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Old 24th Jan 2020, 9:46 pm   #41
ajgriff
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Default Re: 9 volt battery eliminator hum

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Originally Posted by HamishBoxer View Post
Maybe the power supply is not rated high enough current wise for the other set??
The PSU is rated at 500mA which should be fine for the RT7 plus a wide margin. Personally I think the eliminator isn't the problem here and there's some obvious explanation that somehow we're all failing to see. Surely significant capacitor failure would drag down a battery's voltage level to a point where the set wouldn't work.

Alan
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Old 24th Jan 2020, 10:18 pm   #42
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Default Re: 9 volt battery eliminator hum

I suspect Alan is right. It's too late now, if the PSU has been returned, but the next steps I would have recommended are on-load voltage tests at the PSU terminals to see if there was an over-voltage or under-voltage issue. It would also have been informative to measure current drain particularly with the RT7 which seems to be making sounds when switched off. Cheers, Jerry
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Old 24th Jan 2020, 10:20 pm   #43
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Default Re: 9 volt battery eliminator hum

I measured the voltage and it was 19 no load. I will check the loaded voltage in the morning. I have yet to send it back fortunately

James
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Old 24th Jan 2020, 10:51 pm   #44
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Default Re: 9 volt battery eliminator hum

19V seems high for the no load voltage. 12-13V as indicated in Graham's post #18 would be more of the order I'd expect. Interesting that the R500 doesn't seem to mind. Looking forward to finding out what happens under load.

Alan
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Old 25th Jan 2020, 12:21 am   #45
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Default Re: 9 volt battery eliminator hum

Notwithstanding everything else that's been said, it's my experience that most (older?) transistor radios don't work at all well on basic unregulated dc power supplies. Adding extra C may help but the only really effective solution is to add a regulator circuit. There's usually enough headroom on a lightly loaded unregulated supply for the regulator's I/O differential voltage requirements especially if you have a 9V nominal psu and use an 8V output regulator. (8V is well above the end of life voltage of a 9V battery). The jelly bean variable LM317 type with a couple of resistors and capacitors is just fine. You can add the regulator as a "bulge in the wire" between the PSU and the set!
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Old 25th Jan 2020, 1:09 am   #46
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Default Re: 9 volt battery eliminator hum

That O/C voltage isn't surprising on a supply like that with a bridge rectifier and a great big single smoothing capacitor. The rated 9 volts could well be when it's delivering close to its 500mA (originally intended) load.

As said, the pre-made battery snap connectors are always colour coded like that, as they're intended for replacements for the actual radio, so nothing to worry about, although opposite coded sleeving should ideally have been fitted.

There's lots of unanswered questions:

Voltage on load measured with a DVM on each radio? And compare.

Measure current in mA drawn by both radios? And compare.

We still don't seem to know (unless I've missed it) whether the hum is coming from just the power supply itself as a 'physical' vibration, or is actually emanating from the radio as a 'physical vibration, or is actually coming from the radio speaker?

These questions need answering before returning the power supply, otherwise the same problem may well be experienced with another one that replaces it.
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Old 25th Jan 2020, 1:15 am   #47
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Default Re: 9 volt battery eliminator hum

If Chris is right about using a regulator it looks like the PCB of the eliminator could accommodate the components - see first photo attached to post #11. Probably even simpler to use a fixed LM7xxx 9V regulator. There's certainly likely to be enough headroom for a regulator chip as the no load voltage is 19V.

All this led me to compare the schematics for the R200 and RT7 to see if there might be a reason why one works with the unregulated eliminator and not the other. Nothing shouts at me but the audio circuits are really quite different. A more expert eye might be able to spot something.

Alan
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Old 25th Jan 2020, 1:24 am   #48
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Default Re: 9 volt battery eliminator hum

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Originally Posted by Techman View Post
That O/C voltage isn't surprising on a supply like that with a bridge rectifier and a great big single smoothing capacitor. The rated 9 volts could well be when it's delivering close to its 500mA (originally intended) load.

As said, the pre-made battery snap connectors are always colour coded like that, as they're intended for replacements for the actual radio, so nothing to worry about, although opposite coded sleeving should ideally have been fitted.

There's lots of unanswered questions:

Voltage on load measured with a DVM on each radio? And compare.

Measure current in mA drawn by both radios? And compare.

We still don't seem to know (unless I've missed it) whether the hum is coming from just the power supply itself as a 'physical' vibration, or is actually emanating from the radio as a 'physical vibration, or is actually coming from the radio speaker?

These questions need answering before returning the power supply, otherwise the same problem may well be experienced with another one that replaces it.
Agreed, although the no load voltage still seems a bit on the high side to me. Perhaps James will be able to answer the questions before sending back the eliminator.

Alan
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Old 25th Jan 2020, 10:30 am   #49
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Default Re: 9 volt battery eliminator hum

I always buy regulated switch-mode plug-top power supplies from CPC and have found them excellent. I've used 9V 1.6A ones and 12V 1A ones. Regulation is to within 5% regardless of AC input Voltage, which incidentally can range from as low as 90V to as high as 264V. They have over-voltage and over-current protection and short-circuit protection.

I use two to power my homebrew amplified loop antennas - not a hint of hum, and likewise to power transistor radios.

I also use one to power a homebrew tachometer for my woodturning lathe, and a homebrew PCB circular saw.

Here are the links:

9V 1.6A

https://cpc.farnell.com/powerpax/sw3...ted/dp/PW02349


12V 1A:

https://cpc.farnell.com/ideal-power/...power%20supply

They're made in China - where else, but if CPC sell them, that's good enough for me. (The specs are available at the links. Note: 50,000 hours MTB - mean time before failure!). Of course, you may need to cut the 2.1mm connector off and fit new connectors if you need PP3 or PP9 battery snaps, but that's no hardship.

Hope that helps.
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Old 25th Jan 2020, 10:51 am   #50
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Default Re: 9 volt battery eliminator hum

Two possibilities, the most likely is that the radio is pulling more current than the nasty PSU can supply and the other possibility is that there is a faulty diode in the pack.

The radio needs to be fed from a decent, regulated power supply for testing.

Bear in mind that you don't need a specific "battery eliminator". You can run the radio off any PSU of the correct rating that you like. You just need to add a pair of PP9 snaps, or go straight in to the wiring via a 2.1mm power connector.

I am in Gloucester and if you want to drop it in sometime, I will test it and provide you with a suitable PSU if required. AJL Electronics 01452 311031
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Old 25th Jan 2020, 11:03 am   #51
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Default Re: 9 volt battery eliminator hum

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Originally Posted by Techman View Post
As said, the pre-made battery snap connectors are always colour coded like that, as they're intended for replacements for the actual radio, so nothing to worry about
Unless the contacts happen to touch each other when powered on

Forum rules prevent me for saying what I really think about the item...

Lawrence.

Last edited by ms660; 25th Jan 2020 at 11:09 am.
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Old 25th Jan 2020, 12:39 pm   #52
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Default Re: 9 volt battery eliminator hum

I have a Lowe HF225 which runs off the manufacturere's supplied transformer type wall-wart.
This served very well for years, until I physically rearranged the set-up. Afer that, I had quite some hum coming from the speaker of the '225.
The cause turned out to be magnetic field-induced AC in random parts of the receiver from the transformer in the wall-wart.
Repositioning of the wall-wart cured the problem. I wonder if the O.P.'s problem could be something similar? Tony.
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Old 25th Jan 2020, 1:01 pm   #53
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Default Re: 9 volt battery eliminator hum

Quote:
Originally Posted by Scimitar View Post
Two possibilities, the most likely is that the radio is pulling more current than the nasty PSU can supply and the other possibility is that there is a faulty diode in the pack.

The radio needs to be fed from a decent, regulated power supply for testing.

Bear in mind that you don't need a specific "battery eliminator". You can run the radio off any PSU of the correct rating that you like. You just need to add a pair of PP9 snaps, or go straight in to the wiring via a 2.1mm power connector.

I am in Gloucester and if you want to drop it in sometime, I will test it and provide you with a suitable PSU if required. AJL Electronics 01452 311031
A kind offer of help but the power pack is specified to deliver 500mA at 9V which should be more than adequate for the RT7 as mentioned previously. Also one might expect that the R200 would draw a similar current and that works with the pack. Lack of regulation might still be an issue of course.

Alan
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Old 25th Jan 2020, 1:05 pm   #54
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Default Re: 9 volt battery eliminator hum

I have tested the R200 and rt7 with the power adaptor.

The pics are both off and then on.

111454 R200 off
111457 R200 on

111626 RT7 off
111704 RT7 on

James
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Old 25th Jan 2020, 1:22 pm   #55
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Default Re: 9 volt battery eliminator hum

Ditch that PSU and use something more suitable....Also check the output components and B-ve decoupler, the output stages in the RT7 and R200 are different.

The resistance in series with the emitter in the RT7 is less than half of that in the R200 according to the schematics.

Lawrence.

Last edited by ms660; 25th Jan 2020 at 1:35 pm. Reason: addition
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Old 25th Jan 2020, 1:35 pm   #56
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Default Re: 9 volt battery eliminator hum

It looks like the RT7 is drawing far too much current. Can you measure the current James?

Alan
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Old 25th Jan 2020, 1:41 pm   #57
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Default Re: 9 volt battery eliminator hum

That Alan is what I said earlier re the difference in current drawn by the 2 sets.Though no way would I risk an unregulated power supply,which looking at those voltages it is.
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Old 25th Jan 2020, 2:09 pm   #58
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Default Re: 9 volt battery eliminator hum

So do you think the RT7 should be drawing over half an Amp? The PP9 won't last long if does. It looks like the RT7 has a fault which might take us back to a suggestion made earlier by Mike T regarding the possibility of failing electrolytics. By the way, the fact that the PSU is unregulated was established early on in the thread.

Alan
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Old 25th Jan 2020, 2:15 pm   #59
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Default Re: 9 volt battery eliminator hum

My fault Alan,the RT7 certainly should not be drawing that.Not read the whole thread!
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Old 25th Jan 2020, 2:15 pm   #60
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Default Re: 9 volt battery eliminator hum

17.3 volts for a 9 volt transistor radio is potential suicide....I would call it a radio eliminator rather than a battery eliminator.

Lawrence.
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