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PostPosted: 17 Jul 2019, 15:24 
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ILoveHiFi wrote:
I have no idea, but I think maybe you got something else wrong or you've connected something the wrong way.

Unless your being silly delibratley, otherwise I've also never seen transitors break down from excessive base emitter voltage


I have seen a lot of strange things over the years.
In years gone by the AF117 transistor would stop working and when you hit it gently with a screwdriver tip it would start working again.
We didn't know why at that time but since then tin whiskers were found:
https://nepp.nasa.gov/whisker/anecdote/ ... istors.pdf

So I have seen some very strange things in the past.
Another example is graphite build up on I/O control cards on PLC based equipment causing I/O errors.

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M. Gregg

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PostPosted: 17 Jul 2019, 21:09 
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Capacitors have burn in periods and will sound better.

But I always thought transitors are like a dead thing.
Though I can here some diffrence from transitor burn in, but I think majority belive transitors do not burn in.

Growing wiskers inside the transitor, now I'm starting to think of them as more like capacitors and have burn in periods


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PostPosted: 18 Jul 2019, 13:31 
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ILoveHiFi wrote:
Capacitors have burn in periods and will sound better.

But I always thought transitors are like a dead thing.
Though I can here some diffrence from transitor burn in, but I think majority belive transitors do not burn in.

Growing wiskers inside the transitor, now I'm starting to think of them as more like capacitors and have burn in periods


A long time ago as an engineering apprentice I built a digital clock as shown in practical wireless magazine, it used an IC and transistors to drive a seven segment display all on photo copied circuit boards. I powered it up and it worked for a day and a half then the next morning the display was blank (not illuminated).

After hours of fault finding thinking I must have done something wrong, I took it to work and asked a senior electronics Tech to check it out. He said the circuit is fine no problem. I sent the IC back and got a replacement, I had no faith in the circuit I must be doing something wrong. I powered it up and could not believe a week later it was still working. The Tech at work said, " the infancy of electronics is very short if it works for a day it should last a week, if it lasts a week it should last the month, if it lasts a month it will probably last the life of the components. That clock is 45 years old and still works, I never changed or re-soldered any parts only plugged in the new IC.

Solid state electronics have improved beyond imagination and reliability with it. In large manufacturing companies building complex drive systems etc they "soak test" the equipment (power it up and run it on a test bench for a time period). Its sometimes referred to as burn in. NB transistors and IC's were graded in the past some regarded as military grade and some industrial and the domestic after market. I'm not sure how they test "hfe" in production.
A quick google shows:
https://www.quora.com/What-is-the-diffe ... grade-ones


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M. Gregg

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Last edited by M. Gregg on 18 Jul 2019, 14:01, edited 3 times in total.

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PostPosted: 18 Jul 2019, 13:35 
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OK,

I can't make the circuit fail with the new transistor, however I don't like the idea of the charged capacitor on the base of the PNP transistor so I have fitted a safety reverse biased diode across the junction.
(shown in the Pic)
In theory the existing discharge diode should stop any voltage across the junction, but the added 1n4148 is direct across the junction and may not ever need to work. (but..)

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M. Gregg


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PostPosted: 18 Jul 2019, 16:25 
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I do have very old transitors on hand from 1950-1970s, but not massive fan of them

Maybe its common back in the days for faulty transitors, but faulty ones now seem to be rare.


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PostPosted: 18 Jul 2019, 16:42 
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I have kept looking at the circuit,

:|
What happens if the circuit is turned on and then you change the bias pot to zero ohms = 1.25v low output LM317.
The output capacitor will slug the discharge.
Maybe maybe not..

If the time capacitor is fully charged the pot will short the emitter to ground with 12V on the capacitor connected to the base.
The 1n4148 will protect the junction.

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M. Gregg

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PostPosted: 19 Jul 2019, 04:58 
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I have zero exprience with protecting transitors from be overvoltage, not going to comment on that.
And to annalize the current circuit I think you need to know the lm317 internals. Which I only know briefly.

I can fully master the internals if I go into it, but its more consturtive for me to design in other areas.


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PostPosted: 19 Jul 2019, 08:26 
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Just for interest,

BC557 Vebo = 5V
Anyway I though it quite interesting, time to start mains wiring and connecting the two transformers primary.


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M. Gregg


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PostPosted: 19 Jul 2019, 13:18 
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Looking at my mains wiring,

I need to fasten the mains two core in a straight run across the brass support posts.
I reached for the ty-raps (cuz its quick and easy).
But it kinked the cable (not straight).
I remembered years ago learning to use lacing to make harnesses, but I couldn't remember how to do it.
I remember the lacing cord being waxed, and found some thin ribbon (so it doesn't cut into the cable).
Then pulled it across a piece of wax. (yes its sticky and won't slide).
But how to re-learn it, nice video here:
https://youtu.be/oBG30s3PUKw

now to have a go.. :D
It would be nice to not use ty-raps.
Lose the plastic, and its nice to look at...save the planet...LOL

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M. Gregg

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PostPosted: 20 Jul 2019, 07:39 
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Mains in,

Mains input to power switch, fuse and ground/Earth for chassis.

Mains two core to switch laced in to support posts.

Next is transient suppression and inrush limiter.
The slow build continues. :D


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M. Gregg


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