On the 22 I found that the 175Khz-clock signal was coming through the unused pins on RAM/ROM chip (the one closest to the battery compartment and did the following fix: I simply connected one of the unused pins on the RAM chip to ground. (At least some of the unused pins appear to be connected to the substrate of the chip.) In particular, I connected a jumper between pin 7 (ground) and pin 10.
On the 25 this repair did not work, although it too had clock noise coming through on the same unused pins. However, I found that the data signal line was quite noisy too. The fix here was to install a 20K resistor between the data line and ground. The data line is pin 11 of the ACT chip (the ACT chip is the one at the bottom of the circuit board.) and there is a convenient ground just across the chip on pin 12. One fellow collector has reported that this same fix worked as well on the 25C.
On the 25 this repair did not work, although it too had clock noise coming through on the same unused pins. However, I found that the data signal line was quite noisy too. The fix here was to install a 20K resistor between the data line and ground. The data line is pin 11 of the ACT chip (the ACT chip is the one at the bottom of the circuit board.) and there is a convenient ground just across the chip on pin 12. One fellow collector has reported that this same fix worked as well on the 25C.
1818-0550, 1881-0551, 1818-0231, 1818-0232
ROM/RAM ICs
-----------
Vss--| U |-- Sync
Phi_2 --| |-- ISA
Vgg --| |-- Gnd
Phi_1 --| |-- Data
-----------
Vss, Vgg : Power supply
ISA : Instruction and Address
Sync : System bus sync input
Phi_n : 2 phase system clock
Each chip contains ROM and 16 7 byte registers, so a total of 64
registers in the machine. 32 of them are used for 32*7 = 224 bytes
(steps) of user program memory). The other 32 registers form the 26 user
register (P0-P9, S0-S9, A-E and i), the 5 stack registers and one
register containing the user flags, angle mode, decimal setting, etc.
ROM/RAM ICs
-----------
Vss--| U |-- Sync
Phi_2 --| |-- ISA
Vgg --| |-- Gnd
Phi_1 --| |-- Data
-----------
Vss, Vgg : Power supply
ISA : Instruction and Address
Sync : System bus sync input
Phi_n : 2 phase system clock
Each chip contains ROM and 16 7 byte registers, so a total of 64
registers in the machine. 32 of them are used for 32*7 = 224 bytes
(steps) of user program memory). The other 32 registers form the 26 user
register (P0-P9, S0-S9, A-E and i), the 5 stack registers and one
register containing the user flags, angle mode, decimal setting, etc.
