Roberts R200 whistling

[ronbryan]

Dave

I can't get the switch wire info from Howard until he can bring the set over - possibly later this week, maybe tomorrow. Meanwhile, can you produce a diagram of the connnections to and from the w/c switch, with wire colours - i.e. the current position.

Ron

[Hammonds]

Hi Ron

As requested, attached is an image of the switch with the colours of the relevant wires as I receive the set.

I hope this helps.

Regards

Dave

[Hammonds]

Hi Ron

There was a connection ('bare wire to L6' on previous image)), which used to go down to L6 which looked decidedly iffy. I replaced it with a red wire - see the revised switch/connecting wire colour image.

That's the only change I made to the switch connections.

Best regards

Dave

[ronbryan]

Dave

Thanks for the switch detail. I've had a look at it in conjunction with your aerial coil photos and ...

It looks as though this version of the R200 just switches either the LW or MW aerial coil primaries and secondaries into circuit acording to the waveband required. The LW primary is tuned by C1, C2 and C3; the MW primary is tuned by C1 and C2 only.

Roberts appear to have abandoned their initial Trader 1449 circuit arrangement of having the MW coil primary in series with the LW coil primary for LW. They also do not short circuit the LW coil primary on MW, or short the MW coil secondary on LW.

Ron

[Hammonds]

Hi Ron

Thanks for your attached circuit.

I'm hoping that I'm learning a lot for this exercise and thanks again for all your patience.

I've now remade the necessary disconnections and connections. Apart from the car aerial connection, it now looks like when I received the set - LW OK but still no MW.

So does the problem lay with the switch! Should I go ahead and rewire the switch or is there a short cut I should try first?

Do I need an a.m. modulated R.F. generator?

Should the circuit now look like Howard's?

With best regards

Dave

[ronbryan]

Dave

I've photographed Howard's switch and made another sketch of the wiring on his R200 to hopefully explain what is going on, so that you can check your wiring.

What Roberts appear to have done on both your and Howard's sets is to adopt the aerial and oscillator coil circuit of the later middle series of R200, but they have used the 4 pole 3 way switch of the first series rather than the later switch shown in Trader sheet 1602.

The attached pictures show that when MW is selected, an additional contact on the w/c switch is used to de-tune the LW coil by switching the LW oscillator trimmer (C7, C8) in parallel with the LW aerial coil. This is to make sure the LW aerial does not resonate in the MW band under these circumstances. I did not realise this when I made the sketch for post #24.

I suggest that you check that the wiring is as per my latest sketch / photo and then try to confirm that the MW coil is correctly selected by the MW position on the w/c switch.

To perform correct alignment of the RF and IF circuits, you will need to find a modulated signal generator. The generator would help to trace this 'no MW' fault as well. Can you borrow one?

Ron

[Hammonds]

Good evening, Ron

Thanks for your email, the photo and the switch/ circuit info.

I've made the connection you suggested.

I can now receive a signal on MW!! Thank you

It's only one station - "Gold" and very faint compared to R4 on long wave. On the R200 the tuning dial indicates about 210 metres, which corresponds to about 1425 kHz, so it's a bit off

It should be on 1557 kHz according to Google and it's on 1600 kHz on my
R900.

There are some extremely quiet noises as I tune from one end to the other as if it's trying to pick up a station.

I've asked around to see if there's someone near here with a modulated RF signal generator, which I could borrow, but no luck so far. How easy would it be to make one?

Is there anything I can try without wrecking the setup of the IF transformers etc ?

With many thanks to you (and Howard) for your knowledge and patience.

Dave

[Hammonds]

Hi Ron

As an addendum, I have just tried gripping L2. I picked up many stations at very good volume across the dial. So, to my uneducated brain, it looks like some sort of a tuning problem??

Regards

Dave

[ronbryan]

Dave

The lack of gain on MW could be an RF or IF alignment problem, but as I found with Neil Purling's set, low gain can be caused by incorrect bias on the IF stages or inadequate decoupling. I'm not sure whether the LW R4 performance is completely up to scratch, or just a bit better than MW - your opinion would be welcome here.

I've lost track on what components you have changed. You said you have changed C11 the 10uF AGC cap for a 100uF. Not sure whether you actually re-fitted the correct value subsequently. Did you connect it the correct way round? The +ve end goes to chassis.

A photo of the circuit board component side would be useful, as it will show what components you have changed so far.

Alignment of the set can be done roughly using stations at or around the correct alignment point. Do the aerial and oscillator trimmers on the tuning capacitor look as if they have been twiddled, or is the paint locking undisturbed?

You could try adjusting the MW aerial trimmer C2 whilst tuned to 'Gold' on MW and see if that brings the station in stronger. Mark the original position of the screw head with a pencil line, so that you can return to it if necessary and ensure you are on the right trimmer. If this improves the signal strength significantly, a more careful re-alignment will be necessary subsequently, including getting the oscillator frequency correct for the dial position.

Ron

[TrevorG3VLF]

Do NOT use a pencil line on any electronic component, it can give all sorts of problems.

[Hammonds]

Good afternoon Ron and Trevor

I had changed C11 back to 10uF back but had put it in the wrong way round! Now corrected (so used to earth being negative!).

There's no paint locking on C2 or C9 so I've put a couple of spots on the screws and the washer using a very fine felt tip pen, so I can see where they need to back to, if necessary.

Adjusting C2 made no difference, so I've put it back where it was.

I still get a much stronger signal when I touch the MW aerial coil L2.

I'll see if I can get the oscillator correct for the dial position and report back with photos
of top and bottom of the PCB.

Regards
Dave