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Old 11th Oct 2017, 11:57 am   #41
Herald1360
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Default Re: The perils of homebrewing equipment

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Another minor lack of success last night actually. Decided to go for attempt two of a dummy load for RF purposes. All I want is 5W or so, nothing spectacular. So I'm using 20x 0.6W 1K resistors in parallel. Fudged the mathematics and put 9W through them off the bench supply. They desoldered themselves and fell off the BNC connector

Need a better mounting strategy. Seriously just considering buying an expensive TO-220 packaged 50W resistor and mounting that to the box.

Surely 20 * 0.6W is 12W? Why did they fall off at 9W? Bunching effect (like cores in a cable)?
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Old 11th Oct 2017, 12:07 pm   #42
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Default Re: The perils of homebrewing equipment

I built two nearly identical curve tracers in the 1980'2, one from pnp transistors and one from npn transistors. I think it was a Elecktor design. They both worked, I needed the two as the pnp version only tested pnp and the npn version only tested npn. I played wit them for about a week and then put them away for future use, I think they are still in the shed.

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Old 11th Oct 2017, 12:20 pm   #43
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Default Re: The perils of homebrewing equipment

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I played with them for about a week and then put them away for future use, I think they are still in the shed.
That's probably where mine would end up if it worked Dave!
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Old 11th Oct 2017, 12:28 pm   #44
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Default Re: The perils of homebrewing equipment

Here are the other two pages of the original article - not very good copies I'm afraid.
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Old 11th Oct 2017, 12:55 pm   #45
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Default Re: The perils of homebrewing equipment

I think this is the curve tracer I made.

Dave

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Old 11th Oct 2017, 1:39 pm   #46
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Default Re: The perils of homebrewing equipment

Oh dear - don't tempt me!

Noted the 'TUP - TUN - DUG' in the specified parts.

Elektor 'TUPTUNDUGDUS' - t'was once, but is no more.

'Transistor PNP Universal, Transistor NPN Universal; Diode Universal Germanium, Diode Universal Silicon'.

Back then, nothing was sprinkled with 'fairy dust' or endowed with magical qualities.

'It's an NPN transistor innit? Well bung it in then'.

Happy days!
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Old 11th Oct 2017, 2:14 pm   #47
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Default Re: The perils of homebrewing equipment

I think a transistor curve tracer is well worth having - you should definitely persevere with it.

Mine is an old Heathkit that works with an x-y scope and it is really useful for testing and matching transistors.
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Old 11th Oct 2017, 2:24 pm   #48
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Default Re: The perils of homebrewing equipment

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Surely 20 * 0.6W is 12W? Why did they fall off at 9W? Bunching effect (like cores in a cable)?
Sorry should have been more clear above. They were 0.25W not 0.6W. This is where I fudged the maths by calculating for 0.6W as I normally buy them. Ran them at 3/4 power (9W) instead of about 4W.
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Old 11th Oct 2017, 4:49 pm   #49
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Default Re: The perils of homebrewing equipment

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you can take future maintenance into consideration: component selection / availability and physical accessibility.
.... theoretically.

I've opened an item of test equipment that looked homebuilt and commented "hmmm, nicely made...." and took a few minutes to realise that I'd made it! Circuit diagram? Board layout? Not a chance!

In my defence it was over 10 years ago but............
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Old 11th Oct 2017, 4:51 pm   #50
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Default Re: The perils of homebrewing equipment

Ah yes, the good old programmable uni-junction transistor. These seemed to become popular in the eighties in home construction but I haven't seen or heard much of them since. It would appear that I am very like you David. I am an obsessive constructor. Not just electronics but all sorts of things. I've always been like that. Right from early days of making flying models and all the rest of it. I think the posh word for it is "creative"! I have projects dating back at least twenty years, and sometimes when rummaging about for other things, I find a box full of interesting bits which is a complete kit for a half finished something that I have forgotten about. So out it comes and I spend too much time finishing it off. Then the novelty wears off and back it goes on a shelf. I think one attraction of doing this is that you sort of enter a time warp, as it takes you back. I am sure that there is a medical explanation for this but who cares? It's who I am and I am very happy with that.
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Old 11th Oct 2017, 7:10 pm   #51
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Default Re: The perils of homebrewing equipment

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Oh dear - don't tempt me!

Noted the 'TUP - TUN - DUG' in the specified parts.

Elektor 'TUPTUNDUGDUS' - t'was once, but is no more.

'Transistor PNP Universal, Transistor NPN Universal; Diode Universal Germanium, Diode Universal Silicon'.

Back then, nothing was sprinkled with 'fairy dust' or endowed with magical qualities.

'It's an NPN transistor innit? Well bung it in then'.

Happy days!

I always loved Elektor for the TUP-TUN-DUG-DUS simplicity of their designs.

"Nothing special needed here - use what you've got!"
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Old 12th Oct 2017, 3:06 am   #52
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Default Re: The perils of homebrewing equipment

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Seriously just considering buying an expensive TO-220 packaged 50W resistor and mounting that to the box.
I just bought 2 250w 50R resistors off Ebay for the equivalent of ~1 each, postage included.

They are used, but still perfectly serviceable.

Terry
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Old 12th Oct 2017, 6:44 am   #53
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Default Re: The perils of homebrewing equipment

Will have a look on there. That’s certainly less than the 11 I was going to spend on a caddock MP850
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Old 12th Oct 2017, 7:57 am   #54
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Default Re: The perils of homebrewing equipment

Ah, David, sussed what you are doing. You are trying to get us all to debug your curve tracer!
If we all build one and only one works, what will that signify?
But if they all work, you are going to have to make all the lovely wooden boxes for us all, that's only fair.
Does look to be a clever design though, too clever for its own good?
Is there a working example in the cosmos?

Home brews suffers from lack of factory air inside, difficult to capture.
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Old 12th Oct 2017, 4:07 pm   #55
David G4EBT
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Default Re: The perils of homebrewing equipment

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Ah, David, sussed what you are doing. You are trying to get us all to debug your curve tracer
If we all build one and only one works, what will that signify?
But if they all work, you are going to have to make all the lovely wooden boxes for us all, that's only fair.
Does look to be a clever design though, too clever for its own good?
Is there a working example in the cosmos?
Well if anyone did build one and got it to work, I'd happily hand craft a comb-jointed box for them if they could get mine to work Sam!

As to the design being 'too clever for its own good' I have to entertain the possibility that maybe I'm too dim for my own good!

I suppose there must have been at least one working model in the cosmos - that of the author's.
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Old 12th Oct 2017, 4:33 pm   #56
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Default Re: The perils of homebrewing equipment

That's the danger. Someone builds something and it works so they think it's OK to publish.

Full-blooded professional design has to account for the full tolerance spreads of all components, and has to make some hard-nosed estimates of those parameters which are never graced with actual specs. Provided all units are built from in-spec components, all units ever built had better work or the designer gets it.

Some amateur designers do take this level of care, and some professional designers don't.

There are many irreproducible designs in various magazines (Elector seems to be one of the safest) And a few overrun resistors seems to be the hallmark of any Quad amplifier.

Sometimes, to read a magazine article and to be able to decide whether it's OK to build takes as much knowledge as designing your own thing properly in the first place.

It's survivable for hardened constructors, but it can damage the confidence of beginners.

David
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Old 12th Oct 2017, 4:35 pm   #57
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Default Re: The perils of homebrewing equipment

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Home brews suffers from lack of factory air inside, difficult to capture.
Clearly what we have all been forgetting is to inject the correct amount of 'magic smoke' into our homebrew projects to make them work! Too little and the thing does nothing, too much and it'll vent out of somewhere!

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Old 12th Oct 2017, 8:34 pm   #58
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Default Re: The perils of homebrewing equipment

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...As a consequence, I have steadily accumulated homebrew projects which look nice on the shelf, but don't do anything at all. Occasionally, people have said 'that looks really neat - 'what does it do?' to which I can only reply 'It doesn't do anything - it's a dust trap'.


I really enjoyed David's post. I really ought to be a lot further on with project 'if you can meet with triumph or disaster, and treat those two impostors just the same', at my age. On a good day, homebrew projects help with one's humility - one way or another
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Old 13th Oct 2017, 9:21 am   #59
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Default Re: The perils of homebrewing equipment

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Surely 20 * 0.6W is 12W? Why did they fall off at 9W? Bunching effect (like cores in a cable)?
Sorry should have been more clear above. They were 0.25W not 0.6W. This is where I fudged the maths by calculating for 0.6W as I normally buy them. Ran them at 3/4 power (9W) instead of about 4W.
That has a familiar feel to it. I still refer to MRS25 and the like MF resistors as quarter watt types. They're about the same size as the TR4 MOF ones I cut my professional teeth on. ISTR something about their commercial rating being 0.6W and industrial rating being about 0.25W. Modern 0.25W MF jobs are fiddly little things like old style eighth watt CF or CC types.
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Old 13th Oct 2017, 12:36 pm   #60
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Default Re: The perils of homebrewing equipment

I usually use Vishay MRS25's which was actually the problem. Thing is they've got very expensive suddenly (0.08-0.12 each) so I switched to Royal Ohm imports from Thailand (0.008 each) as most of my requirements are pretty modest with respect to power. I've now got an influx of 0.25W ones which are the same size. The Royal Ohm ones are really nice, with decent tempco and nice thick legs. But only if you don't smoke 'em
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