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Old 2nd Dec 2023, 6:59 pm   #1
Silversdream
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Default Roberts R200. Silent.

I have a dead Roberts R200
After connecting the battery and turning the dial to M there is a click from the speaker but nothing else
I have changed all the Mullards Electroltics and still nothing
Now on this forum i read there is a X2 diode inside the tin can that may be the problem
How do i get the lid off the tin can or if its soldered into the circuit board can someone mark up the lugs on a picture of the pcb as i cant tell which are the lugs to unsolder
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Old 3rd Dec 2023, 12:50 am   #2
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Default Re: Roberts R200. Silent.

Some of these had the infamous AF11x series of Mullard transistors.(Or is that the 300? same board pretty much) If you have those then that might be the first place to look. It's usually the mixer oscillator one that goes first in my experience. The older ones with the black OC series transistors are, paradoxically more reliable.
I did one of these with a duff diode some years ago. I proved it by injecting an audio signal into the volume control(ok, thus AF amp works) and having a look for modulated carrier at the second IF with a 'scope. Only thing in between was the diode.
The diode can be replaced with any of the Mullard Oa series, 79,80,81,90,91 etc.
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Old 3rd Dec 2023, 9:26 am   #3
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Default Re: Roberts R200. Silent.

Post 3 at this link might help:

https://www.vintage-radio.net/forum/...d.php?t=205840
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Old 3rd Dec 2023, 10:41 am   #4
60 oldjohn
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Default Re: Roberts R200. Silent.

With failed AF11x or the diode I would expect to hear white noise. Is the AF stage actually working? Easy test is to put a wet finger on the volume control, you should get a 50Hz hum from the speaker with volume turned up if it's working. On some sets this can be confirmed just by working the volume control if it is dirty you would hear crackling through the speaker.

If it does turn out to be the diode, there should be no need to remove the old diode, it will be open circuit. So if you can work out from the circuit diagram where the diode is connected it is possible to solder it flat on the back of the PCB. Trying to remove the IF can may result in the destruction of the early delicate PCB tracks.

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Old 3rd Dec 2023, 9:07 pm   #5
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Default Re: Roberts R200. Silent.

I snipped the Screen leads and now i have white noise
Still no stations and no change in noise level when i turn the volume control
Should i swap the AF117's i have some BC557 which someone suggested elsewhere ?
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Old 3rd Dec 2023, 10:06 pm   #6
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Default Re: Roberts R200. Silent.

Sometimes the whiskers just short the collector bond wire to the case. This is the same as shorting out the battery, so the result is a dead set. At this stage, snipping the screen wire can restore operation.

However, you have now cleared the supply short but in all probability the whiskers in one ore more AF117s have engulfed and shorted two or more electrodes, and the screen snip is no longer enough.

Quite often a sharp tap with the butt end of a screwdriver might give you a "pop" in the speaker or even temporarily restore operation.

I suggest you replace the three transistors one at a time, starting with one that gave a "pop".
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Old 11th Dec 2023, 8:48 pm   #7
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Default Roberts R200 not working

Hi, can anyone advise on a non working Roberts R200 please
Basically i am only getting a click noise when i turn the switch to either M or L
The radio is not producing any background white noise
I have just purchased a Fnirsi DSO-TC3 so i used the signal generator and i am getting a squeel noise from the speaker from all over the pcb and the volume control on all 3 connection tabs.
I then tried the signal generator function (having tested it on a working radio) this gave no reading anywhere on the pcb.
I then tested the voltage and found the 9V reduced to 3V taking a reading from the negative input (black wire connection) on the pcb to S7 on the on off wave change switch. By adjusting R21 i can get the voltage up to just over 7 Volts but still no sound.
Forgot to say i have also changed all electrolytic and polyester capacitors, tested the AF117,s out of circuit, snipped the screen leads and changed X1 to X3 diodes.
Not sure what to try next, any advise will be appreciated
Paul
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Old 11th Dec 2023, 10:08 pm   #8
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Default Re: Roberts R200 not working

Update
I have just tested most of the resistors and a lot are way off the correct ohms.
I did test whilst still in circuit.
Could this be the problem, should i just change all the resistors and see what happens ?
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Old 11th Dec 2023, 10:37 pm   #9
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Default Re: Roberts R200 not working

You can't rely on testing resistor values whilst still in circuit.
You need to make sure you have a schematic diagram, then check all the supply rails.
Next, start at the speaker end and make sure the audio amp is working fine by injecting a tone in to the volume control - or a wet finger (don't do this on a valve set).
Then you need to check the IF amplifier is working by injecting 455 kHz or 470 kHz or whatever it is.
Then you need to move to the local osc. and front end.
These are the ideal ways to test a set, but life isn't always so simple sadly.

You need to double check all you have already done like capacitor polarity etc.
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Old 11th Dec 2023, 10:47 pm   #10
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Default Re: Roberts R200 not working

Quote:
Originally Posted by Cruisin Marine View Post
You can't rely on testing resistor values whilst still in circuit.
You need to make sure you have a schematic diagram, then check all the supply rails.

...and the reason for that is because there are so many parallel paths. I echo what Crusin Marine has said. Always check supply rails first. Note also that cutting the screen lead doesn't always cure the AF117 problem.... If you are getting a click when operating the switch, it probably means the output stage is working but check by touching the centre contact of the volume control....you should hear a buzz. If you do, the amplifier stage is probably OK or at least working to some extent. If you don't get a buzz then check the driver and output transistors....but most importantly check voltages first against the diagram with a good battery.

Go over everything you have done so far.....
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Old 11th Dec 2023, 11:08 pm   #11
Silversdream
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Default Re: Roberts R200 not working

Thank you for the advise so far
I have Trader data sheet 1602 which is correct as serial number is over 70.001
Apart from 9V there are no voltages stated
Where can i find voltages to check against ?
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Old 11th Dec 2023, 11:32 pm   #12
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Default Re: Roberts R200 not working

Quote:
Originally Posted by Silversdream View Post
Where can i find voltages to check against ?
Hi.

In the Trader service information no.1602, check under the section "Second and Third Main Versions" The transistor voltage table gives the expected voltages. Note your voltmeter positive lead connects to the chassis.

Hope that helps.

Regards,
Symon
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Old 12th Dec 2023, 12:07 am   #13
Silversdream
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Default Re: Roberts R200 not working

I would have connected negative to chassis wow thanks Simon
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Old 12th Dec 2023, 12:47 am   #14
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Default Re: Roberts R200 not working

Yes, it does seem odd Silversdream. It's only for the fact that the battery positive connect to the chassis. A lot of early radios with PNP transistors had that polarity. If you're using a digital multimeter no harm will come to the meter should you get the polarity of the test leads wrong. All that will happen is your meter will show a negative reading. An old analogue meter may well object to incorrect polarity of the test leads and the moving coil could be damaged.

Regards,
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Old 16th Dec 2023, 7:56 pm   #15
Silversdream
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Default Roberts R200 unknown Capacitors

I am working to Roberts R200 Trader Service Sheet 1602
There is no value to Capacitors C10 - C14
Where can i find these values ?
Thanks
Paul
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Old 16th Dec 2023, 8:25 pm   #16
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Default Re: Roberts R200 unknown Capacitors

Those capacitors are part of the tuned circuits include 2 trimmers and part of the tuning capacitor.

They are extremely unlikely to be faulty unless physically damaged.

What makes you think they are faulty?

Cheers

Mike T
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Old 16th Dec 2023, 11:35 pm   #17
Silversdream
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Default Re: Roberts R200 unknown Capacitors

Hi Mike
It’s C11 I am trying to find the value, whoever had this R200 previously has removed it then sold the set on eBay. At least it was listed for parts only.
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Old 16th Dec 2023, 11:45 pm   #18
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Default Re: Roberts R200 unknown Capacitors

C11 is C8 on later versions shown as being 160pF

Got to be a good starting point.

Cheers

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Old 16th Dec 2023, 11:48 pm   #19
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Default Re: Roberts R200 unknown Capacitors

There are consequences to changing any capacitors in the RF and LO tuned circuits in a superhet radio.

Firstly, you will need to realign the set after you have changed anything in this area or moved any trimmers. This needs some experience and it's best to start learning on a set which doesn't matter too much. You need a decent accurate signal generator unless you want the job to be a lot more difficult.

Secondly, it is common for capacitors in this area to have carefully chosen temperature coefficients so that their change over the normal range of operating temperature compensate the change in the inductance of the coils. Capacitors were sold in not just capacitance values, but also with the choice of a few different temperature coefficients. Wily designers used stable capacitors in conjunction with controlled temperature coefficient capacitors in order to create wanted values with close-to-optimum temperature compensation. You can no longer get all of these different temperature coefficient parts. The ones in there may actually be irreplaceable as they're no longer made and no-one archived stocks.

For these reasons, the resonating capacitors are normally left carefully alone. Only if one is definitely damaged will most experienced repairers and restorers become desperate enough to try changing one. You can wind up with a set which drifts in tuning if the temperature changing, and one which loses proper alignment if temperature changes.

Sometimes datashhets will have no marked values for such parts, knowing that capacitances and tempcos have been fiddled with over the production life in order to refine the alignment and stability.

Radios can look like simple things, but there are a few invisible subtleties in making a good one and these can trap the unwary and ruin a set which otherwise would have been OK.

On the good side, these sorts of parts have proven to be very reliable and very rarely need changing.

David
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Old 17th Dec 2023, 12:58 pm   #20
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Default Re: Roberts R200 unknown Capacitors

I see on this thread the value is 10uF
https://www.vintage-radio.net/forum/...d.php?t=102471
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