Corroded components on BBC B Motherboard

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VectorEyes
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Corroded components on BBC B Motherboard

Post by VectorEyes » Fri Jun 29, 2018 1:13 pm

Next up in what will hopefully not be an endless list of questions related to my attempt to resurrect some Beebs...

One of my machines seems to have a component that has corroded pretty badly. It's just to the right of IC2 (the CRTC), just underneath the relay:
CorrodedComponentNextToCRTC.jpg
Could anyone tell me what it does and whether I can just order another one and replace it?


I've also noticed that several of the machines have corroding UHF Modulators. I'm unlikely to need to output a UHF signal any time soon, but on the other hand it would be a shame to have an 'incomplete' machine. Is there any way to replace these without incurring vast expense? Or is it generally accepted that you should just get rid of them?
CorrodedUHFModulator.jpg
(And by the way, why is it that the component connector needs to be desoldered before you can remove it from the case? So frustrating when you want to get the motherboard out and give it a good clean!)

Final question relates to a general build up of white gunk on some of the pin connectors (and legs of some of the ICs). Is this just build-up of dust which I can give a good scrub? It looks more like some kind of deposit formed from evaporation (similar to when I was trying to clean up leaked battery fluid in a RiscPC a few months ago). I'm a bit worried that it might be corrosive. Is it safe to try scrubbing it off?
WhiteGunk.jpg
Thanks in advance! I'm looking forward to when I can get these machines working (or at least into a 'known bad' state) and then send them off for some upgrades!

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1024MAK
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Re: Corroded components on BBC B Motherboard

Post by 1024MAK » Fri Jun 29, 2018 1:41 pm

In the first picture, I think you are talking about the manky stuff on the case of the crystal. The crystal is encapsulated in a sealed metal can. I've never seen one go like that before :shock:
And yes, you can replace this. Get the same frequency crystal. It's possible that you may be able to clean this muck off it. But keep in mind the lead out wires are not very strong, so be careful.

The condition of the tin plated steel case does not affect the operation of the modulator or the rest of the computer, as long as bits are not flaking off. If it's just the lid, you should be able to gently prise it off. You can then clean the rust off it and treat it like any tin plated sheet metal. But if you paint it or whatever, keep in mind that the edges of the top have to make electrical contact with the main body of the modulator.

The white stuff on the 0.1" pin headers is not corrosive, failing to remove it does not appear to cause any problems. Feel free to try to remove as much as you can. It affects many, many Beebs.

Mark
Last edited by 1024MAK on Fri Jun 29, 2018 1:45 pm, edited 1 time in total.
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Elminster
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Re: Corroded components on BBC B Motherboard

Post by Elminster » Fri Jun 29, 2018 1:51 pm

1024MAK wrote:
Fri Jun 29, 2018 1:41 pm
In the first picture, I think you are talking about the manky stuff on the case of the crystal. The crystal is encapsulated in a sealed metal can. I've never seen one go like that before :shock:
Also just the Crystal has been eaten on one face, but the rest of the board seems okay. And in the wrong place for battery. Not that there is a battery on Beeb. Unless there was an expansion board with a battery backing at somepoint in the past that leaked.

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Re: Corroded components on BBC B Motherboard

Post by VectorEyes » Fri Jun 29, 2018 2:16 pm

Elminster wrote:
Fri Jun 29, 2018 1:51 pm
1024MAK wrote:
Fri Jun 29, 2018 1:41 pm
In the first picture, I think you are talking about the manky stuff on the case of the crystal. The crystal is encapsulated in a sealed metal can. I've never seen one go like that before :shock:
Also just the Crystal has been eaten on one face, but the rest of the board seems okay. And in the wrong place for battery. Not that there is a battery on Beeb. Unless there was an expansion board with a battery backing at somepoint in the past that leaked.
Thanks again Mark and Elminster!

Interestingly, when I picked up the Beebs I did receive several expansion cards as well, one of which had quite a bit of corrosion in one corner. It's possible that it was originally mounted in that Beeb and whatever leaked from it landed on the crystal. I'll get it out this weekend and see whether the area of corrosion on the card is consistent with it having been above the crystal (and see if I can work out whether it had a battery or not).

Glad to hear that the white stuff isn't corrosive or damaging. I'll still try to get rid of as much as I can though!

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Re: Corroded components on BBC B Motherboard

Post by VectorEyes » Mon Jul 02, 2018 1:56 pm

I wonder whether this was the culprit:

WERamRomBoard.jpg

A Watford Electronics RAM/ROM board, with a highly suspicious leaking battery. There's quite a bit of corrosion:

WEBoardCloseup.jpg

... which has leaked through to the underneath:

WEBoardUnderneath.jpg

I'm guessing that when something's this badly damaged it's likely to be a write-off? (Pins bent as well, and the photos don't show how the pins between the board and the 'pin extender' bit are looking pretty rusty). I'm tempted to gently remove the ROMs and then junk it, unless anyone thinks it might be salvageable?

Is there any point in me removing the other socketed chips for spares?

To be honest, I don't really see how the corrosive bit can have been above the crystal, unless I've misunderstood where it would be placed inside the case, and annoyingly I can't find any pictures online which show one of those specific boards mounted inside the machine. But it's my most likely suspect!

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Elminster
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Re: Corroded components on BBC B Motherboard

Post by Elminster » Mon Jul 02, 2018 1:59 pm

Should be salvagable with a clean and replacement of broken components. Depends how confident you are to do a bit of unsoldering.

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Re: Corroded components on BBC B Motherboard

Post by marcusjambler » Mon Jul 02, 2018 2:22 pm

I'm guessing that when something's this badly damaged it's likely to be a write-off?
I've just gone through the exact same process with one of these boards.
It looked bad but, after carefully removing the battery and processor I managed to clean the PCB ( neutralised with white vinegar - look elsewhere on here there are plenty threads about battery corrosion )
The board is now back in the host fully working, including the 128k RAM drive :lol:

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1024MAK
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Re: Corroded components on BBC B Motherboard

Post by 1024MAK » Mon Jul 02, 2018 3:02 pm

Yeah, compared to some other boards, yours is not that bad.

A lot of the corrosion will come off when cleaned.

Mark
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Re: Corroded components on BBC B Motherboard

Post by cmorley » Mon Jul 02, 2018 3:36 pm

VectorEyes wrote:
Mon Jul 02, 2018 1:56 pm
I wonder whether this was the culprit:
I bought a machine with one of these expansion boards. The chemicals had leaked down onto the main motherboard & destroyed the CPU socket and the two VIA sockets. I also removed, cleaned and refitted the 74 series logic to the west of the 6502 which was also suffering from corrosion. All from the battery on the memory board.

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Re: Corroded components on BBC B Motherboard

Post by VectorEyes » Tue Jul 03, 2018 12:55 pm

Interesting. OK, I'll have a go at cleaning it and see how bad the damage is. Think it might have to be lower priority than actually getting my Beebs running and copying all the 5.25 inch discs I have onto something a bit longer-lasting!

Now... if I need to replace it... where do I get hold of a replacement crystal? As far as I can see, I need a "16MHz crystal oscillator" (the damaged one is marked "X1" which the service manual says is the 16MHz one)... but I'm not sure how to go about identifying exactly what to buy to replace it!
Last edited by VectorEyes on Tue Jul 03, 2018 12:56 pm, edited 1 time in total.

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1024MAK
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Re: Corroded components on BBC B Motherboard

Post by 1024MAK » Tue Jul 03, 2018 2:58 pm

It's just a crystal. CPC sell them, as well as many other electronic parts suppliers.
Here is link to a search at CPC.

Here is a link to a 16MHz crystal at Rapid. They also do the lower profile type as well. But I can't post a link to a search.

Either a crystal in the standard HC49 case or the low profile HC49 case (with through-hole wires) should be okay.

The 16MHz oscillator is made up of the crystal, IC43 and associated components.

Mark
Last edited by 1024MAK on Tue Jul 03, 2018 3:09 pm, edited 4 times in total.
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Re: Corroded components on BBC B Motherboard

Post by DutchAcorn » Tue Jul 03, 2018 5:12 pm

So what are the symptoms? A dead crystal typically results in a bbc without video and no sound at all.

As Mark mentioned, the circuit surrounding the clock is needed for the 16MHz to work:

DAD0626F-1C78-4A2F-B6B4-200FD1747442.jpeg

Can you check if you have a clock on the video chip (pins 4,5,6, 7 and 8 for 1,2,4,8 and 16MHz)?
Paul

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1024MAK
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Re: Corroded components on BBC B Motherboard

Post by 1024MAK » Tue Jul 03, 2018 6:42 pm

Keep in mind that the shape of the 16MHz signal is not much like a square wave...

If the lower frequencies are present and correct (4MHz, 2MHz etc), the the 16MHz master oscillator must be working.

As long as any corrosion has not penetrated the case of the crystal, and the connections to the PCB are okay, and the whole oscillator circuit is free of gunk and damage, it should work (assuming no other problems in the circuit).

Mark
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Re: Corroded components on BBC B Motherboard

Post by VectorEyes » Thu Jul 05, 2018 12:21 pm

At the moment, I don't even know whether the existing crystal has been damaged or not. I need to recap the machine's PSU, then I'll see whether I can scrape the gunk off the case of the crystal, and take a good look for corrosion on the motherboard (I didn't see any last time, but will double-check). If everything looks OK I might just turn the machine on, but if the casing looks severely damaged I'm going to replace it before doing any further testing.

Unfortunately I don't own an oscilloscope or anything else suitable for seeing whether there's an osciallating signal on a particular pin. Is there any way to do it, short of getting an oscilloscope from somewhere? There are some reasonably cheap "combined oscilloscope and multimeter" products available online... But this hobby is rapidly becoming more expensive than I anticipated!

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Re: Corroded components on BBC B Motherboard

Post by 1024MAK » Thu Jul 05, 2018 3:45 pm

You need an oscilloscope with an analogue bandwidth of at least 20MHz for use on a 1MHz or 2MHz system, but this is the bare minimum. Really you are looking for 50MHz or greater.

For general fault finding, the much cheaper option is a inexpensive logic analyser. These don't need to process analogue signals, so use normal logic levels instead. Hence are much cheaper.

Mark
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Re: Corroded components on BBC B Motherboard

Post by Elminster » Thu Jul 05, 2018 4:00 pm

1024MAK wrote:
Thu Jul 05, 2018 3:45 pm
You need an oscilloscope with an analogue bandwidth of at least 20MHz for use on a 1MHz or 2MHz system, but this is the bare minimum. Really you are looking for 50MHz or greater.

For general fault finding, the much cheaper option is a inexpensive logic analyser. These don't need to process analogue signals, so use normal logic levels instead. Hence are much cheaper.

Mark
Did you mean to reference Hoglet's Login Analyser thread?

viewtopic.php?f=3&t=13882&hilit=logic+analyser

Lots of info in there.

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