Roberts R303

[DonaldStott]

I've managed to extract the chassis of this little transistorised set but have a quick question regarding removal of the plastic fascia in front of the tuning dial to facilitate cleaning.

I notice that it is held in place by the headphone socket at one end and the external aerial socket at the other: -




Simple question (hopefully), any clues about how to remove these two sockets - do the metal retaining rings twist off or push off or ... don't want to break or damage the plastic fascia?

Once we've done that we can have a closer look at the electronics but to be fair this radio doesn't sound too bad without any TLC!

[G6Tanuki]

The on/off/bandswitch and the volume-controls definitely need their hexagonal nuts removing in order to release the front panel.

Can't remember precisely about the others - but if you remove the hex nuts the panel might just lift free with a bit of wiggliness.

[ajgriff]

Daft question probably but how can the hex nuts be removed with the perspex panel still in place?

Alan

[music-centre]

If you just want the Perspex panel off don't the two sockets stay attached to it and unsolder the wires from them from the back? - seem to remember it was like this.
Steve.

[G6Tanuki]

From memory - chassis is held into the case by a couple of screws through the side-plates, concealed by the carry-handle [some versions use the same screws to hold the carry-handle to the side-plates].

Remove these, and the chassis drops-out from below.

[DonaldStott]

Thanks for all your quick responses and the points go to music-centre for the correct solution - of course the wires at the back need to be unsoldered first.

The perspex fascia then simply comes off with the two sockets still attached.

[DonaldStott]

Very early exploratory stages but already I've noticed a couple of "anomalies" between Trader Service Sheet 1824 and what I have in my set.

Firstly, the introductory text states that this is a nine transistor set and then goes on to mention five in the audio amplifier and three in the i.f. module: -



Last time I looked 5 + 3 = 8?

The ERT Service Sheet 1637 from 28 March 1968 refers to and "Eight transistor and three diode battery portable"?

Secondly, looking at the circuit board in detail I noticed that we seem to have an "extra transistor" (Mullard OC45) between R1 and RV3: -



Just out of interest, does anyone know when this changed?

[DonaldStott]

And while we have the set disassembled what's the consensus in replacing those electrolytic capacitors which are now over 50 years old. There are nine in total ranging in values from 0.047μF to 200μF?

All that of course before we even consider looking inside the Mullard LP1159/1 IF module at the "infamous" AF117 transistors (x3).

[Ambientnoise]

“Extra” transistors, in marketing-speak, were frequently transistors used as diodes, in the detector or output bias circuits for example.

[David G4EBT]

A few thoughts.

Fist of all, as to the 'extra transistor' the circuit shows a BA114 detector diode which it refers to as 'MR1'. The PCB layout shows it adjacent to RV1, so as 'Ambient Noise' says, maybe they used a transistor instead, possibly with just two leads connected?

As to the electrolytics, they look like blue Philips ones, which tend to leak in the electrical sense. After more than fifty years, it's likely that they'll be well past their best. Purists will say you should release one end, check the capacitance and ESR, and if need be, to replace it. That's as much of a faff as just replacing it anyway.

Assuming the set is working, one way to check if the electrolytics are behaving more like low value resistors than capacitors is to put a meter on milliamps range and connect it in line with the battery supply to check the quiescent current being drawn. (that is with the volume turned down and no station tuned in. The spec is 15mA. If it's much more, that could be due to degraded electrolytics. C14 for example (200uF) is a decoupling cap across the power supply input which - if leaky, would draw excess current.

The ERT sheet circuit has all the voltages shown at key points marked on the circuit itself, so it's worth checking the voltages to compare with the spec. Originally an AVO8 was used, (20,000 OPV), so if you used a DMM, with maybe 10 MegOhm input impedance, the voltages will be a little higher. Hence, if you do have a 20,000 OPV analogue meter, that will give more meaningful results.

The downside of unsoldering the electrolytics is that it's an SRBP PCB typical of the era - not fibreglass, and it only takes a little heat to lift the tracks, so some care is called for. All of the electrolytics have a role to play, and even if the set appears to be working, they'll degrade its performance to some extent. The lower value ones - 2.5uF, 16uF, 64uF will be physically small and will, I think, be dried out by now.

It's you set not mine, and lots of people will prefer to leave things well alone if the set is working. Many threads on here on Roberts, Bush and Hacker sets seem to focus only on cosmetics - 'where can I get a telescopic aerial, how do I clean the Rexine, can I get a new logo, are replacement strap handles available'? etc. It's a personal view, but I don't just want a set to look nice, but to sound nice too. To my mind, just tarting up set to make it look pretty is a bit like putting lipstick on a pig. Others will have a different view, which is fine - I know many people who collect sets to simply display them, and do nothing at all to them, having no intention of every using them.

If you do decide to replace any or all electrolyics, do them one at a time and test after each one, so if you make an error, you only have to check the last thing you did.

The Trader Sheet give a full description of how to set up the audio stage using a scope, 1kHz sine wave audio generator, milliammeter and voltmeter, adjusting RV2 & RV3 for the onset of clipping. It's easy to follow and you've got all the gear. It's likely that it will need adjusting to get the best audio quality, whether or not you do change the caps.

The pics below are of an R505 before and after adjustment.

Pic 1 shows clipping before adjustment, pic 2 shows a clean sine wave after.

Just my rambling thoughts, for what they're worth.

[DonaldStott]

Thanks David - that's a great help as usual and clearly sets out my Work Programme for this week.

A couple of questions: -

1. When measuring the quiescent current should that be on the RED or BLACK wire - I'm assuming in series?

2. I have a temperature controlled soldering iron - what temperature is safe to ensure that the PCB tracks don't lift?

[Herald1360]

For total battery current measurement it doesn't matter which lead the meter goes in series with- what comes out of the battery has to go back in!

There's no "safe" temperature as such. If the bit's hot enough to melt solder it'll melt the glue too. Ideally it should be hot enough and cleanly tinned so that the solder can be melted and cleared with a sucker or wick within a couple of seconds whilst taking care not to physically move the track/pad. Temperature- I normally use a Weller #7 bit which is about 700F or 370C, though a #6 at 600F or 315C will still melt solder for fine work. I would go for the hotter end- say 350C. If the old solder is dull looking, try a two step approach- melt a little fresh solder into the joint, let it cool then go back for the actual desoldering attack!

If things do go wrong, you can retrieve most disasters with some fine tinned copper wire. The fine 0.2mm threads from flexible hookup wire can be very useful here and in short lengths will carry all the current found in a typical tranny and more- fusing current is around 5A!

[DonaldStott]

Thanks Chris - very helpful.

I'm off now to measure the quiescent current for this Roberts R303 - thanks to David G4EBT and the ERT Service Chart 1637 I know that this should be 15mA.

[DonaldStott]

The quiescent current for this Roberts R303 is measuring as 17.40mA - I calculate that this about 16% above what it should be.

[ajgriff]

Presumably you're using a battery as the power source? If so and it's reasonably fresh the voltage will be a bit more than 9V. Service sheets usually quote things like quiescent current based on exact supply voltage ie, 9.0V in this case. Bearing this in mind, I think 17.4mA is quite acceptable and doesn't in itself indicate any significant capacitor degradation.

Alan

[DonaldStott]

Thanks Alan - I'm using a very new battery which has a voltage of 9.77V.

At what point over the specified 15mA would the quiescent current be considered too high e.g. currently 16% over?

[ajgriff]

To me your current variance is marginal bearing in mind the tolerances of all the components in the circuit. I tend to fall into the 'leave it alone unless there's good to reason to believe there is a fault or an imminent danger of one occurring' camp. It's a matter of judgement but I wouldn't be too concerned unless the quiescent current started to approach 20mA or above with a 9.77V supply. It would be interesting to hear other views on the matter.

It would be worth carefully setting the audio output conditions as described earlier by David and then checking the overall quiescent current again. However, you really need to do this with something like a bench supply set at 9.0V.

Alan

[Herald1360]

That's a bit like "How long is a piece of string?"

Probably around 50% high would suggest looking harder at where it was going. A few extra mA won't do either battery life or output stage temperatures much harm!

If you want to check output quiescent current there is a link which can be removed to enable this. The dc tweaking procedure is covered in the Trader sheet, the final clipping symmetry tweak is just to enable absolute max "undistorted" output and can be omitted if the test equipment isn't available.

[DonaldStott]

Thanks Alan and Chris

I really don't think there is much wrong with this Roberts R303 - I'm measuring 9.48V across C14.

I'm leaning heavily towards leaving everything as it is including the AF117s which we haven't even discussed! The set is going back to it's owner so if anything happens in the future they know where to find me!

I'll do a bit more measuring as shown in the Transistor Table in the Trader Service Sheet.



In the absence of a bench supply, that may well wrap things up as far as the electronics are concerned and I can focus on the cosmetics such as the Rexine case, the dial plastic fascia, the knobs and brights, the handle, the brass (?) trim, the Roberts logo and the slight damage to the speaker (easily fixed).

[DonaldStott]

Originally Posted by DonaldStott

I'll do a bit more measuring as shown in the Transistor Table in the Trader Service Sheet.

Having a bit more time on my hands than usual (!) I spent the afternoon measuring voltages on the transistor pin-outs.

Values in BLACK are from the Trader Sheet, values in RED are what was measured: -



Nothing really untoward that I can see here but not sure to what extent the new battery voltage of 9.77V is skewing these figures and of course all voltages were taken with a modern DMM and not a Model 8 Avometer.

I was considering trying the Audio Adjustments described in the Service Sheet but the voltage across C14 is a bit high at 9.48V. Add to this that I don't have a bench supply and just today my ageing Farnell 30-4D scope decided to stop working!