9 volt battery eliminator hum

[ssnjimb]

Hello

I bought a 9v battery eliminator from eBay.

The adaptor is 500ma and 9 volts.

My R200 works great with it.

My RT7 just hums both when switched on or off

I don't understand why

James

[cathoderay57]

The power supply is likely emitting RF signals outside the range of the R200 domestic radio range but within the RF range of the RT7, which I think is UHF? This is quite likely if the 9v power supply is the switched-mode type. You will either need a different type of power pack or else insert some sort of RF filter in the 9v power lead. The other possibility is that whereas the R200 probably has an electrolytic smoothing capacitor on the 9v supply input the RT7 might not? Cheers, Jerry

[Graham G3ZVT]

Are those Roberts model numbers?

Is the PSU you bought regulated? Could you post its make and model, or a link to it.

[ssnjimb]

Hello

I'm not affiliated with the seller but this is what I bought

https://www.ebay.co.uk/itm/PP9-Batte...s/184133860353

James

[Electronpusher0]

Without getting into discussing e-bay items that looks like a switch mode supply so you are probably getting high frequency switching interference.
Try looping the output lead through a ferrite ring a few times or use a clip on ferrite suppressor.

Peter

[ssnjimb]

What is wrong with eBay ?

I have one of those somewhere I will try it.

James

[ssnjimb]

Originally Posted by ssnjimb

I'm not affiliated with the seller but this is what I bought

https://www.ebay.co.uk/itm/PP9-Batte...s/184133860353

Yes both early Roberts transistor mw lw sets.

James

[AC/HL]

Originally Posted by ssnjimb

What is wrong with eBay ?

Nothing, most of us use it. Please review the forum rules section C.

[Electronpusher0]

Originally Posted by ssnjimb

What is wrong with eBay ?

Check the Forum rules regarding e-bay, Rule 4
"No discussion is allowed about the eBay company (including subsidiaries), the eBay website, eBay auction items or eBay sellers."

Peter

Post Crossed with AC/HL

[See_Mos]

James is not promoting the product so I see no problem here?

OK. It is marked 50Hz so probably a transformer supply and not switch mode. The big clue is how much does it weigh?

If it it quite light then it is switch mode and probably electrically noisy as most cheap power supplies are these days.

On the other hand if it feels relatively heavy for it's size it is probably a transformer supply and the regulation might not be so good, I have one here that is DC output but does not have any smoothing capacitor inside so has a 100Hz hum on the output.

You can confirm this by switching a voltmeter to AC and measuring the output. If it shows a voltage then the output is unsmoothed.

Or you can simply add an electrolytic of anything between 220uf and 1000uF of 16 or 25 volts across the output. Be sure to connect it the correct polarity.

[ssnjimb]

Hii

see pics attached of inside the adaptor

it has a 1000 uF capacitor inside

is apparently 9 volts

James

[AC/HL]

Originally Posted by See_Mos

James is not promoting the product so I see no problem here?

Indeed. A question was asked, and we answered it.

[af024]

This doesn't look like a regulated PSU - just 4 diodes and one cap.

[ssnjimb]

Do you have a reccomended psu ?

Something that will work

James

[ajgriff]

Originally Posted by ssnjimb

My RT7 just hums both when switched on or off.

Presumably it's the unregulated linear PSU that's humming (normal) and the radio is doing nothing. The R200 works so there's an issue with the RT7. Does it still work ok with a battery connected?

Alan

[ssnjimb]

Hi

Yes the RT7 does nothing but hum when switched off and on.

It works fine when a 9v battery is connected

James

[ajgriff]

The RT7 originally used a PP10 power pack with connecting pins, not PP9 clips, so what is the arrangement for connecting power (battery or eliminator) now? A photo might help.

Alan

[Graham G3ZVT]

I have a very similar unregulated PSU powering my Bush TR82.

I expected the on-load voltage to be higher than 9V...

...but that's a little too high for comfort. Mind you it's been like that for 20 years

James, I suggest you measure the voltage across the battery connectors with the set switched on.

Does the set that just hums work OK if you revert back to batteries?

Your supply looks as it has unpopulated holes in the PCB for a 78 series regulator.

[usradcoll1]

Originally Posted by ajgriff

The RT7 originally used a PP10 power pack with connecting pins, not PP9 clips, so what is the arrangement for connecting power (battery or eliminator) now? A photo might help.

That's the older style 50 hz transformer type power supply, which is easier to filter, without the strange characteristics of a switch mode type.
Try installing a extra 1000mfd, 16 cap inside the radio and see if there's an improvement. Battery only radios don't have a smoothing capacitor.
Dave, USradcoll1, still trying.

[ssnjimb]

Hello

Attached are pics of the original battery clips. and the power supply clips

It hums on the power adaptor.

It works fine on a 9v battery (pp9)

Yes i know it ran from the pp10 which is no longer available.

Thankyou for your help so far

James