Vertical Bars on Power Up / Solution

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gammaray
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Vertical Bars on Power Up / Solution

Post by gammaray » Thu Nov 15, 2018 12:03 am

I had problems with the refurbished Speccy I got... but I found out the solution!

Using a 12V 1000 mA transformer power supply providing 18V no load, I got vertical bars and the unit started saving routine and had vertical bars.

But...

Using a 9V 1500 mA switching power supply, the bars went away and I got the "Sinclair Research Ltd" message.

My Spectrum still has the heatsink and original voltage regulator so it's replacement may allow the above PSU to work without incident.

These symptoms sound similar to other reports. So maybe this helps.
5-TS1000,UK ZX81<-Sheelagh, US ZX81, 2-TS1500/KDLX , 3-TS2040 printer, 2-TS2020 cassette decks, ZXPAND+AY, ZeddyNET, ZXBlast, UDG, ZX8CCB, AERCO, BUILDS/REPAIRS ZX Spectrum, ZX80 Minstrel, ZXMAX48 v1 v2, 2-TS-2068, ROM, 16kRAM

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1024MAK
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Re: Vertical Bars on Power Up / Solution

Post by 1024MAK » Thu Nov 15, 2018 9:03 am

The ZX Spectrum 16k, 48k and Plus models all have a DC/DC converter/inverter power system on the main board to generate +12V and a -5V supply rails for the 4116 DRAM chips. The +12V is also used by the colour encoder chip (LM1889).

Models using unmodified issue 1,unmodified issue 2, some issue 3 (unmodified) and some issue 4 (unmodified) boards used a version of this DC/DC converter/inverter circuit that would not work correctly if the nominal 9V input voltage was 11.9V or greater.

If a computer was returned for repair or service to Sinclair, or to a a good independent service engineer, the board would be modified to the latest version of this DC/DC converter/inverter circuit.

So unless you know that you have an issue 5 or issue 6A board, or that you have an earlier board that does include the (better) later version of the DC/DC converter/inverter circuit, I strongly recommend that you do not connect it to a PSU that provides an on load voltage of 11.9V or more.

The problem with the earlier version of the DC/DC converter/inverter circuit, is that with the voltage on the nominal 9V input at 12V or more, DC current flows via ‘the coil’ from the 9V rail to the +12V rail. The control circuit sees that the +12V rail is at or above +12V, so turns the switching transistor off. Then there is no AC drive to ‘the coil’. The result is that ‘the coil’ is no longer able to generate the -5V supply. The 4116 DRAM chips (or equivalents) that provide the ‘lower’ 16k RAM then don’t work correctly. Worse, the lack of this -5V bias supply can cause these DRAM chips to self destruct :-(

If the lower RAM is not functional (which means any of the eight 4116 DRAM chips not working), then the system will crash at power up.

The recommended on-load input voltage on the nominal 9V input is between 7.5V and 11.5V for regulated power units, or between 8.5V and 11.5V on-load for unregulated power units.

To reduce the heat output of the 7805 voltage regulator, it is better if the input voltage on the nominal 9V supply is at the lower end of these ranges ;-)

The current requirement varies depending on which issue the board is. But all require 800mA or less. So if no expansions or interfaces are used, a PSU rated at 800mA is fine. If expansions or interfaces are to be used then a more powerful PSU is needed. For simple interfaces like simple joystick interfaces, you need a 1A (1000mA) PSU. Anything else and a PSU rated at 1.4A or greater is recommended ;-)

Mark

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gammaray
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Location: Texas

Re: Vertical Bars on Power Up / Solution

Post by gammaray » Thu Nov 15, 2018 6:37 pm

Which is why it started working on 9V/1500mA.

It is a version 3 selected to match a ULA/Oscillators for NTSC standards.(not installed yet)

Thanks to the maker, that the memory does not seem to be affected.

Is there a Spectrum program to test RAM? RAMTest48.tzx.zip at WOS?

http://www.worldofspectrum.org/infoseek ... id=0011120


I suddenly realized I don't have a SD or USB or other way to get software into the Spectrum. Have not set up any kludge like parallel port transfer...
5-TS1000,UK ZX81<-Sheelagh, US ZX81, 2-TS1500/KDLX , 3-TS2040 printer, 2-TS2020 cassette decks, ZXPAND+AY, ZeddyNET, ZXBlast, UDG, ZX8CCB, AERCO, BUILDS/REPAIRS ZX Spectrum, ZX80 Minstrel, ZXMAX48 v1 v2, 2-TS-2068, ROM, 16kRAM

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