Old TV as a ZX80 monitor may not be optimum

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: Old TV as a ZX80 monitor may not be optimum

Post by 1024MAK »

See this web site for a redrawn ZX80 schematic where the (known) errors have been corrected.

What values are resistors R30 and R32?

Mark
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Re: Old TV as a ZX80 monitor may not be optimum

Post by Creator »

R30 is red red red 2200
R32 is not populated - I have verified that sync signal is getting to the modulator though I haven't looked into how.
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1024MAK
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Re: Old TV as a ZX80 monitor may not be optimum

Post by 1024MAK »

This is my ZX80:
My ZX80 board, showing video circuitry resistors
My ZX80 board, showing video circuitry resistors
(Click to enlarge)

Code: Select all

	U.K. standard 	Recommended (*)
R30 	1kΩ		470Ω
R32 	330Ω		820Ω
(*) Recommended by another web site contributor, I have not tested these values myself.

Mark
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Re: Old TV as a ZX80 monitor may not be optimum

Post by Moggy »

Just taken the top off my ZX80 to checkand my resistors are the same.

R30 1kΩ
R32 330Ω

My ZX80 is fitted with the ZX8-CCB module and works perfectly.
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Re: Old TV as a ZX80 monitor may not be optimum

Post by Creator »

Well that sure is different. I can try that.
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Re: Old TV as a ZX80 monitor may not be optimum

Post by Creator »

Well that works better. I removed r33 & r31, replaced r30 with 1k. putting in r32 causes me trouble, it wasn't in originally. New values make the original rf modulator work better but the zx8-ccb I can just barely adjust in and breathing on either pot cause it all to go away.
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1024MAK
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Re: Old TV as a ZX80 monitor may not be optimum

Post by 1024MAK »

Are R36 or R37 fitted?

Were your tests done with the modulator video input still connected?

On a U.K. board, R36, R37, C3 and TR1 are not fitted. The modulator is type UM1233 which is a Ch36 UHF version.

Mark
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Re: Old TV as a ZX80 monitor may not be optimum

Post by Creator »

I don't like mysteries. R32 is unpopulated on my board. It is supposed to connect the sync signal to the wire OR vin of the Rf modulator. Signal vin has the sync pulse. Tracing around I find sync NOT connects to r36 which connects to base of TR1, collector to vin, emitter to gnd. I don't see this on schematics. Anyone else have this ?

Mark I see you already have info on this. I haven't sorted out r37 yet but it & a cap (22u) are there. Modulator is UM1082 E3 (ch 3)
I had both modulators connected during last tests because I was switching back and forth. Probably a bad idea, I'll unhook the org & retest tomorrow. I've got a crew coming early to remove a tree that fell over
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1024MAK
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Re: Old TV as a ZX80 monitor may not be optimum

Post by 1024MAK »

Unfortunately I can’t find any other original schematics that show the alternative (USA) video circuitry :(

I do however know that the UM1082 modulator needs a different signal compared to the UM1233 type.

It would be helpful if you could trace out a schematic of how your board has the video circuitry connected up. Especially the values and types for the components.

Edit:
I’ve found this topic, is the schematic (this one) shown in the later post anything like how your board is configured?

Mark
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Re: Old TV as a ZX80 monitor may not be optimum

Post by Creator »

ok, my US ZX80 original circuit :
vin to modulator UM1082 E3 (ch3)
signal video thru R30 2.2k wire OR vin
signal sync NOT thru R36 5.6k to base of TR1 collector wire OR vin emitter to gnd
+5v thru R33 5.6k to wire OR
R31 1k to gnd to wire OR
signal level about 1.1v not triggering ZX8-CCB ic1 & ic3

Hope this is clear, I don't want to take the time right now to learn tools to draw the circuit (as simple as it probably is)

Changes made are:
R33 pullup and r31 pull down removed, signal video r30 changed to 1k (signal level 2v) , old rf modulator disconnected from vin, ZX8-CCB connected. ic1 & ic3 now have enough signal level to produce an output.

usually the signal is poor with sync problems or when synced has vertical bars (30 or so ) across the screen. On RARE occasion I can get a decent image when adjusting pots but as soon as I lift the plastic adj tool the image returns to its normal poor state, as if the pots where badly worn/dirty.
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