C-64 Weirdness. - Page 1
Forum section: electronics / repair

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 16, 2019, 07:23:48 am
content: Guys and Gals i am having quite a strange issue with one of my C-64 Motherboards.Datasette do not function reason being a missing 6.9 volt line (From 9Volt DC)Replaced..Q1 CR1 R1Q2 R3 Seem fine.9 volt unregulated is present Q1 Collector and R1 topNo Signal from R1 to base to baseSame issue with R3 +3 present from U6 to the resistor but no signal from resistor to Q3 base.Resistors are fine Transistors are fine Diode is fine.While locating reasons for error i noticed if i remove U7 VIC the circuit all the sudden works perfectly but not if VIC is present.. Question is why ?I see no reason why it would effect the circuit the way it does.Any tips would be welcome.Schematic

author: CJay
author_rank: Super Contributor
timestamp: January 16, 2019, 09:39:26 am
content: You've got the right VIC chip haven't you?ISTR there were later versions for 5V only, early versions used '12V' on pin 13, late versions used 5V and if you plug in the wrong one it could do funny things to the cassette as the cassette power is from the same rail.

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 10:53:42 am
content: No, it's the other way around."In all C64 models VIC-II is socketed for easy replacement, but it is important to notice that 6569, 6572, 6573, 6566 and 6567 use 12 volts and 5 volts when 8565 and 8562 use only 5 volts. Replacing old version with new version without motherboard modification destroys 8565 and 8562 if powered up in the oldest versions of C64 motherboards."Best is to measure Pin 40 to ascertain the voltage present there and determine the model.

author: CJay
author_rank: Super Contributor
timestamp: January 16, 2019, 11:39:23 am
content: Quote from: Armadillo on January 16, 2019, 10:53:42 amUmm, I thought that was what I said?

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 11:56:34 am
content: Oh correction, Pin 13 should be the correct pin to measure VDD pin.Schematic above refers to VIC-II, the 5V version but the mother-board may be the old type, so it's the other way around, 5v type into 12V slot will damage the VIC chip.12V type into 5v slot, the chip should survive.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 16, 2019, 09:31:20 pm
content: The board is 250469 what uses 9 volt only for SID and power Cassette drive and some other portsThe VIC is VIC 2 8565 tested with two VIC2's both produce same problem.Mystery is what can pull the voltages down on those points.. I understand the 9 volt zener combo (to reduce the voltage to 6.8 volts for the cassette drive but what is the 3.3 volt combo coming from the other direction.And how can removing the VIC fix the problem.

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 09:42:36 pm
content: Just Remove Q2 and test it again.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 16, 2019, 09:55:07 pm
content: I will try.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 16, 2019, 10:36:52 pm
content: Quote from: Armadillo on January 16, 2019, 09:42:36 pmI removed Q2 and the cirquit worked as intended (i got the regulated 6.8 to the cassette power pin)Problem the computer do not boot after removal of the Q2 (no display nothing)Put the Q2 pack computer boots but no cassette drive.

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 10:47:43 pm
content: I asked you to remove Q2 just to ascertain the running codes, obviously there must be starting orders.Have you component tested Q2?

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 16, 2019, 11:06:32 pm
content: Quote from: Armadillo on January 16, 2019, 10:47:43 pmSeems test fine but i will order replacement and test.

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 11:15:48 pm
content: Well, better hope that its from Q2, because the only thing that commands it is from the MPU itself [thru one resistor].

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 16, 2019, 11:23:32 pm
content: Quote from: Armadillo on January 16, 2019, 11:15:48 pmLike i said between mpu and resistor there is 3.3 volt that goes to resistor.Between resistor and Q2 there is no voltage.Old resistor reads fine and been replaced.Only thing i do not undestand is if 3.3 goes to resistor and resistor is fine why there is no 3.3 in the collector base leg..

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 11:47:51 pm
content: How do I put it to you?Because your Q2 testing must be very unsure that you didn't find out that the Q2 is shorted from Collector to Emitter or faulty.EDIT [OF COURSE I ASSUME YOU KNOW HOW TO TYPE "LOAD" AND PRESS THE PLAY BUTTON].

author: fzabkar
author_rank: Super Contributor
timestamp: January 16, 2019, 11:53:15 pm
content: The MPU outputs 3.3V to Q2 via base resistor R3. The BE voltage should now be around 0.6V - 0.7V. This turns Q2 on, in which case its CE voltage would drop to around 0.2V. This in turn removes the base drive from Q1, thereby turning it off.So there is nothing wrong in this area. Instead I suspect that the VIC chip may be dragging down one or more of the bus pins. Do you have a spare VIC?

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 11:57:27 pm
content: Read #8

author: fzabkar
author_rank: Super Contributor
timestamp: January 17, 2019, 12:21:48 am
content: That post is confusing. Removing Q2 allows Q1 to turn on (because Q2 is no longer robbing Q1 of its base drive), so it is to be expected that a 6.2V supply (= 6.8V Zener - 0.6V BE) would be present at the cassette interface. However, the absence of Q2 should not prevent the C64 from booting, AFAICT.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 17, 2019, 12:27:36 am
content: Quote from: fzabkar on January 17, 2019, 12:21:48 amThat's right. It is all very weird.Something fubar is going on .

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 12:37:09 am
content: Because it was flagged/interrupted that serial data in coming in and hang there waiting since it was never intended or at least planned for Q2 to be out of circuit.!it's added complexity enough, because "seems" like working is not a test result of Q2 in the first place.Anyway, make sure you press the play button...... otherwise it will be so funny or making at least make sure that the status did pull low for play.OK, is enough for me.Quote from: fzabkar on January 17, 2019, 12:21:48 am

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 17, 2019, 01:21:16 am
content: I was looking replacement for 2SC1815 it being Y model.Would ahttps://www.ebay.co.uk/itm/2SC1815-C1815-NPN-General-Purpose-Amplifier-Transistor-Pack-of-5-10-20-or-50/161833425274?ssPageName=STRK%3AMEBIDX%3AIT&var=460806829647&_trksid=p2057872.m2749.l2648Be fine ?

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 17, 2019, 02:32:59 am
content: I tested the transistor again it seems totally fine..Maybe something else is fubar

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 17, 2019, 02:35:04 am
content: Quote from: fzabkar on January 16, 2019, 11:53:15 pmYes i have a spare VIC same symptoms. I suppose my both VIC's could be broken but they function otherwise totally fine if they are

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 17, 2019, 02:39:27 am
content: I considered this earlier also but i am not sure how to check if some of lines are being dragged down.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 17, 2019, 02:53:12 am
content: Quote from: Bashstreet on January 17, 2019, 02:35:04 amAlso if this is how the circuit is supposed to work why when i insert the cassette drive it gets no power what should then trigger the power on ?

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 17, 2019, 10:19:00 am
content: Inserting cassette and pressing "Play" or "Play+Rec" pulls "CASS SENS P4" low.This is the first condition for the MPU to drive "P5" low and enable cassette motor once a LOAD or SAVE command is used.A shorted motor could bring the 9V line down, but you'd probably notice that due to Q2 overheating.When you say "if i remove U7 VIC the circuit all the sudden works perfectly" does that mean you actually get the cassette to spin? With something in the 6 to 7V range on Q1-Emitter? measuring a voltage there unloaded is bad diagnosis.A wrong model VIC could bring the 9V line down but you'd probably notice that due to VIC overheating.This could be a case of starting at the beginning.Are you sure of your Power supply? Original model or substitute with sufficient 9V AC current? Have you been able to use your PS on another C64 with a 12V VIC?Next check either voltage drops when loaded as you go along, or contact resistances unpowered.Power socket contacts.Weak solders on power socket, rectifiers, L1 choke, fuse+holder resistance.Check C63 is still good enough.Check power switch contact resistance/voltage drop. This would be the perfect culprit IMHO.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 17, 2019, 07:57:26 pm
content: Quote from: shakalnokturn on January 17, 2019, 10:19:00 amOk i think i understand the basic function of the circuit now thank you.I have tried with two other cassette drives they do not function ether.QuoteIf U7 VIC is removed pin 3 in cassette connector gets 6.8 volts and cassette drive functions of course cannot boot the computer without video processor.QuoteBoth VIC's are correct version for the board 8565R2QuoteYes i have tested to board with two power units one original one build the board and other boards work with these units (rebuild one has 3 amp 5V supply and 9V has 10 VA)The VIC (VIC2) uses purely 5 volt line for power. SID of this board uses 9V both lines are fine on on U6 MPU pin 6 and U9 SID pin 28.QuoteC63 Should be fine after all the cassette player works fine (i get 6.8v (constant) on pin 3 of cassette connector and the cassette drive plays rewinds etc just fine after removal of Q2.But the device is not found.Could it be that somehow the MPU do not detect the drive and that is why it do not activate the circuit ? (tested with two drives)What is peculiar as MPU detects my SD reader (it used purely 5 volt supply from the cassette port)

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 17, 2019, 11:11:39 pm
content: Quote from: Bashstreet on January 17, 2019, 07:57:26 pmSure that could be a cause... Even more if your SD reader works.Engaging cassette and pressing "play", can you confirm that the P4 line goes from high to low?You can compare what happens on P4 line with your SD reader. Note that while you " LOAD" or "SAVE" to your SD the 6.8V should appear at Q1-e.Better check the schematic for the tape drive before trying but as P4 line has a pull-up resistor, you can probably force this line low to check.Other than messy power I still don't get why removing VIC would enable the tape motor... I'm no C64 guru though.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 18, 2019, 01:59:36 am
content: Quote from: shakalnokturn on January 17, 2019, 11:11:39 pmThe SD reader far as i know only takes 5 volt from the cassette port (I might be wrong) and communicates through serial bus.I tried pressing play play button but there was no effect what so ever on the lines.But there is difference if i just plug in the cassette drive. Q1's Emitter starts to produce 0.8 volts.. Q2's base still gets the 0.8 volts from R3 Resistor.Any case pressing rewind and forward should work all the time..When i boot if left on rewind button runs for few hundred milliseconds as it gets the power but soon as something happens screen appears it stops and voltage is not provided. (what usually happens if you leave play button on and reboot it moves tiny bit but rewind forward should keep running)Removing VIC is puzzling to me also but it might be that without VIC the MPU do not boot and as result it not to interfere with the Q2 (line stays low and Q1 activates)Quote

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 18, 2019, 11:23:25 am
content: Contact resistances seems fine.

author: fzabkar
author_rank: Super Contributor
timestamp: January 19, 2019, 01:27:11 am
content: ISTM that removing the VIC is a red herring. If the MPU doesn't see the VIC, then I suspect that iatt may halt or hang during the POST, in which case the cassette port may not be initialised.IIUC, the 5V pin supplies the logic in the cassette while the 6.8V (6.2V ?) pin supplies the motor (and analogue circuitry?). Therefore you should see P4 go low when PLAY is pressed. I would think that this pin would be an open collector output from the cassette, in which case you should be able to short it to ground without risk of damage (as already suggested by others). If P4 does not go to ground (as you appear to be saying), then the fault must be in the cassette or the cable. But you have tested other cassettes with the same result. :-?

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 19, 2019, 04:46:51 pm
content: All the datasette schematics checked show that 6.8V is "motor on" (motor supply only).The P4 line is driven low by switch that closes on pressing any key that requires the motor to spin.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 19, 2019, 08:53:42 pm
content: QuoteI agree, i think removing VIC was red herring and it only stopped MPU initializing.Still this lead to situation where C3 pin got voltage cassette drive needs to operate the motor this was "simulated" with removal of Q2.Results of this was cassette player functioned (motor) and play rew etc worked but the cassette player was not detected by computer and could not be used.P4 do not seem to go low with press of play (I think reason is that the MPU?" do not detect there being a cassette player)It might be the core of the problem is MPU do not detect the player (I have tried with 3 players)Quote from: shakalnokturn on January 19, 2019, 04:46:51 pmYes this is confusing me , There should be no need to press play to active Q2 as rewind and forward should work all the time.

author: fzabkar
author_rank: Super Contributor
timestamp: January 19, 2019, 09:16:05 pm
content: If you examine all the schematics URLs on the following page, you will find that the PLAY switch simply connects the P4 pin to ground.http://www.zimmers.net/anonftp/pub/cbm/schematics/datassette/

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 20, 2019, 07:21:21 pm
content: Quote from: Bashstreet on January 19, 2019, 08:53:42 pmConfusing me too actually! I don't know enough on the C64, sure some people out here do though.As the only way to supply motor is through Q1, the P4 line must be pulled low also for REW/FF.Then I'm not sure how this is handled once you have cued the tape and LOAD or SAVE a program or do you just have to: cue, pause, LOAD, un-pause, wait... ?Your problem could very well come from the MPU not detecting the Datasette (therefore no motor power switching), however it is rather unlikely that the CPU itself will stop the P4 line from changing state as all you have there to oppose to that is a pull-up resistor (RP5) and a logic input (if that was destroyed you'd have heat or other problems driving P4 low).If you can get P4 to go low (you can simply force it low for experimentation using a ground strap) and you have checked that the motor power switching works, if you still have no normal operation that leaves you with the possibility of a defective MPU.I suggest you open a Datasette and connect it to the C64 check it's cable continuity including ground and shielding.When Play is pressed P4 must go low, first problem to solve.Very often the supple contacts on tape mechanisms develop a black oxidation giving high contact resistance. (Does P4 decrease at all?)A contact spray and strip of paper as an abrasive is usually enough to do the trick.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 22, 2019, 12:51:13 pm
content: Well the mystery is finally solved.There was nothing wrong with any of the circuitry components IC's or power rails...It turns out Jiffy DOS i installed do not support cassette drive."It is not possible to use a tape drive (Commodore DATASETTE, etc) while the JiffyDOSKernal is selected. If tape access is attempted, an ILLEGAL DEVICE NUMBER ERRORwill occur.It is kind of good mystery is solved i just wish i did not have to waste hours upon hours troubleshooting and desolering and resoldering and lifting one pad..Any case Cheers for everyone for your help.

author: jeffheath
author_rank: Regular Contributor
timestamp: January 22, 2019, 10:38:12 pm
content: Quote from: Bashstreet on January 22, 2019, 12:51:13 pm