Cracking tinnitus, Gromit

[VMSZealot]

Ohh-Kay. So I finally got around to servicing the PSU of freebeeb, and tested the voltages. I’m getting a steady 4.98v on all power lines, with no strange smells or noises. However...

...when I plug the power lines into the motherboard, I get a continuous tone with a shrill edge to it - and no video. I’ve reseated all the chips, and re-heatsinked the ULA, but still the result is the same (I’ve checked the video out port and UHF. Nothing). The LEDs don’t light either.

The beeb is quite pimped. It’s got a sideways rom board and a ton of roms, expanded memory and a WE disk controller. When I tested the power with everything hooked up, the result was the same - steady 4.98v. That said, I can’t help wondering if one of the expansions is the culprit. What do I need to do to return this Beeb to ‘stock’? Presumably one or more of the ROMs might need rehoming on the main board?

Or does anyone have any other clever theories as to what might be amiss?

[1024MAK]

As it could be one of the expansions, the normal recommendation is to disconnect all the expansions. Unplug all ROMs except the OS ROM and the BASIC ROM.

However, before you remove or disconnect anything, please record what is where, and what connects to what. Including taking photos. Then post up the details. Someone here may spot a problem.

It could be something simple like a connection that has come loose.

As some of the expansion boards intercept signals, some links (jumper shunts) will have been removed. Once the expansions have been removed, you will need to reinstate these.

Details of the normal position of the links is detailed in the service manual.

If still no joy, try swapping the 6522 VIA chips.

If still no joy, go for the most minimal configuration. Details in this post.

Mark

[VMSZealot]

Okay. I found another Issue 7 machine (in a Viglen desktop case, but otherwise unmolested) to use as a reference in getting mine back to stock. The only problem with that plan being that the other Beeb doesn't work either - so it might not be the best reference. Having restored mine to what I believe to be stock, I took a few pictures.

The distortion on the final image is because I took it in panorama mode to fit everything in. The board isn't really badly warped - it isn't warped at all.

Despite all this, all the Beeb does is Beep. I can't even get the LEDs to light, let alone get a picture!

Beep.m4v

(1.96 MiB) Downloaded 106 times

[Kazzie]

I haven't watched the video you attatched yet (problem is at my end) but when you say it beeps, is that a continuous beep or the expected boo-bip?

The Caps and Shift LEDs are controlled by the system VIA. If that's dead, the beeb won't boot. So try swapping the two 6522 VIAs over, as suggested upthread, if you haven't already.

[VMSZealot]

No change, sadly. Swapping the VIAs didn't make any difference.

[Coeus]

I haven't watched the video you attatched yet (problem is at my end) but when you say it beeps, is that a continuous beep or the expected boo-bip?

from watching the video, the sound is the first one only, i.e. the one due to the sound chip not having been initialised yet. The ^G clean-sounding beep at the end is missing.

[VMSZealot]

...along with the video and everything else! But what does that mean? What do I need to pull and replace? Could it be two dead VIAs?

[Kazzie]

...along with the video and everything else! But what does that mean? What do I need to pull and replace? Could it be two dead VIAs?

It could be that, but it could also be a different faulty chip. (As you have another beeb, you could swap VIAs with that too, just to see.)

What test equipment (multimeter, oscilloscope, logic probe, etc.) do you have or have access to?

[1024MAK]

In your photos, is a speech system ribbon cable still attached?

Can we please see a photo of the whole of the main board.

Mark

[VMSZealot]

Okay, here's a better picture of the main board. I don't have another BBC at the moment - but I should be able to get it back in December at some point. As to diagnostic tools. I have my eyes, my nose and a multimeter. I've refurbed the PSU and checked it with my multimedia - both loaded and unloaded, and it seems to be stable and happy.

[1024MAK]

Acorn BBC Micro minimal configuration:-

This is to try to get the machine working to a "Language?" prompt.

Disconnect the keyboard.

Then remove all of the following (if fitted and if socketed):
IC3 - System 6522 VIA
IC69 - User 6522 VIA
IC78 - 8271 chip or 1770/2 disk interface module
IC52 - ROM/EPROM
IC88- ROM/EPROM
IC100- ROM/EPROM
IC101- ROM/EPROM
IC89 -68B64 Econet
IC7 - Serial ULA 2C199
IC4 - 6850 serial
IC74 - uPD7002 analogue to digital converter
IC98 - TMS6100 speech system chip
IC99 - TMS5220 speech system chip

Leave in IC51 (OS ROM)

LINKS (jumper shunts)
S9 -assuming IC78 is not fitted/removed, fit a link/jumper shunt or resolder the bridging wire to S9.
S21 - fit two links as follows (if not already fitted): nw to ne AND sw to se (two each East-West).
S25 - note it’s position, then try the machine with it fitted in the south position. If the machine still fails to start, try removing link S25.
.

Link S21 in a BBC Model B, normal position.

.

Link S25 in a BBC Model B, normal position.

Note that in this configuration, the OS is not in control of the sound system, so the speaker will make the continuous tone, so you may want to unplug the speaker.

Mark

[VMSZealot]

So I finally got time to look at this again. I removed all the socketed chips, emeried them, cleaned the circuit board with circuit board cleaner and reassembled as instructed by 1024MAK. Nothing changed. When I plug the speaker in I still get the same tone and no video on either the video out or UHF out ports.
So two questions:
1.) What can I try next?
2.) Is there anyone in the Oxford / Buckingham area who might be able to help me diagnose this?
I realise that this machine probably isn’t worth the effort - but I’d like to fix it if I can.

[tricky]

You might be lucky and find someone with test gear and time at ABUG in January, but it would be worth asking first.

[VMSZealot]

ABUG? What’s that? And where?

[Kazzie]

I realise that this machine probably isn’t worth the effort - but I’d like to fix it if I can.

It's certainly worth the effort: every Beeb is.

ABUG? What’s that? And where?

That'll be the Acorn and BBC Users Group. A room full of all sorts of Acorn machines and users, with all sorts of hardware and software projects on the go for a weekend. The next meet-up is scheduled for January 2019 in Hampshire. See this thread for details, and here's some pictures from the last event.

Getting back to your machine. To recap, the machine starts with a constant beep, no LEDs, and no display. You've checked the power supply, and removed expansions to return the machine to its default state, but no change. (shout out if I'm wrong).

When a BBC is turned on, the very first things the OS does are:

• Set up interrupts and check if it's been turned on (cold boot) or just reset (warm boot)
• Clear the contents of the RAM
• Mute the sound chip, set up the keyboard and LEDs
• ...

So it's failing very early on in the boot sequence. Likely reasons include failing to write zeros to the RAM, or that the CPU isn't able to fetch and execute its instructions properly.

Did you try link S25 in different positions (north, south, removed)? That can help diagnose a RAM fault.

If your multimeter can measure frequency (Hz) you can use it to check that the CPU is getting a clock signal. Particular pins to check include:

IC1 (6502 CPU) Pin 37 should show 2MHz
IC6 (Video ULA) pins 4-7 should show 1, 2, 4 and 8MHz respectively.

[VMSZealot]

Thanks Kazzie. My multimeter, sadly, can't show frequency. And the only setting that I didn't try for S25 was removed - but other than that, you're correct on all points. What does S25 do?

Much of the RAM is socketed (Upgraded Model A? But I didn't know that there were any issue 7 A's!) If there's a jumper to set RAM amount, I could just try removing the socketed RAM…

Finally, although the logic board looks clean and uncorroded, there was a lot of thick black gunk, almost like scorched fat on a frying pan, on the pins of many of the socketed chips on the front edge of the board. Hence the emery session, for all the good it did. A clue, perhaps?

Further, when I removed the speech synthesiser (still removed) using a proper chip puller (just in case you thought I might be the sort of vandal who'd use a screwdriver!), one the pins snapped off. If I can get this beeb going again I'll be replacing that part!

[JudgeBeeb]

Further, when I removed the speech synthesiser (still removed) using a proper chip puller (just in case you thought I might be the sort of vandal who'd use a screwdriver!), one the pins snapped off. If I can get this beeb going again I'll be replacing that part!

Please, please, please tell me it was the TMS 5220 that was damaged and not the TMS 6100.

[tricky]

We really need to persuade someone to make up a batch of Simon's 6100 emulators.

[RobC]

We really need to persuade someone to make up a batch of Simon's 6100 emulators.

Mark@Retroclinic has speech upgrades on eBay at very reasonable prices...

[VMSZealot]

Yeah 5220. But, given that both are optional, that’s not my main concern at the moment!

[Kazzie]

Thanks Kazzie. My multimeter, sadly, can't show frequency. And the only setting that I didn't try for S25 was removed - but other than that, you're correct on all points. What does S25 do?

Much of the RAM is socketed (Upgraded Model A? But I didn't know that there were any issue 7 A's!) If there's a jumper to set RAM amount, I could just try removing the socketed RAM…

That's just what S25 does: it tells the machine how many banks of RAM it has. With S25 north, the machine can use both 16k banks of RAM (i.e. a Model B), but with S25 south, the bank select line is grounded, and only the lower 16k bank (CAS0) is used, as in a Model A.

Because the inputs to TTL chips float up if unconnected, removing the jumper from S25 makes the machine think it's a 16k Model A, but using the other bank - the socketed one in your case.

The fourth combination can be obtained with a 7404 logic inverter and a breadboard to invert the bank select signal on S25, so the machine has 32k RAM but the banks are swapped round. This is useful for identifying which chip in a bank is faulty. (The Beeb can boot with a RAM fault in the upper 16k, but usually not in the lower 16k.)

So having given you a primer on what's going on, I suggest you try booting the machine with S25 removed. If the machine boots, then you've got the equivalent of a working Model A in front of you, and one of the eight soldered RAM chips is faulty.

If S25 doesn't help, there are some other tests that can be done with your voltmeter.

Finally, although the logic board looks clean and uncorroded, there was a lot of thick black gunk, almost like scorched fat on a frying pan, on the pins of many of the socketed chips on the front edge of the board. Hence the emery session, for all the good it did. A clue, perhaps?

I've got some chips in my Beeb (including the CPU) that have that kind of tarnish on them. If it's not on the part of the legs where the socket makes contact. it's generally not a problem.

[VMSZealot]

Okay. Sorry to resurrect a thread that should be resting in peace - but now I have an oscilloscope (one of those cheap DIY jobs off Amazon - but it appears to work). Still no logic analyser - but, given that I now have an oscilloscope (and multimeter, natch), what tests can I do to work out which component(s) on my beeb are fried?

[e668ecp]

DId you ever get this fixed

[VMSZealot]

Sadly no. I’m still at a loss. Does anyone do a repair by Mail service? How much would you want to look at my dead beeb?

[Ronin47]

Just wondering @VMSZealot if you have been round the ICs to see what voltages are there?

Attached is a document with some of the key chips that shows their VCC connections, I'd urge you to check voltages at the chips themselves as I had 4.93v-4.94v at the power connectors, which dropped on load, but at the chips themselves it was either close to the 4.75v lowest level or below.

Here's my results:

VCC 1 - off-load 4.94v on-load 4.83v
VCC 2 - off-load 4.93v on-load 4.76v
VCC 3 - off-load 4.93v on-load 4.76v
-5v - off-load 4.21v on-load -5v

From this we can see that VCC2 & VCC3 are sailing close to the wind with 4.76V at the board.

Further checking some of the main ICs, I got these readings:

6502 - VCC=4.72v
OS ROM - VCC=4.74V
Basic ROM - VCC=4.75V
IC3 6522 - VCC=4.68V
IC69 6522 - VCC=4.67V
All RAM ICs - VCC was between 4.55V and 4.68V
IC2 MC6845 - VCC=4.66v
IC6 - VCC=4.7v

Some notes for the pinouts:

1. For the 4186 RAM, VCC was pin 8.
2. For the 8271 if fitted VCC was pin 30.
3. For the video ULA,IC6 it was pin 24.

Also check some of the 74LS logic chips, they have a standard layout for VCC/GND.

So, with the notch of the 74LS chip in the north position, GND is the bottom left pin, and VCC is top right. If they are below 4.75v as well, it probably will struggle to run.

May help, may not, but worth checking based on the advice I've had from others.