more component questions, t1000 vs zx81

[yalokinh]

so I have a Timex 1000 that I decided to take apart and try to figure things out, and found components C13 and C14. These confuse me because they look just like resistors, but they are labeled as capacitors. can some capacitors be in the shape of resistors? I can't find these in the components list though, it only goes up to C12.

Also found a D9, which isn't used in the british version, do I just assume its the same value as D1-8?

I also found L1 and L2, I understand that these are inductors, but they aren't on the zx81 component list. why are they on the Timex 1000 but not the Zx81?
is there a benefit of having them vs. not having them? can I just leave them out?

When I got the Timex1000 I had no idea what working condition it was in, the outside was pretty rough and they keyboard was pretty banged up, I had no way of testing it either so I decided to open it up and take some ICs and other components out. The RAM and the ROM were both soldered without the sockets (GGRRR), but I decided to take them out anyways using a solder pump. How do I know for sure the heat from the soldering iron didn't fry them?

One more thing, where can I find an old rf modulator? in case the one on the board doesn't work?

[RWAP]

I can help with an RF modulator if you need one - I have one (UM-1233) I salvaged from a QL - I presume the TS1000 uses the same one?

[PokeMon]

so I have a Timex 1000 that I decided to take apart and try to figure things out, and found components C13 and C14. These confuse me because they look just like resistors, but they are labeled as capacitors. can some capacitors be in the shape of resistors? I can't find these in the components list though, it only goes up to C12.

In the 80ies there have been even ceramic capacitors in axial version, like resistors. In my eyes they have been easier to handle this time in automated production. You can use the same machines putting resistors on the board for capacitors as well. The code is easy, like resistors in pF for the smaller ones.
So 47pF is yellow-violet-black which means 47*(10**0)pF. Or brown-black-brown for 100pf= 10*(10**1)pF. The gold ring says typical tolerance of 5%.
The values with letters are same, 473 = 47*(10**3)pF = 47nF.

C13 and C14 are both connected between VIN of 7805 (input voltage) and GND. So these could be used as blocking capacitors any value of a few uF would make sense. But take notice of polarity if you use tantal or classic electrolytic versions. As they are not really needed they have been spared in production. Just a matter of cost. Like all regarding the Sinclair policy.

I think L1 and L2 have been planned for filtering input voltage noise (together with C13 and C14) and are simple wires in my ZX81 ISS3 board.

[yalokinh]

Thanks a lot!
Concerning the modulator, i see a UM 1082 on mine. Would this be the reason why the signal is different on american tvs?

[gozzo]

Yes the modulators are very different, UK and other countries using UHF TV systems had a UM1233, USA versions (maybe other VHF countries as well) had a UM1082LA.

[yalokinh]

I checked out the capacitors and they have color rings of yellow, violet, red, silver, and gold. Im sure the gold means tolerance, but what about the silver?

[iturbez]

Take a look at http://www.electronics-tutorials.ws/cap ... cap_5.html

I hope this helps!
Regards.

[RetroTechie]

The Timex 1000 uses the same board as a ZX81, but with some differences:

• Video modulator is a different type, to output on VHF band vs. UHF
• VHF channel select switch (see underside housing near DC input)
• A metal strip across the board connects ground (0V) to conductive paint on the inside of the case. This is to improve EMC shielding. On the UK ZX81, metal strip + conductive paint weren't used
• A few parts here & there present or removed depending on modulator type and 50/60 Hz selection (see schematic)

I also found L1 and L2, I understand that these are inductors, but they aren't on the zx81 component list. why are they on the Timex 1000 but not the Zx81?
is there a benefit of having them vs. not having them? can I just leave them out?

Near the DC input, right? If so, those are to prevent leaking high frequency noise out of the board, using the DC cord as antenna, and cause HF noise in other equipment. Same as with metal strip + conductive paint. Apparantly, EMC requirements were stricter in the US than UK at the time.

If you would replace those coils with wires, most likely you wouldn't notice any difference. That said: no reason to do so, if they're there just leave 'em. I've removed the metal strip from my Timex 1000 because it was in the way & I didn't feel like putting it back in.

One more thing, where can I find an old rf modulator? in case the one on the board doesn't work?

May I suggest to do a composite video modification? Quick & easy to do, requires a few parts one can buy for pennies, takes the VHF/UHF modulator out of the equation entirely. As a bonus, you get improved picture quality. Modulator case is left in place to make the connection (so looks aren't changed), but now you have composite video out instead of RF signal.