BBC Master Compact - blank screen and beep

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egel
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BBC Master Compact - blank screen and beep

Post by egel » Fri Mar 06, 2020 9:32 pm

BBC Master Compact - blank screen and beep, LEDs don't blink.
p1030145.jpg
p1030144.jpg
A long time ago, something went wrong when I used a wrong power supply.
tricky wrote:
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Nothing can really kill a beeb, even battery damage on a master can be worked around.
I hope it is true. [-o<
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Re: BBC Master Compact - blank screen and beep

Post by DutchAcorn » Sat Mar 07, 2020 8:11 am

What test equipment do you have? Mutimeter, Logic Probe, Logic Analyser, scope? If you say you may have used an unsuitable PSU before. Do you remember if it was wrong polarity or voltage?

Tricky's test rom may help in diagnosing 6522 and memory issues, have a read here.
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Re: BBC Master Compact - blank screen and beep

Post by egel » Sat Mar 07, 2020 8:17 am

Only a Mutimeter.
Wrong voltage, probably 12 V.

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Re: BBC Master Compact - blank screen and beep

Post by CMcDougall » Sat Mar 07, 2020 8:21 am

Totally Fried #-o needs 5v 2amp.
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Re: BBC Master Compact - blank screen and beep

Post by daveejhitchins » Sat Mar 07, 2020 8:50 am

CMcDougall wrote:
Sat Mar 07, 2020 8:21 am
Totally Fried #-o needs 5v 2amp.
Not necessarily! . . . Usually, when an overVoltage is applied one or two devices fail first and clamp the overVoltage saving the rest of the circuit. The real problem is finding the one(s) that died. But, all is not lost - the technique is to apply a current limited Voltage (not exceeding the actual Voltage!) and, with careful measurements e.g. milliVolt range, track down the lowest Voltage on devices. Takes time and patience, however, easier than pulling each device in turn :shock:

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Re: BBC Master Compact - blank screen and beep

Post by Kazzie » Sat Mar 07, 2020 10:09 am

To put a positive spin on things, the fact that you get a long beep means that the sound chip isn't dead. ;)

The list of chips at http://8bs.com/incomp.htm may be useful to help you work through them all. All the smaller 74xx chips will have their 5V supply on the upper left pin (e.g. pin 14 or 16) and ground on the bottom right pin (7 or 8 ). The other chips will vary; you may be able to work out which pins are which from looking at the board or circuit diagram*, or you may need to search for datasheets for the other chips.

*The circuit diagram won't list which pins are used for supplying power to the chips, but it will show you all the pins that are used for something else. So you can often work it out by process of elimination.
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Re: BBC Master Compact - blank screen and beep

Post by KenLowe » Sat Mar 07, 2020 11:42 am

Kazzie wrote:
Sat Mar 07, 2020 10:09 am
All the smaller 74xx chips will have their 5V supply on the upper left pin (e.g. pin 14 or 16) and ground on the bottom right pin (7 or 8 ).
Power upper right, and ground bottom left :wink:

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Re: BBC Master Compact - blank screen and beep

Post by 1024MAK » Sat Mar 07, 2020 12:24 pm

Also, not all of the smaller / 74 series chips have their Vcc (+5V) and GND (0V) pins at the upper right / lower left. Some annoyingly have them in other places. So do check a pin-out diagram, I recommend from a data book or data sheet as not all the images found in a internet search are correct.

Wikipedia has a page with most of the 74 series logic chips, with links to data sheets - link.

In terms of the over voltage, how much damage was done will also be determined by the maximum current (power) that the power supply unit (PSU) could supply. If it could not deliver very much current, then not much damage would have occurred. If however it was a powerful PSU that could supply many amps, far more damage will have occurred.

With a correctly rated PSU, you could also power the computer up for say a maximum of five minutes, and go round with your finger seeing if any chips get hot. Most in a Master Compact should run cool or warm, not hot.

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Re: BBC Master Compact - blank screen and beep

Post by Kazzie » Sat Mar 07, 2020 2:16 pm

KenLowe wrote:
Sat Mar 07, 2020 11:42 am
Kazzie wrote:
Sat Mar 07, 2020 10:09 am
All the smaller 74xx chips will have their 5V supply on the upper left pin (e.g. pin 14 or 16) and ground on the bottom right pin (7 or 8 ).
Power upper right, and ground bottom left :wink:
One of us has got it wrong, but I didn't think it was me. :? I was going by the motherboard pictures that I linked to on 8BS.

Edit: Ah! You probably had a vertically-oriented chip layout in mind. My fault for not being explicit enough. :oops:
1024MAK wrote:
Sat Mar 07, 2020 12:24 pm
Also, not all of the smaller / 74 series chips have their Vcc (+5V) and GND (0V) pins at the upper right / lower left. Some annoyingly have them in other places. So do check a pin-out diagram, I recommend from a data book or data sheet as not all the images found in a internet search are correct.
I'm pretty sure all the 74 chips in the Compact are of the conventional layout. I didn't spot any of those older layout chips (such as the 74x41 and 74x75) in the list of ICs.

But in the general case, yes, it pays to check the pinout of the chips before you try to measure them!
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Re: BBC Master Compact - blank screen and beep

Post by egel » Sat Mar 07, 2020 3:11 pm

1024MAK wrote:
Sat Mar 07, 2020 12:24 pm
With a correctly rated PSU, you could also power the computer up for say a maximum of five minutes, and go round with your finger seeing if any chips get hot. Most in a Master Compact should run cool or warm, not hot.

Mark
Bad chips will get hot?

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Re: BBC Master Compact - blank screen and beep

Post by Kazzie » Sat Mar 07, 2020 3:36 pm

egel wrote:
Sat Mar 07, 2020 3:11 pm
1024MAK wrote:
Sat Mar 07, 2020 12:24 pm
With a correctly rated PSU, you could also power the computer up for say a maximum of five minutes, and go round with your finger seeing if any chips get hot. Most in a Master Compact should run cool or warm, not hot.

Mark
Bad chips will get hot?
Dave Hitchins mentioned earlier that failed chips will have a slightly lower voltage over them. That's because they're conducting more current than normal. A side effect of the extra current flowing through the chip is that it will warm up more than normal.
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Re: BBC Master Compact - blank screen and beep

Post by 1024MAK » Sat Mar 07, 2020 4:07 pm

With a DIL chip, up is normally considered to be the end with either the U notch at the top, or the pin 1 dot on the top left corner. Most 74 series chips have the highest numbered pin as Vcc (+5V) which is top right when the chip is viewed as described above... [Unless of course you prefer landscape view :lol: ]

At this link there is a photo of a Z80 microprocessor showing the numbering of the corner pins.

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Re: BBC Master Compact - blank screen and beep

Post by egel » Mon Mar 09, 2020 8:06 am

Are the OS ROMs of an "ordinary" Master and a Master Compact interchangeable? I tried that, but it doesn't seem like it.

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Re: BBC Master Compact - blank screen and beep

Post by 1024MAK » Mon Mar 09, 2020 8:28 am

No, they are not the same.

They are different capacity chips (the Master 128 ROM chip is a larger capacity ROM compared to the chip used in the Compact) and the OS (and some of the other software) are different versions.

This is needed because some of the hardware is different. For example, the Compact uses a different chip for the configuration settings, as it does not have a real time clock chip (often referred to as the “CMOS” chip in a Master 128).

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Re: BBC Master Compact - blank screen and beep

Post by egel » Sat Mar 14, 2020 12:39 pm

I have now a Hobby Components USB 8CH 24MHz Logic Analyser.

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Re: BBC Master Compact - blank screen and beep

Post by egel » Sat Mar 21, 2020 9:36 am

The row ic35, ic 36, ic41, ic47 gets hot. IC35 and IC36 first. Does that mean these chips are broken or possibly / probably another? Are these one of the most vulnerable chips or are others possibly more vulnerable?
p1030144.jpg
From the BBC Master Series Compact Microcomputer Service Manual: IC 4464 DRAM 120nS 64Kx4 4 IC35,36,41,47

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Re: BBC Master Compact - blank screen and beep

Post by DutchAcorn » Sat Mar 21, 2020 10:49 am

Knowing the possible wreckage caused by the high voltage psu I’d suggest to start with the basics; check for 5V supply on the IC’s on the main board and check if the CPU gets a clock signal (does it have a pulse?).
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Re: BBC Master Compact - blank screen and beep

Post by egel » Sat Mar 21, 2020 1:07 pm

DutchAcorn wrote:
Sat Mar 21, 2020 10:49 am
Knowing the possible wreckage caused by the high voltage psu I’d suggest to start with the basics; check for 5V supply on the IC’s on the main board and check if the CPU gets a clock signal (does it have a pulse?).
Everywhere where I checked untill now was 5 V.
The pulse on the cpu:
65sc12.png

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Re: BBC Master Compact - blank screen and beep

Post by Kazzie » Sat Mar 21, 2020 2:48 pm

Here's four possible approaches for you:

:arrow: Those measurements with your logic probe indicate that the clock signal is good, but the lack of activity on the data lines suggests that the CPU has already stalled. If you probe the CPU's reset pin, and some of the address lines (say A0-A2), you can check if the reset circuitry is working, and if the CPU starts fetching any instructions before stalling.

:arrow: RAM chips can run warm, but I'm not familiar enough with the variety used in the Master Compact to be able to offer much more insight on ICs 35, 36, 41 and 47 off-hand. Examining the activity on the RAM's pins (especially pins 4, 5 and 16, for /write, /RAS and /CAS signals) may indicate whether they're being given instructions that make them consume more current, or whether they've developed a fault.

:arrow: If the board has a good reset signal (as well as the clock), another approach would be to use the Test OS ROM put together by Tricky. This doesn't require functioning RAM, and can show signs of life (LEDs, sound, cassette relay, and display) when the usual Acorn MOS fails to boot.

:arrow: Don't forget the earlier advice to test the 5V supply for each chip (in a detailed manner, using a milivolt scale on a voltmeter, rather than the coarse measurements of a logic probe), to identify which chips are consuming more power than others. My personal preference, in your position of having a board that's been over-volted, would be to follow this step first.
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