MTH PS Smoke Unit Circuitry Advice

hi please show a good picture of the board so we can help you I suspect it is a FET transistor which controls the smoke unit and it probably got so hot due to a smoke issue that the solder got hot enough for the chip to come unsoldered. the ic will need to be replace if it can be replaced but the ps2 5 volt boards are not easy to work on and cannot always be repaired but you can replace those boards with a ps2 3 volt board and just change the sound file to a three volt file and your back in business plus the p s 2 3volt boards can be repaired!

Alan

Schematic with alternate part numbers here.

As for 2 levels of smoke, when track voltage falls below a threshold, the right transistor turns ON to short or bypass the upper smoke heater resistor.   Smoke output does change with track voltage (unlike PS2 or PS3 where smoke output is constant with track voltage) but the transistor switch makes the overall variation much less dramatic.  It's a clever yet simple/inexpensive circuit.

Alan,

Thanks for the reply. That's a good point, since there were scorch marks on the IC board, you're probably right that it got hot enough to melt solder points.

The good thing is is that it was easy enough for me to repair. The smoke unit is working now, I'm just wondering if the bipolar transistor should be re-replaced with an actual FET transistor.

Stan,

So there should be two levels of smoke output, it doesn't appear that there is at this point. So I'm wondering if the transistor is not working properly, or if it isn't the right type.

I'm on my phone right now, so the schematic you linked wasn't legible, I'll have to check it when I get home. If I still can't read it, I'll have to ask for clarification. 

Thanks for the response.

I've had 2 Proto1 engines this week in for repair, and both had the capacitor just fall out due to bad solder job.  Common on these old units.  

When you do Prot2/3 Upgrades on these you no longer use the original circuits anyway.

Jim

2SB772 on left, KTA1266 on right.  Both are PNP transistors.  Note that if you cross-ref the KTA1266, it has the Collector pin in the center which is arguably non-standard for transistors of this type.

To explain a bit more, it's not really 2 "levels" of smoke but 2 "modes" of smoke.  In one mode (at lower track voltages) the rectified DC voltage (after the bridge) is placed across 1 heater resistor (the lower one in the schematic).  In the other mode (at higher track voltages) the rectified DC voltage is placed across 2 heater resistors.

In the first mode, the 2SB772 is "on" which short the upper heater resistor thereby applying full DC voltage across the lower heater resistor.  As track voltage rises and hits the threshold, the left transistor turns on.  This turns OFF the 2SB772 and now the rectified DC voltage is applied to both heaters resistors.  Within each mode, the smoke heat increases with increasing track voltage.

I'm reluctant to specify the crossover voltage as it depends on what kind of transformer (pure sine, chopped sine, etc.) and it can be operating in both modes at the same time near the crossover point.

You can "test" to see if this mode switching is occurring by, say, measuring the DC voltage across a heater resistor:

At low track voltage, the upper heater is shorted so there should be very little DC voltage across it.  As the threshold is reached, voltage will appear and increase with increasing track voltage.  The voltage is then evenly split between the two heaters.

At low track voltage, the lower heater receives all the DC voltage.  The voltage will increase up to the crossover and the voltage will jump down as the available voltage is then split evenly between the two heaters.  As track voltage continues to increase the voltage will again rise on the lower heater.

If for whatever reason you can't find a 2SB772 or replacement, you can put in a PNP Darlington or even a P-FET as long as they have commensurate voltage/current handling capability.

Thanks for detailed reply Stan,

Here are a few photos of the IC board for the smoke unit. You can see the darkened areas under the transistor where it heated significantly and damaged the original. I don't know if that is what weakened/loosened the solder on the recitifer, as well, but there aren't any burnt areas near it.

I didn't get the chance to check the voltages of the heater resistors. When I get the chance I will post the results.

Thanks again for the help.

Tom