Questions on 160/80 metre AM transmitters.

[AdrianH]

I have the circuit here for the AT5, but I am confused over the circuit around V1 the VFO

For the EF80 as shown, I can see that g2 (pin 8) is connected with R2 and C3
g1 (pin 2) is connected to R1, C2 and L1.Anode (Pin 7) connected to R3 and C6..

But the squiggle that shows where C4 and C5 go is unclear. I am guessing that these are connected to pin 8 the g2, so effectively connected to R2 and C3.

Also that g3 is ls strapped to the cathode.

Can anyone verify this is correct, or what things should be connected to.

Lastly am I correct in thinking that the oscillator is running with the g2 as the anode as it were and that the true anode is giving a buffered output to V2, not sure what this modified Vackar VFO is?

Adrian

[frsimen]

There is a photo of the underside of the AT5 chassis on the G4AQB website.

https://g4aqb.files.wordpress.com/20...g?w=1024&h=768

That photo seems to confirm that your understanding is correct.

I think V1 is configured as an electron coupled Vackar. Exactly what the modified bit is, I don't know. The G3PDM receiver used a Vackar circuit with what was described as the original capacitance ratios. This circuit is doesn't use the same ratios, so that could be the modification. I'm sure that someone on the forum will have a more definitive answer.

Paula

[AdrianH]

Thanks I will have a look.

Adrian.

[mickm3for]

Hi a better circuit checked with my at5 Mick

[AdrianH]

Thanks Mick, that is a lot better circuit.

Adrian

[Radio Wrangler]

Electron-coupled amplifier stage (sometimes called 'transitron'). Notice that neither the anode nor the cathode are coupled into the tuned circuit. The tuned circuit is a pi-section which could be as per Vackar, depending on value ratios.

It's quite a neat circuit, getting a degree of isolation to the output more than a non-transitron circuit would give.

The amateur radio world gets a lot too fixated on applying names to oscillator circuits. It's better to look at them and visualise their operation. Also, ask yourself about the usual elephant in the room; what controls the amplitude?

David

[G3VKM_Roger]

Originally Posted by mickm3for

Hi a better circuit checked with my at5 Mick

The cct that Mick has posted was taken from a Short Wave Magazine article. On the copy I have there is a hand-written comment that R1 was changed from 100K to 47K, to improve oscillator starting.

73

Roger/G3VKM

[Terry_VK5TM]

The full text relating to the 47k resistor mod:

Some early AT5s had oscillator starting problems, and the value of the grid resistor, R1, was reduced from 100k to 47k to fix this problem. The resistor, R1, is inside the oscillator coil screening can, and quite hard to change. The easiest option is to put another 100k resistor from the grid to ground on the valve socket to reduce the value to 50k if this has not already been done. The factory fitted 47k resistors in the oscillator assemblies once this problem was identified, so your AT5 may already have the right value of R1. A simple resistance check from pin2 of the valve socket to ground should confirm the value of R1, if you are in any doubt.

Dave Gordon-Smith, G3UUR

[AdrianH]

If someone can let me know which Short wave mag date it is I will try to find it.

The project is going very slow mainly due to other things I have on order that seem to be taking weeks for RM to deliver.

Adrian

[Station X]

Please let's not turn this thread into a discussion about strikes at Royal Mail.

[ms660]

Test report, schematic etc, Codar AT-5, Short Wave Mag. August 1965:

http://www.g4kfk.co.uk/Codar_AT-5_Tr...SWM_196508.pdf

Plus some mod's:

http://www.g4kfk.co.uk/Codar_AT-5_Mo...s_by_G3UUR.pdf

Lawrence.

[mickm3for]

Hi link https://worldradiohistory.com/UK/Sho...WM-1965-08.pdf mick

[AdrianH]

Sorry for mentioning RM, I will consider my knuckles rapped.

Thanks for the links to the magazine and articles.

Adrian

[Bazz4CQJ]

In the 1965 SWM review on page 337, there's a comment that on 80m, the 12W from the AT5 invariably got through OK on contacts which had been kicked off with a 100W Tx, which is telling!

How critical would the matching be of the centre-tapped winding on the transformer used for the modulator? I'm wondering about whether the transformer from a Pye Ranger could be used, using the centre tapped winding originally used by the push-pull EL90's in the Ranger modulator ?

B

[G6Tanuki]

The matching is 'good enough' with pretty much any 1:1 audio transformer. It assumes that the average DC current through the PA is something close to the average DC in the 6BW6 audio-output stage - get around 45mA flowing in them both and the matching is good-enough, and the magnetic fields from the static DC component balance-out in the transformer same as they would in a push-pull audio amp.

Remember we're not talking hi-fi here!

In times-past I did something very similar with a transmitter using an AUY10 as the RF amp and a NKT404 as the audio amp.

Coupled together using a 12-0-12V winding on a little power-transformer, with a neon across the 240V primary to indicate modulation. Yes - Germanium PA stage and class-A modulator! Both nice TO3 cased transistors, whole thing put out about 10W for 3A current draw from a 12V battery. [will remain quiet about the frequency...]

[Bazz4CQJ]

Thanks for that. Somewhere I have three of those oil-filled "bean-can transformers" from early Rangers. They are physically a bit bigger than the small transformer in the original AT5, but seems like they might be OK for a home-brew version. There's a third winding on them, which I think was intended to provide feedback in to the AF pre-amp to provide level-control? I've never seen the circuit for them.

B

[CambridgeWorks]

Bazz, I don't think they were oil filled. Just potted, maybe resin or just shellac coated windings inside the can. A bit like the W30AM ht inverter transformer in a half "coke can" size.
You are correct, the third winding was used for feedback to the af compressor circuit.
Rob

[Terry_VK5TM]

I can't remember where I found this, but I have a note in one of the pdf's re the modulation transformer:

AT5 mod transformer is NOT centre tapped.
It is 0-110-240v, the 0-110 is the modulator load, the 110-240V wdg goes to the TX

A 15VA centre tap transformer works as well as the real thing

Also note the original parts list shows the mod transformer as an auto transformer, and the description of the circuit calls it a tapped auto-transformer.

[Bazz4CQJ]

Re the Ranger transformer, interesting. I think I have one of the half-coke size transformers too, don't think I knew it was an inverter transformer.

Seems though the original transformer was a slightly unusual component(?), but it did not make any special contribution.

[AdrianH]

Another question on the AT5,

Looking at the circuit at the coil and cap on the output of the second EF80, described as doubler for the the 3.5 MHz band.

I have always thought of a coil and capacitor in series as a rejector circuit, in that it would appear as low impedance at it's resonant frequency, in which case if it was tuned at 3.5 MHz it would shunt output to ground, not double it, so wondering if the coil and cap is broadly tuned for 1.9 MHz, or have I got it wrong again?

Adrian