Re: Amplifier transistor substitution.

Mar 19, 2017 6 Replies

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Cursitor Doom 9 years ago

>> The smoke escaped from my old oscilloscope today. I've determined that

>> it came out of R740, to the bottom right of the circuit diagram.
>>
>>
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>>
>> This is because T711 has shorted out.
>>
>> Not very surprisingly I cannot source a replacement BF472, so I need to
>> find a substitute.
>>
>> This is the final X-axis amplifier, so we're not talking high
>> frequencies. Is it likely to be sensitive to the particular transistor
>> characteristics?
>>
>> Sylvia.
>
> Hmm....
>
> They may have supplied a circuit diagram and board layouts in the user
> manual, but this thing is clearly not designed to be repaired. There are
> multiple wires soldered directly to the boards, including some of those
> from the power transformer, and to the side of the board that's not
> readily accessible until after they're been desoldered. How did they
> assemble this thing? With some kind of right-angled soldering iron?

I'm x-posting this to a more appropriate forum, Sylvia, the guys there will be better able to assist you with this...

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jfeng 9 years ago

I would try a MJE350(pnp/300V/500ma/20w/TO-126/hFE=30-240). Anchor Electronics

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wants US$0.95 each.

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Phil Allison 9 years ago

** I have already pointed out that Sydney's biggest spare parts dealer( WES components) has the BF470 in stock for under $1.

Sylvia lives in Sydney.

.... Phil

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Phil Allison 9 years ago

** They have the BF472 as well, same price

Sylvia lives in Sydney.

..... Phil

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jfeng 9 years ago

That was not obvious to me from the posting in sci.electronics.repair. Where was this information to be found in that posting? And if Sylvia is in British Columbia, Anchor Electronics is a plausible supplier.

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John Robertson 9 years ago

You may be able to find your BF472s sourced from India:

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I did a search and no BF numbers showed up, however they did manufacture the device recently according to the referenced PDF. You may have luck tracing the part through CDLI's distribution network. It never hurts to ask!

I would also check T710 (BF458) and T707 (BF199) as well though before ordering anything. A simple gain test should suffice. Try to find a reliable source for these transistors, there are a lot of Chinese (etc.) fakes out there!

Have you upgraded the electrolytic capacitors in your scope? They are most likely well past their Best-Before date!

John :-#)#

(Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out."

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Cursitor Doom 9 years ago

Is Indian stuff kosher? Like others here I've had big problems with fake semis from the far east; voltage regs in particular.

Oh yes, see above!

They were *all* totally fine! I mean I'm sure they're not as they were when new, but none of us are. ;-) The fault with that scope (if it's the one I'm thinking of) turned out to be a diode in the SMPS section that someone had subbed with an inferior part. It needed a fast recovery diode and someone had stuck in the first spare diode the could find I'd imagine. It would have worked fine at 50Hz but this was 20kHz. One of the guys who posts here spotted it from a photo I posted!

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