Help! Master 128 Resurrection (hopefully)

[sfdNut]

Hi All,

Bought my first Master 128 off eBay. Description said "spares - not working" so I knew I was in for a challenging time.
The guy also said it switches on ok but needs a PSU to which I decided he mean't -"you need to replace the blown 30 odd year old capacitors". I'd already encountered (and solved) this problem with my loft based BBC Model B so I felt quite confident. Sure enough one of the 'usual suspect' capacitors was a mushroom shape showing its somewhat crisped innards.
I replaced all the power supply caps (using RetroClinic's full kit), tested the outputs for +5v and -5v, cleaned out the dust filled keyboard, swept the motherboard and discovered that the MOS ROM was missing. Replaced that with another RetroClinic product (4-way MOS ROM + switch) and checked 5v and ground connection to the ICs.
All looked good so hooked up a TV via RGB and switched it on. The left keyboard LED lit but no beeps and no display. CAPS LOCK and SHIFT LOCK lights didn't come on when pressing those keys.
Now I've hooked up a BitScope Micro oscilloscope and am testing the clock signals but I'm getting odd results. Hooking up pin 2 on IC43 to the 'scope did not show a waveform so that might be the main cause of my problem (or I don't know how to measure clock signals). In order to check the latter I hooked up pin 8 on IC42 (VIDPROC) to the scope. When I did this the power relay clicked on and off a couple of times and the speaker started emitting a buzzing noise.

Can anyone suggest why the speaker should be emitting noise?

I presume I'll have to replace IC43 and probably the RAM chips too
Thanks in advance

[dp11]

Firstly are you an experienced scope user ?

What do you see at pin1 and Pin 13 of IC43

[sfdNut]

Firstly are you an experienced scope user ?

No. Just starting out with this.

What do you see at pin1 and Pin 13 of IC43

I see nothing at either.
I checked first to see if the scope was working by hooking it up to a ZX81 ULA video out pin and could see the correct output. Nothing from the Master though.

[dp11]

Can you post a picture of your scope showing nothing?

[sfdNut]

Sorry for the delay. I thought I should learn a bit more first. However the output from IC43 pin 1 is

and Pin 13 is:

These should be showing the oscillator outputs I think. There's something but the auto measure on the scope couldn't resolve it.

[dp11]

Can i have a scope trace of pin 14 too the 5v pin.

To see the clocks the time base needs to be be some like 0.1us per division.

[sfdNut]

Can i have a scope trace of pin 14 too the 5v pin.

To see the clocks the time base needs to be be some like 0.1us per division.

I checked pin 14 at the start. Solid 5v

Thanks for the tip

[dp11]

I'd like to see how the scope displays it. The scope probes should be set to X10. I don't know how the scope does its vertical maths. But what you want is to adjust the the vertical gain and positioning of the trace to be say 5 divisions different from nothing connected to the probe tip. The go back to looking at the clock signals.

[sfdNut]

I'd like to see how the scope displays it. The scope probes should be set to X10. I don't know how the scope does its vertical maths. But what you want is to adjust the the vertical gain and positioning of the trace to be say 5 divisions different from nothing connected to the probe tip. The go back to looking at the clock signals.

Pics from the 5v test:

[dp11]

Excellent can you now go back and look at the clock pins now the scope is setup in a suitable way

[sfdNut]

Excellent can you now go back and look at the clock pins now the scope is setup in a suitable way

OK here we go:
Pin1 (should be a 16MHz signal?)

and pin13 (17.7MHz):

[dp11]

Excellent now try probing pin 2 . If that doesn't give you a signal and there is no obvious signs of damage I think you need a new 74S04 .

[dp11]

just spotted it looks like the scope has a 20MHz bandwidth limit filter now on channel 1 can you turn that off.

[sfdNut]

Excellent now try probing pin 2 . If that doesn't give you a signal and there is no obvious signs of damage I think you need a new 74S04 .

Changed the scope to not limit to 20MHz and saw no real change.

Checked pin2 - got absolutely nothing.

I have 74LS04Ns available. Can I use that instead of a 74S04N?

[dp11]

You do can try a 74LS04 but it might not work, you should really should use 74S04

[sfdNut]

You do can try a 74LS04 but it might not work, you should really should use 74S04

Ok, I've ordered an S04. I'll remove the old chip and add a socket.

Thanks for your help so far. I'll probably be back soon with the next issue

[sfdNut]

Old 74s04n removed and new socket and chip added.
Success! I get the correct beep and a display.

Unfortunately it says "This is not a language" and some of the keys don't work, including 'R" which means I can't reset the CMOS.
I'll do some keyboard cleaning and see if I can resurrect all of the keys.

It's not over yet...

[BigEd]

Progress! It's not unusual for some keys not to work - the remedy is to use them, pretty firmly, for 100 times. They should come good.

[DaveLecky]

Similar to my problem, checkout my experience here.......


viewtopic.php?f=3&t=20565#p288715

Good luck

Dave

[sfdNut]

Progress! It's not unusual for some keys not to work - the remedy is to use them, pretty firmly, for 100 times. They should come good.

Yeah, got some working by doing that so can now reset the CMOS
Some are resisting and one is auto repeating without seemingly being pressed! I have ordered some replacements just in case.

[MJS]

I had some keys that would not work, or did so intermittently. I have a big can of de-oxit that I use for audio work and tried that to see if it'd work. It worked perfectly literally within a couple of presses.