Autohelm 800 power transistors

[winater]

Hi,
Our trusted Autohelm 800 started a mutiny, and after opening her up the problem appeard to be fried power transistors.

Two of the four transistors on the position TR4 and TR8 were fried beond recognition
the others were;
BDW 93B 8923 on position TR7
and
BDW 94B 8917 on position TR 6(?)

My question to the gathered wisdom here is; what were the typenumbers of the unrecognisable transistors?
Bonuspoints question ;-) whith which type of transistors could all four of them best be replaced?

[GordMay]

Greetings and welcome aboard the CF, winater.

[CarCode]

Google is your friend, enter BDW94 or BDW93.
Both are Darlington power transistors, one is NPN and the other PNP.
Use the C version for a higher voltage.
But you cannot use all of one type. You need both types.
Gerhard

[winater]

Thank you Gehrhard but you obviously understand more of this than I ;-) For me it is not so obvious,
my questions are still;
What are the missing typenumbers, where to put them TR4 and TR 8, with which (modern?) alternatives? do I replace all four of them?

[travellerw]

Both of those parts are still sold and manufactured by multiple manufacturers. There is no more "modern" transistor. You should replace like for like. They cost about $2/each from mouser or Digikey.

I will caution though.. Transistors don't usually blow up like that unless there is another issue somewhere. Most likely a power supply issue.

[SteveSails]

As CarCode said these are NPN (BDW93B) and PNP (BDW94B) darlington power transistors in a T0220 plastic package. TR4 and TR8 would be another pair of 93B/94B.
The numbers 8923/17 are date codes, made in 1989 weeks 23 and 17.
Basically the transistors act as a switches to the autopilot drive motor, one pair turns on to drive the motor one way and the other pair for the opposite direction. If one of the transistors does not turn off when it should then it will cause a short with the results as you have seen. Good news is that you can replace the transistors, bad news is they may not be the root cause and will just burn up again. Hard to tell without digging further into the circuit.

Try DigiKey, Allied Electronics, or Mouser for replacements. I'm sure there are substitutes if you can't get the BDW series, perhaps in the TIP series. I can do some research and recommend a sub if you need it, let me know.

BTW Did a fuse or circuit breaker blow? That could be a big clue.

Steve

[winater]

Thank you very much Steve, it is now starting to make some sense to me.
No fuses have blown, but I will take V and A measurements once possible.
Over here (Netherlands) Farnell is probably a good supplier as an alternative they offer a C model instead of B would that be recommendable?

[SteveSails]

The 100V rating of the C version vs 80V of the B will provide some extra protection from voltage spikes but either will work just fine. Be careful when mounting the heat sinks and use thermal grease, a thin coating is all that's needed. The back of the transistor and the heatsinks are electrically connected to the center terminal (collector) of the transistor, be sure they don't touch any other part of the circuit.
Since no fuses blew, assuming the AP is properly fused, then it is possible that one of the transistors was the root cause, no guarantee though. A possible failure mode is a loose or corroded heat sink that would cause a transistor to overheat, an overheated transistor will start to conduct when it shouldn't causing even more overheating, all downhill from there. But that's pure speculation on my part and there still could be other causes. Google 'darlington transistor H bridge' to get an idea how this circuit works.

Steve

[senormechanico]

Also make sure the motor or leads to the motor are not shorted.
Chafe on the motor leads could overload and cause the type of damage you see.