AIUI the 769s were born because someone thought the DC bus down the train was a great way to link the two diesel engines together and provide the "fake" diesel supply to the third rail. Additionally, the class 319 chopper electronics were the most suitable for use on the diesel control side. AIUI the existing 750V DC bus was used, but they had to install another bus anyway because the GWR 769s needed to use the existing 750V DC bus to operate of the third rail. I may have the wrong end of the stick but that is how I understand it, but of course, if 769s were actually any good they should be working the Cardiff - Pompey trains which would run on AC to Filton and DC from Redbridge. We know the answer to that one don't we.
Actually, the 769s were born because at the time there was rising passenger num bers with a corresponding looming shortage of DMU as well as a commitment to remove the pacers that weren't going to be made accessible. Porterbrook who were about to see their 86 class 319s come off lease had an idea that they might be suitable for a conversion to DEMU, whilst retaining their ability to run as straight EMUs. Unlike the other ageing MKIII EMUs (classes 317, 318 & 320-322) they uniquely had a DC bus by virtue of their need to run on 3rd rail 750VDC. (The ac only MKIII classes have thyristor traction control of the DC motors straight from the transformer so there was no DC capable electronics to feed from a locally generated DC supply.)
Whist running as an EMU, i.e. just like a 319, the local DC return from the motors was via the running rails. This return path could not be guaranteed on unelectrified track, so a local return bus was installed.
There was a major control issue during development of the 769 design that resulted in the two gensets hunting whilst trying to power the DC bus. This was fixed with (I believe ) as software modification on the system. There was then an out of gauge issue with the exhaust ducting that required a redesign. This required some mechanical redesgn of the exhaust system, but under current design proving rules, it was necessary to book a retest of the complete system and most of the delay was time spent the queue to access a suitable test facility.
All through the programme there has been the spectre of the units age and ultimately reliability, which once they entered service with Northern, became an operational inconvenience. This of course ran in a period when Northern had other issues, notably driver shortages and an inadequate arrangement for refuelling, especially when they were prevented from using OLE for more of the route than was planned. Similarly there have been failures that are unrelated to their '769 only' equipment that some commentators have rushed to blame on the diesel/genset. As
@FGW_DID says,
"Nobody mentions the runs that occur without problem but are quick to leap on any delays / faults & failures and post about them here, it’s almost as if there was an agenda with some posters!" Then these regular critics vanish into the woodwwork until the next problem with the service occurs, - there have even been posts condemning the 769s when a service delay has been caused by a freight train breakdown!
But in all seriousness that is progress I suppose, but it has been almost 2 years of testing and the main things we have head about their introduction are issues, speculation and now potential for them to be scrapped completely - not very promising.
