BACK AFTER EXTENDED TIME AWAY
My wife planned a big party for my 75th birthday that involved significant planning and setup in advance. Just prior I was down with a long viral infection - not flu nor covid, just a mystery ailment circulating in the area - then a nasty cold struck right afterwards. Add in quite a few other obligations and visits and the result was a long period when I was away from the workshop. Happily I am now back.
SPURIOUS CB TRIPS IN 6V REGULATOR
For some time the 1130 would trip the breaker on the 6V regulator when powering back on after the machine had been off for short periods. I also saw some random shutdowns while operating. I had found a weak circuit breaker and replaced it previously, but the issues did not go completely away.
A power supply in the 1130 takes the input mains voltage and produces unregulated DC (nominally 13V) from a halfwave rectifier that is then routed to the regulator module which outputs a well regulated 6V to power the SLT logic in the machine. The regulator module has a circuit breaker that protects it from shorts on the 6V rail but also has an over-voltage protection circuit.
If the voltage goes too far above 6V, this will fire a silicon controlled rectifier (SCR) to put a dead short across the output of the regulator, forcing the breaker to trip. The SCR fires in tens of microseconds, limiting energy delivered to SLT circuitry in an overvoltage situation. The circuit breaker can shut off in a more leisurely timeframe since the output is clamped to zero.
The regulator module has two SMS cards plugged into it, one that does the regulation and the other that provides the overvoltage protection.
CHECKING FOR EXCESSIVE CURRENT DRAW
I placed a voltage and current meter on the inputs to the regulator - the unregulated DC coming from the power supply - to see whether the draw was close to the limit of the circuit breaker. If it was operating near its trip point, it might be susceptible to tripping on the power up surge. The 6V regulator is designed for up to 24A supply, but when I monitored the operation it never reached 7A, thus I believe the output of the regulator is safely below the rated capacity.
One issue I observe is that the unregulated DC input voltage is just over 15V, but the filter capacitors in the power supply for this input have a 15V rating. Further, the documentation lists the nominal value to be 12V, with the actual about 32% above that.
The power supply has a wiring diagram to support 115, 208 or 230V input power. My workshop is set up for 230V and the wiring is set for that voltage. I have noticed that the unregulated 48V and 12V rails are also running hot by about 10%. Those two are fed from a different transformer that was also wired for 230V.
The higher input voltage means that the power supply would have to drop more power to achieve a 6V output, however the current is still well below the capacity of the regulator. I am mildly concerned that the voltage is above the capacitor rating. There is some safety margin, but I might want to replace the two filter capacitors with ones with a higher rating.
The only mechanism I can imagine for the over-voltage from the power supply is if the primary windings have some turns shorted together, so that the turns ratio increases to drive up the secondary voltage. It would require about 20% of the windings to be bypassed to produce the observed voltage.
The home for this system once I am done with the restoration is in a facility with 208V mains, thus if I leave the wiring as it is, the voltages will be lowered. Nothing I can do in my workshop, however, except to replace the transformer which would be wasteful.
The transformer has two primary windings that are either put in series or parallel to support the 115, 208 or 230V line voltage. For 208, the jumper you can see from 1 to 5 above would instead be connected between 1 and 4 on TB-1. That reduces the number of windings however since we appear to have too few windings as is, I would leave the jumper at 1 to 5.
TESTING OVERVOLTAGE SMS CARD
I pulled the SMS card and hooked it to a bench power supply with overcurrent protection. The strategy was to run up the voltage past 6V until I observed the card clamp the output to a short. The card has a 150A SCR acting as the final clamp, although I set my bench supply to trip out at a much lower current.
The card clamped the voltage at 6.5V, which seemed like a reasonable setting to protect the circuitry and is well above the adjustment of the regulator at just over 6. Since this may be triggering a bit aggressively I altered the potentiometer to increase the voltage where it would fire.
In comparison the card for the 3V regulator clamps at 3.4V. The same percentage over for the 6V supply would be a trip point at around 6.8V. That is what I set.
The problem continued. Whatever causes the breaker to trip occurs even with this higher set point. Thus we have a few possibilities left to chase down:
- Component issues or drifted values on the SLT regulator card cause it to produce a voltage spike above 6.8V as it is powered up with only a short period of downtime (a few minutes or less).
- Some component in the SLT logic downstream from the regulator will short when power returns after a short period of downtime but does NOT short in steady use and clears itself after a few minutes waiting before the next power up attempt.
- Some component on the regulator has an intermittent connection that is opened by the heating of the parts, which works properly when powered but surges on a repower after a brief downtime.
- Some defect in the unregulated power supply produces a surge on a repower after a brief downtime that somehow overwhelms the regulator briefly.
- The replacement circuit breaker I bought on eBay has the same defect as the original part in the regulator, where it heats up inside the breaker during use and makes it susceptible to tripping during a repower.
