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Old 21st Mar 2006, 10:34 am   #1
steve1811uk
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Default Roberts R200 LW reception poor

I have got a R200 and have aligned the MW band the best I can.

I used broadcast stations Capital Gold 1458kHz (set scale to 206m) and RTE 567 kHz (set scale to 529m). I am very happy with the allignment and sensitivity on MW. However when I switch to long wave I find the sensitivity is very poor. I don't have a signal generator only a scope to hand so the only adjustment I could make was to the LW local oscillator trim cap. I adjusted this to get local oscillator range of 630 kHz to 745 kHz which I think is correct.

I believe the capacitor on the tuning rod is the wrong value. It is a small silver rolled polystyrene type (looks modern) and is unlikely to be the correct value of 40pF. Do you think this is the original part ? I aim to replace this with 40 pF or replace with a trim cap and adjust. I need some help with the tuning scale as I can only align on a broadcast station. The longwave scale is not marked with wavelength.

Can anyone tell me two strong stations to align to. Where on the MW tuning scale do they appear ? Any help appreciated. Steve

Last edited by Darren-UK; 18th Jul 2007 at 12:16 am. Reason: General tidying.
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Old 21st Mar 2006, 11:34 am   #2
Darren-UK
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Default Re: Roberts R200 LW reception poor

Hi Steve,

You don't mention which version of the R200 you have; there were three - plus a few changes to the circuit specs within those three main variants. The early ones used OCxx transistors and the little cap you mentioned should indeed be 40pf. Later versions used AFxxx transistors and the cap in these was IIRC 20pf.

Have you tried the simple expedient of shifting the LW coil along the ferrite rod aerial ? Often these coils shift and/or the ferrite rod becomes cracked because careless people tend to remove the chassis by pulling on the aerial rod.

Regarding strong stations, this depends on where you live with regard to MW, but on LW BBC R4 is fairly good regardless of location or if you live in the west of England or Western Scotland RTE 1 towards the high frequency end of LW is a good signal.
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Last edited by Darren-UK; 18th Jul 2007 at 12:27 am. Reason: Clarified model variants.
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Old 21st Mar 2006, 12:05 pm   #3
steve1811uk
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Default Re: Roberts R200 LW reception poor

Darren, I have the early version serial number 8304. The cap should be 40 pF but I think it has been changed as it is a silver polystyrene tubular type and looks modern. I aim to replace this cap with a fixed and variable cap in parallel. I will then adjust this cap to allign as I already adjusted the local oscillator to run at the correct frequency (630 kHz to 745 kHz) using a callibrated digital scope.

I can receive radio 4 but in order to do the LW alignment I need to know where it should appear on the tuning scale. The LW scale is not marked with wavelength. My specific question is this, when tuned to Radio 4 LW what is the reading on the MW scale ? The ferrite rod seems OK and the LW coil has not moved.

Hope someone can help. Steve.

Last edited by Darren-UK; 18th Jul 2007 at 12:18 am. Reason: General tidying.
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Old 21st Mar 2006, 12:15 pm   #4
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Default Re: Roberts R200 LW reception poor

Steve.

When aligning the RF section of a radio you need to use signals close to the ends of the band.

I've just been doing some listening on LW and suggest the following:

Low frequency end. Allouis France on 162 KHz (1850 metres) a 2 Megawatt station. There's a German station on 153 KHz (1921 metres), but it's not nearly as powerful.

High Frequency end. RTE (Irish Radio) on 252 KHz (1190 metres).

In the middle the band there's BBC Radio 4 on 198 KHz (1515 metres).
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Old 21st Mar 2006, 12:27 pm   #5
steve1811uk
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Default Re: Roberts R200 LW reception poor

Thanks again for the help. My problem is that there are no frequency or wavelength markings for LW on the Roberts R200. I need somebody with an alligned R200 to tune to these LW stations and then tell me what the reading is on the MW scale.

It would be a lot easier if I had a signal generator. The allignment procedure says you should tune to 425m on the MW scale and then inject 185 kHz. From this I know that 425m on MW scale corresponds to 1622m on the LW scale. But that doesn't help me.

Hope someone can help me some more with this.

Steve
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Old 21st Mar 2006, 12:56 pm   #6
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Default Re: Roberts R200 LW reception poor

Ok, attached are some pics of my R200. First pic shows cursor on R4 LW; commonly this is stated as being 1500m but in reality it's 1502m. This is close to, cursor wise, 360m MW. Second pic shows radio on RTE1 and cursor is immediately below the 200m mark on the MW scale. The radio was running when these pics were taken a moment ago.

The third pic shows the relevant cap on my radios aerial and so far as I know it's original; the soldering certainly looks to be. Hope that helps.
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Old 21st Mar 2006, 1:11 pm   #7
steve1811uk
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Default Re: Roberts R200 LW reception poor

Darren, that's just the info I need. Thanks a lot. Is that the 40 pF capacitor as my capacitor looks exactly like that. Steve.
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Old 21st Mar 2006, 1:28 pm   #8
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Default Re: Roberts R200 LW reception poor

Yep, it's 40 picos
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Old 21st Mar 2006, 1:57 pm   #9
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Default Re: Roberts R200 LW reception poor

Forgive me for stating the obvious, but don't forget to check the LW ferrite rod coil for continuity and shorts. Also, check the 40pF cap for leakage, and check the wavechange contacts.

Best regards, Paul
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Old 21st Mar 2006, 2:02 pm   #10
steve1811uk
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Default Re: Roberts R200 LW reception poor

Paul, I'll do this as you suggest. I'll try to post back in a few days time with the results. Steve.
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Old 22nd Mar 2006, 9:31 pm   #11
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Default Re: Roberts R200 LW reception poor

Well I did some checks and found that the capacitor and coil were ok. I found that there was a connection missing on the wavelength switch, referring to 'Trader 1449' Switch 2 across L4 was missing. Does anyone know the purpose of this switch as the output of L4 is isolated by S3 in LW mode anyway ?.

I fixed this and then tested. I did an alignment based on Darrens info (adjusting C8) and everything seems OK now (Radio 4 and RTE in the right places). I found that the rotation of the antenna was critical in picking RTE up.

One thing I have found is that some strong MW stations can be heard on the LW setting (Capital Gold, Talk Sport, Radio 5). I am in Manchester and I think the signals are quite strong here. Is this normal for a radio of this design ? Also I found that my antenna is the one where the two coils connect in series for LW when the serial number 8304 suggests that this should not be the case. Has anyone else seen this or could my radio be made up of spare parts?

I am new to radio and appreciate all the help given. Thanks a lot, Steve.

Last edited by Darren-UK; 18th Jul 2007 at 12:23 am. Reason: General tidying.
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Old 24th Mar 2006, 10:34 am   #12
steve1811uk
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Default Re: Roberts R200 LW reception poor

I had another look at this and found that I had adjusted C8 incorrectly (LW Local oscillator). According to "Trader" 1449 the tuning range is 1130 to 1970 metres (265kHZ - 152kHz). A local oscillator range of 735 kHz - 622 kHz was required. I adjusted C8 to give this exact range while monitoring the LOsc transistor collector voltage with a scope. After this adjustment I found the sensitivity and selectivity were the best that they had been. Radio 4 was then found to be exactly on the 400m mark of the MW scale.

Regarding the Capital Gold and Radio 5 breakthrough. I did some reading and found that these are in fact images caused by the LOsc not being a pure sine wave. I found some useful info at

http://www.dbugman.com/handbook/tscmh5.html

To confirm that this was the cause I did some measurements

Capital Gold (1458 kHz) could be heard faintly when the LOsc was at 643 kHz. This was found to be due to LOsc 3rd Harmonic distortion, 1929 kHz - 1458 kHz = 471 kHz, the IF that gets amplified.

Radio 5 (909 kHz) could be heard faintly when the LOsc was at 690 kHz. This was found to be due to LOsc 2nd harmonic distortion, 1380 kHz - 909 kHz = 471 kHz, the IF that gets amplified.

Does anyone else hear these images on a Roberts R200 or other radio using a similar circuit ? or could my LOsc be a little too distorted due to a failing component ?

Steve

Last edited by steve1811uk; 24th Mar 2006 at 10:43 am.
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Old 25th Apr 2006, 2:44 pm   #13
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Default Re: Roberts R200 LW reception poor

I found out that the image frequencies were caused because the LW tuning coil was tuning to the wrong frequency range. The ferrite rod had been wired up according to the schematic for a pre 8200 version. On LW band the LW and MW coils were in series causing too much inductance and wrong tuning. I rewired the switch and coils as they should be for a serial number of 8304 and the tuning improved greatly and the images disappeared. Thanks for all the help. Steve.
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Old 25th Apr 2006, 8:23 pm   #14
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Thumbs up Re: Roberts R200 LW reception poor

Quote:
Originally Posted by steve1811uk
I found out that the image frequencies were caused because the LW tuning coil was tuning to the wrong frequency range. The ferrite rod had been wired up according to the schematic for a pre 8200 version. On LW band the LW and MW coils were in series causing too much inductance and wrong tuning. I rewired the switch and coils as they should be for a serial number of 8304 and the tuning improved greatly and the images disappeared. Thanks for all the help. Steve.
Interesting thread, Steve, thanks. Note how much more difficult a simple repair can become if someone else has meddled first...

My own Roberts R200 stops working completely on long wave when the battery runs down, whilst MW carries on OK. Any suggestions from the transistor experts, please?

Phil
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Old 28th Apr 2006, 1:59 pm   #15
steve1811uk
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Default Re: Roberts R200 LW reception poor

Phil, It sounds like the local oscillator is not running on LW when the battery is getting low. This could be normal behaviour as there is more capacitance added to the oscillator in LW mode and I think this makes it harder to oscillate. As the battery voltage is reduced the base voltage and hence emitter voltage are reduced. This leads to a reduction in emitter current that could cause the oscillator to stop. It could also be due to leaky capacitors at the base or emitter of the oscillator transistor. I will see how my Roberts behaves when the battery goes low but that could be a while. Why not measure the battery voltage, base and emitter voltages with the radio switched on when the LW stops and post that info here. Steve.
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