Re: Amplifier transistor substitution.

[Cursitor Doom]

On Sun, 19 Mar 2017 16:34:43 +1100, Sylvia Else wrote:

> On 18/03/2017 8:27 PM, Sylvia Else wrote:
>> 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.
>>
>> https://www.dropbox.com/s/9053dnyr0izv88r/XAMP.PDF?dl=0
>>
>> 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...

[jf...@my-deja.com]

On Sunday, March 19, 2017 at 7:16:33 AM UTC-7, Cursitor Doom wrote:
> On Sun, 19 Mar 2017 16:34:43 +1100, Sylvia Else wrote:
>
> > On 18/03/2017 8:27 PM, Sylvia Else wrote:
> >> 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.
> >>
> >> https://www.dropbox.com/s/9053dnyr0izv88r/XAMP.PDF?dl=0
> >>
> >> This is because T711 has shorted out.
> >>
> >> Not very surprisingly I cannot source a replacement BF472, so I need to
> >> find a substitute.
> >>
> >>

I would try a MJE350(pnp/300V/500ma/20w/TO-126/hFE=30-240). Anchor Electronics http://anchor-electronics.com/ wants US$0.95 each.

[Phil Allison]

jf...@my-deja.com wrote:


> > >>
> > >> Not very surprisingly I cannot source a replacement BF472, so I need to
> > >> find a substitute.
> > >>
> > >>
> I would try a MJE350(pnp/300V/500ma/20w/TO-126/hFE=30-240).
> Anchor Electronics http://anchor-electronics.com/ wants US$0.95 each.

** 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

[Phil Allison]

Phil Allison wrote:
>
>
> ** I have already pointed out that Sydney's biggest spare parts

> dealer( WES Components) has the BF470 in stock for under $1.
>

** They have the BF472 as well, same price

Sylvia lives in Sydney.




..... Phil

[jf...@my-deja.com]

On Sunday, March 19, 2017 at 6:56:07 PM UTC-7, Phil Allison wrote:
> Sylvia lives in Sydney.
> ..... Phil

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.

[Trevor Wilson]

**I am VERY wary about buying anything from WES now. From their shitty
electrolytic caps (Jamicon - horrible things, one batch of 100 I
purchased had a 30% failure rate - I had to recall half a dozen units I
serviced as a precaution) to counterfeit semiconductors, I've been
caught several times. As have some of my colleagues. I readily admit
that small, cheap devices are likely to be fine though. I purchased some
large, Japanese flatpack (two screw mounting types) from WES last year.
Same type as the ones in the amp. It failed during bench testing.
Subsequent tests on replacement devices revealed a breakdown Voltage
(VCEO) of around 90 Volts. The original devices are specc'd at 230
Volts. I ended up using single screw mounted devices (same ratings),
obtained from an accredited supplier. No problems. A mate did battle
with a TV set flyback MOSFET a few years back. After many hours chasing
his tail, I suggested he stop buying WES supplied parts and go to
Element14 for the MOSFET (less than $5.00). Problem solved. WES were
kind enough to refund his money for the dodgy MOSFETs, but not the
considerable amount of wasted time.

Never again. All my electros are sourced from Element14 or RS
Components. Panasonic is my favourite brand. Phenomenally low failure
rate and well priced. That free delivery from RS is very nice. My spend
with WES has fallen dramatically.


--
Trevor Wilson
www.rageaudio.com.au

[ohg...@gmail.com]

I learned the hard way about counterfeit semis decades ago. Home stereo amps that would blow up in a week after repair, horiz/HV output transistors shorting on power up, hybrid smps ICs that just didn't work or would cause severe oscillation, vertical output ICs running hot and failing in days, PTV convergence hybrid modules that defied alignment...

Here in the U.S., the only suppliers I trust are DigiKey, Mouser, and B&D Enterprises. I'm sometimes forced to buy parts through AliExpress. These are carefully tested. If one is bad, the batch gets scrapped.

[Trevor Wilson]

**Phil was the first person to alert me to counterfeit semis here in
Australia.



--
Trevor Wilson
www.rageaudio.com.au

[Phil Allison]

Trevor Wilson wrote:

>
>
> **I am VERY wary about buying anything from WES now. From their shitty
> electrolytic caps (Jamicon - horrible things, one batch of 100 I
> purchased had a 30% failure rate - I had to recall half a dozen units I
> serviced as a precaution)

** I must have bought hundreds of Jamicon electro fromn WES over the last two decades- never had a single faulty one.

> to counterfeit semiconductors, I've been
> caught several times. As have some of my colleagues.

** WES had a lot of fake TO3 devices - popular ones like MJ15003 and MJ15024 in their stocks. The problem was they were reluctant to admit it or remove them from stock.

The typical attitude of such businesses is " the customer is always wrong ".

> I purchased some
> large, Japanese flatpack (two screw mounting types) from WES last year.
> Same type as the ones in the amp. It failed during bench testing.
> Subsequent tests on replacement devices revealed a breakdown Voltage
> (VCEO) of around 90 Volts. The original devices are specc'd at 230
> Volts.
>

** That sounds like the big Sanken ones, eg 2SC2922 and 2SC3264 etc.

Never had a bad one of them from WES either.

Recently acquired stock might be different.



... Phil

[Phil Allison]

Trevor Wilson wrote:
>
>
>
> **Phil was the first person to alert me to counterfeit semis here in
> Australia.
>

** My god, when was that?

I have been seeing fake MJs since 1980.


.... Phil

[Trevor Wilson]

**A long time ago. I couldn't say when. I buy all my non-Jap semis from
Element14 or RS and have done for a long time. No problems with those guys.


--
Trevor Wilson
www.rageaudio.com.au

[Phil Allison]

** I bought MJs and Hitachi TO3 mosfets from authorised dealers long before the likes of Farnell & RS arrived here.


.... Phil

[John Robertson]

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

http://www.diyaudio.com/forums/solid-state/269984-bf471-472-good-replacement-2sa1381-2sc3503.html

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."

[Cursitor Doom]

On Tue, 21 Mar 2017 08:34:08 -0700, John Robertson wrote:

> You may be able to find your BF472s sourced from India:
>
> http://www.diyaudio.com/forums/solid-state/269984-bf471-472-good-
replacement-2sa1381-2sc3503.html
>
> 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!

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

>
> 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!

Oh yes, see above!

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

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!