Power Supplies

[Ast A. Moore]

The battery provides 5V and 12V on its output port. Would it be a big problem if the Spectrum ran on 12V instead of 9V?

There are drop-in switching replacements for the LM7805 voltage regulator, such as this one. If you’re planning to use an external power supply with a 12V output, you should consider replacing it to eliminate the extra heat the original LM7805 will generate to dissipate all that extra energy.

Keep in mind, though, that if you’re planning to use the edge connector of the Spectrum, it’s best to proceed with caution when using peripherals that obtain their power from it. Pin 4A just passes through the positive voltage from the power supply directly. The Spectrum’s original PSU will drop its output voltage down to about 9V under load. An external switching PSU will easily maintain its nominal voltage for up to its rated power output. I suppose, many peripherals will be able to deal with it, but presumably not all.

Also, the 9V rail is used to energize the built-in speaker via the collector of TR7. Additionally, it’s used as a reference point for generating negative voltages within the Speccy. I don’t know if it’s a good idea to exceed it by as much as 3V (9V vs 12V).

[Ast A. Moore]

The 9-ish Volt supplied to the Spectrum are AC, not DC.

Incorrect. It is DC.

[1024MAK]

Keep in mind, though, that if you’re planning to use the edge connector of the Spectrum, it’s best to proceed with caution when using peripherals that obtain their power from it. Pin 4A just passes through the positive voltage from the power supply directly. The Spectrum’s original PSU will drop its output voltage down to about 9V under load.

The output of the Sinclair ZX Spectrum external PSU depends on the output current. It can be around 10.5V to 11.9V with just a ZX Spectrum connected. When loaded at its rated output current (1.4A) its output should not drop below 9V.

I do agree that using a switching regulator / switching DC/DC converter in place of the 7805 is a wise choice.

Also, the 9V rail is used to energize the built-in speaker via the collector of TR7. Additionally, it’s used as a reference point for generating negative voltages within the Speccy. I don’t know if it’s a good idea to exceed it by as much as 3V (9V vs 12V).

Yes the speaker is driven from the (nominal) +9V rail. But the voltage on this rail will not affect the speaker, as it is fed via a NPN transistor in an emitter follower circuit. The voltage at the emitter cannot (in normal operation) go above the base voltage. However, if the software crashes (or is badly programmed) and the transistor is left switched on, any extra voltage on the +9V rail will increase the power that the transistor has to convert to heat. If the +9V rail is actually at 12V, and the transistor is left switched on, I estimate that it will be running at about 0.7W. It is rated at 1W at an air temperature of 25°C. But keep in mind that the temperature inside a ZX Spectrum (or any enclosed space) may not stay at 25°C and could climb to 40°C or more. If the transistor overheats, it may be damaged.

The +9V supply itself is not used as a reference voltage by any circuitry inside the machine. The +5V supply from the 5V voltage regulator (7805 or any equivalent switching regulator or DC/DC converter) IS used as a reference to generate the correct voltage for the +12V supply (from the internal DC/DC converter formed by TR4, TR5, the “coil” and the other components in this area.

However if the +9V supply is greater than 11.9V, the regulation circuit for the DC/DC converter circuitry (TR4, TR5, the “coil” etc..) may not work correctly. Please read on...

What IS VERY important, is that the input voltage on the +9V rail MUST AT OR BELOW 11.9V unless the board is using the latest modification (see the details here). Issue 5 and issue 6a boards already have this design. Issue 4a and 4b have a variation but may not have the latest version so need to be checked. I have not checked these myself and I have not checked an issue 4S yet (believed to be similar to a 4a). Earlier issue boards are unlikely to have been modified unless they were returned for service/repair. However any board between issue 2 and issue 4a/4b/4S May have already been modified.

If a board does not have the latest version of the DC/DC circuitry, either modify it so that it is brought up to the latest modification version or don’t connect a supply of greater than 11.9V. Connecting a supply of more than 11.9V to a board that is not to the latest modification version may result in the -5V rail being out of specification, which may result in one or more of the 4116 DRAM chips from malfunctioning with possible permanent damage.

The reason that the problem exists in the earlier design of DC/DC converter circuitry, is that there is a DC path from the +9V rail to the +12V rail via the “coil”. When the +12V is above +12V, TR5 will start to switch off causing TR4 to reduce its output power, which in turn reduces the AC voltage produced by the “coil”. The system thinks that the voltage on the +12V rail is too high, so it is trying to reduce the voltage on the +12V rail. But the DC can flow via the coil from the +9V rail to the +12V rail. With a lower AC current in the coil it’s AC output is low or absent, so the system is unable to produce a -5V rail.

Mark

[Mad Fritz]

The 9-ish Volt supplied to the Spectrum are AC, not DC.

Incorrect. It is DC.

Of cause right, too much booze I suppose