spectrum 48k tv chip

suicidal_monkey · August 04, 2003, 10:24:12 AM

anyone know somewhere I can look up the wiring or chip layout of a spectrum system board? My old one is lying in a packing crate somewhere in a garage in scotland, and I know the tv-out chip thing on the corner of the board is a bit broken. I re-soldered it myself years ago, but I'm pretty sure the unit was dead. If not already then by my huge soldering iron :roll:

Would be interesting to know what it is thats broken so I can think about how to fix it...

Cadaver · August 04, 2003, 10:44:53 AM

There's an online service manual for the Speccy 48K at the link below:

http://members.lycos.co.uk/eightbitula/ser...ce/service.html

Section 5 has the bill of materials. Check on the PCB silkscreen for the reference designator of the knacked part.

Having a quick read, if it's IC14 that's gone, it's a National Semiconductor LM1889 TV Video Modulator. Might have difficulty getting hold of one, as they discontinued it in 1998.

suicidal_monkey · August 04, 2003, 05:38:43 PM

ta

I'll have to dig it out of the boxes sometime and have a look now that I'm older and wiser

Might just be a connection to re-do. Might be a broken chip. Might have snapped in half in the packing crate... Now, when am I next dragging myself north :roll:

Stryker · August 05, 2003, 02:22:11 PM

some chips have a soldering life time. Often a chip/component can only be in contact with a soldering iron for 4-5 seconds..... thats cumerative time take to do all the pins 8O

Cadaver · August 05, 2003, 04:07:45 PM

Quotesome chips have a soldering life time. Often a chip/component can only be in contact with a soldering iron for 4-5 seconds..... thats cumerative time take to do all the pins 8O

This kind of figure harps back to through-hole components (i.e. those whose legs went through holes in the PCB) and the wave soldering method of board manufacture. Basically each component had a dwell time in its soldering profile, which is the maximum amount of time all of its legs could be immersed at once in the molten solder bath. Typically 3-5 seconds at 260 degrees C max. Any longer, and the part can't dissipate the heat build-up effectively and you risk damaging the die.

Whilst I wouldn't want to hold a soldering iron on something small, like a modern surface-mount diode, for too long (been there, melted the die), individual pins of an old dual in line package, like this TV modulator is in, should be okay. It's the temperature the iron reaches that will be the main problem. You don't want to be using a 100W electricians iron for instance, far too warm 8O

suicidal_monkey · August 05, 2003, 04:34:52 PM

I've built enough circuitry to have a pretty good idea about soldering

Got a little 18W iron for pcb stuff. 25 would be better as sometimes the 18w struggles with star earths and in places where heavy-duty wiring/components are being used.

you can get round the problem by using a heat shunt on particularly sensitive stuff. A croc clip filled with solder clamped on the component-side of the point you are soldering works pretty well. Kind of like a mini heatsink. General rule is the smaller and more complex a component, the more sensitive it gets.

Oh, and be careful when you say "electrician"

...you mean the guy who comes round in a white van when your oven's broken I assume? Even so, 100W is practically a plumbers soldering iron :roll: and I have one of them somewhere, probably in the same garage in scotland! Unfortunately when the spectrum broke and I made the first fixes I was only about 13-14, and that was before I owned a decent pcb/electronics soldering iron...still, it might be alright. components can be a lot tougher than you may think

suicidal_monkey · August 05, 2003, 04:37:04 PM

Oh, and the cumulative thing can be avoided if you give it time to cool off between legs, ...or you can use chip-mounts if you're really careful

Cadaver · August 05, 2003, 04:59:15 PM

:stupid: on both your points. It definitely sounds like you know what your doing :wink:

Oh, I said 'electrician's iron' as I had a mate who used to use one of those soldering guns regularly in his capacity as site electrician in a Welsh quarry. Borrowed it once - they do get a bit warm don't they :lol:

suicidal_monkey · August 05, 2003, 05:09:22 PM

yeah

Although the big one I have was a kit iron put together by my grandfather (! 8O !) The "bit" at the end is a copper rod of about 1cm dia ground down to a point. Not sure exactly how hot it gets, but it can certainly sustain that heat even when you've got some serious main cabling being soldered to a big iron/steel contact. I used a sander attachment on a drill once to re-grind the point

great fun, surprising I didn't do more damage to myself really. Still not a patch on the impressiveness of electric arc welders and angle grinders however :twisted: Powertools rock!

Stryker · August 06, 2003, 09:08:34 AM

does anyone here remember the soldering iron you had to leave in the gas burner on a stove to heat up?

LOL, twas my dads and the iron he taught me to solder with at the tender age of 10 :lol: