ACS19@UHUPVM1.BITNET (05/10/86)
The problem which many people have been having when they do the one meg upgrade was believed to be a bad MMU. Guess again. We have found that the power supply was in fact the root of the problem. By adding the extra chips, the power drain was more than could be supplied to the chips, and viola, the MMU would heat up and burn out. By increasing the rating of the choke in the power supply, you can allow greater current to pass through. We have had several units not work, but by changing the choke, all was fine. Take note, we have done 1 meg upgrades to Rev. A & B boards without problems.
knnngt@ukma.UUCP (Alan Kennington) (05/13/86)
So, the plot thickens. That explains why my new TOS ROMs blew - perhaps.
When I replaced the old 2 chips with 6 new ones, one (or more) of the new
ones blew. And everything worked okay again with the old ones.
And at the time I thought logically that increased power drain meant
lower voltage, and therefore lower current per chip. The note this
note follows explains that actually a higher power drain means more
current per device. I hope that this new engineering concept will be
explained to novices like myself in due course. How can the MMU be blown
by extra chips on the board???
Respectfully, ak.gordon@sage.cs.reading.Ac.Uk (Simon Gordon) (05/17/86)
In article <3407@ukma.UUCP> knnngt@ukma.UUCP (Alan Kennington) writes: > > > So, the plot thickens. That explains why my new TOS ROMs blew - perhaps. >When I replaced the old 2 chips with 6 new ones, one (or more) of the new >ones blew. And everything worked okay again with the old ones. > And at the time I thought logically that increased power drain meant >lower voltage, and therefore lower current per chip. The note this >note follows explains that actually a higher power drain means more >current per device. I hope that this new engineering concept will be >explained to novices like myself in due course. How can the MMU be blown >by extra chips on the board??? > Respectfully, ak. Some people seem to get their facts twisted. There seem to be two problems 1) Some of the older power supplies cannot supply enough power if you have the extra 16 ram chips to give 1 meg ( my one is black with a lead out of each end, ive been using it for a month since the 1 meg upgrade and rom upgrade which i had done at the same time) 2) With twice as many ram chips, the MMU must do extra work holding the inputs to the ram chips at their normal voltage. The resisters are therefore pull ups and do this job instead. Simon Gordon @ Sibly Hall Reading University UK
jsgray@watdragon.UUCP (Jan Gray) (05/24/86)
In article <171@sage.cs.reading.Ac.Uk> gordon@sage.UUCP (Simon Gordon) writes: >Some people seem to get their facts twisted. They sure do. >2) With twice as many ram chips, the MMU must do extra work holding the >inputs to the ram chips at their normal voltage. The resisters are therefore >pull ups and do this job instead. The resistors go "in series" with the drivers. Then are not pull up resistors; rather they are there to minimize undershoot and dampen the ringing that occurs when driving an inductive/capacitive load. Still waiting for that replacement power supply, Jan Gray jsgray@watdragon University of Waterloo 519-885-1211 x3870
mugc@utecfa.UUCP (ModemUserGroupChairman) (05/25/86)
gordon@sage.cs.reading.Ac.Uk (Simon Gordon) Writes: (in message dated: Fri, 16-May-86 19:57:21 EDT) <> Some people seem to get their facts twisted. <> There seem to be two problems <> 1) Some of the older power supplies cannot supply enough power if you have <> the extra 16 ram chips to give 1 meg ( my one is black with a lead out of <> each end, ive been using it for a month since the 1 meg upgrade and rom upgrade <> which i had done at the same time) <> 2) With twice as many ram chips, the MMU must do extra work holding the <> inputs to the ram chips at their normal voltage. The resisters are therefore <> pull ups and do this job instead. <> Simon Gordon @ Sibly Hall Reading University UK Now from what I read, the resistors went in series between the MMU and the ram chip RAS and CAS lines. If so, how can they be pull-ups then? MMU ]----/\/\/\/\-----[41256 RAM chips resistor is not the same as a pullup configuration: ^ Vcc | | < > < | | MMU ]--------------------------+----[41256 RAM chips