Oh dear, while I was ordering an original 7474 I ordered some other parts that were connected to the same bad chip in case other chips are damaged, and I ordered a Fairchild 74LS08! I will ask them to change it for a Motorola part they also have.
> -----Original Message----- > From: cctalk <cctalk-boun...@classiccmp.org> On Behalf Of dwight via cctalk > Sent: 14 May 2022 23:36 > To: Paul Koning via cctalk <cctalk@classiccmp.org> > Subject: Re: Replacement for a DEC 7474 Chip > > What ever you do, don't use a Fairchild part. When I worked for Intel in the > 80's, we finally band using Fairchild for any latching device. They failed on > pullup current, even when the parts were sent back and they claimed they > were good. We just gave up on them, we couldn't hold production while > they figured it out. > We had a similar problem with PowerOne, a manufacture of power supplies. > Since it was a custom supply, we had to send someone to their plant to fix > their final test. > Dwight > > > ________________________________ > From: cctalk <cctalk-boun...@classiccmp.org> on behalf of Nigel Johnson > Ham via cctalk <cctalk@classiccmp.org> > Sent: Saturday, May 14, 2022 10:50 AM > To: Paul Koning via cctalk <cctalk@classiccmp.org> > Subject: Re: Replacement for a DEC 7474 Chip > > AFAIR LS can only drive one unit TTL load. > > I may have some 7474, even of that vintage, if you cannot find any anywhere > else. > > cheers, > > Nigel > > > Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio, > the origin of the open-source concept! > Skype: TILBURY2591 > > > On 2022-05-14 13:48, Paul Koning via cctalk wrote: > > > >> On May 14, 2022, at 1:41 PM, John Robertson via > cctalk<cctalk@classiccmp.org> wrote: > >> > >> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote: > >>> Hello, > >>> > >>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a > >>> 74<something>74 D flip flop. The problem is I don't know which > >>> modern series would be the best one to replace it with. I am sure I > >>> have seen a list somewhere of modern equivalents for some DEC chip > >>> numbers, but I can't remember where. > >>> > >>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6 > >>> of the schematic (p157 of the PDF). > >>> > >>> Picture of the failed chip here: > >>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.j > >>> pg > >>> > >>> Can anyone tell me what the best modern equivalent is likely to be? > >>> > >>> Thanks > >>> > >>> Rob > >>> > >> You are stuck with using an original 7474 family assuming this is driving > other early TTL. 74LS74, and others simply don't have the drive capability to > work. > > I know LS has less fanout, but is it not able to drive plain 74xx at all? That > doesn't sound right. If the circuit in question runs near the fanout spec of > plain 74 the yes, 74LS won't work. > > > > Spec sheets and the actual schematic will give a definitive answer. > > > > paul > > > >
