MVS MV-1 Repair Logs

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Repairer: channelmaniac

Symptom: Backup RAM Error

Board was giving Backup RAM Error: Written 5555 Read 7255

Battery leaked on the board. Checked the traces to the Backup RAM. They tested good. Removed the battery, cleaned the corrosion, patched 2 bad traces from the NEO-F0 IC to the DIP switches, and replaced the Backup RAM at H6.

Troubleshooting RAM Problems

When troubleshooting RAM Problems on the MV1 the error message (Read 7255) can be broken down into upper bits (72) and lower bits (55)

The bits are kept in RAM as follows:

Backup RAM: Upper H6, Lower H5 Work RAM: Upper G3, Lower G4

The rest of the ICs will get filled in as I troubleshoot other boards.

Symtpom: Audio cuts out

The audio would get very low and sputtery sounding at intermittent times. Board had a cap kit already installed. Narrowed it down to a dirty Stereo/Mono selector switch. Cleaned the Stereo/Mono selector switch and tested.

Symptom: Z80 Error

Swapped the Z80 chip with the proper Z80A. No change. Inspected the board closely and found some corrosion on 4 pins of a surface mount IC connected to the Z80 CPU. Continuity tests showed one trace was open. Cleaned the board and soldered in Kynar jumpers pm all 4 corroded traces to repair it.

Only one was needed to do the repair but by doing all 4 the board will last a lot longer for the customer.

Symptom: Backup RAM Error

Board had a Backup RAM Error at D0000, Written 0000, Read 04D0. Replaced bad 74HC32 chip controlling the enable lines. Board then gave a Backup RAM error at D0008. Replaced one 62256 SRAM chip and tested. Board had no audio. Replaced 2 physically broken 470uf @ 16v caps to finish the repair.

Symptom: Work RAM Error: Written AAAA Read: AA2A

Replaced the SRAM at location H3 and tested.

Symptom: Tinny sounding audio

Cleaned the stereo/mono switch and replaced a bad battery that had started to leak. Tested.

Symptom: Stuck on Green

Board had the battery already removed and had minimal corrosion damage but still had the crystal and D4990 clock chip with some on it. Cleaned the corrosion, including some bridging the crystal's leads and tested the board. The corrosion bridge caused the crystal to not oscillate. Replaced the missing battery and tested.

Symptom: Z80 Error

Board was labeled Z80 error but had multiple issues caused by battery leakage, attempted repairs, and rodent damage. Removed an extra Z80 CPU from the BIOS socket and installed the correct BIOS chip. Removed leaking battery, cleaned board, and replaced crystal to fix calendar error. Patched a bad trace going to the cartridge port. Z80 Error had 2 causes. The first was a corroded trace between the NE0-D0 chip, pin 40 and NE0-B0 pin 41. The Z80 Error returned during testing and the IRQ pins on the CPU had bad signals. Pins 36 and 37 on the NE0-D0 read 50 ohms between them. This drove the NMI and IRQ lines for the Z80 CPU. Removed the NE0-D0 IC, cleaned the rodent damage under it, and reinstalled it and the patch to the B0 chip. Tested.

The rodent damage corroded the solder on the plated thru holes under the NE0-D0 IC. This corrosion was bridging the 2 pads causing the problems.