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#51 tf_hh OFFLINE  

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Posted Sun Apr 8, 2018 12:31 PM

I removed the OS-ROM from the 800XL board and installed a socket. Tested that ROM in the working 600XL and it was faulty. Testing the working OS ROM from the 600XL in the 800XL board and there was still no change so obviously more than one fault to deal with.

 

I have a lot of new 41256 RAM that I bought when I upgraded my XE's to 320K in 2007. Will 41256 work in the XL board? IIRC pin 1 had to be soldered to ground to use them so maybe not.

 

You can replace the 4164 DRAMs on a 800XL mainboard with the 41256 without problems - at all 800XL mainboards pin 1 is connected via pull-up resistor to VCC. Of course only 800XL, when talking about a 800XE, just must connect all pin 1 to ground or +5 volts.



#52 Tezz OFFLINE  

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Posted Sun Apr 8, 2018 1:58 PM

You can replace the 4164 DRAMs on a 800XL mainboard with the 41256 without problems - at all 800XL mainboards pin 1 is connected via pull-up resistor to VCC. Of course only 800XL, when talking about a 800XE, just must connect all pin 1 to ground or +5 volts.

Thanks Jürgen :thumbsup:



#53 Tezz OFFLINE  

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Posted Mon Apr 16, 2018 6:18 AM

I've continued to work on these two spare systems, the spare 600XL also had an AWC keyboard with several keys non-responsive so I stripped that down for cleaning and could see there had been a liquid spill on the bottom right corner. The caps and inverse switches and a wire jumper were all caked in white residue, on the other side of the PCB there was quite bad corrorsion to the switches and most of the tracks in that area. I removed the bad two switches and the wire jumper and cleaned the mess off the board. I replaced the bare wire jumper and the two switches with the ones that Jon kindly provided and then spent some time adding jumper wires neatly between the points of the corroded tracks. The other non-responsive keys appeared to be in good shape so I worked in some contract cleaner to those switches as suggested by Stephen and then testing showed that all keys were now responsive bar one which I'll try some more contact cleaner on later.

With the 800XL I'm still having no luck, I bought a NOS SALLY and OS-ROM and plugged in the new 41256 RAM that I still had stored away. Powered on and .. black screen still, no change at all. I tried another GTIA that I have for the sake of it with no change so I need to stop playing around swapping ICs now and start checking this board with the meter to get to the bottom of the fault/s. I'd used the CPU from that board in another system some time ago so its encouraging that that IC was functional and hadn't been fried by someone for example using a Commodore PSU on the board in the past. The DC socket was damaged when I first looked over the board so I reflowed that and there's no loose connection there now although I should check the voltages coming in. I reflowed all the pins on all of the IC sockets so I should really have tested them at that point with the multimeter but I was expecting it just to be a simple dead IC. I'll remove all the ICs again and check the sockets next as I should have done. What would you suggest to check next on the board if I don't find any problem there?



#54 Tezz OFFLINE  

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Posted Mon Apr 16, 2018 11:40 AM

Going through the field service manual for black/grey screen output I'm starting to check the clock circuit first. I tested transistors Q8 and Q9 with diode test mode on the multimeter. With the positive terminal on the base I get the readings of 488 Emitter / 686 Collector for Q8 and 389 Emitter / 465 Collector for Q9. As much as I remember from repairing some ZX Spectrums in the past I should get a reading between .6 and .9 volts testing transistors this way although they are in circuit so I'm not sure. I've never tested a crystal oscillator before so I'd need to look that up to test Y1. I don't know how likely they are to fail. I haven't yet checked the ceramic cap C109. I've socketed the MMU so I'll see if I have another to test just in case. The AND gate 74LS08 (U18) and Hex inverter 74LS14 (U19) are both non-socketed so hopefully I don't need to remove those also. Can they be tested in circuit?



#55 tf_hh OFFLINE  

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Posted Mon Apr 16, 2018 1:47 PM

Hi,

 

let´s start from scratch. Remove OS-ROM, start computer. You monitor or TV should show a red/brown background. At this state we know: ANTIC/GTIA mostly working fine. System-clock is there. Color output is okay - you should run this test using a CVBS connection (standard DIN 5 pin to SCART etc. cable).

 

Re-insert the OS-ROM, switch on again. When screen turns black, then CPU is working (maybe a little bit, but not totally damaged). Because you change already the CPU and CPU & RAM are the most significant defects I would focus on RAM circuit now.

 

I assume you haven´t an oscilloscope (otherwise you would mention it already), so first idea is to change the following ICs - in exact these order: Both 74LS158, 74LS375, 74LS51. Also the MMU (the only one with 20 pins) could be worth a try. All these parts you have in your working 600XL :)

 

Good luck!



#56 Tezz OFFLINE  

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Posted Mon Apr 16, 2018 2:36 PM

Thanks again Jürgen, I'll try that in the morning.



#57 Tezz OFFLINE  

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Posted Tue Apr 17, 2018 5:33 AM

I tried the 800XL board with OS-ROM removed and still have a black output via SCART and RF. I tried swapping ANTIC with the spare I have but no change. I'll test the GTIA and ANTIC in a working board as I haven't marked whether they were tested so I need to rule that out before anything else. I'd forgotten when I replied last night that I've already done the 64Kb mod on the 600XL board so I can't take those ICs now. Once ANTIC and GTIA are proved to be working I'll order some new 74LS158, 74LS375, 74LS51 ICs and swap those out although shouldn't I be getting a red screen output now even if any of those were at fault?


Edited by Tezz, Tue Apr 17, 2018 5:35 AM.


#58 tf_hh OFFLINE  

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Posted Wed Apr 18, 2018 5:09 AM

I tried the 800XL board with OS-ROM removed and still have a black output via SCART and RF. I tried swapping ANTIC with the spare I have but no change. I'll test the GTIA and ANTIC in a working board as I haven't marked whether they were tested so I need to rule that out before anything else. I'd forgotten when I replied last night that I've already done the 64Kb mod on the 600XL board so I can't take those ICs now. Once ANTIC and GTIA are proved to be working I'll order some new 74LS158, 74LS375, 74LS51 ICs and swap those out although shouldn't I be getting a red screen output now even if any of those were at fault?

 

Hmm - when you remove the OS-ROM, the background should turn red/brown. If not, anything is wrong.

 

I think, before spending more and more money to buy replacement parts, it´s better to get help by an UK hardware professional like FJC. Without scope and the needed skills we could exchange 50 posts and more :)



#59 Tezz OFFLINE  

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Posted Thu Apr 19, 2018 3:57 AM

Hmm - when you remove the OS-ROM, the background should turn red/brown. If not, anything is wrong.

 

I think, before spending more and more money to buy replacement parts, it´s better to get help by an UK hardware professional like FJC. Without scope and the needed skills we could exchange 50 posts and more :)

Thanks Jürgen, yes I'm rather limited for testing equipment. I don't have a scope although I should really buy a logic probe at the very least which would be helpful. Once I know for sure the main ICs are functional in another working board the fault then with this board could be with any of the other logic ICs or components, most of which I'll be unable to determine. The 74LS158, 74LS375 and, 74LS51 are quite inexpensive so I might just try replacing those. It's quite common for certain transistors to fail on the Sinclair boards although that's usually caused by an over voltage due to failed RAM or an expansion being plugged in or out whilst the machined is powered on. On the 800XL Q8 and Q9 are listed as 2N3906 PNP transistors which are also inexpensive so not too much all-in-all to chance replacing all of those I suppose.



#60 Tezz OFFLINE  

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Posted Mon Apr 23, 2018 7:25 AM

I've continued on with diagnosing the fault/s with the 800XL board. Through the SCART I just get a black screen output, through the RF there's a black screen output with faint diagonal white dotted lines.

I bought a Hong Kong 800XL as they're fully socketed to use it as a test board. A single 4464 RAM had failed with that one so it was quickly sorted and ready to be used. I then swapped all the main ICs with the problem board (except for the PIA which I haven't socketed so far) and there was no change .. black screen output (with no OS-ROM installed). I then tried all the ICs from the problem board in the Hong Kong board and all was well so I know now that all of the main ICs with the exception of the PIA are working. I swapped the MMU again from the Hong Kong board this time just to make sure and no change. I've then moved on to Jurgens suggestion desoldering U26 and U27 (SN74LS158N) and installing sockets, swapped those out with the Hong Kong board and .. no change. Next up I'll desolder SN74LS375N and SN74LS51N to install more sockets and test those. I'd like to test the components related to the clock circuit as suggested in the service manual: Y1 Crystal CO16801 4.33618Mhz, Q8 & Q9 PNP Transistor 2N3906, C109 Axial ceramic .1uf (50v). I don't expect C109 to have failed but it would be nice to confirm that the crystal oscillator is functioning. I don't have a replacement for that however. There's also U2 (74LS138), U18 (74LS08), U19 (74LS14) Hex inverter, U20 Hex buffer CMOS (CD4050B) to be tested. I should eventually identify exactly what has failed by process of elimination.



#61 Nebulon OFFLINE  

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Posted Mon Apr 23, 2018 1:51 PM

Here's a what-if scenario.

 

What if the new capacitors that you purchase are not as robust as the ones already in the machine. 

 

Based on the leaky capacitor fiasco in the 1990s, that's always a possibility...

 

On that note, I have a Commodore 1942 monitor that probably needs its capacitors replaced (the screen 'jumps' from time to time). If anyone has any tips on re-capping those monitors, I'm all ears. 



#62 manterola OFFLINE  

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Posted Mon Apr 23, 2018 2:09 PM

Not exactly related, but...
I have 600XL which I am repairing and restoring. The modulator output is very noisy, and the temporary composite out I implemented, as well. I may need to change some capacitors, however I noticed that the common small capacitors used in the XE series between 5v and gnd in some of the ICs are not used in this 600XL.
I was thinking about adding those, at least to the ram chips and maybe to some others 74lsXYZ.
What do you guys think about it...?

#63 tjlazer OFFLINE  

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Posted Mon Apr 23, 2018 8:15 PM

Here's a what-if scenario.
 
What if the new capacitors that you purchase are not as robust as the ones already in the machine. 
 
Based on the leaky capacitor fiasco in the 1990s, that's always a possibility...
 
On that note, I have a Commodore 1942 monitor that probably needs its capacitors replaced (the screen 'jumps' from time to time). If anyone has any tips on re-capping those monitors, I'm all ears. 


I have a Commodore 1942 monitor that needs caps replaced ASAP! I need to try to get a capacitor list so I can order them.

#64 tf_hh OFFLINE  

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Posted Tue Apr 24, 2018 12:40 AM

Next up I'll desolder SN74LS375N and SN74LS51N to install more sockets and test those. I'd like to test the components related to the clock circuit as suggested in the service manual: Y1 Crystal CO16801 4.33618Mhz, Q8 & Q9 PNP Transistor 2N3906, C109 Axial ceramic .1uf (50v). I don't expect C109 to have failed but it would be nice to confirm that the crystal oscillator is functioning. I don't have a replacement for that however. There's also U2 (74LS138), U18 (74LS08), U19 (74LS14) Hex inverter, U20 Hex buffer CMOS (CD4050B) to be tested. I should eventually identify exactly what has failed by process of elimination.

 

The color crystal (4.336 MHz) is only needed for PAL color carrier frequency. The chance that a standard crystal went bad is extremly low. I´ve had a few times a defect 74LS74 when the display hasn´t had colour, just b/w. But in your case there´s nothing on the screen, so mising colour may be the 2nd order issue IMHO.

 

As wrote in other posts, I don´t see the capacitors in scope. Only the 470 uF behind the power connector at XE mainboards or the both 470 uF capacitors behind power connector and OS-ROM at the "Rose" 800XL mainboards (with freddie) should be replaced by default. Never have had the need for others, IMHO in the 80s were better quality used until Tramiel & co. "optimizes" the business.



#65 Tezz OFFLINE  

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Posted Tue Apr 24, 2018 6:39 AM

The color crystal (4.336 MHz) is only needed for PAL color carrier frequency. The chance that a standard crystal went bad is extremly low. I´ve had a few times a defect 74LS74 when the display hasn´t had colour, just b/w. But in your case there´s nothing on the screen, so mising colour may be the 2nd order issue IMHO.

 

As wrote in other posts, I don´t see the capacitors in scope. Only the 470 uF behind the power connector at XE mainboards or the both 470 uF capacitors behind power connector and OS-ROM at the "Rose" 800XL mainboards (with freddie) should be replaced by default. Never have had the need for others, IMHO in the 80s were better quality used until Tramiel & co. "optimizes" the business.

Thanks Jurgen, good to know that the crystals are unlikely to be bad.

 

I removed U28 (SN74LS375N) this morning and tested it into the Hong Kong board which then gave a black screen output. Unfortunately using the good one from the Hong Kong board in the problem board it still did not fix the issue. At least however I have identified one bad component so far. I did have my first PCB damage removing U28, the through hole on the top side of the board at pin 6 came off. It's been a long time since I've caused any damage to a PCB, I'm always very careful removing ICs. I check that all the legs are freely moving before physically removing the IC but there must have been a tiny amount of solder remaining on the top side of the leg that I missed and I didn't use hot air. The underside of the board was fine (see photo attached) and there are no visible traces going to pin 6 so I cleaned up the PCB and installed a socket.

 

U30 SN74LS51N should be next to be tested which is a 14-pin IC, I don't think I have any 14-pin sockets however so I will leave that for the moment. I will compare the readings from the transistors Q8 and Q9 with the working Hong Kong board to see if there's any major difference that could indicate a problem.

Attached Thumbnails

  • U28_bottom_side.png
  • sn74ls375n.png


#66 flashjazzcat OFFLINE  

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Posted Tue Apr 24, 2018 7:39 AM

Possibly a daft question, but have you isolated whether this is purely a video issue yet? Which is to say, are there any non-visual signs of the 800XL trying to boot? Some months ago, when working on a stock 600XL, I had completely forgotten that the neither luma nor chroma were hooked up to the video jack in the factory, and since I hadn't bothered plugging the audio jack into the TV, for a while I thought the machine was dead. Of course, the 800XL has chroma hooked up, but still, it's sometimes worth hookup up to RespeQt via SIO2PC and checking if there are any signs of the OS trying to boot.



#67 Tezz OFFLINE  

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Posted Tue Apr 24, 2018 9:59 AM

Possibly a daft question, but have you isolated whether this is purely a video issue yet? Which is to say, are there any non-visual signs of the 800XL trying to boot? Some months ago, when working on a stock 600XL, I had completely forgotten that the neither luma nor chroma were hooked up to the video jack in the factory, and since I hadn't bothered plugging the audio jack into the TV, for a while I thought the machine was dead. Of course, the 800XL has chroma hooked up, but still, it's sometimes worth hookup up to RespeQt via SIO2PC and checking if there are any signs of the OS trying to boot.

That's a good idea, I'll hook up the SIO2PC later and see if it's actually attempting to boot. Thanks Jon.



#68 Tezz OFFLINE  

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Posted Tue Apr 24, 2018 11:14 AM

It just occured to me that being busy yesterday I couldn't remember if I'd tested U26 and U27 (SN74LS158N) in the working Hong Kong board so I've just tried that now and it doesn't boot if either of them are used so it looks like those are actually faulty too. That means so far the OS-ROM, U26, U27, U28 have all tested bad.



#69 flashjazzcat OFFLINE  

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Posted Tue Apr 24, 2018 11:31 AM

So has every IC on the problem board been swapped out yet and verified working or faulty?



#70 Tezz OFFLINE  

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Posted Tue Apr 24, 2018 11:48 AM

So has every IC on the problem board been swapped out yet and verified working or faulty?

The main ICs GTIA, ANTIC, SALLY, POKEY, MMU have all tested working in the Hong Kong board. The PIA isn't socketed yet so hasn't been tested out of the problem board so far. The OS-ROM tested bad previously in the 600XL so I bought a new one which is now in place. After removing U26, U27 and U30 and they tested bad I've taken the ones from the working Hong Kong board into the problem board and so far still a black screen although I can see a slight pulsing effect to that now via the SCART output that I didn't before.



#71 flashjazzcat OFFLINE  

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Posted Tue Apr 24, 2018 12:32 PM

OK. I wonder if the board took a 9V hit from a C64 PSU or something. It's almost as dead as the XEGS I recently had to admit defeat with (for now).



#72 Tezz OFFLINE  

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Posted Tue Apr 24, 2018 12:44 PM

OK. I wonder if the board took a 9V hit from a C64 PSU or something. It's almost as dead as the XEGS I recently had to admit defeat with (for now).

I'm wondering something similar. I feel like I'll be taking this down virtually to a bare PCB before getting to the bottom of all of it's problems.

 

I was wrong in the earlier post, the MMU from the problem board didn't actually work in the Hong Kong board so appears to be at fault also.


Edited by Tezz, Tue Apr 24, 2018 12:46 PM.


#73 tf_hh OFFLINE  

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Posted Tue Apr 24, 2018 12:47 PM

I removed U28 (SN74LS375N) this morning and tested it into the Hong Kong board which then gave a black screen output. Unfortunately using the good one from the Hong Kong board in the problem board it still did not fix the issue. At least however I have identified one bad component so far. I did have my first PCB damage removing U28, the through hole on the top side of the board at pin 6 came off. It's been a long time since I've caused any damage to a PCB, I'm always very careful removing ICs. I check that all the legs are freely moving before physically removing the IC but there must have been a tiny amount of solder remaining on the top side of the leg that I missed and I didn't use hot air.

 

Such a thing the best professional will happen, never mind. But you can breathe - this side of the 74LS375 is unused at the standard 800XL mainboards. Only on the 600XL mainboard both FlipFlops are used, and at the 1200XL, too. But the 800XL without freddie has the pins 1...7 simple NC - not connected.



#74 Tezz OFFLINE  

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Posted Tue Apr 24, 2018 12:50 PM

Such a thing the best professional will happen, never mind. But you can breathe - this side of the 74LS375 is unused at the standard 800XL mainboards. Only on the 600XL mainboard both FlipFlops are used, and at the 1200XL, too. But the 800XL without freddie has the pins 1...7 simple NC - not connected.

Thanks Jurgen for confirming. That's great, I thought that looked to be the case with the standard 800XL :)



#75 tf_hh OFFLINE  

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Posted Tue Apr 24, 2018 12:51 PM

The main ICs GTIA, ANTIC, SALLY, POKEY, MMU have all tested working in the Hong Kong board. The PIA isn't socketed yet so hasn't been tested out of the problem board so far. The OS-ROM tested bad previously in the 600XL so I bought a new one which is now in place. After removing U26, U27 and U30 and they tested bad I've taken the ones from the working Hong Kong board into the problem board and so far still a black screen although I can see a slight pulsing effect to that now via the SCART output that I didn't before.

 

You should exchange the PIA for test before you change all these small 74s logic chips. The possibility of a defect PIA is much higher. For example, when PB7 (portbit 7) is defect, the whole OS won´t start with a XL/XE operating system. During coldstart they set PB7 to low (mirror selftest in the memory area $5000-$57FF) and then the OS jumps into the memory detection and zeropage/stack-page test routine. If PB7 can´t set to low by a write access to $D301 in case of a defect, then the CPU jumps to elsewhere and hung up - black screen at all.

 

So go ahead with PIA. Or, if you can, burn a EPROM with a OLD-OS from 400/800 series and plug it in. These OS versions doesn´t toggle PB7 and run also with a defect PIA.






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