7. MODE CHOICE MODEL AND SHIPPING LINE STRATEGIES
7.3 M ETHODOLOGY LAY OUT
2.1 Techniques
In a closed loop system such as a computer, because of the inter-dependence of numerous component parts, f a u l t d i a g n o s i s is not necessarily s t r a i g h t - f o r w a r d . In addition, because of the high speed c y c l i c operation, interpretation of any waveforms on c o n t r o l , data and address lines as being v a l i d depends to a large extent on p r a c t i c a l experience of the system. There are however, certain checks with v a l i d waveforms and levels that can be carried out before s u b s t i t u t i n g any integrated c i r c u i t s . Experience has shown that the best method of i n i t i a l l y checking waveforms and levels can be to compare with the same point in a known serviceable board. The f o l l o w i n g pages provide a basic f a u l t - f i n d i n g procedure and furnish a l i s t of p o s s i b l e faults a l o n g with suggested ways of curing them.
W i t h a densely p o p u l a t e d board such as the ZX SPECTRUM, a careful physical examination of the board can sometimes indicate an obvious f a u l t . Burst-out discrete components or an overheated track show up immediately, as do the attentions of an enthusiastic amateur. Bearing in mind the latter, short ^circuits , caused ,by h a i r l i n e solder splatter can be or several ohms resistance and can cause some very m i s l e a d i n g f a u l t symptoms.
Provided first principles are adhered to and a common-sense approach is adopted, it w i l l be found after a short space of time that f i x i n g a faulty Spectrum is very much a routine operation.
4.6
2.2 Power Supply Unit
The unstabilised external power supply unit is a source of some problems. The design is such that, at minimum input voltage (215V a.c.) and 1.4A output, the voltage trough should not be less than 7.0V; at maximum input voltage (265V a.c.) and 60QmA output, the voltage peak should be less than 13V.
2.3 Initialisation
At switch-on the computer should a u t o m a t i c a l l y ' i n i t i a l i s e ' and produce a clear screen with the statement
(01982 Sinclair Research Ltd
d i s p l a y e d in the lower left section of the screen. This indicates that most of the system is working. If the Spectrum does not
i n i t i a l i s e , carry out the f o l l o w i n g basic checks.
Basic Checks. It is d i f f i c u l t to be specific in a f a u l t - f i n d i n g guide because of the large variety of fault conditions which can occur, but the f o l l o w i n g procedure, starting with a table of checks set out in order of priority, w i l l however isolate the major f a u l t area.
FUNCTION CIRCUIT REF. WAVEFORM/VOLTAGE
Voltage regulator +ve side of C50 +9V d.c. ± 2.0V. At less input than +7V the regulator may
not operate correctly.
Voltage regulator +ve side of C34 +5V d.c. ± 0.25V - no output discernable ripple.
Clock pulses IC1 pin 32 14MHz squarewave at +5V TR3 base and amplitude,
collector IC2 pin 6
Address and data RT side of R17-R23 Waveform of amplitude 3.5V lines (following
through to relevant RT side of R1-R8 ICs)
If these tests prove satisfactory check IC1 pin 14, IC2 pin 11 and IC5 pin 28 for +5V. Also check for ground at IC1 pin 40, IC2 pin 29 and IC5 p i n 14. If all is s t i l l satisfactory at t h i s point and IC1 is the p l u g - i n type, replace IC1. If replacement of IC1 does not cure the f a u l t , check the address and data l i n e s of IC1, IC2 and IC5 for active data.
It is possible that one of the Z80A or RAM control lines has become faulty therefore, comparing with a known serviceable board if p o s s i b l e , check the waveforms at the f o l l o w i n g p o i n t s at o r i g i n and 32k of expansion RAM can be isolated. An easy way of doing this, assuming that the ICs are not fitted into sockets, is to remove the +5V from IC25. This is probably best done by very carefully c u t t i n g the track to IC25 pin 16. If this operation clears the fault c o n d i t i o n , the expansion RAM w i l l have to be further isolated. In either instance the point has been reached where it is necessary to start r e p l a c i n g ICs. versions is to make up a 'test 1C' device. This may be done by using an 1C test c l i p , to which is attached a serviceable 1C (of the relevant type e.g. 4116 or 4532), to bridge across each suspect 1C in turn. This method is not guaranteed to work but can often save a lot of time u n n e c e s s a r i l y c h a n g i n g suspect ICs.
Memory Check
Where a Spectrum has i n i t i a l i s e d correctly but a RAM memory f a u l t is suspected, it is possible to f i n d the faulty address and relate it to a faulty 1C by carrying out the f o l l o w i n g procedure:
Key in the instruction:
PRINT PEEK 23732 + PEEK 23733 * 256 The v a l u e printed should be:
4.8
( a ) for a 48k unit - 65535 (b) for a 16k u n i t - 32767
The v a l u e printed in each instance is the last v a l i d memory l o c a t i o n , and in a serviceable u n i t w o u l d be as set out above. Therefore, if a different v a l u e , n, is printed the f a u l t y l o c a t i o n w i l l be n + 1. If the v a l u e returned is less than 32767 the f a u l t l i e s in the o r i g i n a l 16k of RAM. The f o l l o w i n g example i l l u s t r a t e s the method of r e l a t i n g a faulty l o c a t i o n to a f a u l t y 1C.
Example. If a 48k Spectrum is g i v i n g a memory of 25.25k key in the f o l l o w i n g i n s t r u c t i o n :
PRINT PEEK 23732 + PEEK 23733 * 256
Assume the answer d i s p l a y e d is 43200, therefore the faulty location is 43201 (stops at last v a l i d l o c a t i o n ) . Key-in:
POKE 43201,85 : P R I N T PEEK 43201 (= answer A)
If answer A is 85, key-in:
POKE 43201,170 : PRINT PEEK 43201 (= answer B)
If answer B is anything other than 170 look up in the f o l l o w i n g t a b l e w h i c h 1C to change (e.g. if answer B is 234 change IC21). S i m i l a r l y , if answer A is other than 85 refer to the table to find the faulty 1C.
Data 85 Data 170 Size of Error F a u l t y RAM l o c a t i o n if:
IC6-IC13 IC15-IC22 Error Bit < 32767 > 32767 84 171 1 0 IC6 IC15 87 168 2 1 IC7 IC16 81 174 4 2 IC8 IC17 93 162 8 3 IC9 IC18 69 186 16 4 IC10 IC19 117 138 32 5 IC11 IC20 21 234 64 6 IC12 IC21 213 42 128 7 IC13 IC22
If there is more than one f a u l t y RAM location the first f a u l t i d e n t i f i e d w i l l have to be repaired before it is possible to proceed.
4.9
2.5 Keyboard Structure
The keyboard is connected horizontally in eight blocks of five keys and vertically m five blocks of eight keys. Figure 4.1 shows the configuration. It follows that if any block of five keys fail the fault is with KB2 circuitry or the 8-way membrane, and that if any
block of eight keys fail the fault is with KB1 circuitry or the 5-way membrane. Possible keyboard faults are listed in paragraph 2.6
FIGURE 4.1 KEYBOARD FORMAT
4.10
2.6 Fault-Finding Guide envisaged that the ZXTP test tape has been loaded, or an attempt has been made to load the tape, in order to check for a f a u l t y c o n d i t i o n .
AREA SYMPTOM ACTION
TV screen dead, TR4 base-collector Remove TR4 smoke appears on shorted.
switch-on.
Short persists with 1. Check TR5 and C44.
TR4 removed. 2. V i s u a l l y check track.
No video. No video. 1. Change IC1.
2. Check TR1 and TR2.
AREA SYMPTOM ACTION
No video, (contd) No VIDEO INPUT at 1. Check waveform at IC14 IC14 pin 12. p i n 13. If correct check
TR1 and TR2.
2. Check waveforms at IC1 pins 15, 16 and 17:
If correct check VR1, VR2 (Issue 1 and Issue 2 boards).
3. Change IC1.
4. If +5V and VIDEO INPUT correct change modulator.
Corrupt Paper, - 1. Check IC1 socket.
after ULA change. 2. Check PWR socket is not i n t e r m i t t e n t .
3. Check IC1 pin 14 for +5V with no d i s c e r n i b l e
ripple.
No 3 MHz at IC2 1. Low amplitude - check pin 6. R25.
2. Check TR3.
3. Check IC1 pin 32 for 3 MHz.
4. Check IC1 pin 39 for 14 MHz.
5. Check crystal X I .
6. Check IC1 pin 13 for +3V.
AREA SYMPTOM ACTION
Video Incorrect. Dark Screen. 1. Check t u n i n g . 2. Change modulator.
Dim ' S i n c l a i r ' Io9°- 1' Check +12V"
2. Check C65 for high impedance.
3. Check R52.
4. Check +5V on IC14 pin 3.
Wavy lines across 1. Press down on each screen. electrolytic capacitor to see if lines disappear.
If so change that capacitor.
2. Check crystal X2.
3. Check/change TR1 and/or TR2.
4. Check/change IC14.
Self resetting. Check TR6.
Random dynamic Change C54.
squares.
Paper area too low. Change IC1.
Diagonal lines in Change R47.
the border area.
Distorted video. 1. Check for +3V at IC1 pin 13.
2. Change C30.
C o l o u r tuning - V i s u a l l y check IC14 circuits.
faults (Issue 1 and Issue 2 boards)
VR1 or VR2 f a i l s to 1. Monitor IC14 pin 2 adjust IC14 pin 13 trimming VR2.
waveform. 2. Monitor IC14 pin 4 trimming VR1.
3. Change IC14.
AREA SYMPTOM ACTION
Colour t u n i n g VRl or VR2 d.c. Change IC14.
f a u l t s , ( c o n t ' d ) shifts IC14 pin 13 waveform.
Alternate dark lines Change IC14.
on screen.
No c o l o u r after trimming TC2:
ICH pin 17 not 1. Check crystal X2.
o s c i l l a t i n g 2. Change ICH.
3. Change X2.
ICH pin 17 1. Check for broken track, oscillating but 2. Change TC2.
fails to tune. 3. Change ICH.
ICH pin 17 output 1. With 'Sinclair' logo on correct but s t i l l screen ICH pin 2 and no colour (boards ICH pin 4 should be as at a l l Issue Nos). follows:
2. If ICH pins 2 and 4 are correct, change modulator.
Colour faults Pin screen. Change crystal X2.
(Issue 1 - Issue 3 boards.
Blue haze around If untunable Issue 1 and 'Sinclair1 *-™°' issue 2 boardsj c h a g e ICH.
Screen too yellow. Change ICH.
AREA SYMPTOM ACTION
Colour f a u l t s Intermittent 1. Change TC2 (Issue 1 and ( c o n t ' d ) colour. Issue 2 boards).
2. Change IC14.
Red, Blue or Change IC1.
Green 'Sinclair' logo.
Washed-out Check TR2.
colours.
Keyboard faults. KB1 f a u l t . 1. Check for short on 2 or more of the 5 tracks.
2. Change membrane.
KB2 fault. 1. Check for short on 2 or more of the 8 tracks.
2. Change membrane.
Dead keyboard. Change IC1.
Wrong keyboard Change IC1.
response.
Keyboard response Change IC1.
too fast.
Regulator c o i l . Coil excessively Change c o i l . noise.
Speaker. No speaker output. 1. Check load input wave-forms .
NOTE: Figure 4.2 2. Check TR7.
shows typical 3. Check D9.
waveforms in the 4. Change speaker.
speaker circuit
Test program. 'Tape Loading Error' Reload program.
output.
Paper area edges Change IC1.
distort.
Program Loading.
AREA
Test Program.
( c o n t ' d )
SYMPTOM ACTION
Paper area corrupts 1. Change IC1.
d u r i n g tests.
2. Carry out f a u l t - f i n d i n g procedure (paragraph 2 . 2 ) .
Border colours are Change IC14.
s t r i p e d .
F a i l u r e messages. Carry out f a u l t - f i n d i n g procedure (paragraph 2 . 2 ) .
'Test Passed1 Chan5e IC1-message not f l a s h i n g .
Reduced memory 1. Perform memory test, s i z e .
2. Change IC1.
Keyboard does not See keyboard f a u l t s , a l l o w l o a d i n g .
Program f a i l s to 1. Check jack sockets are l o a d , f u l l y inserted.
2. If l o a d i n g stripes in border are u n u s u a l l y wide, check D13.
3. Check/change IC1.
'Program C h e c k / C h a n g e IC2.
appears.
FIGURE 4.2 SPEAKER LOAD INPUT WAVEFORMS
4.17
TABLE 4.1. IC14 (IML889) PIN SIGNALS
Pin No. Signal
1 (CHROMA LEAD) SINEWAVE BOOmV, 0.2yS, d.c. 9.5V 2 (R - Y INPUT)
3 (CHROMA BIAS) d.c. 4.8V
4 (B - Y INPUT)
5 0V (GROUND)
12 (VIDEO INPUT) d.c. 12V unreg.
13 (CHROMA SUBCARRIER)
14 (R.F SUPPLY)
15 (SOUND TANK) ' L I N K E D 1 12V d > c -16 (CHROMA SUPPLY
17 (CHROMA OSCILLATOR OUTPUT) SQUAREWAVE 0.2 us 4V P-P.
D . C . 0.8V 18 (CHROMA LAG) SINEWAVE BOOmV P-P 2 uS d.c. 9.5V 6,7,8 Not Connected
9,10, 11
NOTES: (1) Rails and signals taken on switch-on, computer displaying ' S i n c l a i r 1 y
(2) No keys pressed
(3) All d.c. l e v e l s positive wrt 0V
4.18
3. REPAIR
3.1 Renewal of components should be carried out u s i n g recognised d e s o l d e r i n g / h e a t s i n k i n g techniques to prevent damage to the component or to the printed circuit board. Other p o i n t s to be noted are:
(a) When replacing a keyboard m a t r i x , take care that the ribbon connectors are f u l l y inserted into the board connectors, and are not k i n k e d d u r i n g insertion.
(b) Make sure there is a good contact made between the voltage regulator body and the associated h e a t s i n k in order to ensure
(g) After any component has been renewed the circuit board s h o u l d be examined c a r e f u l l y , to ensure that there are no solder ' s p l a t t e r s ' which may cause short circuits between tracks or connector pins.
4. 16k TO 48k UPGRADE
4.1 Integrated C i r c u i t s / L i n k Settings
For both the Issue 2 and the Issue 3 boards the memory upgrade is accomplished by p l u g g i n g four logic ICs and eight memory ICs into e x i s t i n g board sockets. Depending on the types of memory 1C used, appropriate circuit l i n k s must be fitted on the board. Figure 4.3 shows the layout of the area of the board c o n t a i n i n g the empty sockets. The Issue 3 board is shown which also gives positions (top left of diagram) of the l i n k s .
4.19
4.2 Issue 2 Board
The ICs used on this board are as follows:
1C TYPE
IC15 - IC22 TI 4532-3 or TI 4532-4 200ns (or 250ns) IC23 74LS32
IC24 74LSOO
IC25, IC26 74LS157 (NOT N a t i o n a l Semiconductors)
NOTES: (1) All external RAM ICs must be of the same type (i.e. all -3 or -4 v a r i a n t s ) .
(2) LINK 3 on the board (IC26, pin 10) should be fitted if IC15-IC22 are type 4532-3.
(3) L I N K 4 s h o u l d be fitted if IC15-IC22 are type 4532-4.
.3 Issue 3 Board
The ICs used on this board are as follows:
1C TYPE
IC15 - IC22 TI 4532-3, TI 4532-4 or OKI
MSM3732 200 ns (or 250ns) IC23 74LS32
IC24 74LSOO
IC25, IC26 74LS157 (NOT National Semiconductors)
NOTES: (1) All extension RAM ICs must be of the same type.
(2) Two l i n k s must be fitted on the board (grid located between edge connector and m/c jack socket) depending on memory manufacturer and type (high or low e n a b l e ) . See Figure 4.4 for l i n k positions vs manufacturer and 1C type.
4.20
FIGURE 4.4 ISSUE 3 LINKS vs 1C MANUFACTURER AND TYPE
4.22
SECTION 5
1.1 Parts lists for the ZX SPECTRUM are provided in table form; one for the case assembly (Table 5.1) and another for the board assembly (Table 5 . 2 ) . The latter covers the Issue 2 and 3 Spectrums and is related to the board layout diagrams given in Figure 5.1 and 5.2.
T a b l e 5.2 a l s o l i s t s the a l t e r n a t i v e components w h i c h the service engineer w i l l o c c a s i o n a l l y f i n d on p r o d u c t i o n versions of the Issue 2 board or may introduce retrospectively in order to improve performance. These components are listed in the column headed ' I S S U E 2 MODS' with references to notes a g a i n s t them. These notes are e x p l a i n e d b e l o w
-2. NOTES TO TABLE 5.2
(1) The alternative values given for R47, R49, R56 and T63 are used if the ULA f i t t e d is Type 6C001.
(2) The alternative values for R48, R50, R72 and C65 are introduced to improve the colour q u a l i t y .
(3) C74 is essential on all Issue 2 boards and s h o u l d be fitted retrospectively. At the same time R60 must be replaced with the l a r g e r v a l u e .
(4) The introduction of alternative components for R24 and R27 is e s s e n t i a l . At the same time C67 replaces D14 and R73 is added.
(5) Issue 2 boards fitted with the Type 5C112 ULA have either a transistor (TR6) or diode/resistor m o d i f i c a t i o n (see Issue 2 circuit d i a g r a m for d e t a i l s ) .
(6) On Issue 3 boards o n l y crystal X2 must have a close tolerance s p e c i f i c a t i o n (i.e. 10 ppm a b s o l u t e , 10 ppm 20°C to 60°C, 5 ppm per y e a r . )
(7) On Issue 3B boards, the d.c. converter design has improved r e l i a b i l i t y . R60 is changed from 270n to 68ft and C49 is changed from 47nF to 560pF. Capacitor C77 ( l O O n F ) has been added.
5.1