Question about Roberts "Wall Warts"

[Joxtinkerer]

Hi,

I've been experimenting with a heap of dead, ailing and finally restored Roberts radios - most of which haven't come with power supplies. The PU509 units (9V, neg centre pin) seem to fail over time around the strain relief, so even if you do manage to get one when buying a radio, it's invariably in scabrous condition

I tried several suppliers but eventually found a unit (9V 500ma, 2.11x5.5mm) that was £5.39 and that had the correct voltage, without using polarity reversers. I've been using this now with a variety of sets to see what will work and thought it was worth sharing. Got to be something useful from having too many radios hahaha

So these work with this replacement PU509 type adapter. I've just double checked and tested the following, which work fine:

R250 Revival
R760
R761
R505
RT22
RFM3
R701

However, when I tested a RIC2 and an R505, they just hum loudly. It doesn't appear to have fried them, I'm just not sure why these specific units don't work.

Is there a specific current that these draw or am I missing something important?

I don't know about the rules regarding posting a link to the ebay seller but if it's something genuinely useful for other collectors, I'll let the moderators decide. I do try to reuse 'wall warts' but sometimes a new one is very handy.

Thanks.

C.

[paulsherwin]

No, it won't be anything to do with current consumption.

Are you sure that the RIC2 and R505 expect a negative centre connector?

[Joxtinkerer]

Originally Posted by paulsherwin

No, it won't be anything to do with current consumption.

Are you sure that the RIC2 and R505 expect a negative centre connector?

Ah, that's what I wasn't sure about. If that's the case, I'm glad they haven't been fried Let me do some digging - if anyone knows the answer, that would be great!

[TonyDuell]

You've listed the R505 as one that works and one that doesn't . Which is it, and what should the other one be?

[Joxtinkerer]

Ah, Good spot @TonyDuell

I was going through a heap of radios and you're right - there's one working 505 and the other one does not. I retested two units and both work fine on battery. When the second unit is switched on and tuned to a station, there is a lot of humming but it receives a signal for 2 second and then stops. Must be the radio that is faulty, because the power adapter works fine on the second unit.

So these adapters I have should work for the 505 units. I'll see what I can find out about the RIC2.

Any ideas on what might cause the radio to work flawlessly on battery but not on the DC input?

[paulsherwin]

I can only think that the socket and associated switching is either dirty or faulty. It's true that powering a radio with reverse polarity would normally damage it, but occasionally you can get away with it.

[Cobaltblue]

Does the RIC2 actually have a power socket?

Cheers

Mike T

[Paul_RK]

Originally Posted by Cobaltblue

Does the RIC2 actually have a power socket?

Something like half of them do, it was introduced in the course of the production run.

I've just checked a couple of R505s, the input jacks are clearly marked centre pin negative.

Paul

[Joxtinkerer]

Thank you everyone.

I'll see what specific problem ails this specific 505

[martinhog]

I have had exactly the same situation with an R505 when used with a switch mode power supply, in fact the output transistors got so hot the sleeving in the transistor legs started to melt. Switching back to a “tired” battery pack and it played again very well .
Following this thread.

[David G4EBT]

Before focusing on the power supply of old Roberts radios, in particular the R505, I'd suggest that you first check the current being drawn by the radio. Simply focusing on the power supply is putting the cart before the horse. The quiescent current (the current drawn by the radio with no station tuned in), is stated in the service data, and takes just minutes to check.

If you have a Roberts R505, which has stopped working – typically just a click in the speaker on switch on, maybe two seconds of audio, or motorboating, or if you have distorted audio when powered from a wall wart, do check the current being drawn by putting a meter on the mA range in series with a battery - not powered from a wall-wart at that stage.

With no station tuned in, the quiescent current being drawn by the 505 should be 16mA on FM, and 10mA on AM. If the current drain is appreciably higher than that, the likelihood is that at least some of the four electrolytics highlighted in the annotated audio section of the circuit on below will most likely need to be changed.

If C40 & C41 are leaky, they'll put a short across the battery, pull the voltage down and most likely stop the radio from working. However, if C40 is leaky, that will short the emitter of Tr11 and collector of TR12 to ground via the speaker and R50 and could write off those transistors.

I've had two 505s is recent times with this problem and know of several others.

I did a write-up on diagnosis and repair in the thread below, which also outlines how to remove the radio from the cabinet:

https://www.vintage-radio.net/forum/...d.php?t=194308

I would suggest that only when the radio is drawing the correct current and performing correctly on a battery should a wall-wart be tried. If at that stage there are performance issues, it points to the wall-wart being the problem - not the radio.

If you don't check the current and performance on battery, a wall-wart which may supply 500mA or more, might allow the radio to work without the supply voltage dipping too low to stop the radio from working. But if you continue to use it and the electrolytic capacitors are leaky, you risk wrecking the output transistors. At least on a battery, if - due to leaky capacitors - the current drain is so high that the voltage dips, the radio will stop working.

I don't want to appear to be condemning blue Philips electrolytics out of hand, but they are now more than 40 years old, which must exceed their design life by a good measure.

The 5-page ERT Service Data has a clearer circuit than the Bush data sheet, though the Bush data has an 'X-Ray' of the PCB, which can be helpful. If I had to make do with just one, it would be the ERT.

Hope that might help.

[PaulDarzi]

Wonderful post David, thank you.
I too have a R505 that I'd put to one side as it clearly needed help, your linked thread is just what I needed to take the next steps with it, I won't hijack Craig's thread further but may well need to ask for help if(when?) I get stuck as I'm still finding my feet with the vintage radios - with the holidays (and family stuff) approaching it will likely be in the New Year now before I get enough play time.

[majoconz]

A lot of those 'wall warts' that indicate a voltage at a specific current will give the voltage shown at the current shown, so a 500mA rated supply is liable to give much more than the specified 9volts at only 10 to 20mA.
I have a Zenith 3000-1 that runs very nicely on a 9volt supply even though it is published to run on 12volts - there are two 500uF electros right across the supply rails at the power input that are rated at 12volts! I'm not prepared to risk it!
I also have a Nordmende 7/601 Globetrotter that hums very badly with the factory supply so I made a regulated 7.5volt supply which is perfect - apart from a mechanical buzz from the small transformer.

[PaulDarzi]

Hi Martin (and everyone),

Your comments about off-load voltage from wall warts - especially the olde linear types are well made. As a newbie to this forum and the whole repair/restoration of vintage radios I'm finding it's a new world compared to my previous experience, which has mostly been on the design, test, and implementation of new (much higher frequency, with great test kit to hand too) circuits - looking at old radios I'm finding needs new skills.

Back to topic, I've rummaged through all my wall warts and found a couple of otherwise OK switchers but these sadly are noisy on AM bands. So I've looked at old linears and tried to aquire a new linear wall wart or two, only to find as you mentioned thei regulation is rarely very good, and two in particular (same design it seems, although branded and cased differently on the outside) not only have poor regulation but are also somewhat noisey too.

Am hoping to add some regulation to the unregulated wall warts - if anyone has a favoured circuit or approach I'd love to hear about it, otherwise I'll try either LM78xx or LM317 based design using the appropiate application notes as a starting point and see how that looks in practice.

[Herald1360]

My "go to" for quietening basic linear 50Hz TX wall warts is a 317. For simple transistor radio supplies requiring only a few 10s of mA most unregulated "X" volt supplies have enough low load voltage rise to provide the headroom for a 317 to deliver a regulated "X" volts

[PaulDarzi]

Thank you Herald, I looked at some preamp designs and many used LM317/LM337 in place of the 78xx series so your comment confirms that for low current low noise they seem to be preferred, as for the headroom aspect I wondered how that would go, will give one a try with the latest revived Roberts.

[G6Tanuki]

There's a quick-and-simple Velleman regulated-PSU kit that uses a LM317; I have used quite a few of these to power Roberts radios, they have a bridge-rectifier and big electrolytic capacitor as smoothing, meaning that you can use them with wall-warts that supply AC or DC, and the DC doesn't care about polarity as the bridge will sort that out for you.

See here: https://vintage-radio.net/forum/showthread.php?t=129371 for my writeup of one I built some years back. Still going strong!

[dglcomp]

Note that Roland, Korg, Casio and most guitar effects also use 9V center negative supplies, so there is another supply of PSU's that very much will be designed with audio devices devices in mind, at around the 1A mark there is;
Casio AD-E95100
Korg NT4/KA189
tc electronic PowerPlug 12
Etc.

[PaulDarzi]

Originally Posted by G6Tanuki

There's a quick-and-simple Velleman regulated-PSU kit that uses a LM317; I have used quite a few of these to power Roberts radios, they have a bridge-rectifier and big electrolytic capacitor as smoothing, meaning that you can use them with wall-warts that supply AC or DC, and the DC doesn't care about polarity as the bridge will sort that out for you.

See here: https://vintage-radio.net/forum/showthread.php?t=129371 for my writeup of one I built some years back. Still going strong!

Thank you that's a good approach, great write up at your linked thread too - the "lack of silence in music gaps" is what I'm chasing too, so will try your approach and if I can get the Velleman kit I will, otherwise will try and do an "equivalent" to it, less neat maybe but if a box is needed anyway the end user won't be looking inside it so some vero or perf board isn't too bad, hopefully.