master 128 - 'runs' but no display

[arnoldemu]

I am attempting to identify the fault on a master 128. It's the first time I've done fault finding. I've got a multimeter and a 8 channel hobby electronics logic analyzer.

This master has no cmos connected because the battery leaked a long time ago so much the carrier fell apart.

Inspection of the board doesn't so far show any damage to the pcb.

The power supply in this master was removed so I'm powering it using a usb power device with a raspberry pi 5 power brick with all the spades connected.

Master powers on.
Startup beep stops.
Keyboard responsive and leds can be turned on/off with caps lock/shift lock etc.
ctrl+break beeps.

It seems like it's running but connecting the modulator via aerial cable to a sony crt trinitron 14" shows no image as if there is no signal.
(another beeb I've got powered using the same setup, same television and cable shows an image on the same channel)

(I don't have a rgb lead or something to connect it to.)

I'm currently checking the connections between the vidproc, ic40 and ic43 for 'basic' continuity (beep setting on the multimeter).

Any suggestions on what to home in on?
Would the master be in mode 7 in this state?
Do the hsync and vsync come from the CRTC?

thank you

EDIT: What is VC1? Variable resistor? Can these leak? I did see some 'gunge' around it and cleaned it up.

[dominicbeesley]

You probably need to returne the TV. The modulator frequency is not very accurate. Each machine will be on a different frequency. Some modern TV's really don't want to tune a Beeb: ome because the frequency is not on an official channel, others because the automatic fine tune relies on there being a sound sub-carrier which the bbc doesn't transmit.

All that being said the UHF modulator could be broken as well. However, UHF is probably the worst way of connecting up with poor quality results at best. It was the only option for many in the early 80's but must modernish tellys have composite or scary inputs which will be better.

What model is your TV and what inputs does it have? Composite or SCART can give better results though SCART can be problematic on some sets and need a special cable with a battery.

D

[arnoldemu]

television is a sony trinitron kv-14t1u. it has scart so that is a possibility.

No luck tuning into the signal.

I've checked so far:

crtc clock is fine.
1Mhz, 2Mhz, 4Mhz, 8Mhz to vidproc is fine.
with the analyzer I have I can't tell with 16Mhz. It seems ok.

edit: csync at r150 is good.
when I put the analyzer on r141 I saw something so red? is doing something.

I am thinking the UHF might be broken too.

[dominicbeesley]

That looks like it should be a good TV. An RGB to scary lead should give the best results. There are loads on ebay and amazon, some times TV's don't accept the rgb signal though (viewtopic.php?t=17260)

The other option is to use a disc connector from the composite out to the input on the front of the TV. On your model there looks to be a video in connector on the front bottom left.

D

[WrightStuff]

The other option is to use a disc connector from the composite out to the input on the front of the TV. On your model there looks to be a video in connector on the front bottom left.

D

Yeah you just need a phono lead and a BNC to phono converter plug and you can try the composite output from the Beeb instead of UHF.
Other option is RGB to Scart lead.

[arnoldemu]

BNC -> composite didn't work. I also don't think RGB will work.

When I looked again csync doesn't look good. It's noisy.

Seeing a 50microsecond low, between each of these I am then seeing varying durations of high of 400microsecond, 1000microsecond and 1550microseconds.
e.g.
50microsecond low
400microsecond high
50microsecond low
1550 microsecond high
50microsecond low
400 microsecond high
50 microsecond low
1000 microsecond low
50microsecond low
etc


Hsync and Vsync at IC25 and direct at the CRTC pins look bad too and they are noisy too, similar noise to that at IC25. The noise is obvious.


Very obvious, the timings also doesn't seem to correspond with what I'm expecting. For HSYNC I am expecting a ~4microsecond pulse every 64microsecond, and for vsync I am expecting a varying length pulse (as it does with interlace) but around 4/8 scanlines and spaced every 20ms or so. But I'm not seeing that.

I've tried to continuity check the Vsync and hsync and they do appear to get to ic25, ic40 and one of the 6522s.

As far as I can measure with the logic analyzer clock to CRTC looks regular and good.

So feels like 6845 that is bad and to replace it is beyond my soldering abilities.

A few things to note:
- master beep is a prologned then stops - do vsync/hsync play a part in determining how long this could be?
- On the underneath of the pcb, and in a couple of other places, I noticed the 'green' coating had gone and revealing the silver tracks more so around the crtc. Not sure what would do that but the battery did leak, maybe that is some damage. I beeped a couple of lines for continuity and they seem ok, could these cause any of the described problems.
- the master pcb has a slight curve visible when looking at it side on, it's the left hand side (as you see it installed in the case)
- keyboard works, leds light up
- no battery connected so cmos will be in unknown state?
- g + break and other combinations seem to make no difference to the signals.

Any tips on what else to check? Could this be some signal crosstalk or similar and the crtc actually be ok? Testing at the crtc pin itself will that reduce any signal problems elsewhere?

thank you

[arnoldemu]

I've had another look with the logic analyzer. This time I set the sampling rate much higher (4Mhz, the default is much lower).

csync at ic25 appears to be good. See image.
vsync at ic25 is good. See image.

I can see what I guess is pixels at r141 (red??). See image.

I can see activity on r/w crtc pin 22.

pin 23 crtc clock, 1MhzE, I am seeing regular pulses 250ns of 'high', and 250ns of 'low' but every 400microsecond there is a pause in the clock.
I've attached an image zoomed out to show the pattern. Is this normal for a bbc master? (no idea what mode it is in).

Last time I tried television detected a signal via bnc->compositer but showed nothing.

[1024MAK]

- master beep is a prologned then stops - do vsync/hsync play a part in determining how long this could be?

The first sound is that of the sound chip with uninitialised data. It shuts up only once the OS ROM gets around to writing data to it.

- On the underneath of the pcb, and in a couple of other places, I noticed the 'green' coating had gone and revealing the silver tracks more so around the crtc. Not sure what would do that but the battery did leak, maybe that is some damage.

The green coating is the solder resist. This can flake off on its own. It coming off on its own does not affect the functioning of the board.

The battery pack should not have been mounted on the board. Most of the time, unless the computer was stored on its end, a leaking battery pack would not leak onto the board.

- no battery connected so cmos will be in unknown state?

Without a battery to maintain the configuration settings, the various configuration settings will be random at every switch on.

You need to reset the configuration settings at every power up:

Power up the computer whilst holding down the "R" key. Keep the key down until the message CMOS RAM reset, press BREAK to continue is displayed.

Yes, I know you won't see the message. So just wait for 20 seconds.

Then press CTRL + SPACE + BREAK to get to the "This is not a Language". If the display was working, you could then set up the configuration settings. For example, so that it "boots" to BASIC.

Random configuration settings can affect the display, as the position of the picture on screen is one of the settings. The MODE will also be random until you perform the reset.

Pressing BREAK will not clear the configuration settings.

Mark

[arnoldemu]

thank you. I followed your steps and I got into BASIC

progress.

I can make out words (this is a cheap bnc->composite into a samsung flat screen lcd television). The master is only outputting black and white.
EDIT: It is a bit more obvious when seen from an angle but not a lot. The photo is also not that great.
EDIT: Will try the Sony crt tomorrow.

[1024MAK]

The composite out (on the BNC socket) is a monochrome (black/grey/white) signal if the machine is factory specification. There is a link (LK11) that can be fitted to enable colour.

Acorn did not fit the link as standard, as adding the colour carrier signal reduces the quality of the displayed picture.

Your photo - I'm glad you can read something
It looks like a lack of contrast. Is the +5VA supply rail okay (test pin 6 of the RGB socket, across C71 or the pads of the not fitted C66 or on R113)?

Mark

[arnoldemu]

The composite out (on the BNC socket) is a monochrome (black/grey/white) signal if the machine is factory specification. There is a link (LK11) that can be fitted to enable colour.

Acorn did not fit the link as standard, as adding the colour carrier signal reduces the quality of the displayed picture.

Sorry I forgot to mention this is an Issue 1 Master.

Your photo - I'm glad you can read something
It looks like a lack of contrast. Is the +5VA supply rail okay (test pin 6 of the RGB socket, across C71 or the pads of the not fitted C66 or on R113)?

I agree. I've attached a picture from the crt with brightness set to max.

I'm not sure I've done the tests correctly?

I compared pin 6 of the RGB socket to the 0v wire on the power connectors near the modulator (top-left of pcb) that showed 4.9V.
Should I be comparing it those other components?

I can't find C71 or C66 where are they on the pcb?

thank you.

[arnoldemu]

- master beep is a prologned then stops - do vsync/hsync play a part in determining how long this could be?

The first sound is that of the sound chip with uninitialised data. It shuts up only once the OS ROM gets around to writing data to it.

good to know.

- On the underneath of the pcb, and in a couple of other places, I noticed the 'green' coating had gone and revealing the silver tracks more so around the crtc. Not sure what would do that but the battery did leak, maybe that is some damage.

The green coating is the solder resist. This can flake off on its own. It coming off on its own does not affect the functioning of the board.

The battery pack should not have been mounted on the board. Most of the time, unless the computer was stored on its end, a leaking battery pack would not leak onto the board.

It was found on it's end under a garage roof showing potential signs of leaking. It had it's power supply removed. When I found it I removed the top and removed the battery pack which had leaked but it looked like it might have leaked a long time ago as there was sign the leak went through the grill under the batteries. A lot of things in the garage are covered with a layer of black dirt too so I have already cleaned the case and I've wiped most of the pcb with 99% IPA.

- no battery connected so cmos will be in unknown state?

Without a battery to maintain the configuration settings, the various configuration settings will be random at every switch on.

You need to reset the configuration settings at every power up:

Power up the computer whilst holding down the "R" key. Keep the key down until the message CMOS RAM reset, press BREAK to continue is displayed.

Yes, I know you won't see the message. So just wait for 20 seconds.

Then press CTRL + SPACE + BREAK to get to the "This is not a Language". If the display was working, you could then set up the configuration settings. For example, so that it "boots" to BASIC.

Random configuration settings can affect the display, as the position of the picture on screen is one of the settings. The MODE will also be random until you perform the reset.

Pressing BREAK will not clear the configuration settings.

No problem, I'll do that every time. It's good to know it'll retain it on break. Once I get the display working I'll buy a new battery pack.

[Ukwebb]

Once I get the display working I'll buy a new battery pack.

If you still have the original battery pack, just cut open the heat shrink, give it a clean with IPA, and replace the three AA batteries with alkaline batteries and you'll be back up and running on that front. The only things in the wire are a small resistor and a diode.

[arnoldemu]

Once I get the display working I'll buy a new battery pack.

If you still have the original battery pack, just cut open the heat shrink, give it a clean with IPA, and replace the three AA batteries with alkaline batteries and you'll be back up and running on that front. The only things in the wire are a small resistor and a diode.

Sadly it was so bad it fell apart and the cable broke off from the battery holder.

[arnoldemu]

I've bought a rgb->scart from ebay so I'll see that improves things or not.

But with the bnc->composite I'm not sure if it's relevant but when music is playing there are horizontal lines over the screen. the only way I can describe this without an image/video is it's like c64 loading bars over the top of the entire screen.

I bought a sdcard interface with MMFS and even though the display is not right I'm enjoying the old games

[vanekp]

the BBC RGB output is higher than the normal RGB level 1v p2p as opposed to the normal 0.7v p2p so unless its a RGB cable for hte BBC micro it may need modding.

[arnoldemu]

the BBC RGB output is higher than the normal RGB level 1v p2p as opposed to the normal 0.7v p2p so unless its a RGB cable for hte BBC micro it may need modding.

I should be ok. I bought a BBC->Scart from cool novelties.

[1024MAK]

I should be ok. I bought a BBC->Scart from cool novelties.

That should be okay. I use those.

Do keep in mind that some modern TVs or converters may not be happy as they require the correct SCART function switching signals, but the Beeb only has a 5V power pin on its DIN socket, not a 12V feed.

Mark

[dominicbeesley]

What did it look like in other modes? I.e. if you type MODE 2?

[arnoldemu]

Not sure if this is related to the poor composite image: I checked the voltage between the top of the modulator casing and the nearby 0v supply spade and I see approx 3v but the schematics indicate it should be 0v? Schematics also have a '*' next to them what does that indicate?
With the back plate that the connectors are on being a metal strip I'm wondering if there is some kind of gnd issue.

as I'm not bothered about the aerial output I disconnected the two wires on the left side but that didn't make any difference.

is there a way to easily remove the modulator from the board? any pins I can cut on it?

[arnoldemu]

What did it look like in other modes? I.e. if you type MODE 2?

All modes work (including >128 modes) and they showed about the same kind of image. I could also see shades of grey in other modes.

[1024MAK]

The composite out (on the BNC socket) is a monochrome (black/grey/white) signal if the machine is factory specification. There is a link (LK11) that can be fitted to enable colour.

Acorn did not fit the link as standard, as adding the colour carrier signal reduces the quality of the displayed picture.

Sorry I forgot to mention this is an Issue 1 Master.

It's still possible to modify it to output colour on the composite output, but not as easy.

Your photo - I'm glad you can read something
It looks like a lack of contrast. Is the +5VA supply rail okay (test pin 6 of the RGB socket, across C71 or the pads of the not fitted C66 or on R113)?

I agree. I've attached a picture from the crt with brightness set to max.

I'm not sure I've done the tests correctly?

I compared pin 6 of the RGB socket to the 0v wire on the power connectors near the modulator (top-left of pcb) that showed 4.9V.

That's okay. No other power supply tests needed for this part.

In regards to the battery, you can make your own if you wish. I can provide details if you want.

But with the bnc->composite I'm not sure if it's relevant but when music is playing there are horizontal lines over the screen. the only way I can describe this without an image/video is it's like c64 loading bars over the top of the entire screen.

That indicates that there is something wrong, that's not normal.

Not sure if this is related to the poor composite image: I checked the voltage between the top of the modulator casing and the nearby 0v supply spade and I see approx 3v but the schematics indicate it should be 0v?

With the back plate that the connectors are on being a metal strip I'm wondering if there is some kind of gnd issue.

is there a way to easily remove the modulator from the board? any pins I can cut on it?

The original PSU has the 0V/GND/ground connected to mains earth. Is this the case with your current set-up?

The modulator casing is held to the PCB with tabs that are soldered. Unless you have a powerful iron, modulators are a right pain to remove. Even with a good iron, it's still not easy. These same solder joints are the 0V/GND/ground connections for the modulator. Hence there should not be any significant voltage difference between the modulator case and any other 0V/GND/ground connections/points.

I normally recommend that people leave the modulator in place.

I can't remember if the back metal panel has a specific 0V/GND/ground connection point (I've never specifically looked). Normally the BNC connection provides a 0V/GND/ground connection to it.

as I'm not bothered about the aerial output I disconnected the two wires on the left side but that didn't make any difference.

I presume you mean on the modulator. This will be the video input and the +5V supply. That's fine.

Schematics also have a '*' next to them what does that indicate?

Can't currently find an Acorn reference. But, IIRC, either the value has changed between different issues, is select on test (during manufacture) or the component is not always fitted.

Mark

[arnoldemu]

The composite out (on the BNC socket) is a monochrome (black/grey/white) signal if the machine is factory specification. There is a link (LK11) that can be fitted to enable colour.

Acorn did not fit the link as standard, as adding the colour carrier signal reduces the quality of the displayed picture.

Sorry I forgot to mention this is an Issue 1 Master.

It's still possible to modify it to output colour on the composite output, but not as easy.

Ok that might be a bit too much for me to do.

Your photo - I'm glad you can read something
It looks like a lack of contrast. Is the +5VA supply rail okay (test pin 6 of the RGB socket, across C71 or the pads of the not fitted C66 or on R113)?

I agree. I've attached a picture from the crt with brightness set to max.

I'm not sure I've done the tests correctly?

I compared pin 6 of the RGB socket to the 0v wire on the power connectors near the modulator (top-left of pcb) that showed 4.9V.

That's okay. No other power supply tests needed for this part.

In regards to the battery, you can make your own if you wish. I can provide details if you want.

Ok it depends on what is involved to make it. When the master is working fully I'll decide then.

But with the bnc->composite I'm not sure if it's relevant but when music is playing there are horizontal lines over the screen. the only way I can describe this without an image/video is it's like c64 loading bars over the top of the entire screen.

That indicates that there is something wrong, that's not normal.

Not sure if this is related to the poor composite image: I checked the voltage between the top of the modulator casing and the nearby 0v supply spade and I see approx 3v but the schematics indicate it should be 0v?

With the back plate that the connectors are on being a metal strip I'm wondering if there is some kind of gnd issue.

is there a way to easily remove the modulator from the board? any pins I can cut on it?

The original PSU has the 0V/GND/ground connected to mains earth. Is this the case with your current set-up?

I don't know. I'm using a raspberry pi 5 power brick connected to the usb adapter.

The modulator casing is held to the PCB with tabs that are soldered. Unless you have a powerful iron, modulators are a right pain to remove. Even with a good iron, it's still not easy. These same solder joints are the 0V/GND/ground connections for the modulator. Hence there should not be any significant voltage difference between the modulator case and any other 0V/GND/ground connections/points.

I normally recommend that people leave the modulator in place.

I'm going with your recommendation, desoldering that is beyond me.

I can't remember if the back metal panel has a specific 0V/GND/ground connection point (I've never specifically looked). Normally the BNC connection provides a 0V/GND/ground connection to it.

Not sure tbh. I can see multiple fixing points to the pcb and the tab on the end likely looks like it should connect to the metal cage of the power supply. I don't have that connected.

as I'm not bothered about the aerial output I disconnected the two wires on the left side but that didn't make any difference.

I presume you mean on the modulator. This will be the video input and the +5V supply. That's fine.

yes. great.

Schematics also have a '*' next to them what does that indicate?

Can't currently find an Acorn reference. But, IIRC, either the value has changed between different issues, is select on test (during manufacture) or the component is not always fitted.

ok, what I've just noticed on the schematics and pcb is that the individual gnd, 5v, are labelled A,B,C. I've been checking against A, checking again all seem consistent and all show the same voltage at the modulator.

I've noticed something else.

Either side of PL8 I believe should be 0v but it goes up to 5.7, so seems like there is some kind of power leak somewhere. I've tried to track it back but lost it after R154 so I'm not sure where the origin is and it's labelled '0V' and not '0VA' etc so I don't know which source it comes from.

I'm hoping I'll find it at some point.

[1024MAK]

The 0V/GND pins on PL8 are not true 0V/GND connections. It's a long story with twists and turns, but Acorn designed the board to cater for a rechargeable battery. Hence there is some circuitry associated with this and the battery 0V/GND goes via a transistor. So don't worry about getting strange readings here.

Mark

[arnoldemu]

I've connected the rgb->scart cable to the master and the old sony trinitron tv.

When I press the button on the front of the television to change input from aerial to scart/composite etc I get a momentarily perfect picture with correct colours and sync. This lasts for a second at most. It's going to be a bit difficult to keep pressing it and play a game

Then the tv decides it's not going to accept the input properly and it goes back to aerial display with the on-screen channel number but it's showing a colour image as if the hsync and vsync are being ignored. the image is distorted and rolling. see attachment.

EDIT: Picture is with normal brightness setting on television too.
EDIT: I can keep pressing the change input on the television and it goes to a great display and then back to as shown in the image. Does this indicate the cable is not compatible with the television too?
EDIT: Found button on remote and now it doesn't do this. (displaying teletext over image) so rgb->scart is perfect always but I've got '100' always in top-left of the image.

[arnoldemu]

Yesterday I was attempting to trace the 0v at the modulator casing back to try and work out why the modulator casing was ~3v. I got as far as Q11 and C85 and the thick gnd line seems to stop. (see red line on image)

Then there is a line that comes up to Q10 and that seems to stop (orange line). Looking at other images I've found online there is a thin track that connects them?

blue and purple are the lines related to resistor and capacitor.

Does anyone have a drawing of these or a better photo that shows exactly what is going on?

[vanekp]

Here is a photo from mine if that helps

[arnoldemu]

Here is a photo from mine if that helps
20240409_192512.jpg

Yes thank you. I can clearly see the connections to the resistors.

Would it be possible to confirm if there are connections or not between point A and point B, point A and point C, point A and the track pointed to by the arrow (test at C87?)?

I believe that A is connected to the track but not B and C and I have a break in connection around the capacitor at B,C and the 3.7V I see on the modulator case is 'leaking' via R113->R140->R116.

Thank you.

[vanekp]

Hear is a new picture with the capacitor (C85) bent out of the way a bit and the checked with a multimeter there is no connection to the track that runs past there from the collector of TR11 to pin 7 of IC43

the circuit diagram seems to have a fault in it, it shows TR11 as 1. being a NPN transistor with 2. the Emitter begin connected to ground, but its a PNP transistor and on the computer itself the collector is connected to the track which is ground and emitter is the end away from the ground track shown with a E on the PCB.

And C85 is connected from pins 8 to pins 11 of IC43.

[vexorg]

For reference, on mine the modulator body is a short, 0 ohm, to ground.

Q11 is a BC309, PNP. It could be laziness on the schematic front, I've done that before on PCBs when you can be bothered creating a new component if the footprint is the same. The schematic does say BC309 too.