channelmaniac
Veteran Member
Of all the ones I've picked up from Computer Reset, I've fixed and upgraded 8 of the memory sidecars and fixed 2 of the speech sidecars so far.
The Speech ones gave ERROR J when attached. Both had bad 82C55 PPI chips that needed desoldering, socketing, and replacing. They DO NOT work in conjunction with the SD-Cart, but do work in conjunction with the Jr IDE Sidecar. Something about the SD-Cart gives a "DIVIDE BY ZERO" error in BASIC when running test programs.
For the memory sidecars, here are some things if you're troubleshooting one that the system does not see...
If you're troubleshooting someone else's work, make sure they didn't damage any traces or create solder shorts. I had a card that pins 1 and 2 were shorted together with a solder bridge. Removed that and the card worked.
The Speech ones gave ERROR J when attached. Both had bad 82C55 PPI chips that needed desoldering, socketing, and replacing. They DO NOT work in conjunction with the SD-Cart, but do work in conjunction with the Jr IDE Sidecar. Something about the SD-Cart gives a "DIVIDE BY ZERO" error in BASIC when running test programs.
For the memory sidecars, here are some things if you're troubleshooting one that the system does not see...
- The data to the RAM chips goes through the HC244 chips at the end of the card. Look for outputs on them when the system is trying to determine if a card is installed. The outer one is from the RAM to the bus while the inner one is from the bus to the RAM.
- The /CAS pin on the DRAM should pulse low when the system shows 004K on the screen. If it doesn't, check pins 6, 5, and 4 on the 74HC32 chip. 4 is the output. If it's stuck or dead the card won't be seen.
- If the most of the RAM address lines are dead/stuck on the card, check pin 6 of the 74HC00. If it's dead the card won't be seen by the system either.
- If the 128K card stops counting at 192K then you have a bad chip in the 2nd bank. It's bad enough that the system decides there's no memory between 192 and 256K. Contrast that to an error code which would be a bad bit(s) somewhere inside the chip in the middle of the bank.
- If the 512K upgraded card stops counting at 256K then you have a bad chip in the 2nd bank. It seems that the RAM controller uses the 512K in 4x 128K banks.
- If the computer beeps 2x and doesn't boot, check the 74HC244 2nd from the end. It's probably bad, or you forgot to turn all the DIP switches off after installing the 256K DRAM chips. A bad HC151 can cause this too by enabling the sidecar at the wrong addresses.
- Broken pins on the interface bus CAN be fixed! You just have to have a pin that is long enough. Disassemble the connector and pull the female side off the PCB. Desolder the individual pin and pull it out backwards from the solder side. Insert the new pin and make sure it matches the same length on the male side. Solder in place and trim to length on the solder side. Reinstall the female connector and hardware fasteners.
- ERROR A is a real pain in the rear to fix. I can be anything that corrupts data or causes the memory card to not show in the proper memory locations. A bad 74HC151 or 74LS03 in the memory address decoding circuity can cause this, so can a bad 74S409 memory controller IC.
- If during burn-in testing (looptest.exe on the PCjr Memory Options disk) the screen gets dotted with random characters and locks up, replace the 74HC151 IC.
- Need hardware for the expansion connector? They are SAE thread 2-56 screws, 3/8" long. ACE Hardware has the screws and nuts, but not the lock washers. They do have brass flat washers that are small enough diameter to fit and work.
- LOOPTEST.EXE will try to test a card the system cannot see during the BIOS testing. Use it and your trusty logic probe to dig through the circuits to see what is going on.
If you're troubleshooting someone else's work, make sure they didn't damage any traces or create solder shorts. I had a card that pins 1 and 2 were shorted together with a solder bridge. Removed that and the card worked.
