Alex-2-Graf/Leningrad2-Upgrade-Kit
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> \[English](README.en.md) | \[Русский](README.md)
 
\# Leningrad-2 128K Modification Guide (Classic Piggyback Method)
> \[English](README.md) | \[Русский](README.ru.md)
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).