2 Faulty Beebs

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 6:55 pm

1024MAK wrote:
Kazzie wrote:If IC23's output is static, the system should be stuck in either 1MHz or 2MHz mode. (Mark's suggestion of breaking S19 leaves pin 3 of IC23 floating. Assuming it floats down, it forces IC23 to request 1MHz mode continuously.)
I suggested the middle pin of S19 (and therefore IC23 pin 3) be connected to 0V/GND (in this post), as most floating TTL inputs will float to a logic high.

Mark
Yup, I did that, connected the middle pin to ground.

Still had the same symptoms (clock only appears on CPU while Break is pressed) but this time the CPU got 1MHz instead of 2MHz. So the short to ground, clearly worked.

I installed a header with a jumper, so can easily switch between the 2 now.

User avatar
hoglet
Posts: 7116
Joined: Sat Oct 13, 2012 6:21 pm
Location: Bristol
Contact:

Re: 2 Faulty Beebs

Post by hoglet » Sun Feb 04, 2018 6:59 pm

JannievanZyl wrote: That would be cool, if possible Dave.
OK, both these were captured with BREAK held down. The signals are the 16 address lines.
IMG_1231.JPG
IMG_1232.JPG
About 50% of the time you get continuous activity like this, the other 50% of the time you get a static pattern. So I think it depends on what state the 6502 was in when reset was first asserted.

I've checked with a R6502A and a SY6502A and both do this.

All the data guarantees when RST is asserted is that writes are inhibited:
reset.jpg
Dave
Last edited by hoglet on Sun Feb 04, 2018 7:05 pm, edited 1 time in total.

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 7:04 pm

hoglet wrote:
JannievanZyl wrote: That would be cool, if possible Dave.
OK, both these were captured with BREAK held down. The signals are the 16 address lines.

About 50% of the time you get continuous activity like this, the other 50% of the time you get a static pattern. So I think it depends on what state the 6502 was in when reset was first asserted.

I've checked with a R6502A and a SY6502A and both do this.

Dave
That's good news. Was beginning to think my machine is possessed. Or my scope. :)

I think you're right about the state the CPU is in as I don't see it with working systems.

I suspect something to do with the system starting the reset cycle and then fails in the process which stops the bus and that stops the clock.

Hope Kazzie and Mark spots something, I'm a bit lost at this time :)

Kazzie
Posts: 83
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: 2 Faulty Beebs

Post by Kazzie » Sun Feb 04, 2018 7:12 pm

1024MAK wrote:
Kazzie wrote:If IC23's output is static, the system should be stuck in either 1MHz or 2MHz mode. (Mark's suggestion of breaking S19 leaves pin 3 of IC23 floating. Assuming it floats down, it forces IC23 to request 1MHz mode continuously.)
I suggested the middle pin of S19 (and therefore IC23 pin 3) be connected to 0V/GND (in this post), as most floating TTL inputs will float to a logic high.

Mark
Sorry, I missed that second sentence.
Pudsey - BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

User avatar
1024MAK
Posts: 7421
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: 2 Faulty Beebs

Post by 1024MAK » Sun Feb 04, 2018 7:18 pm

In the datasheet extract that Dave posted, an important point is that it is a positive edge on the /RST (/RES) input that initiates the CPU reset sequence, not the input being low.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 7:35 pm

1024MAK wrote:In the datasheet extract that Dave posted, an important point is that it is a positive edge on the /RST (/RES) input that initiates the CPU reset sequence, not the input being low.

Mark
Also read that. So I suspect by this time the clock had fallen away.

But I'll need to put a logic analyser on to see that. Keen to play with a new toy, never some it before! :)

How do you connect all the signal to a logic analyser? Is there some type of header that one can plug in between the PCB and CPU, for example?

Kazzie
Posts: 83
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: 2 Faulty Beebs

Post by Kazzie » Sun Feb 04, 2018 7:36 pm

From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's clock input stuck being high. (I forgot to ask after IC34 pin 6 and IC30 pin 5, but you should find that they're high(!) and 2MHz respectively.)

When you hold RESET low, that presets IC34 through pin 4, such that IC34 pin 6 (/Q) is held low. This allows the 2MHz clock through IC29 to the CPU.

If you can swap in another 74LS32 for IC29, you should at least get a regular clock signal. (Then we get to discover which other chips are fried!)
Pudsey - BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 7:40 pm

Kazzie wrote:From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's clock input stuck being high. (I forgot to ask after IC34 pin 6 and IC30 pin 5, but you should find that they're high(!) and 2MHz respectively.)

When you hold RESET low, that presets IC34 through pin 4, such that IC34 pin 6 (/Q) is held low. This allows the 2MHz clock through IC29 to the CPU.

If you can swap in another 74LS32 for IC29, you should at least get a regular clock signal. (Then we get to discover which other chips are fried!)
On it!

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 7:43 pm

JannievanZyl wrote:
Kazzie wrote:From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's clock input stuck being high. (I forgot to ask after IC34 pin 6 and IC30 pin 5, but you should find that they're high(!) and 2MHz respectively.)

When you hold RESET low, that presets IC34 through pin 4, such that IC34 pin 6 (/Q) is held low. This allows the 2MHz clock through IC29 to the CPU.

If you can swap in another 74LS32 for IC29, you should at least get a regular clock signal. (Then we get to discover which other chips are fried!)
On it!
/edit

I see I already replaced IC29 but will quickly pop in another one.

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 7:59 pm

Kazzie wrote:From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's clock input stuck being high. (I forgot to ask after IC34 pin 6 and IC30 pin 5, but you should find that they're high(!) and 2MHz respectively.)

When you hold RESET low, that presets IC34 through pin 4, such that IC34 pin 6 (/Q) is held low. This allows the 2MHz clock through IC29 to the CPU.

If you can swap in another 74LS32 for IC29, you should at least get a regular clock signal. (Then we get to discover which other chips are fried!)
I put another 74LS32 in IC29 and get the same results. Checked the one that came our in a logic tester and it was fine.

Also quickly tested continuity from IC29 to all the chips it connects to and all the pins measure out as connected.

I noticed the one gate connects to IC13 and that reminded me that the fault finding text document linked here before (apologies but can't recall the author) mentioned replacing IC12 / 13 as a potential fix for the continuous beep symptoms. I don't have replacements though.

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 8:00 pm

Kazzie wrote:From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low.
Let me recheck that.

Kazzie
Posts: 83
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: 2 Faulty Beebs

Post by Kazzie » Sun Feb 04, 2018 8:01 pm

JannievanZyl wrote:
Kazzie wrote:From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low.
Let me recheck that.
Could you check IC34 pin 6 and IC30 pin 5 as well while you're at it?
Pudsey - BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 8:12 pm

Kazzie wrote:From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's clock input stuck being high. (I forgot to ask after IC34 pin 6 and IC30 pin 5, but you should find that they're high(!) and 2MHz respectively.)

When you hold RESET low, that presets IC34 through pin 4, such that IC34 pin 6 (/Q) is held low. This allows the 2MHz clock through IC29 to the CPU.

If you can swap in another 74LS32 for IC29, you should at least get a regular clock signal. (Then we get to discover which other chips are fried!)
Hi, just rechecked and (as I posted above) pin-8 of IC29 (the OR gate) is low when both inputs are low. So IC29 seems to be working fine.

When I push reset, IC33 pin-2 gets clock and that then also appears on the output of the OR gate (IC29, pin 8) and thus the CPU gets clock.

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 8:12 pm

Kazzie wrote:
Could you check IC34 pin 6 and IC30 pin 5 as well while you're at it?
Give me two secs :)

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 8:14 pm

Kazzie wrote:
Could you check IC34 pin 6 and IC30 pin 5 as well while you're at it?
34-6 - H (goes L on reset)
30-5 - C - 2MHz (stays same on reset)

So, while 34-6 is H, the output of the OR gate (IC29) stays high and there is no clock on the CPU. But while reset is pressed, it goes low, allowing the 2MHz clock from 30-5 through.

Kazzie
Posts: 83
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: 2 Faulty Beebs

Post by Kazzie » Sun Feb 04, 2018 8:49 pm

JannievanZyl wrote:
Kazzie wrote:
Could you check IC34 pin 6 and IC30 pin 5 as well while you're at it?
34-6 - H (goes L on reset)
30-5 - C - 2MHz (stays same on reset)

So, while 34-6 is H, the output of the OR gate (IC29) stays high and there is no clock. But while reset is pressed, it goes low, allowing the 2MHz clock from 30-5 through.
Hmm... I may have speed-read you results. Sorry.

Looking in depth at the rest of your readings, there's certainly something wrong in this area. With IC29-8 low, and thus IC34-6 (/Q) high, the /PRESET on IC31-10 should also be high, and IC31 can flip-flop normally. IC34-9 gives a 1MHz clock output, which gives a clock signal to IC31-11. This should gate in a low value on IC31-12 (from IC33-2), and IC31-8 (/Q) should go high. This in turn should drive IC29-8 high, and the 2MHz clock at IC30-5 (applied to IC34-3) should gate that high into IC34, at which point IC34-6 goes low. But IC29-8 is still low!

You said you had replaced IC34 and IC30 previously. Take a look at IC31. It should have 1MHz on pin 11, and with pins 10 and 13 high, pins 12 and 8 should not be equal. If they are, despite having a clock signal applied, IC31 is faulty.

I may not be at the computer for much longer tonight, but I'll read through any further posts I miss by the morning.

(Incidentally, the other half of IC31 is used for strobing port B of IC3 (system VIA), which deals with the keyboard and Caps/Scroll lock LEDs.)
Last edited by Kazzie on Sun Feb 04, 2018 8:53 pm, edited 1 time in total.
Pudsey - BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 8:52 pm

Kazzie wrote: You said you had replaced IC34 and IC30 previously. Take a look at IC31. It should have 1MHz on pin 11, and with pins 10 and 13 high, pins 12 and 8 should not be equal. If they are, despite having a clock signal applied, IC31 is faulty.

I may not be at the computer for much longer tonight, but I'll read through any further posts I miss by the morning.

(Incidentally, the other half of IC31 is used for strobing port B of IC3 (system VIA), which deals with the keyboard and Caps/Scroll lock LEDs.)
Thanks Kazzie,

I'll check IC31 again. Was tempted to replace it already.

Also need to run but will quickly check this and also pick up tomorrow.

Really appreciate the help!!

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Sun Feb 04, 2018 9:05 pm

Kazzie wrote: You said you had replaced IC34 and IC30 previously. Take a look at IC31. It should have 1MHz on pin 11, and with pins 10 and 13 high, pins 12 and 8 should not be equal. If they are, despite having a clock signal applied, IC31 is faulty.

I may not be at the computer for much longer tonight, but I'll read through any further posts I miss by the morning.

(Incidentally, the other half of IC31 is used for strobing port B of IC3 (system VIA), which deals with the keyboard and Caps/Scroll lock LEDs.)
Just quick feedback.

On IC31, get the following:

12 - L (goes H on reset)
11 - C - 1Mhz (C stays on reset)
10 - H (goes L on reset)
8 - L (stays L on reset)

So, while in non-reset state both 12 and 8 is low and only 12 changes state when reset is applied.

Kazzie
Posts: 83
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: 2 Faulty Beebs

Post by Kazzie » Sun Feb 04, 2018 9:13 pm

JannievanZyl wrote:Just quick feedback.

On IC31, get the following:

12 - L (goes H on reset)
11 - C - 1Mhz (C stays on reset)
10 - H (goes L on reset)
8 - L (stays L on reset)

So, while in non-reset state both 12 and 8 is low and only 12 changes state when reset is applied.
That sounds like a fried flip-flop.

With the clock running, D (pin 12) should propagate to Q (pin 9) quickly, and /Q (pin 8) become the inverse of D. But that's not happening properly.

I see from your board photo that three of IC31's neighbours are socketed. Looks like it's time for it to get one too!

I've really enjoyed helping out with this, let's hope we've nailed this first bug at last.

Good night.
Pudsey - BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

User avatar
1024MAK
Posts: 7421
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: 2 Faulty Beebs

Post by 1024MAK » Mon Feb 05, 2018 12:13 am

Here is a tidied and corrected schematic diagram of the CPU 1MHz / 2MHz clock generation circuits.
BBC B schematic - CPU 1MHz & 2MHz clock generation circuits.png
BBC B schematic - CPU 1MHz & 2MHz clock generation circuits
Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

User avatar
1024MAK
Posts: 7421
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: 2 Faulty Beebs

Post by 1024MAK » Mon Feb 05, 2018 12:34 am

I concur, the 74LS74 D-type flip-flop in position IC31 does appear to be faulty.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Mon Feb 05, 2018 6:06 am

Thanks all,

Will replace it tonight.

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Mon Feb 05, 2018 5:42 pm

Success!! :D

Now get clock on the CPU. Also get CS on the system ROM.

However, still the same continuous tone and no display error. :cry:

User avatar
1024MAK
Posts: 7421
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: 2 Faulty Beebs

Post by 1024MAK » Mon Feb 05, 2018 6:31 pm

JannievanZyl wrote:Success!! :D

Now get clock on the CPU. Also get CS on the system ROM.
Good :D
JannievanZyl wrote:However, still the same continuous tone and no display error. :cry:
Small steps...

Does Tricky's test ROM do anything?

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

Kazzie
Posts: 83
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: 2 Faulty Beebs

Post by Kazzie » Mon Feb 05, 2018 6:49 pm

Great news!

Is there any change in the LEDs' behaviour having replaced that IC?
Pudsey - BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Mon Feb 05, 2018 7:10 pm

Kazzie wrote:Great news!

Is there any change in the LEDs' behaviour having replaced that IC?
No, no change.

But I now see continuous active on the bus as well as the /CS on the System ROM. So nothing is hanging anymore.

But........I need to put this on hold till the 23rd. Flying out to the States in a few hours. Itching to get back into it as soon as I return.

In the mean-time a virtual beer (or six) for everyone!!

User avatar
hoglet
Posts: 7116
Joined: Sat Oct 13, 2012 6:21 pm
Location: Bristol
Contact:

Re: 2 Faulty Beebs

Post by hoglet » Mon Feb 05, 2018 7:20 pm

While you are away, order yourself a cheap FX2 logic analyzer, then you can see what the CPU is actually doing:
http://hobbycomponents.com/cypress/674- ... c-analyser

For more info see this thread:
http://www.stardot.org.uk/forums/viewto ... 48#p182748

The FX2 board can just be attached to the tube connector with ~12 jumper leads (Clk, Data[7:0], RnW, RST, GND).

After post-processing the capture file, you get nice traces like this:

Code: Select all

???? :          : RESET !!       : A=?? X=?? Y=?? SP=?? N=? V=? D=? I=1 Z=? C=?
D9CD : A9 40    : LDA #40        : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9CF : 8D 00 0D : STA 0D00       : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9D2 : 78       : SEI            : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9D3 : D8       : CLD            : A=40 X=?? Y=?? SP=?? N=0 V=? D=0 I=1 Z=0 C=?
D9D4 : A2 FF    : LDX #FF        : A=40 X=FF Y=?? SP=?? N=1 V=? D=0 I=1 Z=0 C=?
D9D6 : 9A       : TXS            : A=40 X=FF Y=?? SP=FF N=1 V=? D=0 I=1 Z=0 C=?
D9D7 : AD 4E FE : LDA FE4E       : A=80 X=FF Y=?? SP=FF N=1 V=? D=0 I=1 Z=0 C=?
D9DA : 0A       : ASL A          : A=00 X=FF Y=?? SP=FF N=0 V=? D=0 I=1 Z=1 C=1
D9DB : 48       : PHA            : A=00 X=FF Y=?? SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9DC : F0 09    : BEQ D9E7       : A=00 X=FF Y=?? SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9E7 : A2 04    : LDX #04        : A=00 X=04 Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9E9 : 86 01    : STX 01         : A=00 X=04 Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9EB : 85 00    : STA 00         : A=00 X=04 Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9ED : A8       : TAY            : A=00 X=04 Y=00 SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=00 SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=00 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=00 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=01 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=01 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=01 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=01 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=01 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=02 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=02 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=02 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=02 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=02 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=03 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=03 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=03 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=03 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=03 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=04 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=04 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=04 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=04 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=04 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=05 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=05 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=05 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=05 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=05 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=06 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=06 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=06 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=06 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=06 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=07 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=07 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=07 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F0 : C5 01    : CMP 01         : A=00 X=04 Y=07 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09    : BEQ D9FD       : A=00 X=04 Y=07 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8       : INY            : A=00 X=04 Y=08 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7    : BNE D9EE       : A=00 X=04 Y=08 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9EE : 91 00    : STA (00),Y     : A=00 X=04 Y=08 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
By comparing with a known good trace, there are usually some big clues as to what the problem is.

Dave

User avatar
vanekp
Posts: 539
Joined: Thu Nov 30, 2000 7:09 am
Location: The Netherlands
Contact:

Re: 2 Faulty Beebs

Post by vanekp » Mon Feb 05, 2018 9:06 pm

I saw in a previous pic you had both VIA's out one needs to be in (IC3) for the system to run.

JannievanZyl
Posts: 165
Joined: Sat Feb 11, 2017 8:56 pm
Location: Cape Town, South Africa
Contact:

Re: 2 Faulty Beebs

Post by JannievanZyl » Tue Feb 06, 2018 9:36 am

vanekp wrote:I saw in a previous pic you had both VIA's out one needs to be in (IC3) for the system to run.
My understanding is that one should still get a display with both out, but I did plug IC3 back in and attached a keyboard. Still the continuous tone and no display.

User avatar
vanekp
Posts: 539
Joined: Thu Nov 30, 2000 7:09 am
Location: The Netherlands
Contact:

Re: 2 Faulty Beebs

Post by vanekp » Tue Feb 06, 2018 6:31 pm

An Extract from the service manual, not sure if you have checked these things (some you have like the clock).

Assuming then that the machine appears totally dead even though the
power supply unit is apparently working, and that you do not have
access to a PET, then here are a number of things you could check:-
i) Check that the reset line on the 6502A (pin 40) is high, and only
goes low when BREAK is pressed.
ii) Check that the IRQ line is not permanently in either a low or high
state. (Pin 4 of the 6502A)
39
iii) Check for the presence of the various clock signals, for example,
the clock input and output on the 6502A (pins 37 and 3), and the 1, 2,
4 and 8 MHz signals on pins 4, 5, 6 and 7 of the video processor (IC6).
iv) A very useful pin to check is pin 7 on the 6502A. This is the sync
pin and, although it is not actually used in the circuit, it gives an
indication of whether or not the 6502A is fetching any instructions. If
this is permanently high or low then the 6502A is totally stalled.
v) Check that the read-write line (pin 34) of the 6502A is working
normally and also check that the same signal, having been inverted and
re-inverted, is available at pin 10 of IC33.
vi) Check for the horizontal and vertical sync signals coming from the
CRTC (pins 39 and 40 of IC2) which will reveal whether or not the CRTC
has been successfu11y programmed at system reset.
If you do detect something abnormal in one of these tests then the next
stage would be to remove from the board any devices in IC sockets which
are unnecessary to the basic operation of the computer. For example,
the 6850 ACIA (IC4), the serial processor (IC7), the ADC Converter (
IC73) and the external 6522 (IC69). Having removed these devices, if
the fault disappears, then it may simply be a case of replacing them
one by one until the fault reappears. If the fault remains, then if you
have any spare ICs, or another machine with which you could exchange
ICs, it would be worth replacing the internal VIA (IC3), the 6502A (
IC1), the 6845 (IC2) and the video processor (IC5).
At this stage the next thing to try is to examine each of the
individual address and data lines to see if one or more of these lines
is permanently high or low. If so, look for short circuits, solder
bridges etc on that line. It is worth checking these lines both on the
6502A itself and also IC51, the operating system ROM.
When looking around the board at various points with an oscilloscope,
try to find any waveforms which either have "slack" edges, ie sloping
rather than square, or which have voltage levels which are not within
the normal TTL range. (Logic 1 must be greater than 2.8V and logic 0
less than 0.8V, though normally one would not expect to see voltages of
less than about 3.4V or more than 0.4V.)
Another very useful test with a model B, is to move link S25 to the
south position to see if the computer will operate in the 16K mode, in
which case, it suggests a problem with the CAS 0 area of RAM. Then if
you remove S25 altogether, it puts the machine again into the 16K mode
but this time with the CAS 0 area enabled and the CAS 1 area
inoperative.

Post Reply