Level42
New member
I bought 3 Berzerks last saturday and I want to get them as much running as possible.
I also got one extra ZPU board with the deal. I will call that board [1] so I don't confuse things too much.
Started work on this board tonight. Of course I first removed all the batteries, neutralized the acid damage with vinegar and tried to clean the boards as good as possible. Two have little acid damage, the others are a bit worse.
This board had Q2 damaged (broken pins) so I replaced it. I checked the tracks since there was some damage there. Turned out the Base and Emitter were fine but the track from the collector to R28 was open. I jumpered that. No other parts look/feel physically damaged.
Powered up this board with a AT power supply. Took me some time to realize that the board needs two +5V supplies (I connected the other voltages to the test-points). The 2nd +5V is specced as "unregulated" on the schematics. Anyway I jumpered the +5V to the pin of the unreg. +5V, I hope and suppose that's fine.
When I did this and powered up I had the LED on for the first time. There is a clock signal, but the reset line was constantly low so no "flashes".
When I forced the reset line high (by shortly shorting it to +5V, I don't know if it's healthy but it's a trick I learned from my Centipede board repair) I got what looks like 2 or 3 flashes, not sure.
Is it normal that the LED goes off after the flashes ?
So I traced back the reset line and figured I'd examine the 74LS00 and 7406 (7G and 8G).
I used the old "Randy Fromm" trick and hook up the + of my Fluke to the GND and tested all the pins of those two ICs. And indeed, I measure a short at pin 10 at 7G that isn't supposed to be there. In fact, that pin is directly connected to +5V !
When I measure resistance between +5V and GND connection, it gives about 50 ohms, that can't be good. But this also means it can come from "everywhere" on the PCB.....
To be sure, I measured the resistance on the other boards and it seems that about 2k is normal.
The easiest first step was to hook up the Fluke to +5V and GND with some clamps and remove the socketed chips one by one to see if the short would disappear. I started at the RAM at E1 and E2 and I got lucky because when I removed E2 the resistance jumped to about 2k ! Bingo !
Measured the 2114 RAM and there's a short between pin 9 (gnd) and the other pins, and the short was on the Write Enable pin 10.
So, swapped the RAM with one from the other boards and had high hopes. Start up, LED on, but no flashes......
Reset still low.....
Traced back reset through 7G and 8G and pin 9 of the NAND port in 7G is constantly low.
So, there has to be an issue with the transistor circuit I guess. Right ?
Can anyone explain how that circuit works ?
I also got one extra ZPU board with the deal. I will call that board [1] so I don't confuse things too much.
Started work on this board tonight. Of course I first removed all the batteries, neutralized the acid damage with vinegar and tried to clean the boards as good as possible. Two have little acid damage, the others are a bit worse.
This board had Q2 damaged (broken pins) so I replaced it. I checked the tracks since there was some damage there. Turned out the Base and Emitter were fine but the track from the collector to R28 was open. I jumpered that. No other parts look/feel physically damaged.
Powered up this board with a AT power supply. Took me some time to realize that the board needs two +5V supplies (I connected the other voltages to the test-points). The 2nd +5V is specced as "unregulated" on the schematics. Anyway I jumpered the +5V to the pin of the unreg. +5V, I hope and suppose that's fine.
When I did this and powered up I had the LED on for the first time. There is a clock signal, but the reset line was constantly low so no "flashes".
When I forced the reset line high (by shortly shorting it to +5V, I don't know if it's healthy but it's a trick I learned from my Centipede board repair) I got what looks like 2 or 3 flashes, not sure.
Is it normal that the LED goes off after the flashes ?
So I traced back the reset line and figured I'd examine the 74LS00 and 7406 (7G and 8G).
I used the old "Randy Fromm" trick and hook up the + of my Fluke to the GND and tested all the pins of those two ICs. And indeed, I measure a short at pin 10 at 7G that isn't supposed to be there. In fact, that pin is directly connected to +5V !
When I measure resistance between +5V and GND connection, it gives about 50 ohms, that can't be good. But this also means it can come from "everywhere" on the PCB.....
To be sure, I measured the resistance on the other boards and it seems that about 2k is normal.
The easiest first step was to hook up the Fluke to +5V and GND with some clamps and remove the socketed chips one by one to see if the short would disappear. I started at the RAM at E1 and E2 and I got lucky because when I removed E2 the resistance jumped to about 2k ! Bingo !
Measured the 2114 RAM and there's a short between pin 9 (gnd) and the other pins, and the short was on the Write Enable pin 10.
So, swapped the RAM with one from the other boards and had high hopes. Start up, LED on, but no flashes......
Reset still low.....
Traced back reset through 7G and 8G and pin 9 of the NAND port in 7G is constantly low.
So, there has to be an issue with the transistor circuit I guess. Right ?
Can anyone explain how that circuit works ?
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