I think that I damaged my ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
luigi
Posts: 4
Joined: Mon Jun 04, 2012 3:21 pm

I think that I damaged my ZX81

Post by luigi » Mon Jun 04, 2012 3:33 pm

Hi from Spain.

Yesterday I was doing the AV mod for display ZX81 in a modern TV. I solded a short cable between the AV conector and the AV signal, previously I had cut the resistence connection. Eerything seemed be fine, but it didn't work. So I saw the connections and I could see the problem... A tin peak was touching the case of the RF Tunner... Oh, my god...

So I put another time the resistence and I tried to turn the computer in a old TV where I knew that my ZX81 could be turned fine, but it doesn't work.

Is it possible that I had damaged the ULA? What can I do?

Best regards.

User avatar
1024MAK
Posts: 2303
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: I think that I damaged my ZX81

Post by 1024MAK » Mon Jun 04, 2012 5:15 pm

luigi wrote:Yesterday I was doing the AV mod for display ZX81 in a modern TV. I solded a short cable between the AV conector and the AV signal, previously I had cut the resistence connection.
Were you trying to just connect a composite video cable to the video signal that comes from the ULA pin 16?
Or are you trying to wire in the transistor "buffer" amp video modification?
luigi wrote:A tin peak was touching the case of the RF Tunner... Oh, my god...
I presume you mean a solder splash or a thin thread of solder :? Always visually check any work and nearby areas, if necessary using a magnifying glass as short circuits caused by solder threads and splashes can cause all manor of faults :(
luigi wrote:So I put another time the resistence and I tried to turn the computer in a old TV where I knew that my ZX81 could be turned fine, but it doesn't work.
:? Not sure what you are trying to say here. I get the part about tuning in an old TV that is known to work with a ZX81. But don't understand the first part.
luigi wrote:Is it possible that I had damaged the ULA?
If you shorted out the circuit connected to ULA pin 16 you may have damaged it. This is why a transistor "buffer" amp is recommended if you want a composite video output.
luigi wrote:What can I do?
Let us have more details. Even better, some nice detailed close-up pictures :mrgreen:
If you have damaged the ULA then there are only three options: Mark

User avatar
PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: I think that I damaged my ZX81

Post by PokeMon » Mon Jun 04, 2012 8:47 pm

luigi wrote: Is it possible that I had damaged the ULA? What can I do?

Best regards.
The easiest way is to watch pin 16 of ULA with a oscilloscope. Do you have one ?
And be sure not to have a shortcut somewhere.
Can find some picture how the signal should look here:
http://forum.tlienhard.com/phpBB3/viewt ... ?f=2&t=588



You had better try out my video circuit for it, is more easy and safe. But could make shortcuts when soldering an pin 16 too. :shock:

luigi
Posts: 4
Joined: Mon Jun 04, 2012 3:21 pm

Re: I think that I damaged my ZX81

Post by luigi » Tue Jun 05, 2012 4:40 pm

Hi, thanks for your responses.

The mod that I did is the easiest: connect directly ULA signal to RCA connector. I followed the next steps (in Spanish): http://www.retrogames.cl/zxav.html

My problem was a thin thread of solder. I didn't check carefully because I though the solder was fine.... yes... yes... I was stupid...

I tried the "faulty" mod in my LCD screen using the AV connector. So I couldn't see nothing then I tried to undo the mod, I connect another time the internal resistence of RF tuner and tried to turn the ZX81 in a old TV where I knew that it worked fine previously.

I don't have oscilloscope. Can I test the pin 16 of ULA using a multimeter?

How is connected the replacement which is sold in http://www.sellmyretro.com? I have seen the photo but I can not understand. Is there any tutorial to explain it?

Best regards.

User avatar
PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: I think that I damaged my ZX81

Post by PokeMon » Tue Jun 05, 2012 5:55 pm

luigi wrote: The mod that I did is the easiest: connect directly ULA signal to RCA connector. I followed the next steps (in Spanish): http://www.retrogames.cl/zxav.html
This describes a mod for a zx spectrum which is not in all case suitable for a ZX81.
Even the pin of the ULA for vid out is #16 (not #17).

Normally the hardware is quite robust for short periods of shortcuts (if you check within a few seconds).
Don't know how it is handled when done for an hour or so for testing with TV.

I had some problem with a temporarily shortcut between 5V and 9/12V for a short moment (a few seconds). The Zeddy did alive, the 16k memory kit died. :roll:

User avatar
1024MAK
Posts: 2303
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: I think that I damaged my ZX81

Post by 1024MAK » Tue Jun 05, 2012 8:21 pm

luigi wrote:I don't have oscilloscope. Can I test the pin 16 of ULA using a multimeter?
I don't have a meter and a Zeddy to test available at the moment, but with the Zeddy just powered on and nothing typed in, just the white screen with reverse K cursor, the voltage at ULA pin 16 should be about 3.8 Vdc. This info from post #40 here. Don't worry about the other details in reply #40 (scroll down) as Andy is talking about an add on video circuit.
luigi wrote:How is connected the replacement which is sold in http://www.sellmyretro.com? I have seen the photo but I can not understand. Is there any tutorial to explain it?
You send your ZX81 to the address given. Your original ULA is removed. The replacement is fitted and tested. Note a number of other alterations are needed (which is why they fit it for you). Your ZX81 is returned to you. The replacement ULA produces a composite video signal compatible with most TV's and monitors so the modulator is disabled and you get a composite video signal output instead.
If you want more detail I'm sure Andy or Rich can jump in and respond to any questions :mrgreen:

If your ULA is still okay, I should say that there are a number of other video corrector / amplifier circuits and modules available.

Mark

gozzo
Posts: 448
Joined: Fri Jul 08, 2011 8:52 pm

Re: I think that I damaged my ZX81

Post by gozzo » Tue Jun 05, 2012 9:48 pm

I would strongly advise NEVER to connect a zx81 ula video output pin directly to a RCA (or other socket) to connect to a composite input of a monitor/tv - there is a fairly high DC level there which apart from the monitor/TV possibly not liking it, the composite input is supposedly 75 ohm input impedance and if a DC level of,say, roughly 3volts is present at the ULA pin, this means a current of just under 50mA!! Not good for the poor old ULA (or TV)! Connect it through a 47uf electrolytic capacitor, positive to the ULA video pin, with a BAT85 or OA90 diode, anode to chassis, on the RCA/composite output side, or even better buffer it with an emitter follower first!!! You could most likely get away without the diode but I would include it to prevent the average DC level from possibly wandering negative..

User avatar
PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: I think that I damaged my ZX81

Post by PokeMon » Wed Jun 06, 2012 12:14 am

Well the capacitor doesn't help much to reduce output current when connected to a 75 Ohm video input as it provides a hf signal and there is no significant resistance of the condensator measureable. So directly 75 Ohm input is too much for the ULA regardless if with or without capacitor. And I would us 100uF up, better 220 I think. And you should as a parallel ceramic or MKT capacitor with about 1-100 nF to better pass higher frequencies which are maybe affected by an electrolytic capacitor as we are talking of up to 10 MHz when calculating 3rd harmonic of highest possible pixel frequency (for a crisp signal). ;)

User avatar
RetroTechie
Posts: 363
Joined: Tue Nov 01, 2011 12:16 am
Location: Hengelo, NL
Contact:

Re: I think that I damaged my ZX81

Post by RetroTechie » Wed Jun 06, 2012 10:21 am

gozzo wrote:I would strongly advise NEVER to connect a zx81 ula video output pin directly to a RCA (or other socket) to connect to a composite input of a monitor/tv - there is a fairly high DC level there which apart from the monitor/TV possibly not liking it, the composite input is supposedly 75 ohm input impedance and if a DC level of,say, roughly 3volts is present at the ULA pin, this means a current of just under 50mA!! Not good for the poor old ULA (or TV)!
Indeed, and if ULA outputs, say, 3V, then (5-3V)*40mA = 80 mW is added to ULA's heat dissipation (since that current comes from the +5V supply, but the 5V - output voltage difference is dissipated inside the chip). That's a -relatively- big addition for a plastic-encased chip which already gets hot under normal operation.

Another reason why not is electrostatic discharge, especially when (dis)connecting big equipment with the Zeddy in between. Say you have a PC soundcard connected to ear input, and you plug in RCA connector to TV. The tip of the connector is normally the first to touch, so any voltage equalization between TV-ZX81-PC then goes through the ULA pin. By the time the outside of the RCA connector (ground) makes contact, the damage is done.

Normally the modulator sits in between, with a [transistor buffer / comp. video mod] that cheap & easy to replace transistor would eat such a discharge.

Same story goes for ear/mic connections, but they include series resistors which makes permanent damage much less likely.
PokeMon wrote:Well the capacitor doesn't help much to reduce output current when connected to a 75 Ohm video input as it provides a hf signal and there is no significant resistance of the condensator measureable. So directly 75 Ohm input is too much for the ULA regardless if with or without capacitor.
Wrong! The other end of the capacitor will go to voltage level it's loaded to. For example schematic of my old TV shows a plain 75 Ohm resistor to ground at its composite video input (SCART). So if ULA pin output averages 3V, capacitor would charge to 3V, other end would average near 0V (and thus little point in adding a diode if your goal was a 0V average), and the ULA pin would behave as if it were loaded with 75 Ohm to a 3V DC source. Supplying current when output >3V, sinking current when output <3V, but on average certainly less than with 75 Ohm to ground. Agree with the value though - 100 uF or more is good, with smaller values you might see some distortions in screens with big black (horizontal) bars & such.
And you should as a parallel ceramic or MKT capacitor with about 1-100 nF to better pass higher frequencies which are maybe affected by an electrolytic capacitor as we are talking of up to 10 MHz when calculating 3rd harmonic of highest possible pixel frequency (for a crisp signal). ;)
You'd think so, but when I tried this once with different capacitor types (like tantalum, which usually has much better high-frequency behavior than aluminium electrolytic), I couldn't find any visible differences. And as long as the pixel frequency gets through, damping higher frequencies may actually improve image quality. :shock: So I guess for this application cheapo electrolytic is good enough, and other factors (ULA, output circuit, power supply, video cable, TV) are more important for image quality.

User avatar
1024MAK
Posts: 2303
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: I think that I damaged my ZX81

Post by 1024MAK » Wed Jun 06, 2012 4:57 pm

This is the circuit that I use for testing to see if the ULA in a ZX81 will produce a picture.
It has more resistors added to provide a bit of extra protection as one of my TV's produces a fairly high (but high impedance) voltage on it's chassis (damn double insulated, ground/earth free designs).
I use test leads to connect the input, +5V and ground to the ZX81 and it has a Phono/RCA connector soldered to the strip-board that I connect a composite video cable to.
ZX81 video transistor buffer amp MAK #01s.png
Schematic
ZX81 video transistor buffer amp MAK #01s.png (3.34 KiB) Viewed 3112 times
All resistors are non critical, suggest 0.25W
Capacitors rated at 16V or greater
Transistor can be any high gain NPN signal transistor (e.g. BC547C, BC548C, BC549C etc)

Here are some photos of my prototype constructed on stripboard:
6890DCAB-79DB-4603-BB06-7519B807AC82.jpeg
Prototype on stripboard
F5A71EC1-D3B4-4067-B7D0-46A84ACA1AB4.jpeg
Prototype on stripboard
2B88E2BC-DBD0-466C-81B5-3D604B86C3C8.jpeg
Prototype on stripboard
D5E50C70-F70D-4260-9E52-79610AF3774A.jpeg
Prototype on stripboard
149FA348-3C8D-44AC-878F-C8623CF9E904.jpeg
Prototype on stripboard
Note that on this prototype, R3 is 150 ohms, C3 is not fitted and Q1 is a BC547C

Mark

Post Reply