Alex-2-Graf/Leningrad2-Upgrade-Kit
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Leningrad-2 128K Modification Guide (Classic Piggyback Method)

This modification allows the stock Leningrad-2 computer to run any official software and games designed for both the **Spectrum 128K** and **Spectrum 48K** models.

Required Components

In addition to the components specified in the main schematic, you will need:

  • **8x KR565RU5 (КР565РУ5 / 4164)** DRAM chips. *Crucial:* They must have the exact same speed/letter suffix rating as the chips already installed on your motherboard.

  • **1x 27256** EPROM chip flashed with the official Spectrum 128K firmware.

Step-by-Step Assembly

1. **RAM Stacking (Piggybacking):** Solder the second set of 8x RU5 DRAM chips directly on top of the original motherboard RAM chips, pin-for-pin.

* **Do not solder pin 15** of the upper chips. Instead, bend pin 15 of all upper chips upward, tie them together with a jumper wire, and connect this line to the **CAS1** signal.

* Locate pin 15 of the lower (original) RAM row. Cut the original trace feeding them and connect this row to the **CAS0** signal.

2. **Address Lines:** Disconnect lines A14 and A15 from the D21 multiplexer chip (KR555KP11 / 74ALS257) on the motherboard. Connect the new **A14+** and **A15+** signals to those freed multiplexer pins respectively.

3. **ROM Switch Signal:** Connect the **CHSR** signal to pin 13 of the D24 multiplexer chip (KP11), replacing its original connection to Ground (GND).

4. **EPROM Installation:** Replace the stock D18 ROM chip with your newly flashed 27256 EPROM. Connect the **A14R** signal to pin 27 of this EPROM socket.

5. **Decoder Modification:** Locate the D15 decoder chip (KR555ID7 / 74ALS138). Isolate/cut pin 1 from the A13 address line. Then, pull pin 1 up to **+5V** through a **2 kOhm** resistor.

Tuning & Calibration

Tuning must be performed using a diagnostic **Test ROM** chip.

The calibration process involves adjusting the propagation delay of the **RAS** signal. To achieve full system stability and eliminate RAM artifacts, find the optimal capacitance value by temporarily placing a small ceramic capacitor (typically between 10pF and 100pF) between pin 4 of any RU5 RAM chip and Ground (GND).