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Old 28th Oct 2017, 11:57 am   #1
rchampkin
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Default Roberts RCS80 query

I'm the proud owner of an ancient RCS80 which is in perfect working order.

Could anybody tell me if the DIN socket can be used for an auxiliary input?

From years back I seem to remember that DIN sockets contain input & output pins, and I'd like to be able to connect an external device to this radio, which has excellent sound quality. There is, however, no switch setting which implies that aux input is possible, so maybe it hasn't been allowed for.

Any expert opinion would be very welcome!
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Old 28th Oct 2017, 12:28 pm   #2
davidgem1406
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Default Re: Roberts RCS80 query

I scrapped 2 of those receivers some while back.
I just checked the input panel and there is just one input on either pin 1 or pin 3.
The socket does not have a changeover switch so it is either auto-sensed or done with one of the panel switches.
Hope that helps a bit.

Dave.
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Old 28th Oct 2017, 12:28 pm   #3
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Default Re: Roberts RCS80 query

Usually, pin 3 can be used as either an input or output and pin 2 is earth. If there is no dedicated setting for an external input, then the radio signal will simply get swallowed up by the detector's own source impedance, if a low-impedance source device is connected (a really strong station could still break through, so you might have to re-tune to a quiet frequency); but if a high-impedance amplifier is connected, that will not pull the signal down.
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Old 28th Oct 2017, 12:40 pm   #4
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Default Re: Roberts RCS80 query

Sorry - it's output-only.
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Old 28th Oct 2017, 2:28 pm   #5
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Default Re: Roberts RCS80 query

Sorry, my previous reply was a bit of a mislead.
Here, in jpg format, is the drawing of the socket and the text, both from the manual and service info.

Dave
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Old 28th Oct 2017, 2:43 pm   #6
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Default Re: Roberts RCS80 query

Quote:
Originally Posted by mhennessy View Post
Sorry - it's output-only.
It certainly looks designed to be output only, but if you connected a line level source it would probably work after a fashion. You would need to tune to a quiet area of the FM band as there is no muting of the tuner signal.

The stereo connection should go to pins 1 and 4, with 2 as the earth (screening). Note that the pins on a 5 pin DIN plug are not numbered consecutively.
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Old 28th Oct 2017, 8:26 pm   #7
mhennessy
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Default Re: Roberts RCS80 query

My earlier answer was uncharacteristically brief because I was on my way out - sorry! Here's a more complete reply:

I tried it previously when I was doing all those loudspeaker measurements...

First, you need a fairly high voltage because those 47k resistors are seeing about 3k3, so you've quite a lot of signal loss... You don't get enough from a headphone output on something like a phone or MP3 player - which is what most people are wanting to use in these situations - but as you say, a full size CD player with a "hot" CD should be OK. Obviously, using pins 1 and 4 as inputs rather than outputs helps because it puts two 47k resistors in parallel, but 3 and 5 are inputs, so it involves a non-standard lead. But if you're really going for it, you might as well put the signal on all 4 pins...

Next, shutting the tuner up is the challenge. FM is out, as the inter-station noise is too loud - the only quiet parts of the FM band I could find went away when I turned off my frequency counter

If you live in a quiet area, AM might be an option, but here, I couldn't find a frequency that resulted in a quiet enough background.

The user interface does not make this easy, BTW.

In short, while "forcing" a signal in via the output pins of a DIN socket is often feasible, it's not the case with this set in this location. For my tests, I silenced the FM tuner with blank carrier from my RF generator, but that was to confirm that the de-emphasis was basically OK. For the speaker measurements, I went in via the aerial as those tests were about end-to-end performance.

Hope this helps,

Mark
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Old 29th Oct 2017, 2:45 am   #8
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Default Re: Roberts RCS80 query

If you didn't mind sacrificing a little originality to practicality, you could reduce the values of the resistors on pins 1 and 4; or fit two new, separate lower-value resistors from pins 3 and 5 (removing the link between these pins) to the junction point. This way you will be able to get a louder volume of sound, by wasting less voltage in the resistor; and possibly improve the muting effect by loading the detector with an even lower impedance. The source impedance and the resistor from the socket to the junction are in series with one another, and this combination is in parallel with the 3.3kΩ of the next stage. According to Kirchof's laws, some of the current from the detector is going to flow this way, not into the next stage; meaning the radio will make less contribution to the overall signal. A stronger signal from the source will need the radio's volume control turned up less, further reducing the contribution from the detector.

You will have to experiment a bit to find the best value to use with your source; probably start witn 4k7 or even 470Ω. If you have a suitable potentiometer to hand, use that, in just one channel to begin with; measure the value to which is set, double it (because in real life there will be two resistors, one from each channel; so the values need to be doubled, to allow the same current flowing into the junction when each channel is making its own contribution to the total) and find the nearest preferred value.

I prefer to think of it as enhancing the usability of the Roberts radio (by allowing you to play music from other sources through the radio's amplifier and speaker, whose sound you find agre.eable) as opposed to spoiling its originality. If you are concerned, tape a note listing your alterations inside the set, where it will not get lost, so the changes can at least be reversed in future, if desired for any reason.
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Old 29th Oct 2017, 9:43 am   #9
rchampkin
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Default Re: Roberts RCS80 query

My grateful thanks to all those who've replied to my question.

It seems that a little experimentation is in order.

I'm no electronics buff, so reducing the value of the resistors may be beyond my skills (I'm a mechanical engineer!), but I will take a look and see what can be done.

Thanks guys, rod
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Old 29th Oct 2017, 1:14 pm   #10
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Default Re: Roberts RCS80 query

Julie, what you say is all good stuff in general terms, and I don't take issue with any of it in that context. But, the trouble with this specific set is simple: the quietest noise that I can get from the radio is too intrusive even when the volume control is right down at "1", so reducing the resistors to get the external signal high enough to swamp that will result in a set that has a very fierce volume control action. It's the wrong approach in this instance IMHO.

If the OP is willing to consider mods, then it would be much better to find a way to mute the tuner.

Luckily, that's not too difficult. The outputs from the IF IC come via diode switching, so we can mute either AM or FM by shorting the junction of a 33k resistor and diode to ground. For the FM section, that's the junction of R61 and D8, and for AM it'll be R67 and D7. Fit a double-pole switch, and you can mute both together. This is easier than breaking the audio path (with a track cut or component lift) and putting a changeover switch in series - though obviously that is an option as well...

The only downside of this is the need to accommodate a small switch somewhere. Yes, it could be done discretely, but I concede that it still changes the originality of the set more than just resistor changes.

A more elegant solution would be to "get at" the transistors that are switching on the AM or FM sections. This would reduce the current consumption when running in "amp mode", though it's likely this set will be running on mains by default (recommended for the greater output power). Unfortunately, the scan of the diagram is incomplete, meaning that when you try to join them together you find that a section in the middle is missing. Plus, it's not the easiest schematic to follow anyway. So that would need quite a bit of investigation, probably for relatively little benefit...



Rod, given the difficulties with this particular set, and in light of your most recent comments, I'd suggest that an FM pantry transmitter might be worth investigating. This has the advantage of being usable with any other FM sets you may have in your collection

All the best,

Mark



PS: I'll add that I now have the set on my bench, so if anyone has any questions about it, I can take measurements. I could also validate my above suggestions if Rod thinks that adding a small switch might be feasible.
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Old 29th Oct 2017, 4:12 pm   #11
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Default Re: Roberts RCS80 query

Thank you once more, guys, for the trouble you're going to on my behalf. It is appreciated.

Mark's comments above seem to me to make eminent sense, but the mods suggested may well be beyond my abilities. However, the following may be pertinent...

There would be no objection in principle to adding a small switch, maybe on the black panel through which the sockets are accessed, or even on the back. The set is a 'working radio' rather than a priceless not-to-be-adulterated classic.

Thinking aloud, might a switch be possible that's actuated when a plug is inserted into the DIN socket? My experience here only extends to jack sockets, which can break a circuit when plugged in, but maybe there's a similar function with DIN sockets?

Both the above assume that I'd be able to find and connect to the appropriate points on the circuit board, and before that actually open the case without doing some damage.

But - Mark's final idea is pretty close to ideal, and is eminently do-able. I confess to not having heard of a 'pantry transmitter' before; my original thinking was to use a Bluetooth link of some kind connecting into the DIN socket, so as to be able to play internet radio remotely from the computer or smartphone. A low-power FM transmitter would remove the need for a physical connection to the radio, which would then remain unmolested.

And it looks like the Belkin Tunecast can be had for under a tenner!

This seems to me the ideal route...which I will be exploring shortly.

Best wishes, and thanks again,

Rod
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Old 29th Oct 2017, 7:34 pm   #12
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Default Re: Roberts RCS80 query

Hi Rod,

Glad it all made sense, and if you do get the Belkin, I and no-doubt others would be interested to hear how you get on with it

Regarding the other points: I have seen DIN sockets with switches, but they are comparatively rare, and might not offer the combination of contacts you need, and they might not fit the existing PCB. But more fundamental than that: you might end up losing the facility to get an output from the DIN socket, should you ever want to do that. It was that point which led me towards a separate switch, as then the DIN socket would be able to perform both functions. You never know what you might need to do in the future.

And you're right that these sets often aren't museum pieces. In fact, the RCS80 is positively modern by our standards! By this stage, Roberts were sourcing their electronics from the far east, and the enclosures were often MDF rather than plywood. The plastics used in the top panels can be hard to restore (mine has lost some chrome paint and there is some discolouring - probably caused by UV). Overall, quite different to the sets from the '60s and '70s, but generally pretty good sound quality and sensitivity, so perfectly usable sets today.

If you're just getting into the old radio scene, I'd recommend a Hacker Hunter RP38A - if you think the RCS80 sounds good, wait until you hear one of those

All the best,

Mark
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Old 29th Oct 2017, 8:56 pm   #13
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Default Re: Roberts RCS80 query

I use a Belkin Tunecast 2, does the job and you can set the VHF FM frequency to a quiet spot on the Band.
You need to have the volume turn up to a reasonable level on the MP3 player or it will not come out of standby, this can be a problem with any music with a high dynamic range, long quiet passages and it can go into standby.
There is no on/off switch just standby, if you don’t use it for a while take the batteries out or they will go flat. I use a seperate battery box with a switch on it, uses AA instead of the AAA that fit inside the Tunecast.

Neither problem insurmountable and for the price well worth trying.
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Old 29th Oct 2017, 9:32 pm   #14
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Default Re: Roberts RCS80 query

Hi Mark,

Investigations on the Belkin FM device have so far suggested that it's largely intended for car use, with consequent short range. I'm looking for 30+ feet, room to room and possibly room to workshop. Any suggestions for a more powerful unit would be welcomed...

With apologies for straying off topic, I'm not actually a collector of vintage radios - I have enough trouble collecting and restoring classic boats and antique outboard motors! But as an aside on the older radio front, in 1969 I saved up from my Saturday job and bought a much-desired Grundig radio, an Elite Boy. This was a superb radio, but after a few years the white plastic chassis that carried the tuning drive just disintegrated, preventing the tuning knob from moving the tuner. I've never felt able to throw it out. Much later, I found a Music Boy in a garage sale for a couple of quid; this is also very good, but kind of expensive on batteries, as PP9s are not so cheap these days. What it needs is a 9v supply and one of the weird Grundig D-shaped DC plugs...

Cheers, Rod

(After typing this, I've seen the post from Nuvistor - any experience of the range would be welcomed)
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Old 29th Oct 2017, 10:29 pm   #15
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Default Re: Roberts RCS80 query

The range of the Belkin is just a few feet, it’s made to comply with the regulations, anything stronger would no doubt breach those regs.
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Old 30th Oct 2017, 5:24 pm   #16
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Default Re: Roberts RCS80 query

Quote:
Originally Posted by mhennessy View Post
Julie, what you say is all good stuff in general terms, and I don't take issue with any of it in that context. But, the trouble with this specific set is simple: the quietest noise that I can get from the radio is too intrusive even when the volume control is right down at "1", so reducing the resistors to get the external signal high enough to swamp that will result in a set that has a very fierce volume control action. It's the wrong approach in this instance IMHO.
Hi Mark,

I was aware it could be somewhat of a balancing act, with levels and impedances. If you've tried it with the actual set and not been able to make it work well, then that's a bit of a shame; but at least it saves the OP from going on a wild goose chase.

In the case of the sets to which I've added auxiliary inputs myself, maintaining originality has not been a major concern; I've usually used 6.3 sockets, for the ease of handling (including the possibility of incorporating a few resistors right inside the plug) and the built-in switching.

Going in at RF by means of a pantry transmitter is also forward-compatible with any replacement set the OP might ever want to use!
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Old 31st Oct 2017, 11:28 am   #17
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Default Re: Roberts RCS80 query

Greetings all,

Since receiving your valued comments on my original post, I've tried to check out the availability of pantry transmitters. It seems that the Belkin is not likely to do the job, although I have come across a couple of hacks which claim to increase its range.

The front runner looks like being the Whole House Transmitter from the US (https://wholehousefmtransmitter.com/...ansmitter-3-0/). I suspect that this would suit the original needs - it claims a 150 ft line-of-sight range - but it's quite expensive - it could be just as cheap to buy a new internet radio! I will however be in the US in January, and might just be able to obtain one and slip it into my baggage...

The search continues, as do my thanks to you all for your assistance.

Rod
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Old 31st Oct 2017, 4:05 pm   #18
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Default Re: Roberts RCS80 query

Just a thought, and I haven't tried it, but how about using a reed switch to disconnect the RF from the audio stage when the external input is in use?

With a bit of ingenuity it might be possible to mount the switch near the socket and the magnet in/on the plug so operation is automatic when the plug is inserted. Otherwise, put the switch on the inside of the top panel and place the magnet near it when required.

No holes in the case and minimal rewiring inside would make the change easily reversible.
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Old 31st Oct 2017, 5:07 pm   #19
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Default Re: Roberts RCS80 query

Most of the FM transmitters sold for a few pounds on eBay have a range greater than the Tunecast - 10 metres / 30 feet is usually claimed. The one I use isn't currently on sale but easily gets from one side of the house to the other.

The eBay ones may well be CE marked. This means nothing, but you can point to it if you ever get an unexpected visit from the authorities. You don't want a transmitter that will radiate significantly beyond your property boundary, as this may cause a nuisance and attract unwanted official attention.
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