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Old 12th Jul 2019, 7:14 pm   #1
ThomasE
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Default Roberts R200 radio not functioning.

Hi All,

I am new here so I hope this is the correct place to be posting this! If not let me know. I recently bought a Roberts R200 radio and it doesn't work, and I am a bit of a novice when it comes to repairing vintage equipment so I was wondering if you would be able to give me some advice on anything that may be worth a check?

It is serial number 41296 so from what I can find it should have reliable transistors unlike newer models which use AF117's. Turning the volume dial results in scratching noise, which did disappear after a couple turns of the knob, and the radio draws around 8 to 11 MA of current so that seems to be within specifications of the service sheet. However I cannot pick up any stations! It appears to have all original resistors and capacitors so I will go about checking these soon as I am sure they are sad after 50+ years. I think that the L3/L5 ? coil on the ferrite bar may be damaged - could this cause the issues that I am seeing (bear in mind that I hear no audio such as static with the volume on 100%, only a hiss as I move the knob to adjust it.) and is there a way to test to see if the coil/s are any good I do not receive anything on long wave either? I cannot see any values on the service sheet for resistance.

Thank you very much for ANY help you can provide, it is much appreciated.
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Old 12th Jul 2019, 7:46 pm   #2
Mike. Watterson
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Default Re: Roberts R200 radio not functioning.

The coils on ferrite rods are probably between 10 Ohms and 60 Ohms depending on number of turns and wire thickness. One end may go to battery + or -.
Touch a 3m long wire to each connection in turn. If you hear interference/static/stations then a coil has an open circuit.

One is for MW and the other for LW. After dark is better for MW. Daytime ought to give BBC R4 about the middle of the LW scale.

If the coil wires are broken then you'll not even hear static. Just a very faint hiss.
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Old 12th Jul 2019, 7:54 pm   #3
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Default Re: Roberts R200 radio not functioning.

L3,L5 are the LW aerial coils, the radio should still work on MW as L3,L5 LW is shorted when switched to MW. The quiescent current seems about right, and scratching from the speaker when the volume control is turned means the audio stage is working. Most likely cause is a fault in the wave change switch, or local oscillator not starting up. Try another working radio close and tune both across the MW band to hear if the 470 KHz IF stage picks up an interference signal.
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Old 12th Jul 2019, 8:22 pm   #4
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Default Re: Roberts R200 radio not functioning.

If your R200 has those brown capacitors marked DCC, they may well be leaky and cause the mixer and IF stages to be incorrectly biased. If the oscillator is not running, the DCC emitter decoupler may be at fault, as it’s value seems critical.

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Old 12th Jul 2019, 8:35 pm   #5
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Default Re: Roberts R200 radio not functioning.

Ideally it should be checked from the detector diode (diode is not faulty) back through each IF to the oscillator and wavechange switch and aerial.
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Old 12th Jul 2019, 10:10 pm   #6
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Default Re: Roberts R200 radio not functioning.

Fantastic, thanks everyone so far. Thanks Geoff, I meant to add that into the original post as I had read about using a seperate radio to test if it was working or not. On AM i don't hear any stations, just silence on the roberts radio. With a working AM radio next to it I definitely get some interference at specific points. For example with the good radio tuned to 550m I hear noises when I tune the roberts radio to 300m. This would indicate some life from the oscilator correct? I will have to measure the coils resistance's tomorrow but from some quick prodding it doesn't seem to be open circuit.
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Old 13th Jul 2019, 10:04 am   #7
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Default Re: Roberts R200 radio not functioning.

I have repaired quite a few RT1 and R200 radios.
What I use to assist in faultfinding is I have working RT1 with a 0.1nF capacitor connected at one end to the collector of TR3. With RT1 tuned to a radio station I connect a fly lead to this capacitor and use it as a signal source to inject 470 kHz signal into faulty RT1 or R200,300 etc. In some cases it may be necessary to connect the ground of both receivers together. It then becomes easy to trace the fault through the IF strip connecting the fly lead to the bases of the IF transistors. It is also good for alignment of the IF strip. Once the fault has been found the repaired radio will pick up the IF signal from the RT1 with just a loop around the ferrite rod aerial. Lengthen the fly lead distance to attenuate the signal. The RT1 IF had been checked for accuracy.
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Old 14th Jul 2019, 11:59 am   #8
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Default Re: Roberts R200 radio not functioning.

Hi All,

Thanks dave unfortuantly I don't have another r200 to test this with. I have cleaned the wavechange switch and still nothing, all the wires look as if they are in the correct positions as I found an image online of it which matched. On mediumwave I hear a quiet whine at around 250m which changes tone if I touch the orange wire on the coil itself (L2) which is used for medium wave. Would the old capacitors be the cause of not recieving any stations / or interference from CFL's? Like I said I can hear an oscilation on a seperate radio so I assume the oscilator is working? Thanks everyone for your help so far!
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Old 14th Jul 2019, 12:48 pm   #9
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Default Re: Roberts R200 radio not functioning.

The wavechange switch should be checked to make sure it opens and closes correctly. S1 S2 short out each aerial coil depending on which waveband is selected. Also look at the tuner to make sure the vanes aren't shorted together and trimmers haven't been tampered with. If the IF and oscillator transformers look intact with no signs of being tampered then voltage checks should be carried out on the osc and IF transistors. Getting noise through shows the detector is working, the detector diode is usually reliable.
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Old 14th Jul 2019, 1:39 pm   #10
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Default Re: Roberts R200 radio not functioning.

If its got the red/black Plessey electrolytics they might be worth looking at.

Regards poppydog
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Old 14th Jul 2019, 5:43 pm   #11
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Default Re: Roberts R200 radio not functioning.

Thanks Geoff, The vanes are fine and not touching at all, doesn't look like the trimmers have been tampered with - it does have the Plessey caps that Poppydog mentioned (A-thank you) I am waiting on a capacitance tester - they do look a bit poorly so no doubt I will have to replace them. Transformers look A-OK as well.

The service manual is not clear as to what pin is which on the switch, I have attached a photo of the switch, have I got the pins numbered correctly? S6? is the red wire which comes directly from the battery. For reference, the yellow and blue are from the Longwave coil, and white and orange are from the medium wave coil.

I do not see any change in the resistance of either coil when I turn the waveband switch. I'm sure the longwave coil is broken but that,s no big deal, medium wave coil looks fine though.
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Old 15th Jul 2019, 9:19 am   #12
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Default Re: Roberts R200 radio not functioning.

On the 200 which I overhauled, literally all the electrolytics were leaky or open. Voltage checks mostly revealed the leaking ones. The wave change switch just needed a clean.

Ken
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Old 15th Jul 2019, 9:51 am   #13
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Default Re: Roberts R200 radio not functioning.

Brilliant thanks Ken, it's looking like that is the problem. I will update this when I get the capacitance tester. I take it that your radio didn't work at all? I would have expected at least a vague reception of any stations which is what threw me off, but I am relatively new to vintage radios. Thank you everyone! Any recommendations on the best/cheapest places to buy new capacitors? Or is that not something I can ask here? (sorry if it isn't!) Not concerned with the original look of the radio so don't mind new electrolytics.
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Old 15th Jul 2019, 11:32 am   #14
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Default Re: Roberts R200 radio not functioning.

Only new ones work, generally.
Farnell is reliable.
eBay requires more expertise and China is 6 to 10 weeks.

You can test electrolytics in a basic way with a 9V battery and a DMM.
Connect to battery via current range. If current doesn't drop to less than 1mA, it's leaky. Typically leaky caps might take more than 5mA.
If charged on battery and not leaky, switch DMM to volts and disconnect battery. If the voltage slowly drops, then it's still a capacitor. If the electrolyte has dried out, then the voltage will almost at once drop to zero.

Yes, there is high ESR, high resistance, which needs a specialist capacitor tester. That's more a problem with newer capacitors, especially in SMPSUs. Obviously a dried out capacitor will read low on capacitance and high on ESR.
A leaky capacitor will give a false higher capacitance reading on a cheap capacitor tester that doesn't measure leakage (parallel resistance, rather than Effective Series Resistance).
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Old 15th Jul 2019, 11:38 am   #15
ThomasE
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Default Re: Roberts R200 radio not functioning.

Thank you Mike, I never heard of using a dmm and 9v to test them that sounds great, I will do that as well as test them with a component / capacitor tester I ordered which should give me a ballpark figure. I'm sure they need replacing anyway but good to check first. Thanks a lot! I will also check a resistor or two, do these usually require replacement or are they generally reliable?
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Old 15th Jul 2019, 11:43 am   #16
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Default Re: Roberts R200 radio not functioning.

My 200 essentially didn’t work at all. Voltage checks showed that some caps were leaking, dragging voltages down. Replacing these fixed most problems but it was still quiet. Replacing open circuit coupling caps fixed that and away it went. I think it was an early version with OC series transistors and an unscreened osc coil, but cannot look as I no longer have it.

Ken
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Old 15th Jul 2019, 11:47 am   #17
ThomasE
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Default Re: Roberts R200 radio not functioning.

Thanks Ken that is pretty much the issue I'm having. I will replace all the caps in it while I'm there, fingers crossed!
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Old 15th Jul 2019, 11:52 am   #18
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Default Re: Roberts R200 radio not functioning.

Resistors rarely need replacing, unless looking burnt. Sometimes even then are fine.
Designs are usually more tolerant than the percent accuracy stated on the resistor.
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Old 15th Jul 2019, 11:58 am   #19
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Default Re: Roberts R200 radio not functioning.

The capacitor testing method should be OK for 4uF and larger, maybe even 1uF if the meter is a 10 M Ohm input impedance.
Even an old analogue 20 K Ohm per volt meter is 1M Ohm on the 50V range, same as 1M Ohm of a cheap DMM. I learnt that trick back in 1970s with an analogue meter. DMMs are often the same input impedance on all lower voltage ranges.
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Old 15th Jul 2019, 4:42 pm   #20
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Default Re: Roberts R200 radio not functioning.

The only electrolytic capacitor in the RF side that could affect the output of the first IF transformer is 10uF C11 part of the AGC smoothing that could pull down the bias on TR2 the first IF transistor.
C22 100uF TR4 emitter bypass capacitor is part of TR3 base bias via the second IF transformer secondary so could affect TR3.
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Last edited by GeoffK; 15th Jul 2019 at 4:47 pm.
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