I have an ongoing issue with my Narco 155.
In Mode Alpha select, the squack code is read as normal, in Mode C however, the number read out, as read by ATC, is a different code. i.e.
Mode A - ATC asks you to squack 2615, In put code 2615 on transponder, read by ATC as expected, 2615.
In mode C, squack 2615, read by ATC as 2766. The number, as read by ATC, appears random.
New encoder has been fitted, new wiring loop, all bench checked as correct, and again checked when installed, all appears ok. Get in the air, and the issue appears. Even the avionics guys I have been using think the issue is unusual.
Any thoughts or comments would be appreciated, before I just go buy a new transponder unit.
The number you set on the transponder doesn't go through any wiring external to the transponder.
The number is encoded and transmitted straight out.
What passes through external wiring is the pressure altitude which comes from some external encoder, and is normally sent via a dozen or so Gray code wires. Sometimes one of these breaks and you get a funny altitude reading returned in Mode C. But that isn't your problem.
It has to be a duff transponder - or something really weird like a bad power connection which causes the power supply to the transponder to dip below spec during transmission.
Narco have a very bad reputation for quality.
Thanks Peter, would that not affect both readouts, Mode C and A? The annoying thing, and the confusing issue, is that one selection works fine, the other not.
Yes, bizzare...
A wild guess: a mode C transmission is not very different from a mode A transmission. Could it be that, when switched to mode C, the transmitter sends out the altitude information in a mode A packet iso a mode C packet?
But, as Peter says, the trouble must be inside the transponder.
would that not affect both readouts, Mode C and A? The annoying thing, and the confusing issue, is that one selection works fine, the other not.
Mode A and C is all about pulse spacing (which internally is timing). The mode A pulse sequence is much shorter than the mode C sequence. Little errors in the internal timing of the transponder may add up to large errors in the mode C sequence that fall outside the tolerance while they are just inside with the short sequence. (Just an attempt for an explanation ... may be something completely diferent though!)
Anyway, I would not spend a single Euro/Pound/Dollar on having Narco equipment repaired. Been there, done it. After the n-th repair you will have to replace it with some decent piece of kit anyway, why not do that right now?
Can you borrow a transponder to see if that helps?
Thanks guys for the advice. The issue is that I have been asked to send the unit to an avionics shop, this will be the third time I have done this, and obviously at the back of my mind I am thinking, lets just buy a new unit.
What next, I take your point. Curiosity though, would the bench tests not show this spacing issue up?? If in fact it was this pulse timing issue?
Given that a A/C transponder will limit your flying pretty much to the UK and that Narco isn't really worth repairing, I'd go for a new transponder. Trig probably.
Curiosity though, would the bench tests not show this spacing issue up?? If in fact it was this pulse timing issue?
I find that also strange. Normally the test equipment should catch this type of fault. But then I have seen quite a few avionics guys use tools and equipment that were in poorer shape than the avionics they were trying to repair...
