Roberts R200 weak on MW

[PaulR]

I was given one of these sets a few weeks ago. It is my first transistor and PCB set, but at least I am not alarmed about zapping myself on the HT! There was the odd whistle as it was scanned across MW and nothing at all on LW.

I have replaced all the wax and electrolytic caps and now it works well on LW but will only pick up a couple of stations on MW - Radio Wales is its favourite for some reason. I have realigned the IF strip but when I try the RF I can only get a very weak response to a 600 KHz signal with the set tuned to 500m as per the Trader instructions.

Many of the resistors measure a long way out, but that is with them in circuit and I have read that you have to disconnect them in transistor sets so I have not replaced any.

Can anyone suggest a way forward? It is the second batch of the first series of sets.

Thanks

Paul

[paulsherwin]

Clean the wavechange switch very thoroughly. Many of these sets spent their working lives permanently tuned to either R4 or R2 and the switch contacts become very badly oxidised.

If that's not the cause then it's just a question of faultfinding as with a valve set. 500M is about 600kHz so the oscillator is running at the right frequency. Can you peak the signal by sliding the MW coil along the ferrite rod? Are there any loose wires?

[PaulR]

Thanks Paul

I have cleaned the waveband switch, but I will try it again It doubles as the on/off switch on these sets so it will have had some movement.

I get odd effects as I bring my hand close to the ferrite rod as if it is trying to pick up something. The coils are fairly firmly attached to the rod and the marks on it suggest they are in the original position, but moving them anyway doesn't improve things. The resistance of the MW coil is about right.

It is running off 9v on a bench supply by the way.

Are transistor voltage readings the next thing to try, then?

Paul

[Ambientnoise]

I overhauled one recently and found it was very easy to crack a pcb track whilst changing the caps. Beyond what has already been suggested about the switch (and after cleaning, check that the static spring leaves are making with the moving leaves) look for track breaks where you have worked around the front end.

Ken

[paulsherwin]

It never does any harm to check the circuit voltages, but I still think you have some sort of RF coil problem. There should be a very obvious peak as the coil is moved about on the ferrite rod, and if you can't detect a peak then the coil isn't resonating at the right frequency.

[ronbryan]

I have had the fine gauge wires on the R200 aerial coils break where they terminate on the tag. It can be caused by impact with the battery or careless handling when servicing.

Ron

[PaulR]

Thanks all

I found replacing the component surprisingly easy with no lifting of the tracks at all. The resistance of the MW coil seems right so I doubt there is a problem with the coil itself. I have checked the tags for poor joints just in case.

I think I will follow my late mother's advice when posed with a problem like this - put it away for a while and look at it afresh and a solution will often appear. In the meantime I could just listen to R4 on it as that comes through very well.

I hate to say it but I can see why these sets were very popular for those having had bulky, immobile valve ones!

Thanks again

Paul

[PaulR]

I have got the set out again. The MW problem is odd (at least to me). It picks up a couple of stations at around 250m quite strongly but the tuning seems "wide" in that they cover a wider band than I would have expected. My pantry transmitter is set to around 450m and I cannot pick that or anything else at that end at all. By contrast my pre-war HMV is picking up loads of stations now it has gone dark. There is still a bit of whistle when it does tuned into a station.

Moving the MW coil on the ferrite rod doesn't improve things and it is fairly clear where it originally was due to the original fixing marks.

I have replaced all the wax and electrolytic caps but no resistors as I couldn't get sensible readings for them in circuit.

Has anyone any further ideas, please?

Thanks

Paul

[G6Tanuki]

This sounds to me like you've got tracking/padding issues.

Check with a frequency-meter or second receiver that the L.O. is tuning the correct range on MW. And check that the MW antenna tuned-circuit is actually being tuned, rather than acting as an aperiodic circuit.

[a grid-dip-oscillator's the ideal tool for this second check].

[PaulR]

I am storing a fairly decent oscilloscope for my son. Can I use that to check the LO?

[G6Tanuki]

It's hard to measure frequency with just a 'scope; you'd need an accurate-to-better-than-0.5% calibration on it, which would imply it's recently been to a decent [expensive!] certification lab and has the timebase-calibration charts to prove it.

A simple HF communications receiver [covering 1 to 2 MHz, with a BFO] or a frequency-counter will be a much easier tool to use to check the LO tracking and coverage.

Know anyone with an AR88??

[Goldieoldie]

Hi,
On the tuning capacitor there are two trimmer caps.
One is for the local oscillator and the other for the aerial tuning
Slowly turn one ,if the station de tunes then it's the osc .Leave it where it was
Turn the other one ,it should peak on a station ( normally set at the higher end on MW about 250 mtrs ).If it does not then there is a problem in aerial tuning circuit .Its quite a small value cap so won't make any difference on LW
Cheers Pete

[PaulR]

Sorted! The LO wasn't tracking properly at all. Several of the leads came off the switches as I was replacing components and I thought I had put them back correctly. I began to wonder about this and found a photograph of the switch connections on another thread. I found I had connected the lead that goes from S5 to L8 to the wrong end of S5. Reconnecting it has greatly improved things and I just need to re align the RF now.

Many thanks

Paul

[PaulR]

Just another problem with this set which is apparent now. There is a continual hiss coming from the speaker which is independent of the volume control setting. I had assumed it was something to do with the RF section but now I realise it must be the AF. Do the transistors go noisy or is it likely to be something else?

Thanks

Paul

[paulsherwin]

Yes, AF transistors can go noisy. Change the first transistor after the volume control and see what happens. Most Ge types will usually work.

[PaulR]

Thanks Paul, I have sent off for a replacement.

[paulsherwin]

I don't have a circuit to hand but assuming it's an OC71 or similar then anything from a scrap radio is likely to work including an OC44/45 or their Japanese/American equivalents. Even PNP silicon will probably be OK.

[PaulR]

It's an OC78D but as this is my first foray into transistor sets I don't have any spares. I will wait for the little packet to arrive from ebay!

Paul

[PaulR]

The transistor arrived today (excellent service) and has been fitted. The background noise has now completely gone. Thank you Paul.

There is a final, cosmetic, issue. The silver has worn off the Roberts logo on the front. I have seen from other threads that these can still be obtained. Does anyone know how? I realise the replacements are gold but that would actually look better on my set.

Thanks

Paul

[David G4EBT]

You can buy badges (and telescopic aerials) from Roberts at:

http://www.robertsradiotechnical.co.uk

Might be a good idea to phone their technical dept on: 03330 142 505 to check which badge would be the best fit for your R200 - I think the sizes vary slightly. No personal experience of buying stuff from Roberts, but I know of others who have.

Hope that helps.

Well done on fettling up the set Paul!