Gary C
Veteran Member
After a bit of a layoff, its time to get back to some of the machines that have piled up
An Osborne 1 arrived the other day. Nice enough condition
Turned on, no display and lots of smoke
RIFA's past their best
A 'compact unit' which makes it a bit more interesting to strip, but upside down and PSU is easy enough to extract
By luck I had the right replacement caps in a drawer, and once replaced PSU is fine. Reading the manual, the display won't work and isn't even powered up without a 20 pin edge connector plug in the front and its missing on this machine. Made up a temporary by cutting down a larger plug while I wait for a replacement from RS and push in bits of ribbon cable to short top to bottom,
And now the display lights up
At least the CRT is in good condition, but the scrambled display means something is wrong. A couple of times on switch on, I got a "no disk in drive A" message so it looks as if most of the system is actually working.
Looking at the circuit, the screen memory data is read and latched to use as the address for the character rom. Probing this area showed sensible signals, but looking at the schematic, the data from the character ROM is passed to a shift register, clocked with a signal called DOT_CLOCK and this 8mhz signal is missing. This in turn is developed by a 4 bit shift counter and three out of the four signals are present, but the dot clock signal is just a very noisy 5v signal
New 74LS161 on order and hopefully that will sort it.
Interestingly, the CPU seems to have a large plastic lump on it filled with a foam like substance with zenith portable computer systems written on it. An early heat sink ? (actually looks like it would insulate it !)
It also has the double density adapter.
An Osborne 1 arrived the other day. Nice enough condition
Turned on, no display and lots of smoke
RIFA's past their best
A 'compact unit' which makes it a bit more interesting to strip, but upside down and PSU is easy enough to extract
By luck I had the right replacement caps in a drawer, and once replaced PSU is fine. Reading the manual, the display won't work and isn't even powered up without a 20 pin edge connector plug in the front and its missing on this machine. Made up a temporary by cutting down a larger plug while I wait for a replacement from RS and push in bits of ribbon cable to short top to bottom,
And now the display lights up
At least the CRT is in good condition, but the scrambled display means something is wrong. A couple of times on switch on, I got a "no disk in drive A" message so it looks as if most of the system is actually working.
Looking at the circuit, the screen memory data is read and latched to use as the address for the character rom. Probing this area showed sensible signals, but looking at the schematic, the data from the character ROM is passed to a shift register, clocked with a signal called DOT_CLOCK and this 8mhz signal is missing. This in turn is developed by a 4 bit shift counter and three out of the four signals are present, but the dot clock signal is just a very noisy 5v signal
New 74LS161 on order and hopefully that will sort it.
Interestingly, the CPU seems to have a large plastic lump on it filled with a foam like substance with zenith portable computer systems written on it. An early heat sink ? (actually looks like it would insulate it !)
It also has the double density adapter.