Re: Replacement for a DEC 7474 Chip
On 5/14/22 15:35, dwight via cctalk wrote: >>> 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. >> I've got a whole parts drawer full of 74S74s, which I believe exceed the output current capabilities of the original 7474. There's also the 74ABT74, if you want a beefer BiMOS version. Heck, I can probably come up with a couple of crufty old 74H74s... --Chuck
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 on behalf of Nigel Johnson Ham via cctalk Sent: Saturday, May 14, 2022 10:50 AM To: Paul Koning via cctalk 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 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 >>> 7474 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.jpg >>> >>> 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 > >
Re: Replacement for a DEC 7474 Chip
On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote: AFAIR LS can only drive one unit TTL load. paul LS is 4 TTL, 4 ma low. Was there a trick of forcing the output of D flip flip to clear it? I was wondering if this is what kills all the 7474's? Ben.
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 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 7474 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.jpg 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
Re: Replacement for a DEC 7474 Chip
> On May 14, 2022, at 1:41 PM, John Robertson via cctalk > 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 >> 7474 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.jpg >> >> 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
Re: Replacement for a DEC 7474 Chip
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 7474 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.jpg 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. You can use 74S74 or 74F74 as they have the same output current, the "S" is Schottky, and the other is "F"aster. I find that https:///unicornelectronics.com is a reliable source of TTL. John :-#)# -- John's Jukes Ltd. 7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 Call (604)872-5757 (Pinballs, Jukes, Video Games) flippers.com "Old pinballers never die, they just flip out"
Replacement for a DEC 7474 Chip
Hello, I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a 7474 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.jpg Can anyone tell me what the best modern equivalent is likely to be? Thanks Rob
RE: DEC H7822 power supply
On Thu, 12 May 2022, Peter Coghlan via cctalk wrote: > > Wayne: I just got an Dec AlphaStation 200. Look like its running NT > > though. > > > > It's probably ok but check the power supply for leaky capacitors! Definitely, watch out for possible damage! Prompted I had a peek in mine (an Astec SA180-3505) and it does have lots of the dangerous Chemi-con LXF parts! While mine looks clean and tidy yet (LXF series seem sturdier than older SXF ones), I need to get this sorted sooner rather than later. Thankfully last time I booted it a couple weeks ago it worked just fine. (Strictly speaking I have either an AlphaServer 300 or an AlphaStation 250 system, which are hard to tell apart and the only difference mentioned in DEC documentation between the two mainboards is the maximum amount of DRAM supported, 512MiB vs 256MiB. However both are essentially the same, up to the dedicated sound connector (unused with the AlphaServer), so it's not clear to me if it was a manufacturing option such as a wire bridge put somewhere into the PCB or just a matter of software support. Both kinds of systems use the same firmware, which does not report the system model! And in any case my mainboard has a 54-23787-01 designation, which I'm told is an AlphaStation 250 (an AlphaServer 300 would be 54-23787-02), and also an early version, dated week 51 of 1994, S/N 60, and with lots of hardware patching present, however without the sound option and with an AlphaServer 300 badge and a 1997 date code on the case. Which in any case is exactly the same as with the AlphaStation 200, so the PSU is likely the same as well, so again, do check yours!) > Have a look around for batteries that might leak while it was/is in > storage too! There's a 2032 Lithium coin cell in these systems only, so while it might have been depleted it won't do any harm. Maciej
