TS1500 no video

[bola_dor]

Yeah, sorry about being terse. It’s lack of time you see. I was intending on posting some more help today, but have not had much spare time. And then today, the hot water and heating system in the house failed , so that took priority...

It failed while I was at work. The outside air temperature in my part of the U.K. is about 7 to 8°C, so the family were not exactly happy about this.

After investigating, I found the natural gas boiler had shut down. After a bit more fault finding, I found that the extraction fan was not spinning even though it had 240V AC on its connection terminals (which are not insulated which is a bit naughty ). Anyway, I isolated the mains supply. When I tried to turn the fan blades, they were very stiff.

The coil measured okay on resistance, so I applied some solvent cleaner to the bearings. That helped a little. So more solvent cleaner until I drowned it. Then some WD40. That’s much better, it spins freely now. Finally a few drops of 3-in-1 light mineral oil.

After putting everything back together, time to power it up...!

And whoosh, the boiler fires up

It did take a couple of hours before the house warmed up though (which is normal when the entire hot water and heating system is starting off cold).

Now I’m knackered...

Sorry that became rather longer than intended...

Mark

I can give you some of our 38°C 88% humidity
(How do you post those unicode chars?)

[Spinnetti]

Brr! I've had the same thing happen to my furnace in the winter before.

I'm in the USA, so the TS1500 is a 60hz one.

All, thanks for the guidance.. I'll need to figure out which pin off the ULA is video then start checking the output at the ula and memory chips voltages next.

[Spinnetti]

Here you can see how I modified video output of mine with the Mark's buffer mod. You can check ULA video output from the pictures.
viewtopic.php?f=7&t=3771&p=38736&hilit= ... ite#p38736

Thanks... That newer board is much more compact for sure... So, if I got the right one, 8 pins up from the lower left corner (with the edge connector pointing away at the right top) and I'm only getting .2v off the ULA. So, does that mean my ULA is toast? (sad face)

[1024MAK]

(How do you post those unicode chars?)

That was done on an iPad.

Mark

[bola_dor]

Here you can see how I modified video output of mine with the Mark's buffer mod. You can check ULA video output from the pictures.
viewtopic.php?f=7&t=3771&p=38736&hilit= ... ite#p38736

Thanks... That newer board is much more compact for sure... So, if I got the right one, 8 pins up from the lower left corner (with the edge connector pointing away at the right top) and I'm only getting .2v off the ULA. So, does that mean my ULA is toast? (sad face)

0.2v at the video output ? Sounds very low..
But. Before evicting it. Check clock. Z80 and ram voltages.. as far as I know there in no easy replacement for that ULA..
It's Sad because I have a goog TS1500 board but in bad cosmetic conditions.. but I am really far away in South America....

[1024MAK]

So, does that mean my ULA is toast? (sad face)

Far too early to reach such a conclusion.

Can you please do the following tests:

You can use use the heatsink tab of the 7805 for 0V/GND, use this point for the black/negative meter lead unless I specify a different location.

Please check the +12V and -5V power rails. These are available on the pins of one of the 4116 (or equivalent) DRAM chips, for example U8.



A copy of the full data sheet is here

Pin 1 is VBB, which is -5V (-5.5V to -4.5V).
Pin 8 is VDD, which is +12V (+10.8V to +13.2V).
Pin 9 is VCC, which is +5V (+4.75V to +5.25V) (the output from the 7805 regulator is normally very close to +5V).
Pin 16 is VSS, which is 0V (ground/GND) Test this by measuring between this pin (black test lead) and pin 1 (red test lead).

If the above voltages are okay, test the following Z80 pins:
Pin 4 (A14)
Pin 5 (A15)
Pin 6 (clock)
Pin 11 (Vcc/+5V)
Pin 18 (/HALT)
Pin 19 (/MREQ)
Pin 21 (/RD)
Pin 26 (/RESET)

A diagram of the Z80 is here

Report back your results.

Mark

[Spinnetti]

Mark,

Thanks for the hope... Given the voltage issues, maybe issue is upstream of the ram?

Pin 1: .21-.22 on all the ram
Pin 8: 8.30 on all the ram
Pin 9: +5.16 on all the ram
Pin 1 (red, Pin 16 (black): .22V on all the ram

[1024MAK]

The DC-DC converter (labelled ‘switching power supply’ on the schematic) circuitry has failed. With a low +12V supply (the main supply for this type of DRAM chip) and a missing -5V supply (required to bias the internal semiconductor junctions) the DRAM chips will not work correctly. Worse, the loss of the -5V supply with the positive supply voltages still present can cause damage to this type of DRAM chip.

It’s also possible that a DRAM chip failed and overloaded the switching transistor (Q2) in the DC-DC converter circuitry.

I’m not convinced that the part of the redrawn schematic for the TS1500 that shows the DC-DC converter circuitry is correct, as it does not make sense to me.

Until the DC-DC converter circuitry has been repaired, please don’t run the machine for any longer than needed to carry out any required voltage tests.

The first thing to do, is to renew transistor Q2 (type ZTX750). This can be replaced with another ZTX750, or a ZTX751, ZTX752 or ZTX753 (which are all higher voltage versions of the ZTX750). I recommend buying at least THREE spares.

Mark

[bola_dor]

Testing Q2 with the multimeter and replacing electrolytics C20, C24, C25, C30, C31 won't harm I guess.
https://www.electronics-notes.com/artic ... r-test.php

[1024MAK]

Testing Q2 with the multimeter and replacing electrolytics C20, C24, C25, C30, C31 won't harm I guess.
https://www.electronics-notes.com/artic ... r-test.php

I suspect Q2 is now toast.

According to the schematic, the values of the capacitors are:
C20 - 22uF
C24 - 22uF
C25 - 100uF
C30 - 1uF
C31 - 1uF

Also while Q2 is out, test the following diodes:
D1, D3 to D9 keeping in mind that D1 and D9 are zener diodes.

Mark