PRAVETZ 8A – restoration and cleaning

After long years of searching, we finally acquired not one but two very well-preserved computers. Pravetz 8A is considered to be one the rare models here in Bulgaria, mostly because most of them were exported to the USSR. Also, they were not sold freely here, only certain institutions had access to owning them. Although it is mostly the same with the previous 8M model with the exception of the removed integrated Z80 CPU and the added expansion slot that give the ability to upgrade the RAM to a maximum of 1080 kb. Pravetz 8A comes with a CM630 (6502 clone) CPU and base RAM of 64k and the same case as older models. There are some differences in the keyboard, like added four-way arrow keys and two function keys at the bottom of the keyboard.

In this article we will teardown one of our computers and we will try to clean and restore it.

Pravetz 8A before cleaning. The case is slighly yelowed on the sides and is dusty with left overe sticker residue.

At first glance there is a stuck key on the keyboard.

Time to remove the top cover and reveal what we have inside. Here we can see the added expansion slot and many of the similarities with the previous model 8M. The keyboard buttons are with reed switches. Our computer has a serial number 1625 and is manufactured in 1988.

Removing the keyboard and revealing the motherboard. Here we can see that the main board is completely redesigned, most of the TTL logic is reduced and replaced with Application specific integrated circuits (ASIC`s). Centered on the board is the expansion slot allowing the RAM to be raised to a maximum of 1080kb. Also there were available combined RAM expansion+80 comumn cards for the 8A.

After removing the motherboard all plastic part we cleaned with hot water and soap. Then the borken parts were glued together. The broken reed switch in the keyboard was replaced with a spare one.

The next step was to open the power supply and replace all of the capacitors which were 30 years or older. This is done as a preventative maitenance, because these power supplies were imported from Tiawan and were generaly built with bad quality parts.

The final step was to put everything back together, reinstall the power supply and test the computer.

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