Here we go I finished the design of a working RF generator nutting out each section.
I'll send the usual PDF to Brad covering part 1 the design
I learnt a lot doing this, RF is a bit different to AF, i'm learning!

Fred.

RF signal generator - Part 1
RF signal generator - Part 2

Document uploaded.

Hi Fred,

On several occasions you have asked readers for critical commentary of your posts. I finally have a critical comment.

Your description of modern plastic switches as "crap" is far too generous.

Hi BBTV, I had to laugh!!!!!!!

Great commentary!!!!!!

Yes, I actually used a rotary switch from Jaycar in something about 10 years ago and the !#$%^ing thing fell apart after about 10 clicks with pieces of broken plastic bits left hanging by the wires.
I put the !#$$%ing thing on the product managers desk on a monday morning and gave him a free "customer complaint" and "product advice" all rolled into one, about what to do.
I do not know if it did any good, but anything I built after that used a circa WW2 bakelite Paton or similar switch rescued from old equipment.
You can run amps of current and 240 volt AC through those things without flinching.

Fred.

Yes, no surprises there Fred.

I bought five of the damn things plus twice as many pretty coloured plastic SPST switches. I have nothing nice to say about them. All but one of the rotary switches fell apart. The pretty SPST switches did not fare much better. I cannot rely on them.

Their terminals will not take solder unless you scrape away whatever the coating is to make them look shiny. I destroyed several just by leaving the iron on a millisecond or two longer, the terminals melt into the body. The ones that survice fail later by sticking intermittently.

When I saw your attenuator resistor network I knew immediately why you chose "real rotary switches". The extra heat required to solder multiple resistors would have destroyed the joke switches.

There is a proximity effect with some of those switches & they flash over. RF is used in TIG welders to carry the arc over. I know of one accident in a fuse box in a transmitter hall where RF caused a flashover aided by a mere screwdriver.

Some Digital multimeters will flashover in the chip, with RF & die. A DC switch is entirely different to AC thats why you see the lower DC rating. DC will often follow the switch contacts and flash over.

Previous post: AVO 7X in normal useage should not flashover, under 3KV.

Marc

Marc,

Rest assured the switches I refer to will never be exposed to anything beyond a 9 Volt battery. And even then they fail. They stick and stay ON, they stick and stay OFF. You have to toggle the damn things 8X a second before they free up. God knows what type of crap the contacts are made from, but hell will freeze over before I ever allow them anywhere near 'real electricity'.

...where RF caused a flashover aided by a mere screwdriver.

This is the unfortunate thing about electricity. I've seen real electricity (240/415) do the same thing and the result is not pleasant.

I was working at a hospital in 1993 as my apprenticeship was drawing to a close and someone came by to commission a new power factor correction unit. He was not an electrician but decided to try and be one during the commissioning of the machine. The fuse block would not close correctly (or the technician was trying to close it the wrong way) so he grabbed a screwdriver and gave the fuse block a bit of persuasion and this was the last thing he did before spending a few weeks in the burns unit at Concord Hospital.

Third degree burns to 30% of a human body is considered automatically fatal and he was up to around 50%. That was the first workplace fatality associated with anywhere I have worked though I have seen quite a few workplace accidents involving various scenarios. I guess that all it takes to avoid an accident is a bit of planning and thought before diving in at the deep end.

As a guide to what type of switch to use with mains power, I take notice of two things - the rating on the switch - natuarally this will be 250V or higher and as a guide to quality I only use switches that have spade lugs on them. The smaller ones which have solder tags I won't bother with due to the issues mentioned above where they just drop their guts without warning. I've had that happen and to me, earthing the body of the switch isn't enough to inspire confidence.

Well I have got the RF generator to a working point.
All systems are go, so I halted the build short of final dress up and will swap back to bringing the uV meter to the same stage.
I met a few challenges on the way particularly to do with the oscillator coil.
That item needs a lot more work on, but I have stopped at the point where the coil works ok.
On reflection I really should have used a ferrite core former and wound on a secondary winding as well to couple the signal to the following valve. Thats the sort of thing you find out as you develop a design.
I will send the usual PDF for part 2 to Brad to attach to the thread under the first part.

Fred.

Coupling oscillators can be tricky and that also applies to some signal generators. Just coupling, can pull the oscillator off frequency. With one crystal locked one I built using a dual gate FET as a modulator. It used a Quad NAND gate; Two as oscillator and one as the buffer amp.

Marc

Document uploaded.

A good job as always, Fred. I admire the effort and time you take to build these projects.

Thanks Brad it is quite absorbing makes valves do basic things.
I hope showing the rough and tumble of building something from scratch encourages others to give it a go as well.

Come on you radio guys, how about a few more "little generals" or some more rebuilds of existing chassis?
Or even a scratch build of an instrument, meter or some such.
Its easy to document the process and take a few photos and make up a story.
I find it fascinating what other enthusiests do.
Fred.

Something I've wanted to try for years but now working full time in a design role with cutting edge stuff....

Just doing a retrospective - something that could have been done back in the day but no-one ever did.

I'll probably never get the chance.

A one valve superhet using a 6Y9 dual pentode. Arrangement thus:

Small pentode - autodyne converter.
Power pentode - reflexed IF and audio output.
It's permitted to use back to back diodes to implement AGC.but no fets or transistors.

Or

A 2 valve battery portable using a 1A7GT and a 1D8GT.battery triode-pentode. Triode either neutralised and reflexed IF or a regen stage (too easy)
Would have made a pocket radio possible before the 7 pin battery valves appeared.

Hi Ian, the one valve superhet sounds like a challenge.hmmmmmmmmmm.

I might give that a go after I finish the latest project, the mechanical sweeper unit to attach to the RF signal generator.

I'll direct drive a spinning tuning gang with a permag motor.
Should be a piece of cake, what could go wrong?
Already under way, have to machine up a coupling to link the motor and a gang and trial that first.

Fred.

I like your wobbulator idea,

Maybe somehow add a ball race to the other end to replace the thrust ball, which I suspect would wear through the brass quickly. Depends on how long you want it to last!

I recall an AWA wobbulator that used a large ferrite slug, spring loaded with a make-break contact and energising coil, it would work a bit like some types of battery electric clocks and oscillate backwards and forwards with a mechanically linked slug tuning a coil,