US BBC micro restoration - help (and PSU) needed

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steve3000
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US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Sun Aug 28, 2016 1:43 pm

I picked up my first ever American-spec BBC micro a few weeks back and finally had some time yesterday to start exploring it. It's in need of some serious repair - so I'm starting a thread, in the hope that one day I'll get this up and running :)

Thankfully I knew about the condition before purchasing - this computer was purchased by the previous owner in the early 90's with a view to using it as a second computer, but he had 'problems' with the PSU. He could not recall what the 'problems' were (I'd guess possibly just conversion to UK voltage?) but apparently it needed to be replaced, which never happened, so this became a donor machine to keep his other BBC running until the late 90's - hence the missing ICs. He's not seen the PSU has since the early 90's and suspects it was binned at the time. :(

Time for some 'before' pictures.

On arrival, at first glance it looks to be in great condition:
tn_DSC_2148.JPG
Outside
Apart from some missing ICs, at a glance, the inside kindof looks OK...
tn_DSC_2123.JPG
Inside
...until you take a closeup look around the TV modulator!
tn_DSC_2131.JPG
Closeup
The corrosion-like mess in the third picture looks very concerning, but having washed and scrubbed the PCB yesterday I suspect this is actually the result of the Beeb having been used as a mouse-toilet for the last few years! :shock: In fact, after cleaning there doesn't appear to be too much track damage, but the modulator may have to go. More pics of the cleaned machine, later today.

For now, I need some help - does anyone have details and schematics of the US beeb? In particular what changes are needed to convert back to UK spec? (so I can check if this has been done - but as it still has the US operating system chip, I guess not?).

Also, I'm fairly certain I can get hold of missing chips (pics from Chris' Acorns should help identify these) - but I'm more worried about the PSU and what can be done to source a replacement, because I'd doubt anyone has a spare US beeb PSU lying around... Any thoughts?

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daveejhitchins
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Re: US BBC micro restoration - help (and PSU) needed

Post by daveejhitchins » Sun Aug 28, 2016 1:52 pm

The power supplies are universal, in that there's a 115V link to make for that Voltage. Contact Mark, at Retroclinic, he'll have a spare power supply. He may have some/all of the ICs you need too - and maybe a replacement modulator.

The fun will start when you have everything assembled and turn on for the first time . . . Good luck and don't forget to keep everyone undated with your progress.

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ARA III, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

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Re: US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Sun Aug 28, 2016 2:47 pm

daveejhitchins wrote:The power supplies are universal, in that there's a 115V link to make for that Voltage. Contact Mark, at Retroclinic, he'll have a spare power supply. He may have some/all of the ICs you need too - and maybe a replacement modulator.
Hi Dave, I had hope that was the situation, but unfortunately not. The US Beeb PSU had a different casing design which forms part of the rf shielding that encases the pcb.

See here: http://chrisacorns.computinghistory.org ... BBCUSC.jpg

Unfortunately the original beeb PSU just doesn't fit (I've tried :() and if it did, it would be open on the right hand side exposing the live psu components.

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Re: US BBC micro restoration - help (and PSU) needed

Post by daveejhitchins » Sun Aug 28, 2016 3:00 pm

steve3000 wrote:but unfortunately not.
:oops: Ooops! . . . What do I know . . . Mark may be able to help, anyway?

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ARA III, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

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Re: US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Sun Aug 28, 2016 3:31 pm

daveejhitchins wrote::oops: Ooops! . . . What do I know . . .
Haha, well I had assumed the same too, and had got a far as pulling the psu and most of the ICs out of my issue 3 beeb to try on the US Beeb, when I noticed the problem!

I'll rig up a bench supply to test the US Beeb some time later this week, but will drop Mark a line in case he had some ideas, and still hope to find an original US PSU as a long term solution... :)

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Re: US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Tue May 29, 2018 8:47 pm

Well fast-forward almost two years, and I now have one of these: viewtopic.php?f=41&t=5594&p=203516#p203516

...so time to resurrect this resurrection thread :)

First a comparison between the American Beeb PSU (left) and the UK Beeb PSU (right):
20180522_231441.jpg

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Re: US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Tue May 29, 2018 8:53 pm

As you can see from the photo at the top of the thread, the insides of this US Beeb were a little nasty, having been used as a mouse-toilet for several years!

I cleaned the PCB up back in 2016, with a nice soak in hot soapy water and a good scrub, but forgot to post the result, so here it is:
20180520_155306.jpg

And close-up:
Clean US beeb modulator area.png
But we're still missing 4 important ICs: 6502, 6522, 6845 and VidProc

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Re: US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Tue May 29, 2018 8:57 pm

After fitting the American Beeb PSU, and a quick loan from my working issue 7 beeb - we have all the missing ICs and a source of power :D

I couldn't find a really clear picture of an American Beeb online (at least not one where I could easily read the IC numbers!), so hopefully this will fill the gap:
20180529_124819.jpg
Only one thing left to do (after fitting the 0v cables and keyboard)... Power on!

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Re: US BBC micro restoration - help (and PSU) needed

Post by VectorEyes » Tue May 29, 2018 9:09 pm

The suspense!... :)

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Re: US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Tue May 29, 2018 9:09 pm

So first thing I did, was to switch on without a monitor connected, as I didn't have one nearby at the time...

And Brrrrrr-BEEEP! :D :D :D

It works! =D>


...or at least it worked...briefly...!

Plugging a monitor in revealed the following:
20180529_100817.jpg
Not bad I guess for the first time powered up in 25+ years (this computer was used as a donor computer in the 90's when previous owner had issues with the PSU - hence missing ICs, but it may also have had fault(s) at the time...).

Looks like a RAM fault with Bit 6 of the upper 16kb, but what could be causing the instability of display in MODE 7 (inverted colour and raster-bar background lines)? Other modes looked fine, up to the start of the upper 16kb, when partly broken vertical lines were present - indicating the bit 6 issue, but no background effects...

Anyhow, the computer no longer functions :( sadly it only booted up 3 times, and now gives a long Beeeeeeeeeeeeeeep of doom. Possibly relating the above memory issue or further issues?

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tricky
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Re: US BBC micro restoration - help (and PSU) needed

Post by tricky » Tue May 29, 2018 10:47 pm

Might it have had its ULA swapped at some time?
Does it even have/need the invention mod?

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Re: US BBC micro restoration - help (and PSU) needed

Post by steve3000 » Wed May 30, 2018 6:28 am

tricky wrote:
Tue May 29, 2018 10:47 pm
Might it have had its ULA swapped at some time?
Does it even have/need the invention mod?
Good question. Fitted a ULA to get it running.

No idea if it has or needs the mod - pcb is a different layout which might make it difficult to tell, and dates from 1983, if that helps? What do I need to look for?

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Re: US BBC micro restoration - help needed

Post by steve3000 » Wed May 30, 2018 10:00 am

Also, does anyone know of any fault-finding information for the US Beeb?

The PCB layout is completely different, along with the IC numbering and link numbers, and very few chips are socketed - similar to issue 7 beebs. So if there's a list somewhere of US Beeb IC numbers or link numbers, that would be really useful.

I'm assuming the circuit itself is pretty similar, if not identical, given that it runs normal Beeb software...(at least I'm hoping it will, when it is working!?).

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tricky
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Re: US BBC micro restoration - help (and PSU) needed

Post by tricky » Wed May 30, 2018 1:05 pm

Some of the ULA models have inverted output compared to another, but I don't know which are which. I believe the mod is to flip the invert signal coming in, but I could be completely wrong about all of this.
I think my"debug OS" room image would still work, but I don't know how the RF and composite encoders in a us beeb will like pal 50p timings. I can't find the link, but if you can burn an eprom, it displays different modes while playing sound and toggling the cassette relay and LEDs for the first few seconds. It's most useful for spotting dodgy ram chips.

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Re: US BBC micro restoration - help (and PSU) needed

Post by 1024MAK » Wed May 30, 2018 1:42 pm

More info on the Video ULA / Videoproc here.

Mark
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Re: US BBC micro restoration - help needed

Post by flynnjs » Sun Jun 03, 2018 9:48 pm

steve3000 wrote:
Wed May 30, 2018 10:00 am
I'm assuming the circuit itself is pretty similar, if not identical, given that it runs normal Beeb software...(at least I'm hoping it will, when it is working!?).
I know for a fact the ROMSEL circuitry is different to a regular beeb as
I found when repairing my German Beeb (same as a US Beeb).
I was unable to find a schematic. I guess it would be fairly easy to at
least make a IC number translation table for the 90+% which is the
same as a UK Beeb?

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Re: US BBC micro restoration - help needed

Post by steve3000 » Tue Jun 05, 2018 10:31 pm

flynnjs wrote:
Sun Jun 03, 2018 9:48 pm
steve3000 wrote:
Wed May 30, 2018 10:00 am
I'm assuming the circuit itself is pretty similar, if not identical, given that it runs normal Beeb software...(at least I'm hoping it will, when it is working!?).
I know for a fact the ROMSEL circuitry is different to a regular beeb as
I found when repairing my German Beeb (same as a US Beeb).
I was unable to find a schematic. I guess it would be fairly easy to at
least make a IC number translation table for the 90+% which is the
same as a UK Beeb?
Ah, that makes sense - I've already started a list of ICs and links, mapping them back to those on an original Issue 7 beeb - most of the ICs are easy, but the links are more challenging! In fact I think the MODE 7 issue I have may be due to incorrect link settings for the VidProc, but I can't find what the links should be anywhere. I've tried looking at all the photos of US beeb PCBs I can find online, but none are high enough resolution to see all the links and determine their positions.

Do you have a high resolution picture of your working German beeb PCB with links in place? This could be really useful. [-o<

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1024MAK
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Re: US BBC micro restoration - help (and PSU) needed

Post by 1024MAK » Wed Jun 06, 2018 10:43 am

It's likely that 95% of the circuitry of a USA Beeb is the same as a U.K. Beeb.

If you assume the above, you could printout a A3 (or larger size if you can) copy of the U.K. Beeb schematic, then use a multimeter on the continuity range, Mark each connection on the schematic as you go. Of course, this will not be totally accurate if there are extra connections...

This of course would take a long time. So you may want to start off by tracing the connections for the links first.

Mark
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Re: US BBC micro restoration - help needed

Post by flynnjs » Sun Jun 10, 2018 7:53 am

In fact I think the MODE 7 issue I have may be due to incorrect link settings for the VidProc, but I can't find what the links should be anywhere.
It may be that this PCB is only laid out for later versions of Vidproc just
like the UK beebs that had the "hidden" wire on the underside.
Do you have a high resolution picture of your working German beeb PCB with links in place? This could be really useful. [-o<
I'll take one when I get chance.

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Re: US BBC micro restoration - help (and PSU) needed

Post by klintworth » Tue Jun 12, 2018 5:31 pm

Here's a photo of a Canadian Beeb motherboard. I swapped it out with an issue 4 UK board which is working well in the North American case with 115V power supply. Hope this may help! Paul :D
IMG_0396.jpg
BBC B Issue 4, BBC B UNB09 Issue1 (USA), Electron & A7000+

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Re: US BBC micro restoration - help (and PSU) needed

Post by klintworth » Tue Jun 12, 2018 5:39 pm

And here's a photo of the UK motherboard in the North American case with 115V power supply. All working well! :D
IMG_0376.JPG
BBC B Issue 4, BBC B UNB09 Issue1 (USA), Electron & A7000+

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Re: US BBC micro restoration - help (and PSU) needed

Post by klintworth » Tue Jun 12, 2018 5:50 pm

And I put the speech ROMS in the UK motherboard - no Kenneth Kendall, it speaks with an American accent! :lol:
BBC B Issue 4, BBC B UNB09 Issue1 (USA), Electron & A7000+

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Re: US BBC micro restoration - help (and PSU) needed

Post by KenLowe » Tue Jun 12, 2018 8:46 pm

Those are impressive GND wires!

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Re: US BBC micro restoration - help (and PSU) needed

Post by klintworth » Tue Jun 12, 2018 9:53 pm

Maybe big GND wires to do with all the shielding
BBC B Issue 4, BBC B UNB09 Issue1 (USA), Electron & A7000+

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