Video mod for TS1000

[bmwhitetx]

I recently purchased a Timex Sinclair 1000 that appears to be in good shape. It has the 210E ULA with a 1982 date code. This forum has been helpful in me getting a good picture using the composite video mod so I wanted to share my success and failures hoping it would help others. Without the mod, the TS1000 produced a picture on a very old Sony tube TV using a 75 to 300 ohm adapter connected to the RF input but it was very poor quality. So I ripped the guts out of the VHF modulator and installed the transistor resistor mod shown here:http://www.zx81.de/english/video_e.htm. It produced a beautiful picture on the tube TV so I tried it on three LCD flat screens in the home with various results:

1. An Insignia (Best Buy brand) circa 2009: Would not detect any signal.
2. A Sony Bravia circa 2007 ; white screen with faint gray characters. Could not improve using brightness and picture controls.
3. A LG circa 2016 - crisp black/gray characters but after a minute it started to cycle to a black screen and back to a nice screen.I thought maybe I had a loose connection but nothing helped.

Going back to the Sony I remember reading about DC offsets and a decoupling capacitor to help with washed out signals. So I cut my video cable and added a 100uF cap in series. In the schematic referenced above, this is like adding a capacitor between the emitter and video out with the cap (+) on video out and cap (-) on the emitter. Results:

1. Insignia - still no picture
2. Sony - very very nice picture. Clear blacks and no interference
3. LG - very nice picture and no cycling (only tested for a couple of minutes)

When I get a chance I'll reopen the TS1000 and add the capacitor internally but I am trying to minimize alterations because of concern of breaking the keyboard membrane connector strip. I remember it being very fragile. Still not sure why the Insignia refuses to detect any signal.

One other note - I originally picked up the video input from the video modulator's video input and got a very dark gray screen on the tube TV. I eventually realized that the US version has a resistor divider circuit and a diode between ULA 16 and the modulator. So I moved the input wire back to the diode which is before the resistors and got the great black and white screen.

[1024MAK]

Hello and welcome to our forum

Yes, the DC levels do have different affects on different TVs. I use the circuit in this post for testing.
Another way to reduce the DC output level without using a series output capacitor, is to include one or more signal diodes (e.g. 1N4148) in series with the base terminal of the transistor.

If you want to spend some money, there are two different designs available on Sell My Retro:-
ZX8-CCB - video output for ZX81
Sinclair ZX81 ZXVid - Composite Video/ULA Fix (self-fit)
Although mainly made to add a back porch signal level to the video signal to make a composite video signal more compatiable with modern TVs, they also clean and filter the signal, and of course provide the correct output level for TVs.

Mark

[dessony]

1024MAK (Mark),

Here are two pictures (based on your own schematic) I made a small video board on a stripboard to fit within the gutted-out RF Modulator metal box area. The small board measured around 20 MM x 27.5 MM. So it fits!

Video board Before Installation.JPG (127.94 KiB) Viewed 6350 times

Video board After Installation.JPG (176.07 KiB) Viewed 6350 times

Since I am on the American side of the Atlantic Ocean, I use a NTE123AP. I understand that 100nF means .1uF or marked as on the capacitor as 104. The small video board works fine here!

[1024MAK]

That's nice and neat, well done

Yes, 100nF = 0.1uF, the marking of 104 means a value of ten with four more zeros - 10 0000pF, which is of course 100nF

Mark

[mhudson52]

I've looked around but haven't run across the schematic for this board. Could you please post a link? Thanks!

[bmwhitetx]

Did you click the first link in 1024MAK's post above for the test circuit? I believe that's the schematic.

[1024MAK]

Did you click the first link in 1024MAK's post above for the test circuit? I believe that's the schematic.

Yes, the schematic is in a post in another thread, follow this link (same as in my first post in this thread ).

Mark