poorly Master in need of assistance

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bob147
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poorly Master in need of assistance

Post by bob147 » Thu Jun 13, 2019 7:16 pm

Hi guys, as mentioned in the 'introduce yourself' thread, I'm attempting to restore a second hand master back to working order and was hoping to get some advice from the kind people of this community. I fear the answer may well be "start learning how to solder", but we'll see!

Symptoms
Similar to the 'long slow beep of doom' thread underneath this one (minus the sound)
-the system powers on, and the 'power on' LED comes on
-a flashing cursor appears in the top left hand corner
-there is no sound of any kind from the speaker, complete silence
-besides the power LED the keyboard is completely unresponsive
-one odd thing I noticed, one time when I turned it on, white vertical bars appeared briefly. Then back to the flashing cursor.

Things I've attempted
There was a few things on the fault finding pages that I could try easily.
-The PSU was recapped before the machine was sold to me
-The CMOS battery has been replaced by a retroclinic modern version
-I've unplugged and plugged back in the 2 keyboard connectors
-I tried replacing the MOS rom chip with a replacement from retroclinic, as the pins on that one looked quite corroded and it was already in a nice socket.

No change in behaviour observed so far after doing any of that. Any other tips I can try before I invest in some equipment? Sounds like a multimeter would be a good place to start, equipment wise!

Thanks in advance,
Rob

bob147
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Re: poorly Master in need of assistance

Post by bob147 » Thu Jun 13, 2019 7:20 pm

I've attached a photo of the internals (taken from phone, hopefully high enough quality to be of use!)

Thanks,
Rob
Attachments
master1.jpg

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flaxcottage
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Re: poorly Master in need of assistance

Post by flaxcottage » Fri Jun 14, 2019 7:17 am

I know you said the keyboard does not respond but does power on with the R key held down work? It could be corrupted data in the CMOS RAM.

After that check the 6522s. The system VIA is needed to boot and controls the keyboard, I believe.
- John

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dixiestoat
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Re: poorly Master in need of assistance

Post by dixiestoat » Fri Jun 14, 2019 1:31 pm

I sold my old Master on ebay years ago because of precisely those symptoms. Apart from Powering on, thats all it did... silence... flashing cursor... completely dead keyboard... no response to any wiggles or removing / replacing Roms... sold it and got a lovely "new" Retroclinic one..!
Didn't know about the clever Stardot people then though.... =D>
If in doubt, CTRL-BREAK thou should clout..

bob147
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Re: poorly Master in need of assistance

Post by bob147 » Fri Jun 14, 2019 2:10 pm

That doesn't fill me with a huge amount of confidence but I'll persevere. Will attempt a cmos reset this evening. Have a feeling the VIA chips are soldered directly on so out of my league. Will have a careful look though. Thanks for the suggestions

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danielj
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Re: poorly Master in need of assistance

Post by danielj » Fri Jun 14, 2019 4:01 pm

Bob, where abouts are you based?

Boydie
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Re: poorly Master in need of assistance

Post by Boydie » Fri Jun 14, 2019 5:06 pm

What's the weird replacement chip in (I think) IC32?

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marcusjambler
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Re: poorly Master in need of assistance

Post by marcusjambler » Fri Jun 14, 2019 5:10 pm

What's the weird replacement chip in (I think) IC32?
SAA5050 (Teletext Character Generator) SMD on a carrier by the looks of it.
Last edited by marcusjambler on Fri Jun 14, 2019 5:12 pm, edited 1 time in total.

bob147
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Re: poorly Master in need of assistance

Post by bob147 » Fri Jun 14, 2019 5:37 pm

Hi Daniel, I'm based in Hampshire. Basingstoke to be precise

bob147
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Re: poorly Master in need of assistance

Post by bob147 » Fri Jun 14, 2019 6:38 pm

I can report that attempting cmos reset did nothing. Which leaves the system VIA as the next suspect. Unfortunately that one is soldered directly. Will have to have a think about how far I want to go with this.

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flaxcottage
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Re: poorly Master in need of assistance

Post by flaxcottage » Fri Jun 14, 2019 9:38 pm

If the Master is proving too much for you I feel sure Mark at Retroclinic will be able to repair it for you. He is good at that sort of thing. :D
- John

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1024MAK
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Re: poorly Master in need of assistance

Post by 1024MAK » Sat Jun 15, 2019 10:06 am

In terms of general fault finding, it starts off like this:
  • Get a good quality multimeter, test the +5V (VCC) supply voltage where the leads from the PSU connect to the main board. Check these connectors are making a good firm connection. Test the +5V (VCC) supply voltage on a major chip on each corner of the board.
  • Test the “CMOS” back-up battery voltage. If less than 1V, disconnect it and leave it disconnected.
  • Get a logic probe, logic analyser or oscilloscope to trace / check activity on the digital lines (address, control and data busses). A logic probe is the cheapest, but most basic. A logic analyser (minimum 16 input type in order to be useful) can, with the aid of some clever software, tell you how far the CPU gets into the ‘boot’ sequence, which may give us a clue what the problem is. An oscilloscope is the most expensive out of the three. But using this, you can see a visual representation of the electrical signals (it shows the signal waveform). So you can see if the waveform reaches the correct voltage levels (e.g. a logic high is actually above the minimum acceptable voltage level). You can also see if there is noise on signal lines.
  • When a suspected defective chip is found, get a replacement and a suitable socket. Then cut the legs on the old chip near the plastic package, desolder the remains of the pins from the PCB and clear the holes of solder. Solder the new socket in. Plug the chip in and try the machine.
  • If no progress has been made, rinse and repeat.
The other thing that is possible, is to replace the OS megaROM with a test ROM that does not need working RAM or a working 6522 VIA. That can then tell you if the CPU is alive and working.

The fact that you get a flashing cursor is however a good sign. It means that at least some of the circuitry is working.

Mark

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DutchAcorn
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Re: poorly Master in need of assistance

Post by DutchAcorn » Sat Jun 15, 2019 11:18 am

The Master should boot without a keyboard so you could see what it does without the keyboard connected.

If you remove the OS ROM you should get a continuous beeeeep (no cursor). What do you get if you switch her on without the OS ROM?
Paul

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