ZX Spectrum Issue 3 repair

[Tezz]

It turns out that I had a spare issue 4a board here with a working 6C001E-7 ULA and this 3b board is now working although I still need to install the lower RAM module and replace the old capacitors. It seems both the issue 3 and 3b ULAs were faulty. I'll get back onto the repair of the issue 3 board next, according to some info I read online yesterday, if the board doesn't show +5V input it says to first check for a voltage drop across R64, then check +5V regulator O/P and +9V regulator I/P (which Mark advised that I did) that was showing a low value with and without the regulator so perhaps I should check R64 next, it then says to simply check that the power socket is not shorted. I should check that again with the new socket and track repair that I did to make sure.

[Tezz]

I continued with this issue 3 repair today, I've replaced all of the capacitors and carried out the tests again giving the same results as before. All of the voltages compare correctly with the now working issue3b with the exception still of no 5v showing. The 3b still has its regulator whereas the issue 3 hasn't yet been replaced. I'm still rather in the dark with multimeter usage. Should I still see 5v showing with no regulator installed? Will the readings come from the test voltage supplied by the meter? I'm going to compare the dc sockets next to make sure that I haven't made a mistake or missed a bridge.

edit: tested the dc socket compared to the 3b and everything looks as it should there. Had a look with the mag glass for any stray solder but can't see any problems there either.

I can't spot R64 on the board, perhaps one that isn't marked on the screen print. I'll take a look at the schematic and try to figure it out. edit: I can see it now above r69, maybe I should replace it next to see if there's any change. It's listed as an R15 resistor, I don't think I have one of those.

[Tezz]

Would these resistors (selecting 15R) be a suitable replacement for R64?

http://www.ebay.co.uk/itm/50-pcs-Throug ... oEb4UH3wDg

[1024MAK]

Without a 7805 voltage regulator chip (or modern switching regulator chip) fitted, there will be no +5V supply.
With no +5V supply, the switching power supply based on TR4, TR5 and the coil (actually a small transformer) will not work correctly. It is this circuit that generates the regulated +12V and the -5V supplies.

Note that even if the switching circuit based on TR4, TR5 is not working, the nominal +9V supply (which may actually be between 10V and 12V) will leak through one of the windings of the coil transformer to the +12V line, making this look okay-ish. This comment applies to early issue boards that do not have modified circuitry, not issue 4 and later boards.

When you use a multimeter on the resistance range, it outputs a small current through the test probes/leads so that it can measure the value of the resistance/ resistor under test. Different multimeters use different test currents, so the voltage in this mode depends both on the meter and the value of the resistance that is being tested. But with modern multimeters it should not be more than about 0.5V to 2V.

Why do you think there is a fault with R64?

Mark

[Tezz]

Why do you think there is a fault with R64?

Hi Mark, I'm following the information on this guide http://www.retroisle.com/sinclair/zxspe ... ooting.php ..what to check when no +5v is present. If you recall I've had no +5v since the beginning of the diagnostics with this board when the regulator was still in place, after reading this and then looking at the schematic I wonder now if R64 was damaged and is left open circuit? I thought it would be worth replacing and retesting.

[1024MAK]

If R64 is open circuit, only the 5V supply after it (feeding the modulator and associated video circuitry) would be unpowered. This would not cause a short circuit between the +5V line and the 0V line.

If R64 is short circuit, it may indicate another short circuit in the video circuitry. With both short circuits, this would show as a fault between the +5V line and the 0V line. But small resistors mostly go open circuit, not short circuit when overloaded.

If you use the 200 ohm range on your multimeter and put the probes on each lead of R64, you can tell. If the reading on the meter is approx 15 ohms, it is okay (actually displayed result may be a bit lower).

Also, I have just added a bit that I missed off my earlier post today (above).

Mark

[Tezz]

If you use the 200 ohm range on your multimeter and put the probes on each lead of R64, you can tell. If the reading on the meter is approx 15 ohms, it is okay (actually displayed result may be a bit lower).

Thanks Mark for the further info, I didn't know I could test the resistor in circuit, I'll check that out in the morning.

[Tezz]

Well, R64 shows as 15.6 so it looks to be fine. Should the next step be to install the modern DC converter and retest? Without the regulator or DC converter in the circuit, should the +5v be showing as 0.0 or can that only be because it's shorted to ground somewhere?

[1024MAK]

Without the regulator or DC converter, even with no short circuit, it will measure 0V due to the load formed by every resistor, and semiconductor that it connects to.

Yes, fit a the modern DC converter and power up. Have your meter ready, and quickly test the +5V line. If it is in the normal range (4.85V to 5.15V) then that's good. If the voltage is lower or higher than normal, quickly switch it off.

Mark

[Tezz]

Is this the correct orientation to install the DC converter? I assume it's correct with pin1 being the input indicated by the dot but thought I'd better ask to make sure before soldering it

tsr_1-2450.png (436.89 KiB) Viewed 2918 times