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Member postings for John Olsen

Here is a list of all the postings John Olsen has made in our forums. Click on a thread name to jump to the thread.

Thread: Anyone good at fault finding with amplifiers here?
18/08/2019 01:34:19

I haven't attempted to fix a direct coupled amp like this for a while now, but anyway...Because they are direct coupled and have DC feedback as well as AC, faults can manifest themselves in subtle ways. With one I worked on, replacing a cooked output transistor just lead to the immediate demise of the replacement. I ended up checking the transistors one by one with the power off to localise the culprit. That was one that had been built from a kitset by a learner, so I had to check everything. You can sometimes check them OK in circuit, but sometimes it is better to take them out so you can be certain.

Other than that I tend to agree with Simon's reasoning up above. Bear in mind that something like a leaky capacitor could put the circuit out of balance enough to possibly take out one of transistors in the early stages, so there may be more than one fault.

Also unplugging the preamp does not do anything with respect to C204 being possibly faulty. If C201 was leaky unplugging the preamp would help.


Thread: What 3 Words
16/08/2019 22:04:30

It's all fun and games until they turn the servers off...

Thread: Myford ML7 - Size of Mandrel Through Drilling?
11/08/2019 04:35:25

Mine, of about 1953 vintage, measures at .590 inch. That's using a digital caliper, maybe + or _ a thou or so on that. It is of course too small, but anything will always end up too small because the jobs expand to exceed the capacity of the lathe.


Thread: Dam Solution?
08/08/2019 23:40:02

Properly built earth dams are actually very good, and there does not appear to be anything wrong with the basic construction of the dam itself here. Earth dams have the advantage of being able to absorb a bit of movement in the underlying material, whether from natural faults of mining subsidence. However they are vulnerable if water is able to overtop them, and the situation with this dam where the spillway concrete had failed would have much the same effect. Water running over the dam is quite capable of eroding the dam quite quickly and catastrophically.

At Oroville, the failed emergency spillway was not built over the dam itself, so there was natural rock under the vanished concrete. There was some concern about the nature of the rock and its ability to withstand the erosion from the overflowing water for very long, and of course if they had not spilled water fast enough the dam might have overtopped at the earth dam itself, which would be bad news. They did have the advantage of being able to pass as much water as possible through the hydro electric plant there. But mainly they had to keep using the emergency spillway despite the damage it was doing to itself and the surroundings.


07/08/2019 05:32:37

Going right back to Neil's original suggestion, I wonder if he has really thought it through? So OK, we know now that a big enough siphon pipe could shift enough water to help...but how does he plan to get the siphon started? I don't think any of us have a thumb big enough to block the end of the immersed pipe, nor would sucking on the bottom end be much fun. So it seems to me that getting the siphon started is going to need a big pump. So since you are going to need a pump anyway, why bother trying to set up a siphon?


04/08/2019 01:10:42

I can't see why anyone should have the slightest difficulty in understanding how a siphon works. Atmospheric pressure can support a column of water about thirty feet high, which is why a pump can only lift water from that far below itself. Really the pump just removes the air from the pipe, and the atmospheric pressure pushes the water up the pipe.

So OK, we want to do without the pump and start a siphon. For the siphon to work, the low end must be below the level of the water in the reservoir. The highest point must not be too high above the reservoir, eg about 30 feet for water, or 29.6 inches if you are siphoning mercury. You can start the siphon by filling the entire length of the tube with the working fluid. With a hose, this can be done by immersing the whole length, then blocking one end and transferring that end to the low point, then unblocking it.

If you were to measure the absolute pressures along the tube, you would find that at the inlet, the pressure was about 14.7 psi. At the highest point it would be less, a minimum of zero psi. So there is a pressure gradient causing the water to flow towards that point. From the high point to the outlet, it is downhill, so the well known tendency of water to flow downhill accounts for the flow there.

Overall, you can calculate the flow on the basis of the difference in height between the two ends and the resistance of the total length of tube. A siphon will not work if there is a vacuum above the fluid in the top reservoir. It will also not work if the whole system is in free fall. You can still use capillary forces in these situations, but that is a different can of worms.

Do not try to siphon petrol by sucking on the hose. You are likely to get lungful of petrol vapour, which can cause your lungs to go into spasm and can be fatal quite quickly. One guy was found lying dead beside the car with the siphon still running onto the ground. The same would apply to any other volatile liquid.


Thread: 316 Stainless
04/08/2019 00:31:53

Stainless steel is a relatively poor conductor of heat. You need to keep the speed down to control the heat build up. This is a particular problem with drilling, where you need to withdraw the drill frequently. It is a good idea to use coolant, even if all you can do is squirt it on from a plastic bottle.


Thread: Myford 7 Capacity Check
02/08/2019 22:37:25

Bear in mind that you can't usually get right to the maximum size since you have to be able to hold the job somehow. So it is generally better to have something with a bit to spare.

However...if it is only going to be one or two parts of a job that won't fit the machine, sometimes it is better to just find someone cooperative to help with those bits. Usually best to make a start on the bits you can do, then when people ask how you are going to do the big bits , say you are hoping that something will turn up. People are more likely to offer to help if they can see you are making a good start. This worked for me with my compound launch engine, where some of the bits were too big for my ML7. Speaking of which, by the time you have a face plate on there is not much gap left on the Myford anyway.


Thread: IC engine tractor conversion query
01/08/2019 05:43:49

Husqvarna do an electric ride on mower, as well as their robotic ones where you don't even need to be there at all. The ride on one is about NZ$7500 which would translate to about 300 to 4000 GBP I guess.


Thread: Which Stuart models can be done on a 7x14 lathe with no mill?
26/07/2019 00:01:35

You are wise to plan a bit of practice work before starting on the good stuff. Like Jason above, I made a 10V on my Unimat...the castings came with the lathe..but before starting I made a simple oscillator for practice. If something like that does not appeal, why not make a few simple accessories for the lathe, like a tailstock die holder, knurling tool, and so on. There are plenty of simple things that will come in handy later, and it is better to make your first mistakes on something easily replaced.


Thread: Pendulum enquiry
24/07/2019 00:42:57

What about a Foucault pendulum? That is the one that changes its plane as the earth rotates. Well, strictly the plane remains the same while the earth rotates under it, so after 24 hours it is back to the plane where it started. The pivot at the top must somehow allow for that, and I am not sure how the driving arrangements work. Quite an interesting device while not actually a clock.


Thread: Crucible for annealing cast iron piston rings
19/07/2019 01:48:50

I made rings for my steam launch engine using this method. The rings were sprung over a cylindrical piece with a key set in it with the right thickness, then the whole lot was enclosed in an airtight container made from stainless steel tube with cast iron end caps, held on by a bolt through the middle. That helps keep the air out, which will reduce scaling. High temperature anti seize compound can be used too. I had to make several sizes, as there are four sizes of ring in the engine, ranging from 1.625" up to 5 inches. (low and high pressure pistons, and low and high pressure piston valves.) Each container was then heated up to a dull red heat and then kept there until I was confident that the heat would have soaked all the way through

Apart from the rings themselves, I don't think the material is critical provided it won't melt on you. The resulting rings are working well so far, I've taken the covers off enough to peer in at the bores and it all looks pretty good. The ring proportions are actually based on IC engine practice. The ones in the valves are apparently happy running over the ports.


Thread: Forging brass; how easy would it be?
10/07/2019 23:40:35

Some years back, I visited a place on a club visit. They were making brass fittings like the ones used with LPG bottles. They had a whole lot of power presses. The pieces of bar stock were being heated up with gas burners, and the press operator would pick one up with the tongs and put it on the lower die. He would hit the pedal, the upper die would come down and go back up, and a nicely shaped piece would then be extracted from the die. This worked well enough to reproduce letters on the finished piece, eg the makers name or whatever. It all looked pretty straightforward. I suspect the main things you would need to be concerned about would be whether the brass you had was a suitable grade, and then getting a suitable temperature, hot enough to soften it but not so hot as to make the brass hot short. I don't think work hardening is a concern if the brass is hot.


Thread: Cross Slide Lock
24/06/2019 07:47:29

I did what GHT described in one of his books. You drill and tap a hole like the existing gib screws use, and fit it with a suitable screw, with the end rounded a bit so it bears nicely on the gib strip. When you tighten it, it pushes the gib strip harder against the slide, which locks it. I've done that to both the cross slide and the top slide. I used allan screws since that is easier than working a spanner in a confined space.


Thread: HSS hire 500 kg folding engine crane
22/06/2019 23:07:14

I have lifted heavy items without a hoist by using a pig sty. This is where you lever up one end and put some packing under it, then lever up the other and repeat. The idea is to build up the packing in small stages, ideally with pieces in each direction crossing to make a stable structure. You don't need any fancy equipment other than a prybar and lots of good packing material, such as off cuts of four by two maybe salvaged from a builders skip. The job is always supported, so as long as you make a good job of the pig sty it should be safe. It helps if you have two people, one to lever it up and the other to put in the next piece of packing. Never use bricks for this sort of thing.


Thread: Large scale hit and miss engine castings
18/06/2019 02:31:20

PM research still do their 1/4 scale Redwing, I picked a set of castings up from their place in upstate New York last year. Of course that might not be big enough.


Thread: Quicksilver
04/06/2019 22:19:54

You might want to check, but last I heard the way of dealing with small spillages of mercury is to scatter flowers of sulphur over the spill and mix it up well. (Flowers of sulphur is finely ground elemental sulphur) The sulphur and mercury react to form mercury sulphide, which is a stable non soluble solid. Not an organic compound so non toxic.


Thread: Myford boring bar help
30/05/2019 22:23:35

Usually the holes would intersect, and one of them would be tapped for a grubscrew. That is how it works on the ones I have inherited and made anyway.


Thread: Adept and Super Adept Register
24/05/2019 04:50:57
Posted by David Standing 1 on 23/05/2019 18:21:40:
Posted by Neil Wyatt on 23/05/2019 17:09:10:
Posted by Brian Oldford on 23/05/2019 11:27:09:



Bugger that, that's what electric motors were invented for! devil

Beats pushing a file though, and more accurate too. Seems a bit spendy though, and missing the actual tool post.


Thread: Interesting??
24/05/2019 03:52:44
Posted by Chris Trice on 24/05/2019 01:33:44:
Posted by Neil Wyatt on 24/05/2019 00:38:16:

Approximately half the population are of below average intelligence.. most people think they are above average...


By definition, "exactly" half the population are below average intelligence. That's the definition of average.

Actually not so, this is only true if the population truly follows a normal distribution. If it does then the median will be the same as the mean. If the distribution is skewed, with say a very few ultra high intelligence people and then a lot of people just below the mean, then you would have more people below average than above. For example, if we had 100 people with an IQ of 99, and one with an IQ of 200, the mean would be 10100/101, or 100. 99 people would be below average and one would be above. That is an extreme somewhat contrived example but many things do not actually really fit a normal distribution, but they are often close enough for practical purposes, especially in large numbers of samples.

The median value is the one where half the population are above and half below. The mean, or average, is the sum of all the scores divided by the number of samples.


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