Bomb Jack: Fix logs and other information

BOMBJACK (Tehkan)

1. Picture will not sync correctly also no sound: [Jammajup Repair Logs]

SOLUTION: Picture had sync problem, looking at schematics i could see a 7408@1T was controlling sync output. I used a logic probe and checked the outputs of the IC and discovered one output @ pin 8 was stuck low. Replacing the 7408@1T restored the picture.
Replacing the 51516 audio amp restored sound.

2. Black screen with the odd flecks of colour. [Pinx]

The game runs with 2 clocks (crystals) a 4.000 MHz at J4 and a 12.000 MHz one at between T4 and T5, using a logic probe, i checked the J4 one for a pulse and this was fine, but probing the one at T4 / T5 caused the game to boot up and play ok. The only thing i could think of was that either of the two capacitors in that circuit were leaky, so i replaced both the 103Z and 221J caps at T4 and this cured the problem.

3. After only 3 or 4 minutes of playing the sound would deteriorate until there was no sound. [Pinx]

The first thing I did was touch the amp to see if it was getting very hot and it was, but as soon as I did this, the coolness of my fingers made the sound work again, so I used some freeze spray on it and it made the sound last 6 or 7 minutes. So at first I added 2 small heatsinks to the amp, this made the sound last for 15 minutes or so. In the end I replaced the M151516 amp and this fixed the problem.

4. Missing the green and blue colours and sound was stuck in a constant repeating loop from power up. [porchy]

The RGB, sync and video ground go to a 5 pin header on this bootleg, not a 6 pin header, the previous owner was clearly not had an adapter or plug for anything on this board and had soldered the wires directly on, had also melted all the plastic supporting these 5 pins so a couple were shorting to ground, easy fix. The sound had a corrupt EPROM, I erased and burnt a new one, still no change. The RAM had already been socketed so I tested it and it came up clear. Checked the Z80 with the logic probe and found the address lines were all high except for a couple that were pulsing. Desoldered it and tested it, it failed on pretty much everything, replaced it and now the sound was working but seemed to be lagging behind when compared to MAME. After much head scratching and some very bad words I tested the RAM again on a continuous cycle in the ChipMaster tester, whilst it was testing I put in a new one so I could continue testing the board and fault fixed! Swapped the RAM back and fault is back. This RAM works fine in my other boards so am putting it down to RAM speed.

5. Video reddish tinted all over. [DarrenF/KLOV]

Diagnosis: scoping video output; red video signal is always high (except during vblanks).

Further investigation: following schems backwards from R output, it comes almost directly from a color palette RAM (as opposed to a color PROM, which is more common). The signals on the data lines of this RAM looked different than those for the Green or Blue outputs. Piggybacking a new RAM clears up the problem. [Interestingly, the power-on self-test does check RAM, but apparently not this RAM, as it did not report any problems at bootup.]

Solution: Replaced 2114 RAM @ 6A

Result: PCB now works fine.

6. Three horizontal lines of white pixels towards the bottom of the display (bottom board) [Vectorglow]

Diagnosis: Thought it was probably a video RAM problem but decided to see if the fault changed as the chips warmed up. Sure enough some of the pixels faded away so applied freezer spray to the suspects on the lower board. Sure enough the faded pixels came back good and strong when freezer spray was applied to the 6148 at location 4B. Tried piggy-backing a 2148 (I didn’t have any 6148’s) on top of that and that ‘fixed’ the fault, so removed the old 6148, popped in a socket and a new 2148

Cure: Replaced faulty 6148 RAM at location 4B

7. All moving sprites are ‘shredded’ into vertical bars and data in one ‘bar’ is produced in the rest (bottom board) [Vectorglow]

Cure: Pin 10 stuck low on the LS283 at location 5E – replaced

8. Horizontal lines (about half a character high) throughout the background graphics [Vectorglow]

Cure: Bad output on pin 15 of the LS194 at 7K – replaced

9. No video. Game and sound seem OK. [Paul Swan]

A logic probe indicated that R, G and B were always low but the Sync was running. The colour signals came from a couple of resistor networks and two LS174’s. A frequency meter indicated a 6Mhz pixel clock on the CLOCK pin and a 57Hz clock on the CLEAR pin of both devices. The inputs to the LS174’s came from three 2114 RAM devices (to give a programmable colour palate, one 2114 per colour giving four bits per colour), and the inputs were shared with the CPU via a couple of LS245 buffers. The RAM data pins were pulsing, as was the buffer control lines and CPU data bus on the other side of the buffers. The address and control lines on each of the 2114 RAM’s were pulsing. Using a pulse injector on the 2114 data pins affected the video and sometimes caused moving colour bars to appear on the monitor, that cleared shortly after. This indicated that the LS174’s must working, and at least some of the 2114 RAM locations. Setting up a logic analyser on one of the 2114 RAMS revealed the the CPU was writing valid data to valid locations in the RAM, but most of the address lines were always low when the data was read out during read cycles, resulting in the read out data being all-zeros for all three colours. The address lines were fed by a bank of LS157 two input multiplexors. The A-side inputs were probably fed from the CPU address bus and were pulsing. The B-side inputs of some of the multiplexors were always low. The mux control lines were pulsing. The B-side inputs were in turn fed from am LS273. The corresponding inputs to this device were always low, and it’s clock and control line were pulsing. At this point I noticed some damage to some traces near by. Three traces were damaged and two were severed. Connecting the two disjoint traces together had no effect on the fault (ho hum). Continuing tracing back indicated that the LS273 latch bank was being fed from a bank of LS153 four-input multiplexors. These probably acted as the video mixer, selecting pixels from the three (presumably) video planes (one input was unused). Examining two devices in detail indicated the all the inputs, outputs and selects were low. All these inputs were fed from various sources. The SELECT inputs were fed from a LS148 priority encoder, the inputs to which were all low. Some inputs were fed from a couple of LS174’s and some from an LS27. It looked as if an entire section of the video circuitry wasn’t running. Pin 10 of one of the LS153’s had a resistance of 15 Ohms to ground, and this was selected as a fault (even it it wasn’t THE fault). The suspect net also went into an LS27 at pin 9 (an input). Another input (pin 10) of the LS27 had a resistance of 70 Ohms to ground. Tracing the second faulty net revealed that both of the faulty nets were the two that had been severed. Checking the third of the damaged traces revealed that it had a resistance of 6 Ohms to ground. Whatever had damaged the three traces originally must have had a high voltage on it such that it had destroyed some devices attached to the nets (operator probably caught the board on a live monitor chassis whilst removing it or something). All three damaged traces went off card onto the CPU board and disconnecting the ribbon cable cleared all three resistances on the video board. The faults were on the CPU card. Two traces originated from an LS157 and the third went into an LS04 near the ribbon connector and an LS08 at the other end of the board. Removing the LS157 and the LS04 cleared the resistances and replacing them fixed the game completely.

10.

Technical Notes:

1. RAMs [reference JammaPlus]

Top board:

3 x 2114 rams at A6, B6 and C6 = colour RAM (1 each for R,G,B) which is checked on boot

2 x big Toshiba TMM2016 rams = RAM #4 (5th RAM checked) is the 6116 equivalent at 6L

Bottom board:

4 x MBM2148L-55

2 x 2114 rams at E3 and F3

2 x big Toshiba TMM2016 rams.

2. Bootleg vs Original

The most obvious differences are the lack of Tehkan markings and the dodgy ROM labels.
Others include; the video connector (original is parallel to the PCB not at right angles), no power LEDs, no heatsink, 2114s/2148s not socketed, Toshiba 6116s…