Nascom 2
|
Bought as 'not working or Parts'. When it arrived it looked like it had been stored in a pigeon loft for 40 years.... Not really, but it was very dirty, so I did wear mask and gloves when I cleaned it outside.
My plan was to recap where necessary, replace the missing IC's, and individually test all the other IC's in the Retro Chip Tester Pro, before any attempt to power it up. |
|
|
Transformer:
Dirty and rusty, cleaned and de-rusted with 'Evapo-Rust'. To avoid the casing rusting up again I gave the outer a coat of Hammerite (oddly enough my tin of Hammerite was about 40 years old too, from when I did this sort of stuff before.) For the historic record the windings measured as follows: Resistance Inductance Rated Voltage Measured Voltage(OC) Primary: 42R 7H 240v Secondary 1: 0.8R+0.8R 55mH+55mH 15-0-15v 16.94v-0-16.94v Secondary 2: 0.1R 17mH 9v 10.25v |
PSU Board:
Started off with the main capacitors...
C1/C2 3300uF/16v, signs of corrosion at the ends and measured at 4000+uF with ESR of 0.44R, on the high side both for capacitance and ESR so changed for Nichicon 4700uF at 50v.
C6/C10 1000uF/25v, looked OK and they were Rubycon but they measured slightly high at 1265uF with an ESR of 0.27R so swapped these out for Nichicon 1000uF at 50v. All 4 above were axial capacitors mounted right next to heatsinks, so I fitted radial.
Bridge rectifier:
temporarily removed and tested for reverse leakage/forward voltage drop, all OK so refixed and heatsink remounted with Thermal Grizzly paste, later changed for a higher capacity one plus mounted to the aluminium case side.
Voltage Regulators:
All removed and tested on bench supply, refixed with Thermal Grizzly paste.
Looking at the traces on the PSU, some of them has signs of corrosion, my main concern was that at least one of them was on the high current +5v rail, so after cleaning off the solder mask I flowed solder along the length of the trace.
So..... the only untested components at this stage were the tantalum capacitors, as I didn't have any of this value I thought, 'what could possibly go wrong'..... I reconnected the board to the transformer and filled the room with capacitor smoke (just after 2 of them exploded!). As I have to wait for the new ones to arrive I removed all the others and put them across the bench supply, gradually increasing the voltage up to the max of 35 (rating of the capacitor), all but 1 exploded at just above 12 volts! (Never trust old Tantulums!)
Started off with the main capacitors...
C1/C2 3300uF/16v, signs of corrosion at the ends and measured at 4000+uF with ESR of 0.44R, on the high side both for capacitance and ESR so changed for Nichicon 4700uF at 50v.
C6/C10 1000uF/25v, looked OK and they were Rubycon but they measured slightly high at 1265uF with an ESR of 0.27R so swapped these out for Nichicon 1000uF at 50v. All 4 above were axial capacitors mounted right next to heatsinks, so I fitted radial.
Bridge rectifier:
temporarily removed and tested for reverse leakage/forward voltage drop, all OK so refixed and heatsink remounted with Thermal Grizzly paste, later changed for a higher capacity one plus mounted to the aluminium case side.
Voltage Regulators:
All removed and tested on bench supply, refixed with Thermal Grizzly paste.
Looking at the traces on the PSU, some of them has signs of corrosion, my main concern was that at least one of them was on the high current +5v rail, so after cleaning off the solder mask I flowed solder along the length of the trace.
So..... the only untested components at this stage were the tantalum capacitors, as I didn't have any of this value I thought, 'what could possibly go wrong'..... I reconnected the board to the transformer and filled the room with capacitor smoke (just after 2 of them exploded!). As I have to wait for the new ones to arrive I removed all the others and put them across the bench supply, gradually increasing the voltage up to the max of 35 (rating of the capacitor), all but 1 exploded at just above 12 volts! (Never trust old Tantulums!)
|
|
|
|
|
After some thought I changed my mind about the power supply PCB, there was so much corrosion on some of the tracks that I could actually measure up to a 0.5v drop along them under dummy load. A new PCB was ordered from CodeSqueak and mostly new components fitted. I added pins to the led points as I wanted these on the front panel. |
|
|
I later changed my mind on this, and went all out and installed voltage monitors on each of the supplies, colour coded to the same colour of the main power wiring. |
|
Keyboard
I don't know where to start! The keyboard was filthy, so needed a good scrub under the tap, even this didn't touch a lot of the corrosion. The 1,2,G,Y,D,T,H,X,C keys were permanently stuck down, and in some cases could not be lifted! It made me wonder if someone had previously tried to change a few of the keyswitches as some of the pads were lifted/missing. There was a broken track between 'L' and ',' so I jumpered that. Initial test showed a fully unresponsive keyboard, no clock signal showing, traced to a short between pin2 of IC24 and ground, found to be a solder bridge. |
|
|
First step was to desolder all the keyswitches for a closer look. dismantling was fairly straight forward but revealed the disaster inside, almost all the switches had corroded away and in some cases the steel wire loops were just rust. This is just an example of what came out of them, the top left corner shows 2 new steel wire loops that I made, 90% of them needed new loops! Also, I wondered why they had used steel loops... I remade 2 switches, one with Steel loops and the other with Copper loops. In testing (see below) I found that there was absolutely no difference in the response characteristics, so, the decision was made to use Copper from then on, as this was easier to form. |
The keys work as follows:
With the key unpressed, any pulses on the scan loop are blocked because the 2 magnets are surrounding and saturating the ferrite core, when the key is pressed the magnets are moved out of the way and incoming pulses are passed (albeit inverted) on the sense loop.
To test each key I injected a 500KHz waveform in, and monitored the output, the two traces show as follows:
Key unpressed, the input waveform (yellow) is dampened because the ferrite core is saturated, when pressed the input waveform is unconstrained and also shows an output waveform (Green).
With the key unpressed, any pulses on the scan loop are blocked because the 2 magnets are surrounding and saturating the ferrite core, when the key is pressed the magnets are moved out of the way and incoming pulses are passed (albeit inverted) on the sense loop.
To test each key I injected a 500KHz waveform in, and monitored the output, the two traces show as follows:
Key unpressed, the input waveform (yellow) is dampened because the ferrite core is saturated, when pressed the input waveform is unconstrained and also shows an output waveform (Green).
|
|
|
To assist with faulting later on I decided to desolder all IC's and individually test them, 4 of the IC's failed, IC1, 5, 6 & 9, odd that these form the main matrix for the keyswitches (perhaps more likely that the other 7400 were more tolerant)
While there I tested the only tantalum on board, it was short circuit so, upgraded it from a 22uF/10v to a 100uF/16v The keyboard steel mounting plate was very rusty on both sides, losing a lot of its paintwork, so, sanded back, treated with kurust then spray painted matt black. With the mounting plate refitted with bolts/nuts it was time to reinstall the switches. Once all the switches had been installed I checked all suspect traces for continuity and all adjacent points tested for shorts, then the IC's were reinstalled, then the board was cleaned with IPA. Keytops refitted after an extra wipe with Aerospace 303. The top of the PCB was treated with silicone conformal coating, and the bottom with acrylic conformal coating. Finally gave the ribbon cable a good clean. |
I took the opportunity to replace the NAS-SYS1 rom with a NAS-SYS3 EPROM, and also added a Graphics EPROM
I fitted 8 * 4118 RAM IC's to give the main board 8k RAM and rewired LKS1 to accommodate. I later changed the Z80 to a Z80A. |
Main PCB Needed a good clean down with IPA to begin with. I removed all the IC's, many of them had pins that were so corroded that they did not have any contact with the chip body, as a result when I removed them the pins just stayed in the sockets. Where there was only 1 or 2 pins lost I resoldered new pins from a good donor IC. Any more than that and I replaced the IC with one from stock. The following had pin loss: IC:12,13,14,16,17,57,58. 57 was so bad that all but 2 of the pins were broken! All IC's were then tested in the chip tester, with the following failures: IC: 3,4,23,24,30,63 All sockets were cleaned thoroughly with DeOxit, the pin headers were 'crusty' so needed a good clean and pins straightened. |
Also added was the Screen Snow eliminator board (from CodeSqueak again)
|
|
The BASIC ROM had several broken pins, these were actually snapped off at the chip body, so, using an engraving drill I removed the epoxy above the missing pins and added new ones, back filling with new epoxy.
IC56 pin 1 and IC35 pin 4 were not soldered, redone. There was a 6.8nf capacitor (or what was left of one) between IC71 pin 6 and 0v, this is the LD line going to IC65 pin 1 (LS165), not sure what this was for, nothing in the schematic and could not find anything on the web so I removed it, I can add it back later if necessary. All Tantalum capacitors were then changed, in many cases for higher voltage versions. The board was tested at this point.... No clock as the crystal was faulty so this was changed. The wire on IC11 was a temporary power on reset line (later removed) as the keyboard reset was not working, this was down to a faulty LS221 which was ordered and changed (interestingly the LS221 actually tested OK on the RCTPro) |
I think this is the first hint that I am using Lego for parts of the case! :)
|
|
Backplane: The original backplane was a 2 slot veroboard, this was scrapped though I did recover the 2 edge connectors for reuse. I purchased a new 5 slot backplane (CodeSqueak yet again) and added the edge connectors. |
Memory board(A)
Originally strapped as:
P4-5 MEM Select 0000-0FFF
P5-10 4k ROM Blocks
P6 unconnected
P7-1,6,7,8 IC4..11 1000-4FFF
So, this looks like one of the early Nascoms that didn't include the main board 4118's, as I had a box of 4118's I planned to equip both the main board memory and completely fill the expansion board.
All tantalums changed though I wasn't going to power it up before a few more issues addressed...
Originally strapped as:
P4-5 MEM Select 0000-0FFF
P5-10 4k ROM Blocks
P6 unconnected
P7-1,6,7,8 IC4..11 1000-4FFF
So, this looks like one of the early Nascoms that didn't include the main board 4118's, as I had a box of 4118's I planned to equip both the main board memory and completely fill the expansion board.
All tantalums changed though I wasn't going to power it up before a few more issues addressed...
|
The board was filthy, all IC's removed, crusty legs cleaned and board and sockets washed in a bath of IPA. |
|
|
I was not happy with the mods on the back so I removed them all and rewired them. |
|
|
I also added some recommended mods in the form of resistors on the top of the board and small capacitors on the tracks side. The board was 'memoried' up and initial tests showed the memory board as dead in the water. IC's 1,2,3,20,21,22,23,24,25,26,31,33,34,35 all tested faulty in the RCTPro! Something serious had hit this board at some point to destroy that many IC's!. Oddly, the 74157 tested OK in the RCTPro but failed on the board, when I put this on a breadboard and clocked it at 1MHz it was not happy! I wondered at first if the RCTPro was acting up so I checked them in the T48 programmer (which also tests logic IC's, and they failed in that too) New IC's fitted. Found a track with a hairline cut so rejumpered. I decided not to use this board for any EPROMs as I had a better solution. |
So, after all that it still had issues, inconsistent addressing, traced to a high resistance R9, changed.
Now it booted up and 'worked', but had a 'tap' problem... if I tapped the board it reset the Nascom. Bloody single wipe sockets again, thoroughly cleaned with neat DeOxit, cured!
Now fully working. I did have to fit 74HC157's temporarily as I didn't have any 74157's, and 74LS257's did not work there, but later on I replaced them with true 74157's
Now it booted up and 'worked', but had a 'tap' problem... if I tapped the board it reset the Nascom. Bloody single wipe sockets again, thoroughly cleaned with neat DeOxit, cured!
Now fully working. I did have to fit 74HC157's temporarily as I didn't have any 74157's, and 74LS257's did not work there, but later on I replaced them with true 74157's
|
Eprom expansion board (yet another CodeSqueak board! :)
Brand new board, I added all the Sockets, etc, populated it up and programmed 2*2732's Giving me NAS-DIS, ZEAP This now fills the entire 64k memory map...(apart from B000-BFFF, which I have pre-jumpered to Socket 1) Also, I replaced the BASIC ROM first with an eprom but this ran a bit slow so replaced this with a AT28C64B EEPROM. [Eprom Board Review What a beautiful board, very clean and well laid out. Oh, and it is PURPLE! Everything fitted perfectly and the instructions were clear.] |
|
I mounted the keyboard in a metal case found on ebay.
The rest of the electronics you may have noticed the increase in the use of LEGO!, The base was 4mm aluminium sheet which made it very sturdy (and heavy), the corners were castellated Lego 2x2 bricks, these formed a convenient slide to hold the sides, back and front panels. The top was a 2mm aluminium sheet. |
|
Final Memory Map
0000-07FF NAS-SYS3 2k 0-2047
0800-0FFF RAM WORKSPACE AND VIDEO 2k 2048-4095
0800-0BFF VIDEO 1k 2048-3071
0COO-0C7F OS WORKSPACE 128 3072-3199
0C80-0CFF EXTENDED OS WORKSPACE 128 3200-3327
0D00-0F7F WS FOR FW OR PROGRAMS 640 3328-3967
0F80-0FFF STACK 128 3968-4095
1000-2FFF Main Board RAM 8k
3000-AFFF RAM Expansion board 32k
B000-BFFF Unallocated 4k
C000-CFFF Debug/NAS-DIS On eprom board 4k
D000-DFFF ZEAP On eprom board 4k
E000-FFFF BASIC ROM (Main Board) 8k
+++++++++++++++++++++++++++++++++++++++++++++++++++++
Additional Notes
PROMS
IC9 N2DB/3L IM5623CPE 7937 7611-5
IC26 N2IO/1 IM5600CPE 7907 7603-5
IC47 N2MD/3 IM5600CPE 7907 7603-5
IC59 N2V/2L IM5610CPE 7949 7602-5
IC20 UART SMC COM 8017 is a 6402
IC32 ETA-94 is a 4049
MK36000 available as MCM68766C EPROM
0000-07FF NAS-SYS3 2k 0-2047
0800-0FFF RAM WORKSPACE AND VIDEO 2k 2048-4095
0800-0BFF VIDEO 1k 2048-3071
0COO-0C7F OS WORKSPACE 128 3072-3199
0C80-0CFF EXTENDED OS WORKSPACE 128 3200-3327
0D00-0F7F WS FOR FW OR PROGRAMS 640 3328-3967
0F80-0FFF STACK 128 3968-4095
1000-2FFF Main Board RAM 8k
3000-AFFF RAM Expansion board 32k
B000-BFFF Unallocated 4k
C000-CFFF Debug/NAS-DIS On eprom board 4k
D000-DFFF ZEAP On eprom board 4k
E000-FFFF BASIC ROM (Main Board) 8k
+++++++++++++++++++++++++++++++++++++++++++++++++++++
Additional Notes
PROMS
IC9 N2DB/3L IM5623CPE 7937 7611-5
IC26 N2IO/1 IM5600CPE 7907 7603-5
IC47 N2MD/3 IM5600CPE 7907 7603-5
IC59 N2V/2L IM5610CPE 7949 7602-5
IC20 UART SMC COM 8017 is a 6402
IC32 ETA-94 is a 4049
MK36000 available as MCM68766C EPROM