TK85 (Brazilian clone) white screen trouble shooting

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bola_dor
Posts: 398
Joined: Wed Oct 02, 2019 5:32 am

TK85 (Brazilian clone) white screen trouble shooting

Post by bola_dor »

Well this is a TK85, a Brazilian clone with 16Kb internal RAM and a rubber keyboard so I think it is more like a TS1500 clone, Rom is not identical as have some differences but compatibility is almost 100%.
This particular model has a Ferranti ULA, pin 22 is grounded so it runs at 60Hz (Brazilian PAL-M is 60Hz) and has 4x 4116 and 4x MM5290N (4116 compatible) RAM IC, I don't know which of them are original. Luckily almost all IC are on sockets..
20210626_193856.jpg
On first review RAM voltages were not good. 12v line was like 9v and -5v line was -8v. DC DC circuit is completely different from that used by Sinclair or Timex. I had to discover that 12v comes almost directly from the PSU through some filtering and a 12v Zener diode, so I am using a 12v regulated PSU and inserted a diode at the input. I have 11v with the load. -5v are generated by a lm555 oscillator, some capacitors, diodes and a 5v Zener. I replaced almost all components and now I have good -5v.

I tested all 8 RAM chips on another board with sockets and discovered 2 of them were bad so I replaced them with two modified 4164 ..
But still I have a white screen without the [K]...

I tried with another Z80 with no change. CLK is OK and control lines are OK but /HALT is held High and noisy. /WAIT is pulsing..
20210626_184403.jpg
20210626_184325.jpg

so I located the Tr1, R1,C1, R17 equivalent circuit. Continuity/resistance to ULA pin17, 5v and lines to Z80 pin 18 and 24 are good.. resistors measure good.. Tr1 tested (on board) with the diode meter looks good as NPN..
(I noted That the ZX81 schematic gif around the net has miss labeled Z80 /HALT as pin 19 instead of 18)
2021-6-26 18-32-56.jpg
Perhaps next obvious step is to replace that transistor and may be the ceramic capacitor... but what else sould I check for?
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
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1024MAK
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Re: TK85 (Brazilian clone) white screen trouble shooting

Post by 1024MAK »

Keep in mind that /HALT is an output from the Z80A. The processor normally holds the /HALT line high. But when it executes a HALT instruction code, it stops incrementing the program counter (PC) and acts as if it has fetched a NOP instruction. It changes the logic level on the /HALT output from high to low to indicate that it has halted. It then goes back and fetches the /HALT instruction (because the PC is still pointing to the memory location where the HALT instruction code is) and does the same thing all over again. The only way to break it out of this ‘loop’ is either reset it or have interrupts enabled (*1). When it detects an interrupt (non-maskable or a maskable interrupt), it will service the interrupt. Then after the return from the interrupt, normal operation will resume. The /HALT pin having gone back to logic high shortly after it detected the interrupt.

So if the /HALT output is always high, it’s highly likely that the processor is executing garbage. Or it is not receiving any interrupts. Does the Z80A work in other machines? If yes, and if the ULA is known good (*2), then it’s either a problem with the ROM, or there is still a problem with the RAM. If all the RAM chips test okay, then it could also be a PCB track problem. Either an open circuit / break or dry solder joint. Or a short circuit (maybe a solder splash).

*1 the maskable interrupt (/INT) control is part of the Z80 and is enabled with a software instruction. It is essential for the way that the machine generates the video display. The non-maskable interrupt (/NMI) control is part of the ULA. The software has to write to the ULA to enable or disable it. This /NMI circuitry is the extra circuitry that ZX81/TS1000 etc. machines have compared to the ZX80 and clones, and it is this that provides the ‘display and compute’ or SLOW mode.

*2 note that it is possible for the the ULA to be partly faulty. It’s the ULA that generates the /NMI interrupts. The /INT input is driven from the Z80A address line A6.

Mark
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bola_dor
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Joined: Wed Oct 02, 2019 5:32 am

Re: TK85 (Brazilian clone) white screen trouble shooting

Post by bola_dor »

Hello MARK,
I was reading other posts these days and had come to a similar conclusion. So I left the transistor alone. Pin 17 of the Z80 is pulsing. I tried another ULA and another Z80 with no change. I removed the ROM and there was no change either. The RAM had already been tested in a TK90X (spectrum clone) which has a similar structure with the same PSU and it works although I omitted to do the reverse test.
IMG-20210628-WA0002.jpg
The 5v line had a lot of noise, it doesn't have any decoupling capacitor on the ram so I installed a 100nF ceramic one on the integrated ones. It improved but not completely so I soldered another 1uF multilayer together with a 100uF electrolytic between +5 and GND in another part of the board and now all the lines are clean of noise. But it didn't solve the problem either.
Finally I tried a ZXPAND+ which I know replaces the RAM and I think partially the ROM. And HALT started pulsing low at 15.6KHz slightly out of phase with the ULA pin 16 pulse. And I can see video signal as well as horizontal sync... I guess it's the ZXPAND menu. The ZXPAND board obstructs the video output so I couldn't see it on the screen. I'm going to try the printer connector so I can connect video.
Anyway, if it is working like that. It still remains to be seen if it is a RAM or a ROM fault , or the board...
I'll try a rampack to rule out RAM.. AND post some pictures...
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
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