Roberts R200 whistling

[Hammonds]

Good morning all

I'm trying to sort out a Roberts R200 for a friend. I think it dates from about 1961.

when I first connected up to my 9.00 volt power supply and switched on, a very loud whistle appeared on LW, regardless of the tuning position. MW was silent. A dose of Servisol to the wavechange switch resulted in the loud whistle on both wavebands!

Disconnected the slider from the volume control - no change, with total current being drawn at about 45mA.

Reconnected volume control.

Replaced C11 (Trader 1449)with 100mFd 16V - whistle level much quieter and current now 23.68mA.

Measured transistor voltages:

emitter base collector
TR1 OC44 1.520 1.459 7.46

TR2 OC45 0.404 0.582 7.45

TR3 OC45 1.131 1.292 7.45

TR4 OC81D 1.292 1.445 8.53

TR5/6 OC81 0.126 0.159 8.91

So, the voltages look pretty much like those shown in the Trader sheet, so does that mean that all the transistors are working OK?

I noticed that the tone varied considerably as I brought a voltage probe close to L6/7/8.

Can anyone help me, please as to what to try next?

Best regards

Dave

[ronbryan]

Try changing (or checking) C20 (100uF 16V). This is the HT decoupler for the RF and IF section and if faulty could give unwanted feedback.

In addition, any brown DCC capacitors will be leaky and may prevent the RF and IF sections in the set from working properly

The normal current for the set should be about 9mA with volume at minimum.

Ron

[TrevorG3VLF]

The high current could be due to the whistle, the power has to come from somewhere.

Try running on a PP3 battery. A switch mode supply can give a whistle.

[Hammonds]

Many thanks for your help, Ron

Changing C20 and the brown caps has got rid of the whistle.

I'm left with a gentle hiss at max volume.

On MW, when I tune to about 500 metres, there is a click from the speaker. If I leave the tuning at that position and turn on a nearby light there is a definite click from the speaker. (so it's picking up RF?).

What do I do next? Does it need re-aligning?

Thanks again

Dave

[ronbryan]

You will have to see if the local oscillator Tr1 is running, as it does not seem to tune. A click as you move the tuning control may indicate a short on the tuning cap at that point.

If you have a scope, connect it to the emitter of Tr1 (i.e. junction of C5 and R3) and tune around to see if you can detect any oscillation on either band.

Otherwise, it should be possible to detect the oscillator using another MW radio placed close by (try this with another working set if in doubt). The oscillator will run at 470kHz above the frequency shown on the tuning dial.

If the oscillator is running, set the volume to max, tune to your local strong station and try to increase the signal strength by touching the appropriate aerial coil on the ferrite rod. It is not unknown for aerial coils to have a broken wire at the tag, due to collisions with the battery.

Obviously, if you have a signal generator, use it to trace the signal through the IF and RF stages.

Ron

[Hammonds]

Hi Ron

Thanks once again.

Tried the portable radio set to MW trick and get a very strong whistle on that set at one point on the R200 tuning dial, but not vice versa. So I presume that points to the local oscillator on the R200 working.

I'll have a look at the continuity with the ferrite rod aerial.

Cheers

Dave

[Hammonds]

Hi Ron

I've checked the leads from the ferrite rod for continuity and shorts - all OK.

Re-checked the bias on TR3 got the same voltages as before, as per your earlier post.

Tuned to our local MW station on 300M but not a peep (it's very strong on my R900).

Without a af modulated RF signal generator what can I try next, please?

Best regards

Dave

[ronbryan]

Can you re-check that the oscillator is running at the 300m dial position, using the procedure as before. If it is ok, check that the detector diode X1 conducts in one direction only - you may have to lift one end to confirm this.

On your first post you said that you replaced C11 with 100uF. The agc decoupler should really be 10uF, although I doubt if that is anything to do with the current problem.

Can you confirm that your R200 is the one with single tuned IF transformers and no adjustable bias pot underchassis for the OC81 pair - i.e. serial no. less than 36,546.

You could build a signal injector like this to help trace the signal path.

http://www.zen22142.zen.co.uk/Circui.../sig_trace.htm

Ron

[Hammonds]

Hi Ron

Thanks again.

Local oscillator still running, but the emitter of TR1 is now -1.510V and the base is -1.517V whereas on the trader sheet it's shown as 1.5V and 1.43V respectively.

Checked the OA 70 with my Fluke - 8.38k in one direction and 104k in the other and set to the diode setting - voltage drop in forward direction of 0.293V and shows OL in the reverse.

I haven't got a modulated RF signal generator, but I have got an old Philips oscilloscope, so I could probe around, if you think that would help.

Best regards

Dave

[Hammonds]

Hi Ron

I re-soldered all the connections to L1 and I guess there must have been a dry joint there because the set came to life!! But only on the long wave!!

Any ideas, please about bringing medium wave back to life?

I really appreciate your help.

With best regards

Dave

[ronbryan]

It's surprising that re-soldering L1 made any difference as it is the coupling coil for the car radio aerial and would have no effect unless it was short-circuited.

For the missing MW problem, try cleaning the wavechange switch contacts again with a little Servisol on a cotton bud and check all the wires are still correctly connected. Check that the S1 and S3 contacts do switch correctly with a meter, confirming that the LW coil is shorted for MW operation by S1 and that S3 closes for MW.

Use your 'scope to look at Tr1 emitter to confirm that the oscillator frequencies generated for MW and LW are different.

Look with an eyeglass for dry joints on the circuit board. Re-solder suspect joints.

Try building the signal injector/tracer.

Ron

[Hammonds]

Hi Ron

My mistake - I should have said the L9/L10 transformer.

Tr1 frequencies are different and look like a reasonable sine wave.

I'll get hold of some Servisol and check the selector switch again.

Many thanks.

Best regards

Dave

[Hammonds]

Good evening, Ron

Thanks for being so patient.

I think this set has been "got at" and the problem appears to be related to the connections on the aerial coils and the selector switch.

The junction of L2 and L4 have been connected to earth! On my Trader sheet 1449 there is a black blob between the car aerial input and the junction of C1 and L2, should there be a connection between the two?

I'm amazed I can get reception even on LW.

As usual, I would value any help you can offer.

Best regards

Dave

[ronbryan]

Dave

I believe that all versions of the R200 fitted with OC44/45 transistors have the car radio coil L1 isolated from the chassis.

Perhaps you could quote the serial number of the set, because on later versions of the first version, a different wavechange switch was fitted, with different connections to the aerial coils (from s/n 26705). See Trader sheet 1602 third page. The coil component references are also different, to further complicate the issue, so maybe the aerial coils are different too.

Ron

[Hammonds]

Good morning, Ron

Thanks for your prompt reply

The serial number is SC 83038, but the switch definitely looks like the switch shown on the 1449 part of the trader sheet i.e. 4 pole 3 way. On your attached thumbnail, maybe I'm having an off day but it's not immediately obvious to me where the poles of the switch are.

To me it all looks a bit strange and complicated seeing that there are no
AF117s in my set.. Maybe I should revert to the original 1449 layout and rewire the switch.

With best wishes

Dave

[ronbryan]

Dave

The serial number you have quoted is a bit strange. Have a look at the white plastic label pinned to the inside of the rear door. This should be just a numeric value, with no letter characters.

Before you alter any switch wiring, I suggest you document exactly where each wire goes. Perhaps you could post a photo of the switch and aerial coil assembly, if you have a camera that can focus on close objects.

The poles of the modified switch are the longer ones, that continuously wipe the moving contacts. Although the switches are shown on the circuit diagram as individual single pole on/off, I believe they are constructed as a single pole change-over, with a blank in the middle for the off position. Anyway, it seems as if the early switch is fitted, which appears to be a more conventional 4 pole 3 way type.

Ron

[Hammonds]

Hi Ron

Thanks again for your speedy reply.

I can see where the serial number confusion arose. Serial number 25469 should have been the one I should have spotted - sorry.

Also attached are the pictures , which you requested, of the MW and LW aerial coil and the switch.

I hope that helps.

With best wishes

Dave

[ronbryan]

Dave

The serial number (less than 26705) and photos confirm that you have the first type of band switch as you indicated earlier.

I don't have an R200 here, but from the photo, the MW coil (the single layer coil on the ferrite rod) L2/L4 common point is missing its connection to the 'top' connection of the LW coil L3 (the wave-wound multi-layer coil on the ferrite rod). It looks as if the short black wire should link the MW common point to the LW coil, but if the long black wire then goes to chassis, it will earth both coils, which is not as the Trader 1449 diagram shows.

I'll try to get a look at Howard's similar R200, and confirm the wire colours and 'as installed' wiring detail - it is not unknown for Trader sheets to have small errors or for manufacturers to implement changes in production which are not fully documented.

Ron

[ronbryan]

Dave

Howard has confirmed by phone that the middle tag of the MW coil has the two black wires attached to it. It looks as if the Roberts idea of series connecting the LW and MW coils when on LW (as on Trader sheet 1449) was abandoned, certainly by serial number 14000. The aerial coil wiring will be more like the middle R200 version, but with the original 4 pole 3 way w/c switch.

If the black wires are soldered as detailed above, are you back to the previous no MW situation, or will it now work?

Ron

[Hammonds]

Hi Ron

I've now made the connection between the L2/L4 common point to the 'top' connection of the LW coil L3.

The common point of L3 and L5 is shown as connected to earth on the 1449 sheet, unless I'm missing something.

At the moment the 'hot' end of L3 is connected to S2 and not S1!

If you could find some information from Howard as to what coloured wire goes to which tag on the selector switch, I sure that would be huge help.

Many thanks in advance

Dave