Radio Shack CCR-81 Reference Handbook

in a conductive tube or in conductive foam.
DO NOT USE
STYROFOAM! Unless special-
ly
treated, styrofoam eats MOS devices like
candy. It can generate tremendous static
charges. Do not use cellophane tape to hold
RAM's in sets of eight. The process of remov-
ing
tape from the roll will act as a handheld
static generator.
RAM problems are slightly more difficult to
troubleshoot because of multiplexing of the
address inputs. Aside from addressing differ-
ences,
the RAM's are checked like the ROM's.
Use SCQATS
to dig out memory errors if pos-
sible. Also, be
sure you check the three power
supply voltages going to RAM; they are: -5, +5
and +12 volts. Insure that all voltages are present
on all RAM's. Also check for bad RAM insertion
- pins
not in sockets or loose RAMs due to ship-
ping
vibration. Check for activity on RAS, CAS,
and MUX going to RAM and the multiplexer. In-
sure
that all specified addresses do indeed go to
the multiplexer. RAM problems are most likely
suspect after a 4K to 16K conversion. Be sure
that DIP shunt Z3 is programmed correctly for
the amount of RAM in the system.
If
you have a RAM problem and the system will
not load SCQATS, you can replace the eight
RAM's with a known good set. If this fixes the
unit, start replacing your standard RAM's with
the parts you took out, one by one. Power up
after each exchange to see if you still have a
"Ready". Continue this process until you have
isolated
the bad RAM (s).
Address Decoder
A problem in the address decoder section will
probably point you in the memory direction.
For example, if the ROM is never addressed with
ROM*, you'd think you have ROM difficulties.
Therefore, if you suspect one of the "memory"
locations, keep in mind that the address decoder
sources the memory selects. The select inputs to
the different memories should be the very first
thing you check.
Failure of address decoding will usually be asso-
ciated with one of the higher order address lines.
You should check Z21's inputs for activity, pay-
ing particular
attention to pins 14 and 2 of Z21.
Z21 's outputs also depend on the status of the
shorting straps on DIP shunt Z3. Remember,
you cannot test an open collector output unless
there is a pull-up resistor attached to it.
If Z21
and the DIP shunt appear OK, look for
activity on the other inputs. For example, A 12
and RD*. Maybe the CPU is constantly address-
ing
one memory all the time, but is not getting
any data in return. This is especially true of the
keyboard "memory". If the CPU thinks there's
a key pressed, it may lock itself into a loop, try-
ing
to isolate a phantom key.
Keyboard
Difficulty with the Keyboard is usually mechani-
cal.
Sticking keycaps, bouncy keys and a broken
Interconnect Cabie are common. Shorts in the
Keyboard matrix are usually easily detected. If
you find an alphanumeric character displayed
right
after the > , that particular key, or PCB
run, may be shorted. A completely "dead"
Keyboard could be caused by lack of power, a
broken I nterconnect Cable or the address
decoder is not supplying KYBD*.
If
you have a weak space bar spring, replace it
with a 3 oz spring. If it still sticks, and there is
no plastic flash that could cause sticking - give
the spring more muscle by stretching it a little.
If
you lift a sticking keycap and find mangled
switch
contracts, don't scrap the whole key-
board, replace the contacts. The following is a
step-by-step
procedure for contact replacement:
1. Disassemble the unit.
2. Remove the keycap and keycap plunger.
3. Remove the spring.
67

Radio Shack CCR-81 Specifications

General

Brand	Radio Shack
Model	CCR-81
Category	Desktop
Language	English