BBC Master Issues with Replacement CMOS Battery

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
Clem
Posts: 6
Joined: Fri Nov 17, 2017 10:56 am

BBC Master Issues with Replacement CMOS Battery

Postby Clem » Fri Nov 17, 2017 12:12 pm

Hi All,

Usual story - an old BBC Master from my childhood recovered from the loft to be resurrected for a bit of nostalgic gaming time ...

First issue I came across was that the CMOS battery was dead (naturally) and so I bought a replacement.

I fitted the replacement. Went through the process of typing in all the configures (on the slightly iffy keyboard) which all went in OK. I started to work on setting the time which was naturally giving out junk results. I could not set the full timestring but I could get the hours mins and seconds to go in and stick. After any TIME$ command the next command input (usually this would be PRINT TIME$) would lock up partway through output, pressing escape would allow it to complete the output and reset the cursor. Still I was unable to set the date. Also when i turned off the power, the CMOS was reset.

Turns out the diode was in the wrong way ...

So i resoldered the diode, checked the voltage (4.8V at the pack, 4.1V at the plug on the motherboard) and powered the computer back up and input all the settings. This time around I can set the full time and date! Still locks up the first PRINT TIME$ command though ... Also if I turn it off for more than a few seconds it loses all its settings still!

So I came here for some help ... I'm a bit stuck. The CMOS batteries are clearly giving some power to the system but its unable to hold the settings for any time. Also the RTC seems to be giving some odd results when its set (it does hold the time while power is still connected).

Other info that may be of use - My turbo MMC rom is only usable after I set the configs to point at it with the CO. FILE 8 command and then turn the power quickly off and on (CTRL- BREAK is not sufficient).

User avatar
danielj
Posts: 5364
Joined: Thu Oct 02, 2008 4:51 pm
Location: Manchester

Re: BBC Master Issues with Replacement CMOS Battery

Postby danielj » Fri Nov 17, 2017 12:14 pm

Are you sure +ve and -ve to the motherboard are the right way around?

If so, the next suspect is the CMOS IC itself... (HD146818)

d.

Clem
Posts: 6
Joined: Fri Nov 17, 2017 10:56 am

Re: BBC Master Issues with Replacement CMOS Battery

Postby Clem » Fri Nov 17, 2017 12:46 pm

I did the polarity by the instructions that came with the AA battery kit.

This says to put the negative on the top pin (furthest from the keyboard) of the three on that header. Positive on the middle pin and the bottom pin is empty.

Whereabouts is that CMOS IC located. I can go and check its properly seated/soldered ...

dominicbeesley
Posts: 466
Joined: Tue Apr 30, 2013 11:16 am

Re: BBC Master Issues with Replacement CMOS Battery

Postby dominicbeesley » Fri Nov 17, 2017 12:53 pm

The CMOS IC should be soldered in.

Have you got any test equipment, that sounds like the RTC's clock oscillator may have stopped it would be worth having a prod to see if the RTC clock is runing. I had similar problems with my homebrew machine's RTC clock dropping out causing reading time to hang. You should be getting a nice 32.768 kHz squarewave at TP1.

The little watch crystals are quite robust but are high impedance parts meaning they can easily be overwhelmed by a bit of muck/damp/coffee/dead spider on the board leading them to stop. A wash round that area (both sides of motherboard) with some IPA might wake it up

D

Clem
Posts: 6
Joined: Fri Nov 17, 2017 10:56 am

Re: BBC Master Issues with Replacement CMOS Battery

Postby Clem » Fri Nov 17, 2017 2:35 pm

Cool, I can give that a look. Gives me an excuse to use my PICO scope :)

User avatar
vanekp
Posts: 345
Joined: Thu Nov 30, 2000 7:09 am
Location: The Netherlands

Re: BBC Master Issues with Replacement CMOS Battery

Postby vanekp » Fri Nov 17, 2017 6:04 pm

I would 1st do a rest of the cmos Hold "R" while powering the master on then try setting it up, or else it will have random information in there and could confuse the config when trying to setup things like the time.
Peter.

Clem
Posts: 6
Joined: Fri Nov 17, 2017 10:56 am

Re: BBC Master Issues with Replacement CMOS Battery

Postby Clem » Wed Dec 06, 2017 9:01 pm

Its been a while but I know how annoying it is to find these threads with no "it fixed" post

Looks like my battery box was a friday afternoon job. The crimp at the plug end fell apart and then the wire fell out where it joins the diode (this one may be my fault).

So I soldered the original diode, resistor and wires/plug from the dead battery to the 9V style plug that came with the new battery box and it WORKS

Tests were performed by probing the top right and bottom left pins (taking the little dimple to be the top) of the HD146818 so that bit of info was super useful!

Now on to making the turbo MMC work for all the games I remember (repton 3 seems to be dead in the included games)


Return to “hardware”

Who is online

Users browsing this forum: No registered users and 13 guests