PDP 11/24 - A Step Backwards - test-drb@ccmp.vtda.org

28 Mar 2022

Its 600mV, but it is more of a spike than a ripple. Here is a trace:
https://rjarratt.files.wordpress.com/2022/03/pin-1-5v-ripple.jpg

Regards

Rob

...
  -----Original Message-----
 From: Wayne S <wayne.sudol at hotmail.com>
 Sent: 28 March 2022 23:15
 To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>; General
 Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
 Cc: Chris Zach <cz at alembic.crystel.com>
 Subject: Re: PDP 11/24 - A Step Backwards

 How bad is the ripple?
 Anyone on the list know what?s acceptable?

 Sent from my iPhone

  On Mar 28, 2022, at 14:46, Rob Jarratt via cctalk
<cctalk at classiccmp.org>  wrote:

  -----Original Message-----
 From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Chris Zach
 via cctalk
 Sent: 28 March 2022 20:57
 To: cctalk at classiccmp.org
 Subject: Re: PDP 11/24 - A Step Backwards

  I don't think the CPU is working at all. The
reason being that there
 is absolutely no LED activity. Including an LED that is supposed to
 indicate a clock. Having hopefully eliminated all the power voltages
 it left me wondering if there was a fault on the CPU or in the PSU.
 Having had activity on those LEDs recently it seems most likely that
 it will be the PSU, particularly as *something* in there blew up.
 The only signal that I can identify that seems likely to have this
 kind of effect is LTC, but I don't know enough about LTC to know if
 its absence could cause the CPU board not to work at all, although I
 see above that you think it unlikely. I suppose the fault could be
 something I can't see on the CPU board, particularly as there do
 seem to be some quite large spikes, otherwise I am not sure if there
 is anything else from the PSU that could prevent the CPU getting going. 
 I'm on a nice long train trip right now but I recently got my 11/24
 running again. One thing that baffled me was it would not do anything
 on the serial port. No ODT, no nothing.

 Turns out you really need to make sure the slots are filled properly.
 The CPU top, then the memory map, then for the next 4 boards one has
 to be either a properly configured MS11-PL (the 128kw board) or the
 memory boards specific to that type of 11/24. Or you have to put
 G7273's in the CD slots.

 I have been reluctant to put everything back in, in case the PSU fries  something.
And the ripple I noticed is certainly something that bothers me.
 Previously I had a burning smell from the memory board. I have since
 replaced all the electrolytics on the memory board, but I have not tried
 putting it back in the machine since. Just checking my notes, it seems I have
 had *intermittent* lack of activity on the CPU LEDs before, so it may be
 worth trying to put everything back in, although the ripple makes me
 hesitant to do so. For the record, right now I have only the M7133, M7134
 and G7273 installed.

> Next you need proper devices or G7273's in the next two slots and a
> proper terminator in the left sockets of the last slots and a G7273 in the 
center slots.
 > Only then will ODT work.
>
> Another oddity is that the 5.25 inch box has +5 and +12 I think and
> the
> 10.5 has +5 and +15. There are different memory boards that work in
> one and not the other, or both depending on jumper settings that have
> to be right. Unibus drives me bonkers sometimes with the number of
> different voltages requires (+5, +12, +15, +20, -15, etc....) It
> doesn't help that the +15 and +12 are on the same pins.
>
> Plus it's possible someone screwed with some switches, make sure they
> are set properly (ie: default is a good start).
>
> If you're still stuck next week drop me a line and I'll fire up my
> 11/24 and see if I can replicate your failure.
>
>>
>>>
>>> The first will tell you that i) the CPU is basically functional,
>>> executing
>> micro-
>>> instructions, etc; ii) that the bus is basically functioning (for
>> master-slave
>>> cycles; DMA and interrupts will remain to be checked out); iii)
>>> that the console port is working. (Yes, on the KDF11-U, the console
>>> is on an
>> internal
>>> bus, and so in theory a machine could have the ODT 'front panel'
>>> working, _and_ still have a problem with the bus, but depending on
>>> the exact details of said problem, maybe not.)
>>>
>>> So, hook up a console, set the machine to 'halt', and power on. Is
>>> console ODT working? If so, congrats, you win, go to stage ii) below.
>>
>> I had a console attached. There is nothing on the console. When I
>> first got the machine I did get output on the console. But that was
>> before the PSU first failed on me, which is quite a few years ago now.
>>
>>>
>>> If not, you have a reduced area in which you have to investigate -
>>> and
>> you'll
>>> need a 'scope of some kind to make any progress. (If you don't have
>>> one, you're SOL. Get one.). In order i) is the CPU's internal clock
>>> (and thus, probably the microcode) running? (At this point you will
>>> need to consult
>> the
>>> "PDP-11/24 System Technical Manual", EK-11024-TM.) If so, is it
>>> trying to
>> talk
>>> to the console's registers? (See Section 4.6 of the TM, "Internal
>>> Address
>>> Decode".) If so, is the UART working properly? (4.7 of the TM,
>>> "Serial
>> Line
>>> Units".)
>>>
>>> If so, console ODT is running, you're now at stage ii): you can see
>>> if the
>> CPU
>>> will run. Deposit a 0777 ('BR .') in a likely location (I usually
>>> use
>>> 0100 or 01000); read it back to make sure the write succeeded. (If
>>> not,
>> likely
>>> either the UNIBUS or the main memory has a problem.) Start the
>>> machine; the 'Run' light should come on - if you're lucky!
>>>
>>> Depending on which bin you wound up in, further assistance s 
available.
 >>> :-)
 >>>>
 >>>>      Noel
 >>>
 > 