A Marine Phone

   

  I received this odd-looking box purporting to be a telephone the other day with the request to repair it as spare parts are no longer available. In fact, besides the box, there were three identical circuit boards in newish condition and one with really bad water damage (below).
 
   

 I was also supplied with the picture below which shows the main equipment with which the phone connects.

 

 Three boards plus one in the box looked like this and all had the same fault.

 

 

 Below, looking very innocent, are the failed components. These are marked IR1J (Schottky diode = 10BQ100 made by International Rectifier) and 2597H M-5.0 (Buck switcher = LM2597HVM-5.0 made by National Semiconductors). Note that I've shown later data sheets not those available when the parts were made (ie.manufactured prior to the specific published datasheets). It's possible therefore that the failed parts in question do not have the exact specs to those in the datasheets. I'll mention this later. Click for Nat Semi chip markings.

Manufacturing dates for the two chips appear to be 2001 and 2003 respectively.

   

 

In all four cases of the newish-looking boards The Schottky diode was short-circuit and the switcher chip had a bad leak or short between at least two of its pins. The various capacitors all measured close to perfect. The circuit, parts and its (critical) layout follow fairly closely to those given in the manufacturer's data sheet but with the 24 volt Schottky diode shown as 1N5817 replaced with a 100 volt type.
 

 Interestingly, I've encountered this type of failure in the past when one particular type of a numeric indicator board for a lift had very similar faults, but I recall that in those examples the fault was most likely due to a bad capacitor. Nevertheless the Schottky rectifier and its associated voltage regulator chip were always bad. I never figured out the reason for their failure but I'm tempted here to try harder.

 

 My plan is to work out a way of repairing and testing without shutting out any chance of getting the phone to work (spares are no longer an option and the microprocessors are critical as they carry proprietary firmware). In several cases of the numeric indicators I've repaired in the past the incoming high voltage had punched through the 5 volt regulator and blown up all the chips.. typically an SN74HC14 SOIC.

The question is therefore.. are the three surface-mount chips on these boards OK? They're all complex types viz. an ATMEGA128L, PSB21373 and a MAX202. The first is not available as a programmed chip.

I applied a low voltage to the repairable boards and found that one had a very hot processor so I removed the three complex chips from that board and then the three corresponding chips from the scrap corroded board and fitted the three "new" complex chips to the repairable board. I removed the bad regulator and Schottky diode and measured the current to the board using an external 5 volt supply. This was much less than the previous current draw and the processor ran cool. This board requires a new regulator and diode. Call this Board "B".

Next I repaired a second board (still with its original complex chips) by fitting a new voltage regulator and diode. As the former is not readily available I used the regulator from the scrap board but fitted a better Schottky diode. One of the potential reasons for power supply failure was a weak Schottky diode (100V and 1A) so I found one with a better rating (100V and 2A) and fitted this. Call this Board "A".

 

 My repair philosophy is to fix two boards and test with the least risk of destroying the last working microprocessor.. These are "A" and "B" boards.

Board "A" has a new power supply and original complex chips. If this works then two of the remaining repairable boards should work once new power supply parts have been fitted. The fourth board "B" with salvaged complex chips should also work once new power supply parts have been fitted. The repair of these three will have to wait until new parts arrive from China. These are LM2597HVM-5.0 and STPS2150A (150V and 2A).

Good news... board "A" went off to the customer and it worked! With a bit more luck three repaired spare boards plus extra power supply chips, in the event of future failures, should be available by mid-May.

Below, repaired board "A" fitted into its case.
 

 

 

 Now some of the theory concerning the fault

One board can of course be discounted as the problem is water damage which has destroyed the carbon pads and made a mess of lots of solder joints. The other four including that fitted in the metal box all have much the same fault which is failure of the 5-volt regulator circuit. Examination of the circuit board reveals that the designers have essentially used the circuitry recommended by the chip manufacturer so in theory all should be well and the regulator will be reliable.. but clearly it's not.

The diode used is a 10BQ100 which is rated at 100V and 1A against the 1N5817 (24V and 1A) used with a supply of 12 volts.. This change is of course necessary because of our 48 volt supply. The regulator chip is the LM2597HVM-5.0 which is rated by one manufacturer as having a max input of 57 volts and another at 60 volts. The original Industry chip was an LM2597-5.0 (missing the "H" in the coding) rated at 40 volts so at first sight unless the chips have been wrongly coded with "HVM" they should work fine with the system voltage of 48 volts DC.

 The 48 volt supply to the circuit board is fed via a pair of "Line" wires (two of the black wires in the picture above). My first suspicion for the failure was reversal of the plus/minus wires (you can see in the above picture that this would be an easy mistake to make).

 

However, supply reversal, which would have destroyed the regulator and diode, isn't important because, on examining the circuitry (right), the designers had included a full wave bridge comprising four small surface-mounted diodes (D1-D4). This will guarantee the correct connection of the 48 volt supply to the regulator circuit. On all boards the diodes were found to be OK.

On the right you can see I've removed the bad 5-volt regulator U1 and the Schottky diode D8 together with L2 (for access) and R6 which became unsoldered with U1.

The smaller square black chip marked "GFX39" is the SMCJ48CA TVS diode.

Also note that blue capacitor C3 which will be mentioned below.

 

 

 What about the effect of an excessive supply voltage which might result from a fault in the main system equipment? In this case, to protect the circuitry the designers have included a TVS diode at the input to the bridge rectifier. This is an SMCJ48CA (the "CA" means the TVS will work on either plus or minus 48 volts). It has a stand-off rating of 48 volts so should be virtually invisible to a good DC supply voltage. Its critical parameter is that it will break down at a voltage of between (plus or minus) 53.3 to 61.3 volts. This means that it will protect a circuit from a voltage of anything higher than a figure between these two values. It will do this by shunting up to 1500 watts through itself with the effect that a fuse (if it's fitted) will blow before it gets destroyed. So given a TVS meeting the "max" figure, we should be able to guarantee that the regulator will never see anything greater than 61.3 volts. This figure might be higher if the worst case tolerance of the TVS isn't met. This raises the question.. do the numbers in the manufacturers spec include manufacturing tolerances and ageing and even if this is true will these figures be met over a long period of storage or use?

According to published specs the regulator chip will definitely work OK up to 57 volts and nominally up to 60 volts so this means that in a worst case scenario the chip might fail because the TVS limit is greater than 57 volts (ie. 61.3 volts). Of course for this to occur the system voltage would need to be high by say a nominal 13 volts making it 61 volts.

If we look further into the circuit board design there's one factor which may have been overlooked by the designers.

The full wave bridge used for polarity reversal protection has a prime feature of rectifying AC. Although the 48 volt system supply voltage is DC (see the picture of the main equipment above) the telephone "Line" circuit may be relatively long or unscreened and might induce interfering noise, which if powerful enough would be added to the voltage seen by the regulator and exceed its maximum rating. This noise would need to be outside the normal operating characteristics of the TVS diode which is fitted before the bridge (eg. a high frequency). In fact after studying the design of the line circuit I see a 470nF capacitor C3 (that blue component) is fitted across the output of the bridge rectifier. That capacitor will raise the composite DC plus AC noise to a peak voltage level whose amplitude will be determined by the load power. In fact load power is pretty low so it wouldn't take much induced noise to kill the regulator. Another factor to bear in mind is that the layout of the circuitry may well result in a high susceptibility to local VHF signals (ie. a nearby VHF transmitter or perhaps radar may raise the DC input to the regulator to something greater than 60 volts).

Further testing on a repaired board disclosed another interesting factor. The regulator chip works with power rather than voltage. This means that given an input voltage of something above the minimum required to supply 5 volts output (in fact circa 8 volts) it's the power input which results in the power output. The 5 volt drain was measured at about 80mA meaning a power requirement of 400mW. At the minimum nominal 8 volts input the current drain from the incoming supply will be 400mW divided by 8 volts or 50mA. In fact due to less than 100% efficiency I measured this at about 100mA. As the input voltage was increased to 30 volts I measured 20mA or 600mW. At the system voltage of 48 volts I assume the board would draw 600 divided by 48 volts or between 10 and 15mA. This makes the input impedance of the order of 5Kohm. This is high enough for the line circuit to be easily susceptible to RF pickup from the ships radar or VHF transmissions.

Remembering that the input bridge will convert incoming RF to a peak DC voltage added to our 48 volts it would only require say 10 volts RMS of RF pickup to damage the board ie 48+10 x root 2= 62 volts.

 

 I was able to source new regulator chips plus 2A x 150V Schottky diodes from China and once these had been fitted the boards worked OK with all drawing similar currents. Also, reducing the current limit to the repaired boards I found the input voltage varied cyclically which is indicative of microprocessor or similar switching activity. Once the current limit was raised the input voltage remained stable. If more failures are met I can redesign the regulator circuit.

 

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