Q*bert knocker repair

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May 162020

For those that don’t know what I’m talking about, a knocker is basically a bit of a gimmick that was installed into a Q*bert cabinet. The idea is that when Q*bert falls to his death off the pyramid, the game gives out a thump as he hits the bottom of the arcade cabinet using a coil to fire the knocker. Clever huh?

Since I’ve owned this cabinet I have never been able to get the knocker working, so this weekend I decided that this was going to change.

I had previously purchased a replacement known working knocker, as the plunger that makes the knocking noise, on my original knocker, had totally seized up. When I got the replacement I hoped that it would be a case of just swapping the part and enabling dip switch 6 on the main PCB and that was it. Sadly I was very wrong!

So the first thing I did was grab a copy of the manual and have a look a the wiring diagrams for the knocker area. I also confirmed that the plunger was freely moving, which it was, and that the 1 amp fuse at F6 had not blown, which it hadn’t

Next I started to look at what voltage was required to power the knocker, and started to trace it all the way to the PCB from the Power supply. I attached my black multimeter wire to the ground strap and then I could see +30v DC at the soundboard (A6, J1-P1), and the sound worked ok, and from there went backwards through all these locations:

  • Video Power supply pin 6
  • Filter board (A8, J3-P3) pin 10
  • Filter board (A8, J10-P10) pin 10
  • Knocker connector (A7, J1-P1) pin 2
  • Knocker connector (A7, J1-P1) pin 1
  • Filter board (A8, J10-P10) pin 9
  • Filter board (A8, J3-P13) pin 9
  • Main PCB (A1, J1-P1) pin 6

At every location I could see +30v DC so was a bit stumped at this point as to why things weren’t working. What I decided to do next was to verify that everything worked beyond the PCB, and thus proving that the PCB had a fault, so what I did was ground the tab of the 2N6044 transistor @ Q2 on the main PCB, and nearly got the shock of my life when then coil activated and the knocker fired. This confirmed that the fault was on my main PCB.

I didn’t know if the fault was now a logic one, or an issue with the transistor at Q2. So I hooked up my logic probe to the main PCB and attached it to the 74LS377 @ A8, pin 15. This is the chip which sends a signal to the transistor in order to activate the knocker. I then played the game and watched my logic probe as Q*bert fell of the pyramid. I could see that the logic changed from a low state, to a high state confirming that it was trying to activate the coil and knocker.

This then led me to believe that the transistor @ Q2 had failed and this was where the problem was. Unfortunately I did not have any spare 2N6044 transistors but noticed that the same transistor existed on the PCB @ Q1 and Q4 and these were optional transistors for coin 1 and coin lockout, so I used one of these to replace Q2. I also confirmed with a tester that Q2 had failed, and it had:

Once the new transistor was in place, I put the main PCB in the cabinet, powered up and the game, and was immediately greeted with the sound of the knocker. I then played a game, and when Q*bert fell off the pyramid was delighted to hear the sound of him hitting the bottom of the cabinet. Knocker fixed!

Juno First Bootleg Repair

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Mar 312020

I picked up a Juno First bootleg PCB at the end of December but have only had chance to take a look at it in the last two weeks.

On initial power up the game would not boot. It was displaying a solid colour with lines covering half the screen. On checking the 68A09E CPU on the bottom board I could see the CPU was watchdogging (constantly resetting).

The first basic check that I performed was to verify all the roms on the upper and lower boards.  All of them checked out ok. Next I noticed a capacitor was missing on the lower board in the top left hand corner. Based on the schematics of the original PCB and the other capacitors that appeared on the bootleg PCB, I guessed that this was probably a 220uf 16v capacitor. I also noticed a missing tantalum capacitor, but found pictures online which confirmed it was missing on other boards. I also noted a missing transistor on the top board but this was only to do with a coin counter so was not important.

Next up I started checking the clocks on the processors.  I always get confused with the 6809’s and it’s variants so I wasn’t sure what pins the clock should be on.  It could be either 34/35 or 38/39.  On my board the clock was only showing on pin 34 which I thought was a bit odd.

My full results were:

39 – L
38 – H
35 – H
34 – P

I asked a fellow collector to check his bootleg PCB which partially worked, to see what he had on his pins.  He confirmed the following:

39 – L
38 – P
35 – P
34 – P

So I knew my first issue was one with the clock on the 68A09!

I checked the actual clock was working by looking at one of the pins on the crystal before I started looking any further.

The next problem is that there are no schematics for a Juno First bootleg, only an original board.  Luckily Porchy had previously had an issue with the clock on a Juno First bootleg and mapped out a schematic here:


This made it VERY easy for me to track down where the problem was.  I started at pin 35 at the CPU and then went backwards.  I could not see the clock at pin 8 (E6) but I could see the  clock at pin 9 (E6).  As E6 was an LS14 hex inverter I would expect the same results at pin 8 that I could see at pin 9, and clearly this was not the case, so it seemed as if this hex inverter was not working correctly. I swapped it out and was pleased to see the game boot!

Just when you think the board is 100% and you try and play a game I noticed only partial sound when I started the game. Before digging too deep into this problem, the first thing I did was swap the AY-3-8910 in case this is where the problem was.  I’m glad to say swapping that restored all the sound and the bootleg board was fixed 🙂

Atari Dunk Shot to Tehkan World Cup Update 3

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Mar 022019

I’ve got a few minor things to do but I’m classing this project as finished.

Outstanding jobs are

  1. Fit Power Switch
  2. Swap Opto boards on the player 2 side as the controls are a bit jittery
  3. Mount the PCB in the cabinet
  4. Take a look at the PCB on a test bench as it has developed a small graphics issue

Paperboy PCB Repairs

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Feb 202019
Paperboy on the bench

A fellow arcade collector asked me to take a look at a Paperboy and Championship Sprint PCB as their cabinet had been standing idle for a long time. I intended to look at my own Championship Sprint and convert it to Paperboy so it seemed the ideal time to look at all three.

The first task was to visually inspect the boards and I found a number of problems, and ended up replacing corroded connectors, missing clock crystals, broken roms, dead rams, and various problem sound chips.

The second task was to setup a test bench without the need for an Audio Regulator 3 and Atari power brick which takes up lots of space! This was challenging because the PCB uses odd voltages like +15v, -15v and +10.3v DC. I had previously been told that if the +15v was not exact, this would stop the board from booting.

After a lot of experimenting I found that the PCB would run off a single switcher for test bench purposes. +12v DC is fine for both “Power OK” (10.3v DC), and as a substitute for +15v. In order to get sound, and for inputs to work, you need to have -5v DC which is derived from -15v on the PCB. To bypass the -15v you can clip -5v to the -5v end of the capacitor marked C140 and this worked fine. I then added a small +5v amp that connected to the audio pins at J15 on the CPU board, so I could hear the sound.

The next challenege was to find a way to display the output of the game as this uses a medium resolution monitor, and they are not common. I found that it was possible to use an Acorn CRT monitor (AFK18 or AFK52) or an LCD like a NEC AccuSync LCD 1770 nx

I also found that for the different monitors you also had to put inline resistors on the Red/Green/Blue to get the perfect image.

For the NEC – 270 Ohms
For the Acorn – 33 Ohms

I was now in a position to power up the boards and see if they worked.


On my own PCB I had a number of issues.

1) The sound was not working, and sometimes I would get a 6502 communication error – The 6502 error was caused by a bad rom socket @ 2C.  If the roms or sockets are faulty @ 2A-2C it can generate this error.

2) On the self test, parts were missing on the scrolling playfield test, there were no colour gradients visible and I was missing the individual colour grey. In-game the same parts were missing – This turned out to be a fault with the Q2/Q3 transistors in the colour intensity section.

Paperboy was looking better!

The next problem was the audio. I had music and sound from the audio chips but it was distorted. Initially I thought this was an issue with the LM324 @ 9J/K.  I tested pins 10 with an audio probe and the sound was good, but was distorted on pins 8 and 9.  I isolated the pin 8 output by lifting the leg of the cap @ C64 and used an audio probe on the other leg of the cap.  The music was still distorted. So I replaced the Op Amp, but still had the same issues! The problem could only be in a small number of components and in the 2.5v adjust section. I measured the voltage at one end of resistor R105 and got 5 volts as expected.  I then measured the other side expecting something like 2.5v though a 1K resistor, but got 0.64v, so this is where the problem was. I had a look at components R105, R104 and C91, and all of them had corroded legs so decided to replace them all, and sound and music was now as expected but the speech was twice as fast!

This one took me quite a while to figure out.  I checked pin 8 on the Speech chip with an audio probe and the speech was also twice as fast at this point, so confirming that the fault was somewhere before the LM324 amp.  I thought it might be a fault with the speech chip itself but it was good, and all voltages to it were fine.

Obviously this was a timing issue, but I had no clue where to look, as it wasn’t clear to me on the schematics how the clock circuit in the speech section was all tied together. I brought up the datasheet for the TMS5220C and noted all the pins used by Paperboy.  The one that really stuck out for me was the OSC @ pin 6 which was the oscillator input, as it looked like the clock pin at pin 3 was not used by Paperboy. The datasheet also mentioned that the oscillator needs to run at 640khz. 

So I got the scope out and connected to pin 6 and measured the wave and it was reporting as 1.25Mhz, which when converting to Khz is twice as fast, so I was on to something! I then looked in the Tclk, and T1clk section with the scope taking measurements and this was still reporting as 1.25Mhz to the LS163 @ 4D.  I also confirmed that both pin 2 was 5Mhz and pin 12 was 1.25Mhz.

At this point I had a look at a working Speech board, and it was 1.25Mhz @ pin 12 so this was correct.  However it then went to the LS74 @ 4C/D.  This had a clock pin @ pin 11 from the LS163 which was 1.25Mhz, but the output pins @ 8, 9, and  data 12 showed 640Mhz, whereas my faulty board was showing 1.25Mhz at all these pins! 

This would conclude that the fault was at the 74LS74 because this should be dividing the clock signal and it is not doing so.  I changed the LS74 but this sadly did not fix the problem! The fault actually turned out to be the chip before the 74LS74 @ 4C/D. The 163 at 5D had a missing 1.5mhz signal @ pins 9 and 15, and also double speed @ pin 13 (2.5mhz instead of 1.25mhz). Replacing this chip fixed the issue

I then noticed a problem where the left coin input was permanently on. I tracked this down to no voltage beyond the resistor at R118, so input was permanently low. Replacing R118, and the input was now working as expected.

Strykr’s boards

Strykrs Championship Sprint PCB had a couple of faulty roms, but nothing major at all to report. His Paperboy CPU board was also good, but the video board had a number of problems after replacing a faulty rom:

(Blue actually looks Purple on screen but blue on camera!)

I checked the transistors in the blue, green and intensity section and found no issues, so I was a bit stumped as I was sure this was a straighforward colour problem. The resistors also checkeded out ok. So I thought I would independently check the colours to see what was what. All the solid colours checked out ok, on each output, but I noticed that both red and green were broken on the intensity section, when looking at them independently. Blue was fine. Also when all the colours are combined blue, and green show incorrect colours, but red is ok. So as the problem seems to be shared by red and green I thought I would check to see which chips it might use for both. I came up with 12L and possibly 13K thinking one of these must be at fault, but found no issues with those on a logic probe and piggy backing them. So I started tracking backawards looking at the independent colours R0-3, G0-3, B0-3 on the 374’s @ 12K and 13H I initially found no issues with those using a logic probe so dug out my scope and starting looking at the signals.

What I noticed was that R0-R3 had a very weak signal, so I tracked that back further to CRD12-15 on the 374 @ 12J, and then back to the color ram addressing and data buffers. I checked pins 11-14 on the 2149 @ 11H and found the same issues, and then tracked back further to the 244 buffer @ 10H and found that this chip had a decent input from the playfield section but had no output at this buffer as you can see below:


I piggybacked a 244 @ 10H hoping this would resolve the issue but sadly it didn’t although I know piggybacking buffers generally does not work. The good thing is I could clearly see an issue, so I thought I would also try and piggyback the 2149  colour ram @ 11H to see if this was affecting the buffer output rather than the buffer being faulty and voila…..colours corrected!

Unfortunately changing the colour ram only fixed half of the issues.  The intensity section was still broken and the in-game colours were still wrong….but looking better!

This made me think that *potentially* other colour rams were faulty, so I started to look at them and found that 11J next to the faulty ram also had weak signals, so I decided to replace both.

2149 ram is really expensive and looks like it is hard to get hold of so I thought I’d try some 2148-45 in replace of the 2149-2’s

I was pleased to see that on power up the video board looked to be 100% fixed.  Both in-game colours were correct, and the video intensity check looked right.