In time this page will evolve into a more detailed description of the work the Yahoo Super 80 Group has done to create the “V1” and “V2” reproductions of the Super 80. But for now this is just a scratchpad where I am capturing my notes that are relevant to this exercise. For this reason there is no real narrative or flow to the content below.
Transformers and Power Supplies
The transformer used in the original Super 80 was Dick Smith Electronics’ P/N M2325. This transformer (according to a schematic that I have) describes it as having a 12-0-12 winding rated at 1 Amp, and a 7VAC winding also rated at 1 Amp. Unfortunately I’ve not been able to find a suitable replacement from any local Australian store.
When I started looking for a suitable replacement, I came across Signal Transformer’s P/N A41-80-512 on Digikey’s website. The datasheet (which is no longer on Digikey) shows that this transformer is designed to deliver (through a power supply with linear voltage regulators) +5V at 3.5A and +/-12V at 600mA. It sounds ideal. Back in 2014, this was priced at around US$50. Now (Jan 2018) the Digikey price has halved to US$25, but they have no stock. I wrote to Digikey and they say its 12 weeks lead time. So I have ordered 2. I hope this is not a discontinued product. I also wrote to the manufacturer (Signal Transformer) to check whether this is a discontinued product, but they have not responded.
If a transformer with dual secondary windings cannot be sourced, it will be relatively easy to use two separate transformers. But I hope to avoid that.
Another option is to feed DC into the S80 PCB (but without modifying the S80 in any way). I figure this can be done with the following Meanwell Products:
- NES-100-7.5: Adjustable 7.13V to 8.3V at 13.6A; or
- NES-100-9: Adjustable 8.55V to 9.9V at 11.2A
- NET-75C: 5V/6A (which we would leave disconnected), +15V/2.3A and -15V/0.5A
The NES-100-7.5 or NES-100-9 could be used to drive the +8V rail (via diode CR3). The NET-75C’s +15V and -15V outputs could drive the +16V and -16V rails (via diodes CR7 and CR5 respectively). I haven’t tried this out yet. I suspect the NES-100-7.5 might not deliver a high enough voltage to the +8V rail, due to the 0.9V drop across diode CR3. So the NES-100-9 is probably the better choice.
Another option is to bypass all the rectifier diodes and voltage regulators on the S80 and drive the +5V rails (there are two of them) and the +12V and -12V rails with a multi-output switch-mode power supply. Meanwell’s RQ-65B would be ideal for this: +5V/6A, +12V/2A, -5V/0.5A, -12V/0.5A.
There are many other Meanwell power supplies that could be useful, depending on how you want to power the S80. Keep in mind that if you chose to bypass the S80’s on-board voltage regulators then you will not be able to supply the higher “unregulated” DC voltages delivered to the S100 slot. So you will also need to bypass the regulators on any S100 board that you insert. Here are some of the more relevant models:
- PD-2515: Open frame +/-15V supply. Low current rating
- TP-75C: +5V/6A, +15V/2.5A, -15V/0.5A
- TP-75B: +5V/7A, +12V/3A, -12V/0.4V
- MPT-65A: +5V/5.5A, +12V/2.5A, -5V/0.5A
- T-50A: +5V/7A, +12V/2.5A, -5V/0.5A
- NET-35B: +5V/3A, +12V/1.0A, -12V/0.5A
- RQ-85B: +5V/7A, +12V/3.1A, -5V/0.5A, -12V/0.5A
- RT-85C: +5V/7A, +15V/3A, -15V/0.5A
- EPS-25-7.5: 7.5V/4.7A
- PT-65A: +5V/5.5A, +12V/2.5A, -5V/0.5A
Summary of changes
- Updated footprints for large electrolytic capacitors (C1, C2, C3 & C4)
- Replaced 2513 Character Generator with 2764 EPROM
- Implemented Technical Bulletin 63
- Implemented Technical Bulletin 53
- Implemented high-speed cassette interface (3,000 baud)
- Added a RESET push-button switch
- Updated the relay footprint (to suit available 5V/30mA coil relays)
- Updated footprint silkscreen for 20-turn variable resistor (POT1) to better match actual part
- Modify the LED footprint so a 2-pin header can be optionally fitted instead of a LED
Changes still to be made:
- Provide option for use of 4164 RAMs
- Provide option to switch ROMs out of circuit (for CP/M)
- Update the keyboard keyswitch footprints
- Consider allowing for 2732 ROMs
- RCA jack footprint (Digikey RCJ-024)
- Implement un-connected pin D3 of /INPORT
- Check whether the LED 0.8mm holes are sufficient for a 2-pin header
C1 and C3 (5600uF Capacitors)
C1 and C3 on he original board are 5600uF electrolytic filter capacitors. The voltage rating was not specified, but presumably they should be at least 16V (they filter the input to the 7805 voltage regulators). My original Super 80 is fitted with 5600uF 40V Elna capacitors, which fit the 3 holes in the PCB perfectly. The assembly manual says that C3 is only required if an S-100 card is being used. Taking measurements from the PCB, these capacitors were 1.2″ (30.5mm) in diameter, and the main two pads were 0.6″ (15.24mm) apart. There is also a not-connected third pin which presumably was to physically stabilise the capacitor.
I have not been able to find any suitable capacitors that have 15mm pitch between the pins. Suitable replacement capacitors (5600uF or 10000uF, 16V or 25V) typically have a pitch of 7.5mm or 10mm. So I have modified the footprint for C1 and C3 to accommodate the following:
- Pitch=10mm, Pad=4.7mm, Hole=2.0mm
- Pitch=7.5mm, Pad=3.2mm, Hole=1.2mm
- Pitch=0.6″(15.24mm), Pad=3.2mm, Hole=1.2mm
Suitable currently available replacements for C1 and C3 are (prices exclude GST):
- EPCOS B41231B5109M000 (Digikey P/N 495-6158-ND). 10000uF 25V Snap-in. Diameter=25.4mm, Height=30.0mmm, Lead Spacing=10.0mm. Leads=1.5mm x 0.8mm. Price=AU$2.60/ea
- EPCOS B41231A4109M000 (Digikey P/N 495-6140-ND). 10000uF 16V Snap-in. Diameter=22mm, Height=25mm, Lead Spacing=10.0mm. Leads=1.5mm x 0.8mm. Price=AU$2.08/ea
- Nichicon UHE1C562MHD6 (Digikey P/N 493-1543-ND) 5600uF 16V Radial. Diameter=18mm, Height=31.5mm, Lead Spacing=7.5mm, Lead Diameter=0.8mm. Price=AU$2.47/ea
- Generic 10000uF 25V (Futurlec C10000U25E). Diameter=21mm, Height=41mm, Pin Spacing=10mm, Lead Diameter=Unspecified. Price=US$1.70/ea
C2 and C4 (1000uF Capacitors)
C2 and C4 on the original board are 1000uF electrolytic filter capacitors. The voltage rating was not specified, but presumably they should be at least 25V (they filter the input to the 7812 and 7905 voltage regulators). My original Super 80 is fitted with 1000uF 25V Lihyeh capacitors, which fit the 2 holes in the PCB perfectly. Taking measurements from the PCB, these capacitors were 0.75″ (19.05mm) in diameter, and the main two pads were 0.345″ (8.76mm) apart.
I have not been able to find any suitable capacitors that have 8.76mm pitch between the pins. Suitable replacement capacitors (1000uF or 4700uF, 25V) typically have a pitch of 5.0mm, though a few have a pitch of 7.5mm. So I have modified the footprint for C2 and C4 to accommodate the following:
- Pitch=7.5mm, Pad=3.0mm, Hole=1.2mm
- Pitch=5.0mm, Pad=3.0mm, Hole=1.2mm
Suitable currently available replacements for C2 and C4 are (prices exclude GST):
- Nichicon UVR1E472MHD (Digikey P/N 493-1068-ND) 4700uF 25V Radial. Diameter=16mm, Height=31.5mmm, Lead Spacing=7.5mm. Lead Diameter=0.8mm. Price=AU$2.04/ea
- Nichicon UVR1E102MPD (Digikey P/N 493-1065-ND) 1000uF 25V Radial. Diameter=10mm, Height=20mmm, Lead Spacing=5.0mm. Lead Diameter=0.6mm. Price=AU$0.64/ea
- Generic 1000uF 25V (Futurlec C1000U25E). Diameter=10mm, Height=20mm, Pin Spacing=5mm, Lead Diameter=Unspecified. Price=US$0.35/ea
High-speed cassette interface (3,000 baud)
The PCB now incorporates the MDS Fast Cassette System. This gives the S80 3.000 baud cassette read/write functionality. The existing (low-speed) cassette circuitry has also been retained. Two jumpers (JP4 and JP5) need to be set to the “L” or “H” position, to select the appropriate cassette interface. Note the high-speed cassette uses the same I/O pins as the existing S80 cassette system. This differs from the MDS approach, which used Port 7 for input and Port 5 for output. So some changes will need to be made to the MDS firmware to make the on-board high-speed interface work. The upside of this approach is that the two Port pins continue to be available full-time for other uses.
RESET push-button switch
The standard S80 has two pins available for connection to a RESET switch, but no switch was fitted to the PCB itself. In V2, the two pins are still available, but I’ve also added a small on-board RESET switch in the top-right corner. It is a 6mm tactile push button.
Updated the relay footprint (to suit available 5V/30mA coil relays)
The standard S80 used a modifed 0.3″-wide DIL IC socket footprint for the cassette relay. The S80 energises the coil from +5V via drive transistor Q2. There are relays commonly available that will fit that hole pattern, but the silkscreen perimeter is way off. Another problem is that the 5V relays that suit this hole pattern typically consume 70mA to energise the coil. That’s a lot of energy just to switch the tape deck motor on and off. So I’ve replaced the relay footprint with a 0.2″ spacing pattern. This accommodates 5V relays that typically consume 30mA of coil power, and have an outline of 0.5″ x 0.3″. Samples include the TSC-105L3H from element14, or the JRC-23F-05 from Futurlec.
Updated footprint silkscreen for 20-turn variable resistor (POT1) to better match actual part
The silkscreen pattern (but not the hole placement) has been updated to suit currently available parts, such as Futurlec CERMR20K or Bourns 3006P series trimpots.
Updated LED footprint
Minor changes have been made, to allow a 2-pin header to optionally be fitted instead of the LED. This is so the LED can be mounted external to the PCB. The spacing between the pins had to be adjusted slightly (from 105 mils to 100 mils) and the hole sizes were increased to 0.8mm.