System clock

discuss both original and modern hardware for the bbc micro/electron
Post Reply
marcelaj1
Posts: 120
Joined: Wed Apr 29, 2020 5:07 pm
Contact:

System clock

Post by marcelaj1 » Wed Sep 23, 2020 12:53 pm

How "solid" should the 16 MHZ clock be, when I monitor mine on IC6 pin 8 or any pin of IC43 my scope shows a 16 MHZ signal that twitches down to 15.9MHZ intermittently, this has an impact on the outputs from IC6 pins 4,5,6 and 7.
I had a problem back in May with my screen "twitching" and the advice was to change the X1 crystal which I did. This did not clear the problem but a change of monitor did, could this be the root of that problem?
Before changing components just thought to check if I actually have a problem or this is normal.

UPDATE:
I read the manual that came with the scope and found out how to record some frames....
ClockNormal.jpg
This is the normal 16MHZ clock signal - to watch it moves from side to side a bit but stays at 16MHZ
ClockDrop.jpg
This is the intermittent drop in frequency, the drop always seems to be to 15.82417MHZ, sometimes it is a single drop occasionally I get two drops very close.
Last edited by marcelaj1 on Wed Sep 23, 2020 2:30 pm, edited 1 time in total.

dominicbeesley
Posts: 1163
Joined: Tue Apr 30, 2013 12:16 pm
Contact:

Re: System clock

Post by dominicbeesley » Wed Sep 23, 2020 2:29 pm

Certainly if that isn't stable many monitors will be upset by any change in clock.

I had one machine that was bit twitchy after being damp. I can't remember what cured it but I think I replaced some small value capacitors and then gave it a good clean with IPA around the clock generator circuit. I can't remember which cured it though

It might be worth giving it a clean first and then try subbing C37 and C42?

D

User avatar
BeebMaster
Posts: 3628
Joined: Sun Aug 02, 2009 5:59 pm
Location: Lost in the BeebVault!
Contact:

Re: System clock

Post by BeebMaster » Wed Sep 23, 2020 10:15 pm

I don't think it is all that accurate/stable, at least on a Master. The AIV SCSI board has its own 16MHz crystal replacing the motherboard one otherwise the Domesday System can't genlock properly. I built my own replacement crystal for a Master without the AIV interface so I could use it with Domesday controlled from the 1MHz bus:

Image
Image

dominicbeesley
Posts: 1163
Joined: Tue Apr 30, 2013 12:16 pm
Contact:

Re: System clock

Post by dominicbeesley » Thu Sep 24, 2020 11:45 am

True, they're neither accurate nor stable. But I wouldn't have thought they would suddenly jump, drift maybe. Sounds to me very much like an intermittent capacitor.

BTW: where are you measuring the frequency? Your probes could be upsetting things I would measure one of the divided down outputs of the ULA and check that for accuracy. The crystal circuit itself will be pretty sensitive to any small amount of capacitance.

marcelaj1
Posts: 120
Joined: Wed Apr 29, 2020 5:07 pm
Contact:

Re: System clock

Post by marcelaj1 » Thu Sep 24, 2020 12:42 pm

dominicbeesley wrote:
Thu Sep 24, 2020 11:45 am
BTW: where are you measuring the frequency?
Good point I will have a look on IC6.

marcelaj1
Posts: 120
Joined: Wed Apr 29, 2020 5:07 pm
Contact:

Re: System clock

Post by marcelaj1 » Thu Sep 24, 2020 3:38 pm

Went to the 1MHZ output from IC6 and have the same issue...
1MHZ_OK.jpg
Normal 1MHZ
1MHZ_Drop.jpg
1MHZ having a wobbly

I have ordered a set of caps to replace capacitors C37, C40 & C42

UPDATE: I have changed the capacitors and although the pulse now seems a little "squarer" it has not made a difference to the frequency drop, it still drops to 15824.176KHZ.

Post Reply

Return to “8-bit acorn hardware”