AVR Freaks

Marvellous.  Thanks for the link.

 

So the 'S' would be the single?  While that makes sense, two things make me question that:

 

1) The MEGA catalogue shows the M1WA_SR2 series to be dual output devices, with the single output version in the M1WB_LSR2 series.

 

2) While the TRACO power datasheet shows that, for single output devices, the centre pin (pin 5) is absent, for the dual output devices that pin is present and is the common to the -ve and +ve outputs.  What's more, I've measured -5V between pins 5/4, and +5V between pins 5/6.

 

The TRACO or the MEGA?

 

I can characterise MEGA parts neither as high-quality, nor as cheap n cheerful, as I have no experience with them.  However it would seem to me odd for a single-output device to fail over voltage at exactly x2 the rated output, and to somehow grow a common.

 

So I've heard. 

 

I may consider replacing all 4 transceivers (the isolated 'in' and the three 'outs') with the Linear part, but the reality is that the theatrical environment just isn't as demanding as your typical industrial application.  I've rarely seen better than the 75176B in any of the gear I've opened up, whether a desk, a dimmer card, or a fixture.  There was a Radiance hazer which had a different transceiver.  I don't think it was a Linear part.  Whatever it was, it died an awful death for no discernible reason (possibly when it was rented out to another theatre).  I replaced it with a 75176 I had in my parts bin last year and it has been happily running ever since.

 

Many thanks for your help.  I'm still uncertain what to conclude.  A curiously over-volting converter, or a particularly tenacious 75176B which has survived a really stupid design error in an old, low-volume product.  From my experience with this kind of gear, I'm inclined to go with the latter.

It may just be my own bias, but this is looking more and more likely.

 

I'm not sure why I didn't clue into it before, but the other repair I made might be related.  And a number of anomalies, in my mind, just reinforce this conclusion.

 

The muscle in the PSU is provided by what look like three off-the-shelf SMPS units, each bolted to the chassis.  They provide 24V to the main accessory power bus, via three beefy diodes, I suppose so that ripple and switching from one doesn't affect the other two, and so that a failure in one or two of them allows the unit as a whole to continue to work.

 

The 24V is regulated down to 5V through a peculiar arrangement of diodes and regulators.  First, the 24V passes through a zener, then into a 7815 with heat sink  The output of the 7815 passes through another zener, then into a 7805 with heat sink.  The 5V output drives the converter, the three output transceivers, a 6N137, and the 'DATA' LED.

 

Now, the zeners are a guess.  Those original parts aren't even on the board any more.  There is a silk-screen profile of them suggesting quite beefy through-hole parts, and the cathode marks are clearly on the high-side, suggesting they were zeners meant to dump a fair bit of heat, rather than standard diodes meant to protect against polarity reversal.

 

Those parts are gone.  It looks like a pretty inexpertly done repair, but the first zener has been replace with a low-value, high-wattage resistor in series with a what looks like a normal, small glass-package diode, forward biased, being pushed beyond it's specs.  The voltage drop across the diode is about a volt, and the drop across the resistor is about 3.5 volts.  The zener between the 7815 and the 7805 has been replaced by two small glass-package diodes in series, one of which is appears to be a zener, the other of which is likely a normal forward-biased diode.  The voltage drops are similar.  It was the mid-air solder joint between these two diodes which failed and resulted in the unit ending up on my bench.

 

Your hint that the 75176 failures result in excessive current draw makes me conclude the following:

• Design flaw on PCB, or incorrect MEGA part (pulled from wrong bin by hand-assembler, or mislabelled by manufacturer, or shipped in error by vendor and not caught during assembly)
• Excessive current drawn by 75176 caused eventual failure of beefy zeners
• At least one past repair to address a frequently failing unit.  I expect the 75176 has been replaced at least once, as it (and one of the 'send' transceivers) is a SN75176BP, while the others are DS75176BTN.  I'd posit that one of the first repairs may have replaced the dead DS75176BTN with one of the send transceivers, leaving one of the three sends unusable.  A later repair, after that one also failed, would have replaced both units.

 

Who knows.  All the evidence is circumstantial, of course.  I've sent an email to MEGA in the hopes that they can give me specs on this specific part number.  For now, I'm inclined to replace all the transceivers, modify the PCB to power the isolated one from 0V/+5V instead of -5V/+5V, and possibly replace the mess of diodes and linear regulators with an off-the-shelf SMPS module to drop the 24V to 8V or so, and then use a fresh 7805 to get a clean 5V.

 

Oh, and the silk-screening on the PCB is reversed!

 

Not a single tranzorb to be found, by the way.  I'm not likely to go to the trouble of installing any.  I'm not getting paid enough.

The pic is from the intertubes for reference, not a photo of the part in front of me:

http://www.jotrin.com/technology/details/1_W_converters_target_IGBT_driving_circuits

The SIP form factor is the same, but the part number is not.

 

 

It was late when I sent them an email yesterday.  I've called today and spoke with a rep, and am now waiting for him to either call back or email me some info.  Frankly, I'm not holding my breath since it doesn't represent any new business to them (in response to my initial question about specs, the first words out of the rep's mouth were "how many units are you looking to purchase").  At best, if I conclude that their component does indeed need replacing, they'll sell me one piece.  That won't even cover the cost of time they've already spent talking to me on the phone.

I did notice the rise, but what concerned me was later reading this:

Requirement on output load
    To ensure this module can operate efficiently and reliably, During operation, the minimum
output load is not less than 10% of the full load, and that this product should never be
operated under no load! If the actual output power is very small, please connect a resistor
with proper resistance at the output end in parallel to increase the load, or use our company’s
products with a lower rated output power (MW2A_S /MW2B_LS series).

Overload Protection
    Under normal operating conditions, the output circuit of these products has no protection
against overload. The simplest method is to connect a self-recovery fuse in series at the
input end or add a circuit breaker to the circuit.

Their emphasis.