Help Identifying transistor

[tobytreacher]

Hello all,
I am looking for help identifying a replacement power transistor. It is from an old spark eroder machine (EDM). The machine in basic terms produces a square wave DC voltage to make a spark which is connected to a copper electrode. The electrode is held just off the work piece and the resulting spark erodes metal away from it.
The transistors have no markings on them, they are in a TO3 package. I have made a circuit diagram of the board they are on, as you can see it is very basic stuff. I have also ascertained they are NPN and have a Hfe of 22. There are three of them on a board and are switch in in parallel to increase the power to the electrode. Each one gives you about 3 to 4 amps. This is done by reed relays. The input to the board at 12 is some sort of frequency control, I have not investigated this circuit as it is OK.
I have not as yet properly identifying the device marked BD232 or 6D232 but this is of no concern at present.
If someone could recommend a replacement device for the three main power transistors I would be very great full. Any other thoughts welcome.

 

[shrtrnd]

How old is your equipment?
It was pretty common to use 2N3055's (hfe 20 to 70).
The problem nowadays is the early 2N3055's were very robust, and the newer 2N3055's are not. Some kind of design change to save money that made the 2N3055's just not
as tough as the ones built in the 60's and 70's.
I've found in a lot of the old gear I repair, that the newer 2N3055's just won't last as long
as the older devices.
I've been using MJ15003 as a more durable replacement for old 2N3055's, when I can't
find any old date code ones (hfe 25min at 5Adc).
This may not be the right transistor, but if you've got an older piece of gear, it's a strong
possibility.
Good luck.

 

[duke37]

The 2N3055 is limited to 100V which is at your limit.
The 2N3442 (140V) is used as a more robust replacement in audio amplifiers.

I would make a little circuit modification and use FETs. This would be easier on your reed relays and reduce the heat dissipated.

 

[tobytreacher]

Hi Thanks too you both for your replies.
The machine was made in 1977. What I was doing was was to search RS for devices in a TO3 package with a min dc gain of 20. The 2N3055 is there as well as a few others, I had sort of thought of trying the BUX10, is there any reason why this is not suitable? This is were my electronics knowledge reaches it's limits. I was shying away from ones that had for audio use in their description, is that not a problem?
The MJ15003 looks like a good contender so dose the 2N3442.
Another question - is the price of them relevant?
Thanks again for all you input.
Cheers Toby

 

[duke37]

Audio transistors need linearity, this is not needed in your situation, what you need is a low saturation voltage at the current that is demanded. The BUX10 or MJ15003 (more modern?) would doubtless do.