128k Composite out problem after upgrade to 2amp voltage regulator

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pkupcik
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128k Composite out problem after upgrade to 2amp voltage regulator

Post by pkupcik »

Hi,

I have ZX 128K toastrack (PCB version 6U), I changed the original 7805 1amp voltage regulator for 2amp Traco TSR 2-2450, the computer is working fine, but now composite output is unstable, switching from B&W to color constantly. I also tried using 78S05 2amp voltage regulator and same problem. The original 7805 1amp voltage regulator does not exhibit this problem.

RGB output works correctly, only composite is affected. Composite output is provided by the RGB socket via DIN to composite cable adapter, that I bought online many years ago.

Any ideas what could be wrong? I haven't done any other mods to it yet, all capacitors are original and looking ok.

Thank you.
cj7hawk
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by cj7hawk »

Hi,

The Toastrack will lose colour output when the 12v input voltage to the TEA2000 chip falls below a certain level. IIRC, it needs around 10.8v stable minimum at the input, or there will be no colour.

Of course, the ZX Spectrum 128 has it's own 12v generator, so why is this an issue?

There could be something wrong with the 12v circuit. A schematic can be found here: https://spbnick.github.io/assets/2018-0 ... poster.pdf

That oscillator on the right produces 12v from 9v. If it's faulty, it will drag down the input voltage, depending on how much of a resistor TR4 has become, but TR5 will still provide 5v, give or take a bit, to the 12v line. This isn't much current, but it might be enough to make the video encoder work - though I think the video chip also doesn't work if this is below around 8-9v, which brings up another possibility.

12v is only needed for the video chip and RS232 on the toastrack. If so, then I'm guessing that;

1) The 12v circuit was faulty.
2) It's been repaired before and;
3) Someone just ran a wire from the 9v input to the 12v line, bypassing the circuit instead of replacing TR4 and possibly TR5 to fix the circuit.

This works, but will result in poor video quality, and loss of colour if the input drops below 10.8v or thereabouts.

So, I'm going to go out on a limb and suggest that this might be the case. Check the 12v line to make sure there's 12v on it, because that's quite possibly the fault. If it's lower than 12v, check if someone has bypassed the oscillator previously. If so, either "up" the input voltage to 12v ( A Toastrack is well equipped to handle it, and you can amaze your friends showing them why it's called a heated toastrack ). or better still, replace TR4 which will probably fix the issue. Remove any bypass first, or you'll overload the oscillator, unless the bypass was created with a diode.

If that's not it, then the fault is probably within the video circuit. Hopefully the schematic will help you troubleshoot further. Let me know if I guessed right that someone bypassed the oscillator directly from the input power rail, or if the oscillator was just faulty. :)

Regards
David
pkupcik
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by pkupcik »

cj7hawk wrote: Thu Jan 06, 2022 2:30 am ..
Check the 12v line to make sure there's 12v on it, because that's quite possibly the fault.

Regards
David
I checked pin 11 on TEA2000 and it reads:

11.23v with the original 7805 voltage regulator.

11.39 with the replacement TSR 2-2450 voltage regulator.

So my guess is that's not the problem.

Any other ideas?

Thank you.
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1024MAK
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by 1024MAK »

DC/DC converters or switching regulators (like TSR 2-2450) do generate more electrical noise than series pass regulator chips like the 7805.

It may help if the electrolytic capacitors in the power supply sections are renewed. But before doing that, if you have a couple of 100nF (0.1uF) capacitors, temporarily connect one across the +5V and 0V/GND pins of the TSR 2-2450, and connect one across the input (+9V rail) and 0V/GND pins. See if that helps.

Mark
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pkupcik
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by pkupcik »

1024MAK wrote: Thu Jan 06, 2022 2:50 pm DC/DC converters or switching regulators (like TSR 2-2450) do generate more electrical noise than series pass regulator chips like the 7805.

It may help if the electrolytic capacitors in the power supply sections are renewed. But before doing that, if you have a couple of 100nF (0.1uF) capacitors, temporarily connect one across the +5V and 0V/GND pins of the TSR 2-2450, and connect one across the input (+9V rail) and 0V/GND pins. See if that helps.

Mark
Thank you, I'll give that a try, but need first get some more capacitors. I think I have exactly one 100nF somewhere.

Btw, I have the same problem when using 78S05 voltage regulator, not just TSR 2-2450.
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1024MAK
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by 1024MAK »

pkupcik wrote: Thu Jan 06, 2022 8:30 pm Btw, I have the same problem when using 78S05 voltage regulator, not just TSR 2-2450.
Yeah, but I currently can’t think of anything else…!

Mark
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by zx64 »

You can also try to connect external 12v power supply and just bypass the 12v generation circuit. If problem is resolved, then you know for sure it's the power issue
pkupcik
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by pkupcik »

1024MAK wrote: Thu Jan 06, 2022 2:50 pm ... if you have a couple of 100nF (0.1uF) capacitors, temporarily connect one across the +5V and 0V/GND pins of the TSR 2-2450, and connect one across the input (+9V rail) and 0V/GND pins. See if that helps.
I tried the suggestion above and it didn't make any difference.

I also tried another voltage regulator, this time TSR 1-2450 and same problem.

I then proceeded to recap few capacitors, I'm still awaiting delivery of most of them, but went ahead to replace 1uF electrolytic capacitors that I already had on hand and add ceramic capacitors to memory chips to remove jailbars.

I replaced C27 with 1uF - composite problem was still there. Reminder, no problem was observed with original 7805 regulator.

I replaced C123, C127 with 1uF electrolytic and replaced C7 and C8 with 1uF Ceramic. The result of this is that now I don't have any composite output, RGB still works and jailbar effect is reduced. Somehow I screwed up the composite output. Visually everything looks ok on the PCB. I also tried switching to original 7805 voltage regulator, but that didn't help, still no composite. Any thoughts on what to check or what I've done wrong?
cj7hawk
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by cj7hawk »

Sounds like it's oscilloscope time... Would be very difficult to fix when you can't see what's going on.

Alternately, check each and every component between the TEA2000 and the video output.

But if you can get hold of a scope, here's an idea of what to expect on each of the pins.

https://www.youtube.com/watch?v=3ehCIXH_d1M
pkupcik
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by pkupcik »

cj7hawk wrote: Mon Jan 17, 2022 9:50 am Sounds like it's oscilloscope time... Would be very difficult to fix when you can't see what's going on.
I think you're right, that's going to be my next step. Thank you.
pkupcik
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by pkupcik »

Would this type of oscilloscope be sufficient?: https://www.owon.com.hk/products_owon_v ... cilloscope

The VDS1022 model is about $100.
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by 1024MAK »

Are you going to use this for other fault finding/work/projects?

An analogue bandwidth of a ‘scope should where possible be ten times that of the fastest frequency of the signal you want to test/examine. It’s even more important for digital signals where the translation is a very rapid rise or fall (square edged pulses).

The sample rate is also important. Too low and it may miss the change of state of a digital signal. Or miss a pulse entirely…!

For video signals, a bandwidth of at least 20MHz is recommended. For digital signals for a ZX Spectrum, you definitely want a greater bandwidth.

I have a PC/USB ‘scope from Pico Technology. The model I have is however discontinued.

I also have a Rigol, they are stand alone ‘scopes and are good value for money. But do buy from a U.K. official dealer otherwise you the warranty will not valid.

Mark
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pkupcik
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by pkupcik »

1024MAK wrote: Mon Jan 17, 2022 5:45 pm Are you going to use this for other fault finding/work/projects?
I would use it for troubleshooting the various retro computers I have, both internal parts and video outputs, custom cables, etc. I mostly have multiple Sinclairs, Amiga 1200, 386, 486, Pentium 1 systems, etc.

How about something like Rigol DS1202Z-E? 2 channels with 200mhz bandwidth?
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by zx64 »

pkupcik wrote: Tue Jan 18, 2022 12:02 pm I would use it for troubleshooting the various retro computers I have, both internal parts and video outputs, custom cables, etc. I mostly have multiple Sinclairs, Amiga 1200, 386, 486, Pentium 1 systems, etc.

How about something like Rigol DS1202Z-E? 2 channels with 200mhz bandwidth?
This would be more than enough for spectrum and amiga. Enough for 386. And probably for 486, Pentium
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Re: 128k Composite out problem after upgrade to 2amp voltage regulator

Post by cj7hawk »

I use one of these: https://www.youtube.com/watch?v=5iwtDwJlbWk ( 100 MHz ??? scope.. Cheap... Battery powered ) for general stuff.

I also have a hantek one which is better. But somewhere I even have a tiny pocket sized scope I bought for $30 a few years back made from an old MP3 player that would even handle video ( I did read some USB 1.0 signals with it ).

For video, 5MHz is enough. Or even 20 Msps would probably get you through.

But it's also a chance to consider whether you might need to fix other things in the future and want a better scope given how cheap they are now.

The one you linked to looks OK, but given how cheaply you can get pocket and tablet oscilloscopes now, which are standalone, I'm not sure that a PC scope has any real advantage unless you do a lot of capture.

The cheapest one I could find looking is about $60 that meets the minimum requirements.

David.
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