Issue 4A with no colour!

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Liveinabin
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Issue 4A with no colour!

Post by Liveinabin »

Hi all. I’ve exhausted my wildly inadequate electronics knowledge and I’d appreciate some advice.

Found a Spectrum issue 4A, mostly working. Just needed a RAM chip changing at IC15. Did that. Worked beautifully.
So...for a lap of honour, thought I might use my nice new soldering iron to recap the whole thing and composite mod it.
Did that with one of the little sets from Retroleum (also replacing TR1 and 2).

Fired it up and..monochrome! Well, there’s intermittent colour but it’s VERY faint and fleeting. Initially, I had a capacitor as part of the composite mod but I removed that. I even tried going back to RF but that just gave me an unstable (and still mono) picture.
I’ve looked around for any damaged traces but I can’t see anything. Wondered if it might be the LM1889 video encoder chip. I only have a multimeter and I checked pin 14, getting about 12v so it’s alive. More than that, I can’t say.

With hindsight, I should have tested after each component change. Oh well, lesson learned :)
Any thoughts on what could be wrong with this? Thanks in advance.

Image

Image
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DouglasReynholm
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Re: Issue 4A with no colour!

Post by DouglasReynholm »

There's some really dedicated Spectrum HW people on here that will be able to help you I'm sure, but as an EE myself, I can't help but point out that just because an IC has the right rail voltage, it doesn't necessarily follow that it is 'alive'! Good luck.
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Ast A. Moore
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Re: Issue 4A with no colour!

Post by Ast A. Moore »

First, reseat the ULA. Then make sure C25 is installed correctly and soldered properly. Next, check TR1, TR2, TR8, and TR9.
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Re: Issue 4A with no colour!

Post by Liveinabin »

Will do. I can see myself having fallen at either of ‘installed correctly’ and ‘soldered properly’. :D
I’ll try that out and get back to you.
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1024MAK
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Re: Issue 4A with no colour!

Post by 1024MAK »

Are the replacement transistors the correct way round?

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Re: Issue 4A with no colour!

Post by Liveinabin »

Well I wondered that. These are BC549C replacements (supposed to make the picture brighter) and the instructions were to install them the other way round from the original set.
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black »

Same question from me to, about TR1 and TR2. If the transistors are the BC549C, they are fitted correctly (opposite way). But if they are the ZTX313 or the MPS2369 they fitted wrong.
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Re: Issue 4A with no colour!

Post by Muttley Black »

Ha! I was a bit late :)
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Liveinabin
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Re: Issue 4A with no colour!

Post by Liveinabin »

Definitely BC549C
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Re: Issue 4A with no colour!

Post by Liveinabin »

Okay. Reseated the ULA, checked the polarity of all the capacitors and reflowed/resoldered. Transistors are in the right way round (and I reflowed solder and checked some continuity) but I don't know how to test transistors in situ.
Still mono :(
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black »

Look at the video below. JoulesperCoulomb shows the way you can test transistors. He do it on Tr4 Tr5 that is for dc converter. Do the same test on TR1,TR2,TR8,TR9 and post the results here.

[media]https://m.youtube.com/watch?v=IzgCmldm2H4&t=33s[/media]
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black »

You change the TR1 & TR2 with BC549C just to give more brightness to your screen, am I right? If yes, that’s mean that you still got the 2x ZTX313 and they are in working order. Have you tried to put them back and see the results?
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LET REPLY$="OFF-TOPIC"

Post by Guesser »

PRINT COMIC$="GREAT"

:)
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Liveinabin
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Re: Issue 4A with no colour!

Post by Liveinabin »

Muttley Black wrote: Sun May 05, 2019 10:17 pm Look at the video below. JoulesperCoulomb shows the way you can test transistors. He do it on Tr4 Tr5 that is for dc converter. Do the same test on TR1,TR2,TR8,TR9 and post the results here.
Superb video. Thanks. I'm getting around .7v on those transistors so, according to that guide, they check out, yes?
One alarming thing. Early in that video, there is a test for the voltage regulator. Should give a reading of >100 kOhms. I get 76.
Last edited by Liveinabin on Sun May 05, 2019 11:10 pm, edited 1 time in total.
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Re: LET REPLY$="OFF-TOPIC"

Post by Liveinabin »

Guesser wrote: Sun May 05, 2019 10:58 pm PRINT COMIC$="GREAT"

:)
FOR A=1 TO 10
PRINT "THANKYOU!!"
NEXT A
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Re: Issue 4A with no colour!

Post by Liveinabin »

Muttley Black wrote: Sun May 05, 2019 10:45 pm You change the TR1 & TR2 with BC549C just to give more brightness to your screen, am I right? If yes, that’s mean that you still got the 2x ZTX313 and they are in working order. Have you tried to put them back and see the results?
Sorry, just spotted this. Um, no. I changed them a couple of weeks ago and stupidly didn't keep the bits. I do have 4 more unused BC549Cs and a single (new) ZTX313 but I could easily pick up more if you think it's worth try. They're not expensive :)
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black »

Basically, this video talking about some test we need to do before switch on a ZX Spectrum. Mostly because the most common problem on ZX's are the transistors TR4 /TR5 that was used on dc circuit to provide the -5volt & +12volt that 4116 lower ram need. Missing the -5 volt can easily blow away all the 4116 ram. In the other hand a faulty 4116 ram can easily blow TR4 transistor. But all this is not part of your story. Anyway, is always good to test the voltages on your ZX just to be sure about them. Pins 1,8,9,16 of any 4116 ram can give you this results. About 7805 voltage regulator, the only thing it does is to convert the (about) 9volt to 5volt.

Now about transistor test, you need to do them with your multimeter set on "diode test". See the test on the video carefully. You should post 4 measurements for every transistor.
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Re: Issue 4A with no colour!

Post by Liveinabin »

Oh right, of course. I’ll post results tomorrow. Thanks for all of your help tonight. Sorry to take up so much of your Sunday evening 😊
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Re: Issue 4A with no colour!

Post by Liveinabin »

Readings taken (I am learning a LOT about electronics here)
Four readings for each:

pos at base
1) neg at collector
2) neg at emitter
neg at base
3) pos at collector
4) pos at emitter

TR1 (BC549C)

0.63
0.70
1.73
1.68


TR2 (BC549C)

0.70
0.70
1.19
1.30


TR8 (BC184)

0.75
0.75
open
open


TR9 (BC184)

0.75
0.75
open
open
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black »

TR1 looks faulty to me. Cause with neg at base, you should get "open", both on collector and emitter if i am not mistaken. If you are sure that you measure TR1 as TR1 and TR2 as TR2 then replace TR1. I say that because your TR1 measurements fit more to TR2 transistor. Anyway, to be 100% sure replace both TR1 & TR2 with new BC549C transistors and tell as if the problem solved. The other transistors looks ok.

Also post your voltages just to have a picture about them. With neg probe to ground measure voltages on pins 1,8,9 and 16 from the IC6 and post them here.
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Re: Issue 4A with no colour!

Post by 1024MAK »

Yeah, but keep in mind that with any in-circuit testing, the results may be affected by the other components on the board.

Mark
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Re: Issue 4A with no colour!

Post by Muttley Black »

That's true. But TR1 should give you this measurements ("open", both on collector and emitter with neg at base) in-circuit testing. I also test it on my spectrum's after your post.
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Re: Issue 4A with no colour!

Post by Liveinabin »

Muttley Black wrote: Mon May 06, 2019 9:40 pm TR1 looks faulty to me. Cause with neg at base, you should get "open", both on collector and emitter if i am not mistaken. If you are sure that you measure TR1 as TR1 and TR2 as TR2 then replace TR1. I say that because your TR1 measurements fit more to TR2 transistor. Anyway, to be 100% sure replace both TR1 & TR2 with new BC549C transistors and tell as if the problem solved. The other transistors looks ok.

Also post your voltages just to have a picture about them. With neg probe to ground measure voltages on pins 1,8,9 and 16 from the IC6 and post them here.
I’ll check my measurements again and I’ll change both components (it’ll be a good chance to have a proper look at the tracks as make sure I haven’t shorted/damaged anything). I’ll report back tomorrow night with findings and more readings.
Thanks so much for helping me out here. Really appreciate it.
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1024MAK
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Re: Issue 4A with no colour!

Post by 1024MAK »

If you are getting a good monochrome picture. By that I mean good contrast and brightness, then there can’t be much wrong with TR1 and TR2, as the monochrome (‘luminance’) signal passes through both of these.

Also TR1 has nothing to do with the colour part of the signal. The encoded colour signal comes from pin 13 of IC14 (LM1889) via C65 (100nF) to TR2 base, where it is mixed with the ‘luminance’ signal.

An oscilloscope would be a great help with fault finding this problem.

Mark
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
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Re: Issue 4A with no colour!

Post by Liveinabin »

The mono image is perfectly fine (well, for a Spectrum).
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