ZX Spectrum Bits & Bobs Bytes
|Opening the case||Pre Power Checks||Powered Up Checks||Lower RAM test|
Do a general check and remove anything that is clearly wrong or replace anything that is clearly broken
Do a full capacitor recap. Whilst you're doing this check the board for (and repair) obvious other soldering faults such as dry joints, solder bridges and damaged tracks.
Perform pre-power checks for resistances (see Maintenance Checks page)
Replace any components that failed these tests. If TR4 or TR5 need replacing then do both at the same time as they often cause each other to fail.
Perform the Power On voltage tests (see Maintenance Checks page) (potentially remove ULA until sure OK). If removed, add ULA, test in socket.
Complete composite Mod if required.
Power on and check visual
Run diagnostic ROM, perform soak test, load and run a game.
When all OK/happy, remove ULA socket from board, solder ULA directly to board and add heatsink
Turn the unit over and place it keyboard down.
You should see 5 crosshead/phillips screws, 2 on each side and 1 on the top edge in the middle (this may be covered with a sticker).
Unscrew all 5 case screws.
Hold the unit by the edges, keeping the 2 halves together and turn over to the right way up (i.e. keyboard up!)
Carefully lift the keyboard section off the base.
Note that the keyboard membrane will be connecting the 2 parts together and is easily damaged
Carefully wiggle the keyboard membranes from the connectors. Be slow and careful here as they are fragile and can easily be broken (if they aren't already!). Lift the keyboard away from the base and place to the side keyboard down to avoid stressing the membrane.
You're ready to check out the insides!
Notes taken from this video by JoulesperCoulomb.
NO POWER SUPPLY CONNECTED
Multimeter on 200KOhms
Positive (red) probe onto left hand terminal of voltage regulator - should build up to over 100 in a few seconds. Open circuit maybe OK. If < 100 the regulator is probably faulty
Switch multimeter to minimum 500ohm range
On the lower left RAM chip test the following pins in turn with the positive multimeter probe
Upper right most pin - should be close to zero
Lower right pin = +5v connection : should show approx 300 ohms, maybe 400-500. Much below something wrong
Upper left pin = -5v : min just under 300 ohms - often 400-500. Much less probably a fault
Change the multimeter to the 20KOhms setting. Lower left pin = +12v : 2.6 to 2.8 kOhm on an issue 3+, or 800-900 Ohm issue 2
Disconnect the multimeter common probe from the heatsink
Check the Inductor coil
Find solder pads on the back of the board where the inductor is connected
Measure either end of each winding (each side, top and bottom solder pads - indicated with the red lines and #1 in the picture) - very low / close to zero. High or open circuit indicates probably fail
Between 2 windings (top left and right / bottom left and right - indicated with blue lines and #2 in the picture) - open or very high circuit of several Mohms
(NOTE Issue 4S board difference: TR4 has provision for 2 types of transistor. If outer (right) set of pads then same as below, if inner (left nearest keyboard membrane connector) set then base will be the top leg nearest the back of the board, middle collector and front emitter.
Hold positive probe to center (base) leg.
Other legs are Collector and Emitter as shown above. Try each in turn with negative probe- reading should 0.6 to 0.9V
Replace positive probe with negative on center leg and repeat - should indicate open circuit.
Hold negative to base center leg. Positive to other legs should show 0.6 to 0.9V on collector and emitter legs.
Swap to positive on base center leg and test again to see open circuit on collector and 1.2v on emitter.
If any of these tests fail DO NOT connect power!
Notes taken from this video by Sabretooth Barnacle - from around the 6 minute mark, until around 9 minutes
Set multi-meter to measure DC Voltage of 20mA.
Connect multimeter common (black) probe to the inner/tip of the power connector socket (which is the negative side)
Power connected ....
Check voltage on power connector. Inner/tip is negative, outer is positive (NOTE this is the opposite to modern PSUs so be careful!!!)
Barrel jack, positive probe to the barrel via the pin on the outer left hand side. Should be around 9 volts, but maybe up to 12 if you have a higher rated power supply.
Check the Voltage regulator:
Bottom pin should give the same value as the power socket test above (around 9 volts)
Middle pin = ground = 0
Top pin = regulated = 5v
Test the RAM chips - pick any chip, bottom left corner probably the easiest to access.
Top Right pin = Ground - Hold the Neg/COM probe on this one and then place the Positive probe to the following pins in turn ...
Top Left = -5v
Bottom Left = +12v
Bottom Right = +5v
An individual 4116 (Lower RAM) chip can be tested out of circuit with the following tests performed using a digital multimeter switched to the 20k ohms range (or equivalent range).
Test between pin 1 (VBB, -5V) and pin 16 (VSS, GND). You should get more than 11k ohms.
Test also between pin 8 (VDD, +12V). and pin 16. You should get more than 20k ohms.
Any that test below these values are probably defective.