the light show usually happens after 1 minute you got plenty of time and this way you don't waste any more fuses. Just as a sanity check for now.

    Without the SID the current should almost be zero...

  • Hm. Pin 1 -> Pin 10 (which is 9VAC1) should be 560 ohms (no k, no M, just ohms). It's this part in the schematic:

    ok, it was .500 K Ohm, it was.

    I measured the ampre flowing throug the fuse, but it is steady at 0,07 A (70 mA).. so nothing (Bitte melde dich an, um diesen Link zu sehen.).

    It realy looks like something is drawing more current when something is triggert.

    Regards,

    Jeroen.

    [Bitte melde dich an, um diesen Link zu sehen.]

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  • it was .500 K Ohm

    Ah, okay. Everything fine there.

    steady at 0,07 A (70 mA)

    More than I expected, but yeah, virtually nothing, as it should be.

    Now put back in the SID and measure the current again. Should not exceed ~300mA, maybe 500. But maybe the SID is broken after all...

    Did you have a Monitor plugged in? Everything still normal?

  • Yes, monitor was connected.

    I put the SID back in and did a new test.

    This time current was about 120mA.

    Bitte melde dich an, um diesen Link zu sehen.

    You can hear the SID at the end of the video (running Diag 586220).

    as I didn't want to film for 3 or 4 minutes, I decided to put some more load on the system.

    I attached a known working harness (Sven Peterson), I only didn't connect the joystick connectors, they would interfere with the test probes.

    the result was a system that is drawing 1.8 A current, too much for most fuses.

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    edit: 2.1A, my DMM went back to DC after powersave.

    The harness was used for repairing about 10 systems this week after I had some fuses blown by this system.

    I put the system aside as I couldn't find the fault and had those other systems to work on as well.

    so, I rule out an issue with the harness

    it just looks like some component is conducting if it gets more load to handle.

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  • I rule out an issue with the harness

    Maybe not the harness itself. If it - whatever it is - is only conducting when the harness is connected, it must have something to do with it.

    Did you always have something/everything connected when the fuses lit up? It's obviously not the Control Ports. But you could try now piece for piece to narrow down which of the other component is needed for the immense current draw. (I'm guessing User Port and/or Tape Port)

    Those 1.8A would explain why the fuse wouldn't always blow immediately since its just above the threshold. Its probably limited by your power supply.

    I would start with cartridge only, then dongle for the Serial Port, keyboard - if that doesn't change much leave them out and try cartridge + Tape and so on.

    Another, more laborious but safer angle:

    measure resistances as above with only one dongle plugged in at a time, machine switched off, no PSU attached, no fuse, no DMM at the fuse holder.

    A current of 1.8A at 9V should equal a resistance of around 5 Ohms which is essentially a dead short. Maybe one of the dongles is creating a connection where no connection should be. (again most probably at the Tape and/or User Port)

    This way you don't stress the PSU that much.

  • dearr advotax,

    I think I found something... :thumbsup:

    look what happens with the resistance between userport pin 10 and gnd when I push the serial port.

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    tomorrow I will remove the connector and will look how circuit board is shorting.

    thank you for all your help.

    Jeroen.

    [Bitte melde dich an, um diesen Link zu sehen.]

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