Issue 4A with no colour!

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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black » Wed May 08, 2019 9:23 pm

1024MAK wrote:
Mon May 06, 2019 11:44 pm
An oscilloscope would be a great help with fault finding this problem.
Mark
I bit off topic but, can you suggest a value for money oscilloscope? Just for spectrum repairs. What is the minimum requirements for? Thank you.
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Ast A. Moore
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Re: Issue 4A with no colour!

Post by Ast A. Moore » Wed May 08, 2019 10:47 pm

Muttley Black wrote:
Wed May 08, 2019 9:23 pm
What is the minimum requirements for?
For an analog scope, I’d say at least 20–40 MHz of bandwidth. If you’re looking at a digital scope, you’ll need something like 50–100 MHz of bandwidth with 200 MS/s and enough memory depth.

A two-channel scope is often beneficial, but even a single-channel scope will do the job nicely.

An aftermarket analog scope can be obtained dirt cheap and is a pleasure to use. If it works, that is. You might end up with something you’ll spend more time repairing than working with.

Cheap digital scopes have gotten better, but decent ones can still be quite pricey and take a little getting used to.
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Re: Issue 4A with no colour!

Post by Liveinabin » Thu May 09, 2019 8:06 am

So given that I'm currently bereft of any other test equipment; am I right in assuming the component we really needed to test was the LM1889 video encoder chip (as I'm figuring that it is responsible for the colour, mostly)?
Reason I ask is, I'm about to order some bits anyway (keyboard membrane, voltage regulator..) and a new LM1889 is only £3. If we're even vaguely suspicious about the chip, I'll happily pay £3 to find out :)
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Re: Issue 4A with no colour!

Post by Ast A. Moore » Thu May 09, 2019 8:13 am

Liveinabin wrote:
Thu May 09, 2019 8:06 am
If we're even vaguely suspicious about the chip, I'll happily pay £3 to find out :)
In that case, go for it, definitely.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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Re: Issue 4A with no colour!

Post by 1024MAK » Thu May 09, 2019 9:52 am

The cheapest will be a used analogue ‘scope. I recommend one with a minimum bandwidth of 25MHz. Or higher. Ideally the analogue bandwidth should be at least ten times greater than the frequency of the signal you want to measure/see. Especially with digital waveforms that are supposed to have fast rise times, fall times and are supposed to be so called ‘square waves’ (but are actually pulses).

The best value digital ‘scopes are made by Rigol and their basic (cheaper!) models are popular amongst the home ‘retro’ computer community. The greater the sampling rate and the greater the memory, the better the resolution of the displayed signal waveform.

Note that if you do want to buy a new Rigol, buy it from an authorised dealer, as otherwise the warranty will not be valid.

Dual/two channel ‘scopes, with a separate trigger input are good. Four channel can be very useful (but cost a lot more). And if you are feeling flush with money, you can get ‘scopes that include multiple digital inputs like a logic analyser. These are known as MSO - mixed signal oscilloscopes.

As well as stand alone digital ‘scopes, there are USB PC ‘scopes. I use one from pico. These have the high frequency circuitry in a box that you connect to a PC. Then you run an application on the PC, and get a display on the PC of the signal along with a control panel. Most waveforms that I post online are from my pico. Note that despite using a PC, these are also not cheap.

In answer to your question about buying spares, yes, if you can afford it, it’s always a good idea. It’s not often that the LM1889 chip fails, they are normally reliable. I suspect one of the peripheral components or connections.

If you are buying spares, buy a new crystal for X2 (4.43361875MHz).
See the service manual for more about the sub-carrier (colour) circuitry (here).

Mark
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Re: Issue 4A with no colour!

Post by Liveinabin » Thu May 09, 2019 10:54 am

1024MAK wrote:
Thu May 09, 2019 9:52 am
In answer to your question about buying spares, yes, if you can afford it, it’s always a good idea. It’s not often that the LM1889 chip fails, they are normally reliable. I suspect one of the peripheral components or connections.

If you are buying spares, buy a new crystal for X2 (4.43361875MHz).
See the service manual for more about the sub-carrier (colour) circuitry (here).

Mark
Oh yes, I forgot about the crystal. Right, all bits ordered including a matching crystal for X2 and a new LM1889 (although, as has been said, it's probably fine). About the crystal, is there a 'right' way to orient it? Do these things have a polarity?
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Re: Issue 4A with no colour!

Post by Ast A. Moore » Thu May 09, 2019 11:21 am

Liveinabin wrote:
Thu May 09, 2019 10:54 am
Do these things have a polarity?
Nope, they don’t. Although, their physical orientation in space will have a small effect on their operation causing deviations form their standard frequency. (Just a technicality; no need no worry about it.)
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Re: Issue 4A with no colour!

Post by Liveinabin » Thu May 16, 2019 7:32 am

Oooookay! Update time.

Got a new LM1889. Fitted that. No difference (didn't think there would be but worth a pop, although desoldering that was a pain)
New crystal in X2. Still mono.
Reflowed and, in some cases resoldered and replaced some of the new capacitors around the video bit. Nope. Still greyscale.
Desoldered the BC549Cs at TR1 and TR2 (mainly to have a good look at the traces around them) and fitted new ones.
Added a 100uF transistor into the composite mod (which does indeed brighten things up)
Went over the whole board with a big magnifying glass, cleaning up as I went.

Still black and white! Gah!

I do have one more thought. I swapped TR1 and TR2 for BC549Cs at the start of this escapade (sadly ditching the old transistors) and I've tried this Spectrum on a couple of TVs: My reliable LG is greyscale and a little (also LCD) Samsung shows, it turns out, no picture at all!
Tried my other working, comp modded Spectrum on both displays (a Samsung 3B with the original TR1 TR2 transistors) and it displays on both: Well on the LG and poorly on the Samsung TV (but then the Samsung TV IS crap).

Might it just be the BC549Cs not being compatible with my TV? I read that the 'improvement' they give may not work with flat TVs. Might it be worth me fitting a couple of ZTX313s instead?
That's really all I can think of to try now.
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Re: Issue 4A with no colour!

Post by 1024MAK » Thu May 16, 2019 5:43 pm

The “monochrome” parts of the video picture require a larger bandwidth (meaning higher frequency) than the colour carrier, so as I said above, I’m not convinced. But you are free to try.

Mark
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Re: Issue 4A with no colour!

Post by Muttley Black » Thu May 16, 2019 6:58 pm

You have already checked if 12 volts present on LM1889 (pin14), yes?
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