Author Topic: How to replace Ge transistors with Si in test gear?  (Read 787 times)

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Offline 001

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How to replace Ge transistors with Si in test gear?
« on: July 17, 2018, 05:37:35 am »
Hi!

I have old 15MHz millivoltmeter based on Germanium transistors. Some transistors are dead.

Can I replace it with common Silicon 2n3904 bc547 etc?

Thanx
« Last Edit: July 17, 2018, 07:50:57 am by 001 »
 

Offline PhilipPeake

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Re: How to replace Ge transistors with Si in test gear?
« Reply #1 on: August 04, 2018, 01:26:50 am »
Old post, but here is my answer anyway:

Yes, but probably not easily.
First, most old germanium transistors are pnp and most modern silicon transistors are npn. You can't reasonably substitute npn for pnp.
Next, the bias setup will probably need to be adjusted. The base bias voltage is around 0.3 volts for germanium and 0.6 volts for silicon.

If you have a circuit diagram it will help a lot. If not, its going to be more work.
You might want to see if you can identify the transistors and try to locate some NOS somewhere.
 

Offline Gyro

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Re: How to replace Ge transistors with Si in test gear?
« Reply #2 on: August 04, 2018, 11:08:12 am »
There are nearly as many PNP Silicon transistors as there are NPN, that's not a problem. The real issue is getting the biassing right as PhilipPeake says, it might be as easy as just adjusting a few biassing resistor values.

If it's a relatively simple circuit then you should be able to reverse engineer a schematic. The PCB will probably only be single sided with hand placed track layout.

The other problems you might encounter are that Silicon replacements will almost certainly have much higher gain and also higher Ft. That might possibly cause stability problems but hopefully not insoluble.
Chris

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Offline Wolfgang

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Re: How to replace Ge transistors with Si in test gear?
« Reply #3 on: August 04, 2018, 02:42:46 pm »
Which ones are dead ? Maybe you can still get some on eBay. Ge transistor gains are normally varying a bit more than Silicon,
you might have to watch gain groups (if present) and do some adjustments. Much luck !
 


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