Spectrum Computing

Thanks for these replies and for all the advice on installing a heat sink and replacing the 7805. Very much appreciated!

I think I’ll try with a heatsink first and will replace the regulator if this doesn’t help.

While I wait for those parts to come, I won’t fire up this Spectrum - especially as it doesn’t have a heatsink in it right now. Does it make sense to you that it would power on okay, then the regulator would shut down when it hit a certain temperature, causing the edge connector to fail (which my ZX-HD is plugged into)? When I let it cool down and power up again, it starts up fine until the regulator gets insanely hot, then the HDMI picture shuts down again.

Interestingly, the picture isn’t lost if I just use composite output, even without the heatsink. Maybe it would just take longer for this to happen?

Does this make sense?

Contact Sam Cruise wrote: ↑Mon Nov 25, 2019 8:41 pm Interestingly, the picture isn’t lost if I just use composite output, even without the heatsink. Maybe it would just take longer for this to happen?

It’s quite possible the ZX-HD is drawing extra power from the 5V and 12V rails, which are all generated by the DC-DC converter circuitry after the 7805. (Well, the 5V rail is supplied by the 7805 directly.) Other than that, it’s all just pure speculation.

I’ve never heard of a 7805 “self-regulating” itself down upon overheating. It either outputs a stable 5V or it overheats and dies. Note that a heatsink is necessary if the output current draw is over 40–50 mA. The Spectrum itself draws about 600 mA (or more). Any additional peripherals will increase that requirement. So, it’s best not to experiment without a proper heatsink.

Thanks again! I will try again with a heatsink.

7805 voltage regulator chips are part of a wider range of series pass voltage regulator chips.

Most include over temperature protection. When they get too hot, they reduce the output voltage to reduce the current demand by the load (the circuitry powered by the output terminal). The actual output voltage is not specified when this happens, as there are a number of variables involved. But it is often a fairly low voltage and (for a 5V 7805 type) will be well below the minimum acceptable value of 4.75V. By reducing the output current, the chip can reduce the power that it has to dissipate as heat. Hence reducing the temperature rise.

The 7805 provides the +5V power supply rail. In a 16k, 48k or plus model, everything except parts of the video circuitry requires the +5V rail. The specification for a +5V rail in a computer like a ZX Spectrum is it has to be between 4.75V and 5.25V. Although some circuitry may kinda work at lower voltages, this is outside the specifications.

There is also a DC/DC converter circuit that converts the nominal 9V input to a +12V rail and a -5V rail. The DC/DC converter circuitry uses the +5V as a stable reference voltage in order to correctly control the voltage on the +12V and -5V rails. So if the +5V rail voltage drops, the voltage on the +12V rail will also fall, and the voltage on the -5V can also go out of specification.

The +12V rail is used by some of the video circuitry and both the +12V and the -5V rails are used the first 16k bytes of RAM chips (the ‘lower DRAM’ is formed from 4116 or equivalent DRAM chips).

The 4116 or equivalent DRAM chips get very unhappy if the -5V is not present and if there is still a high enough voltage on the +12V rail, they can self destruct.

So, to sum up, if no heatsink is fitted, don’t power the machine up. In the same way, you don’t run an internal combustion engine (that is normally water cooled) without a radiator connected.

Mark