Saturday 11 January 2020

XV Ignition Intermission - Fundamentals (pt.1)

Well about time to leave the comfort-zone of mechanical engineering and swan-dive into a matter that has become a bit more pressing in the elementary squish-boom-bang-cycle and as usual it has become more pressing, because it started acting up.

But let's start at the start and not somewhere in the middle. After I fried my Ignitech, I installed a stock ignition box and the bike idled fine again. After one or two test-rides, I did some rejetting. (Going pretty much back to the specs I had with my round-slides a while back, which again led me to believe that the old Ignitech probably had taken a hit a lot longer ago, than I really wanted to admit.)
The thing is, the old girl, even with a fresh engine feels notably down on power, especially at lower rpm. And that's when I decided to dig into the stock ignition box (TCI) and see what's wrong with it.

As a first step, I did what any sensible engineer would do in the light of such an endeavour: I bought a second TCI that was in known (good) working order and as life decided to get in the way, I had to open this one up to at least have a glimpse inside. (And that's where I am stuck at the moment and why there wasn't a post last week, etc.)

When it comes to (early) Yamaha XVs there's basically two ignition boxes:

1) The standard metal cased one as you see above, stamped J4T00671 or J4T00672 (I haven't worked out the difference yet, but 671 was only available in 1981 and for 1982 to 1984 672 was available.)

2) and secondly and I hadn't been fully aware of this box for quite a while, on the so called Midnight Special models and very early XV750 and XV1000 twin-shock models, so from 1984 to 1986-1987, there was a second ignition box, marked 20x-20 J4T00871.(This box was kindly donated by a TR1-Forum-member as his bike only ran on one cylinder with it.)

I include it in this lineup, because with a home-made sub-loom it is perfectly usable even on the earlier bikes, because up to 1986 or 1987 the XV1000 models were still equipped with the older style twin-ignition-pickup system.

 One of the more interesting discoveries that came with taking both boxes apart is that one can witness the micro-processor revolution of the 1980ies right in from of himself.

First let's have a look at the original, metal-box TCI (warning big pictures ahead, so you can zoom in and snoop around to your heart's content):

Doesn't look very neat, does it? Well that's because as you will see in the next post(s), this one still has all the logical components living in the open, which in this case means spread out over the main and daughter board.

Hard to believe, but the later box does the same with just two Mitsubishi M59201P chips and as such wasn't only a lot cheaper to manufacture, but more than likely also should be a lot more reliable. (As is perfectly contradicted by this one being broken...)

My main focus in the coming posts will honestly be on the metal enclosure TCI, because first it's the one I run on both TR1s at the moment, second I already identified all the capacitors for it and I also got some replacement transistors (as I suspect one of them being broken) and lastly because the second type is both rather uncommon in Europe (at least I haven't encountered it a lot) and you can still buy it new for a very low price. (Which I may do as the Mitsubishi IC isn't available to the public anymore easily and I would really like to see what the manufacturers use to substitute it with...)

Next post will be on how to free the boards from its metal confines and a bit more in-depth analysis.


  1. Hi, I found your blog when looking to find out more about my XV750SE TCI unit which may (or may not) be ok. Yes, its the metal box type and I have already replaced all the electrolytic & mylar caps and was just wondering about the ETD51s and whether there is a still available (& better?) equivalent. Curiously, your blog is the only place on the web I've found a reference to these - although I remember them from my years as an elec tech. So thanks for pointing me in the right direction.

    The problem I have is backfiring (it has always done it - and after the cap change still does) - but only on the rear cylinder. Checked for exhaust air leaks (cured one), complete double carb rebuild with new jets etc, checked sparks (both orangey blue but rear one 'thinner'), ignition coils, harness wiring etc. So now I am stuck... :-/

    1. Hi Chris, to be honest, I have yet to be forced to search for an actual replacement for the ETD51-030ies as they are still available via Infineon. That being said, I have you tried to scope their switching behaviour, i.e. whether they still create a sharp flank, when switching?

      That one side - you did ditch the the auxiliary breather on headstock, which goes to one of the cylinders? Also with the stock exhaust, there's always going to be the odd pop and crackle. On top of that: the carbs are sync'ed properly? If you want, feel free to drop me an e-mail... I somewhat monitor the comments, but only infrequently on older posts.

    2. That being said, I just went through my notes and the substitute I chose was to use the BU941ZT, which is also a Darlington transistor with relatively similar characteristics and it's hardened for automotive use.

  2. Thanks for the extra info. Yes, I got rid of the aux breather (aka mixture control valve) and did many other tweaks and changes such as carb jets, one plug lead, one plug cap, pilot jet adjustment, one coil and, finally, carb syncing - which entailed trimming the adjuster spring cos slack in the linkage put the whole thing out of adjustment range - interesting! Each of these 'improvements' made a slight, incremental reduction in the back-firing problem so it seems there was no single solution - just the gradual ravages of ageing and wear. I have yet to try the transistor subs though. They are not cheap and I can live with things as they are for now. Cheers,

    1. Glad to hear everything progressed in the right direction. :-)

  3. Hi again. Due to increasingly bad performance - nearly all on the front cylinder failing to fire reliably (and a weak spark on both cyclinders) - I have finally resorted to changing the transistors for the (expensive) BU941ZTs. The result is that a weak spark has now been downgraded to no spark at all! :-( Both the (carefully) removed EDT51s and the replacement BU941s still appear to be functional, from a static meter test POV. However, now I am stuck. Literally! Before proceeding further, I have been unable to find a datasheet for the EDT51s to confirm that the pinout is actually the same as the BU941s. I have assumed it is and a brief pcb check suggests this is so (ie the case/middle pin goes to the coils and the 'emitters' are commoned whereas the bases seem to be fed from different smaller transistors). So any idease? :-/