TS2068 input voltage...

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Spinnetti
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Joined: Sat Sep 12, 2020 11:29 pm

TS2068 input voltage...

Post by Spinnetti »

Hi,

Anybody know why the TS2068 needs 15v in instead of the 9v of everything else (except the QL)? We are working on a universal power supply to get rid of the rats nest of walwarts, and the TS2068 complicates things.
Zeddy: ZX80, ZX81/ZXpand, TS1000/ZXpand, TS1500/Zxpand+
Speccy: 48k, +, +2, +3, TS2068, "Bare Metal" Pi, Next KS2, IF1/Microdrives/Vdrive/Light Gun/VGA-Joy
QL: Minerva/QL-VGA/Custom PSU
C5: 24v, LiFE battery, Disc brakes
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1024MAK
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Re: TS2068 input voltage...

Post by 1024MAK »

Because it uses a 12V series pass voltage regulator. This needs its input to be 15V DC nominal.

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Spinnetti
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Re: TS2068 input voltage...

Post by Spinnetti »

Thanks. Is there an ignorant person translation? So, instead of a typical 7805 going from 9v to 5v, this is going from 15V to 12V? If so, what's the different requirement that it needs 12v? Sorry, I've only the minimal understanding of circuits.

Thanks.
Zeddy: ZX80, ZX81/ZXpand, TS1000/ZXpand, TS1500/Zxpand+
Speccy: 48k, +, +2, +3, TS2068, "Bare Metal" Pi, Next KS2, IF1/Microdrives/Vdrive/Light Gun/VGA-Joy
QL: Minerva/QL-VGA/Custom PSU
C5: 24v, LiFE battery, Disc brakes
jdfan1000
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Re: TS2068 input voltage...

Post by jdfan1000 »

Yes: the 78L12 needs 15 volts to deliver 12. The 12v is mainly for the video chip. Even so, the it’s under sized. Replacing the 78L12 with a 7812 will improve the video a bit.
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1024MAK
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Re: TS2068 input voltage...

Post by 1024MAK »

The chip that performs colour encoding and modulation of the video signal (for the colour composite video and the RF outputs) requires a stable, regulated +12V DC supply.

In the Sinclair ZX Spectrum, a step up DC-DC converter was used to convert the +9V DC input to +12V for the chips that form the ‘lower’ RAM (where the screen memory display is located) and for the chip that performs colour encoding.

A separate circuit based on a 7805 series pass voltage regulator chip converts the +9V supply to +5V, which then supplies all the digital logic chips, such as the ULA, Z80, ROM, ‘upper’ RAM etc.

In the Timex TS2068 they are using completely different power circuitry. Instead they are using a 15V external supply, then using a 78L12 series pass voltage regulator chip to convert the +15V supply to +12V. This then feeds the chip that performs colour encoding and modulation of the video signal.

A separate DC-DC converter circuit converts the +15V to +5V for the all the digital logic chips (Z80, ROM, RAM etc.).

All the 78xx series pass voltage regulator chips (like the 7805, 78L12, 7812) need at least 2.5V headroom on the input terminal above the output voltage that they are designed to provide. It’s a bit like a gravity fed water system. The water tank has to be higher than the water tap/faucet, plus some extra height so that there is enough pressure for a reasonable water flow.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Spinnetti
Posts: 253
Joined: Sat Sep 12, 2020 11:29 pm

Re: TS2068 input voltage...

Post by Spinnetti »

Awesome,

Thanks for the education
Zeddy: ZX80, ZX81/ZXpand, TS1000/ZXpand, TS1500/Zxpand+
Speccy: 48k, +, +2, +3, TS2068, "Bare Metal" Pi, Next KS2, IF1/Microdrives/Vdrive/Light Gun/VGA-Joy
QL: Minerva/QL-VGA/Custom PSU
C5: 24v, LiFE battery, Disc brakes
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