I think that should be a suitable replacement. While looking at data sheets I did notice that the KT825 (With no "G" at the end comes in a TO220 package so make sure you get the verion with the G which is in a TO3 package. I think the power rating is OK. The minimulm beta value is slightly lower than the BDX66 but I don't think that will be a problem. (The beta value is the gain of the transistor which is the ratio of collector current to base current.) It will just mean the drive current (Via the white wire.) may be slightly higher. What drives the signal to the white wire ? (A PLC, a relay, a transistor or something else.) It may be worth buying more than just the one transistor that you need in case you need some in the future. I can find no other suitable replacements on the Farnell (UK) website. (I assume you are not in the UK or Europe from the times of your posts.)

Les.

Yes spares sound like a very good idea, thanks for your thoughts on that as a replacement. I don't know what drives the white wire signal and presently not sure how to find out. The unit was made in 1988 which I think would rule out a PLC. Is it still worthwhile checking the current draw on a working valve? And I'm in Melbourne, Australia...

Thanks again,
Jerry

 

If it was me I would check the current in the red wire of a working valve with it still in the machine. I am assuming the vaves are actuated for long enough to read the current. I would have thought PLC were around in 1988. (It is over 50 years since I last worked in a factory environment. Then the logic was done with dozens of relays.) I would also measure the current in the white wire to see if mine and Alec's theory is correct that the white wire is only a control signal. I see it is late for you (Near midnight.) It is about 13:00 here in the UK.

Les.

 

Red wire?

 

Hi Alec,
Thanks for spotting that. I meant the green wire.

Les.

 

Bad back prevents further investigation at the mo. I'll be back on it as soon as. In the meantime, is it a Zener diode I'm looking for?

Cheers
Jerry

 

I see a 2A rectifier diode was suggested - there seems to be many types though....

 

I think any with 2 amp or more rating will do. I think I would look at any of the 1N5400 series of diodes. They are rated at 3 amp with a 200 amp surge rating. The last digit of the part number indicated the peak reverse voltage rating. As there will only be 12 volts across them and the lowest rating one (1N5400) is rated at 50 volts ant of the range will do. Your choice will depend on what you can obtain easily. The diode will be passing 10 amps for a few mS just after the power to the valve is switched off.

Les.

 

I think any with 2 amp or more rating will do. I think I would look at any of the 1N5400 series of diodes. They are rated at 3 amp with a 200 amp surge rating. The last digit of the part number indicated the peak reverse voltage rating. As there will only be 12 volts across them and the lowest rating one (1N5400) is rated at 50 volts ant of the range will do. Your choice will depend on what you can obtain easily. The diode will be passing 10 amps for a few mS just after the power to the valve is switched off.

Les.

Thanks for that Les. I got 1N5404 diodes and the power transistor replacements arrived from Germany today. Assembled it and all working as before. The valves can be open for 10 secs or more so I'll still measure the coil current and post that after they're reinstalled.

So thanks to everyone for the great response. I learnt a lot and have working valves again!!