Regentone ARG209 output transformer

[stuart_morgan_64]

Advice please, I have no output, and no HT beyond C50, the primary of the output tx is reading around 1.5M from centre to outside. I assume it should be in the K range?

Is this part critical, would another tx from a similar radio work? Or what would I search online for.

Thanks

[G6Tanuki]

That's a classic "hum-bucking" output transformer schematic: the transformer's not anything special.

You could replace it with a conventional non-tapped transformer - something like this - https://uk.rs-online.com/web/p/audio...rmers/2106475/ connecting the end of the resistor that currently goes to the output-transformer instead to the top of C50, and if hum is a problem increasing the value of C49 to compensate.

[ms660]

I would have thought that increasing the value of the reservoir capacitor would have a better effect of reducing ripple hum caused by the output stage if fitting a standard output transformer.

Lawrence.

[Silicon]

Which part of the primary is open circuit?

If it is the Anode winding you will need another transformer (or a rewind).

If it is the 'hum-bucking winding, you could modify the circuit. Add another resistor and smoothing capacitor between C50 and R27. Something like a 1K Ohm 2W resistor and 47uF 350V capacitor.

[G6Tanuki]

Originally Posted by ms660

I would have thought that increasing the value of the reservoir capacitor would have a better effect of reducing ripple hum caused by the output stage if fitting a standard output transformer.

Lawrence.

Yes but you need to remember to stay within the maximum specified reservoir capacitance of the rectifier to limit the peak inrush current to whatever the rectifier-manufacturer has defined.

The output-stage is relatively-immune to HT-line hum (largely because there's no gain downstream of it!); the RF/IF/low-level-audio stages less so.

For the low-level circuits supplied by the smoothing-resistor - you can throw hundreds of Microfarads around with impunity because the smoothing resistor moderates the current-peaks that the rectifier has to handle.

[ms660]

So far as I'm aware the hum cancelling winding is for cancelling the ripple that's developed due to the current being passed by the output valve, I don't think it cancels any hum induced in previous stages, the extra bit of winding on the transformer is small and any choking effect of that extra bit of winding on the HT supplied via it to the earlier stages is minimal so far as I know.

I always refer to the extract below from my copy of Radio And Television Engineers Reference Book for info on that circuit configuration.

Lawrence.

[Silicon]

When they talk about the 'AC due to hum' I think they mean ripple voltage and current.
This would normally pass through the output transformer and appear on the speaker winding.

The inevitable ripple on the anode current is cancelled by the smaller ripple current going to the earlier stages of the radio. The transformer ratios takes care of the magnitude and phase so that they cancel out when they get to the speaker winding.

My suggested alteration would reduce the HT ripple. If the primary of the output transformer was connected to the additional capacitor, there would be less hum appearing at the speaker.

[stuart_morgan_64]

Well some good news, just come back to it, found a scrap chassis in my junk box, thanks for the info too.

[Herald1360]

Pentode/tetrode output stages are relatively immune to to HT ripple voltage because they act more like current sources than voltage sources (high Ra) so the current through the O/P TX primary is controlled far more by the grid voltage of the valve than by the anode voltage. Triodes with their relatively low Ra are affected much more by HT ripple.

The hum cancelling output transformer probably cost less than better HT filtering would have when the design was found to be bit a bit "hummy" in development.

[stuart_morgan_64]

Thank you for all the info all now working.