Roberts R300

[Uncle Bulgaria]

A friend's given me this to repair. An obvious problem - C17, a 2µF Elkomold, had parted company with one of its leads.

I wonder if anyone can tell me why this capacitor is rated 150VDC when as far as I can tell it's just for interstage coupling?

[Radio Wrangler]

Probably what they had in the parts bins, or they wanted to stick to a standard body size to suit manufacturing automation, so rather than letting the case size go down, they opted to let the voltage rating go up.

At the voltages an OC81D and its friends run, there won't be enough DC bias for reliability with 150v electrolytics, some will start to depolarise.

David

[Paul_RK]

Wondering whether the high rating was usual, I took a look in the first R300 that came to hand. Can't readily see the values and ratings of all the electrolytics, one or two at least are 10V, but there's a 2µF 300V on view! Curious, I don't think Roberts would have had automated board assembly back in '64, and as far as I know their only products at the time and for some years previous were battery powered transistor radios.

Paul

[Uncle Bulgaria]

Thank you both - intriguing. Certainly most of the Elkomolds are 16V, then a couple are much higher without apparent reason. I've ended up having to replace all the capacitors on the board. The smaller non-polarised reddish brown Hunt's were way out of tolerance, and the Elkomold electrolytics didn't respond to reforming.

I've used some parts from the parts bin of varying voltages, but none as high as 150V! I'm going to do the alignment procedure from the service sheet, which is not something I've attempted before. The instructions make it look quite straightforward though.

I need to make a coupling coil from 'approximately 14 turns of 18SWG wire on a 1" former'. It seems solid core mains wire fits the relevant hole in my wire gauge, so I was planning to use that.

Is the 1" former crucial? I have a small bottle that's slightly larger than that, and am planning to wind it on then leave it as an air core, connected to my Lyons SQ10 oscillator, which apparently goes up to 10,000kHz. Is this sufficient?

[ms660]

According to what I'm looking at your signal generator isn't suitable as it only goes up to 100kHz.

If you happen by a suitable generator (one that covers the required frequencies and has internal AM modulation or an input for AM modulation) you will have to be sooooo careful with any transformer slug adjustments, the right type of adjustment tool is required....and....the slugs must not be tight otherwise they might crack, if they seem to be stuck and you try to use force then it's possible that you can end up damaging the coil formers and snapping the transformers Litz wire from the terminals.

The dimensions of a coupling coil aren't that critical, neither is the type of copper wire that's used, eg: solid or muti-strand will be ok.

Lawrence.

[Uncle Bulgaria]

Have I read this wrong then? The generator (as I interpret the dial) has a 100kHz multiplier, and a 1-10 dial, so I could do all except the two instructions that need 1360kHz.

I don't expect it to be out of alignment as I've only changed capacitors not transistors, and was told it was working until the capacitor lead broke. I've been able to tune in to a little, but haven't tried outside (I've been working on it in a semi-underground room, so reception's not great at the best of times).

[AD360 Rob]

Your generator isn't suitable for alignment as apart from not having the required frequency range, it isn't modulated. Ideally you need something that will go up to a few Mc/s (mHz)and has AM modulation capability. you don't need to worry too much about accuracy as long as it's close, you'll be ok. I use an old Advance E2. Also, I'll re-iterate what has been mentioned about the slugs in the I/F transformers. You need to be super gentle with them as they will fracture very easily. Having said that, if the I/F transformers don't look to have been fiddled with, chances are it will not need that part of the realignment procedure.

[ms660]

Originally Posted by Uncle Bulgaria

Have I read this wrong then? The generator (as I interpret the dial) has a 100kHz multiplier, and a 1-10 dial

Sorry, missed the multiplier, still not much cop as it stands, Advance E2, Taylor, Avo All Wave etc were the most popular in workshops back in the day, by the way you don't have to adjust the IF transformers to check their alignment, just do a manual sweep around the IF and see what frequency the output peaks at.

If you ever do need to adjust and the slugs are slotted all the way through then a hardened copper or phosphor bronze blade set in a plastic body is the type of trimmer to use like the one in the link below......Page 7...2nd photo....Top left, but remember as I said earlier that they're easily damaged:

http://eddystoneusergroup.org.uk/Res...0Alignment.pdf

You can do alignment without a modulated generator but you would have to monitor the output of the detector with a 'scope or a sensitive meter.

Lawrence.

[Uncle Bulgaria]

Thank you for the thoughts. It looks like I'll just clean the case up and see how she finds it with the out of tolerance components replaced - alignment seems like the next big area to tackle. Perhaps if I can get to Retrotech there'll be a modulated generator there with my name on. The ones you mentioned look like characterful pieces of equipment, Lawrence!

[Uncle Bulgaria]

It seems a Taylor 68A has my name on it thanks to Sinewave, if the FCS purrs along nicely.

Until then, I have been cleaning up the case, and applying some 'Cherry Blossom' green shoe cream, which I and the man in Timpson's decided was the best match.

I have sent a PM to Peter Kinnaird, as there was a 2011 thread in which he'd said how he'd had a number of brass inserts for the R300 knobs made, but I haven't had a response as to whether he still has any. I've got a bit of thin brass sheet here, but making a decent circle without a central hole seems a tall order.

The handle's very grotty. Has anyone had any luck re-stitching the leather? The holes look very small for my leather needles. I see a number of threads about new handles from Roberts, but these all date from about ten years ago and the Roberts Spares site doesn't list them any more. The underlying plastic is fine, so I don't need a whole new handle anyway.

[Andy 1964]

Here is my case restoration, the gold trim around the speaker and dial aperture is car interior molding trim from that well known rain forest website. The P shape molding is exactly same dimensions as the original, just needed to trim the recess strip slightly. The plastic wire insert is resprayed, but have since found crafting wire in a gold finish or the correct diameter.

[Uncle Bulgaria]

The radio's come back and now has not a peep out of it. I've replaced two of the AF117s with freshly-zapped ones that are recognised correctly on my transistor tester. However, still no sign of life. There's a small 'pop' from the loudspeaker when the set's turned 'on' to either MW or LW, but no hiss nor reception.

I have injected a signal at the volume control and the AF section appears to be working. Next I'll put a 'scope on the preceding stages. I have no experience with RF so I'm not sure what to look for beyond a signal of some sort that passes through the AF117 stages. I have the Service Data sheets, and now a Taylor 68A signal generator! I hope to check the alignment once it's picking something up.

[Uncle Bulgaria]

I'm powering this with a Thurlby PL310, to save the battery. As the positive terminal of the battery is grounded through a diode and the volume control, am I correct in thinking that, as the Thurlby's instructions say:

"The load should be connected to the positive (red) and negative (black) terminals marked OUTPUT. Both are fully floating and either can be connected to ground."

then it is not a problem to use it as the oscilloscope reference ground just as if the set were being powered by a battery?

[ms660]

PSU +ve to +ve battery connector.

PSU -ve to -ve battery connector.

PSU Earth/ground....No connection needed.

'scope ground to the cathode side of MR3 (the thick Black line rail in the ERT Service Chart schematic)

The pot. across MR3 is for adjusting the quiescent current for the two output transistors.

Lawrence.

[Uncle Bulgaria]

Thank you, Lawrence.

I've made up a little coil as per the service sheet so I can see if the first stages are picking anything up. The signal generator has a coaxial connector, so I assume I make up a cable with the two ends of the coil connected between the centre and the braid.

I feel like there must be a failed component or a broken connection somewhere.

Oh, I checked the output stage by measuring the current at the centre tap of the output transformer primary. There's a wire link there. It was way down, at less than 1mV. Am I measuring at the right place for the 4mA quiescent?

[Uncle Bulgaria]

I think I've tracked it down. Despite checking the AF117s I put in, it turns out TR2 was faulty. I measured the EBC voltages and found TR2 was very different from TRs 1 & 3. Replacing it with a new one that measures open circuit to the screen has restored hisses and squelches as I tune the dial.

I can't pick up any stations with clarity, but there is R4 longwave under the hiss at one point. This could be the room, or the proximity to various electronic bits and pieces that could be causing interference. I'd still like to check the alignment if someone could give me pointers on how to hook up the signal generator as in my last post. (I see my post above should have read '1mA' when I was measuring the current at the OPT centre tap!)

I notice there are some cracked areas of the rings that hold the wires on the ferrite aerial. The wires appear intact. Should I just glue this back down or is the orientation critical?

[Nuvistor]

I would melt the wax and push the broken piece into the wax, that should hold it in place and allow adjustments in future if required.
The instructions for alignment should be ok and how you have made the coil seems correct. The signal generator may have lower output that the one used in the service sheet so move the coil closer to the ferrite rod. The cores in the transformers are brittle, you must use the correct tool or risk breaking the core. Depending on if the phantom fiddler as been there they should be very close to their correct position.

[Uncle Bulgaria]

Thanks Nuvistor. I'll try to warm the wax gently (I didn't realise that's what was on the aerial)!

I am still uncertain how to connect the coil to the coax I have coming out of the generator, as per post #15.

Is it a reasonable check to apply a modulated signal at various frequencies and see whether the radio picks it up at the same positions on the dial?

[Radiocruncher]

I’ve used a small cable tie to hold the clip onto the ferrite coil in the past. The way you have suggested connecting the coil to your sig gen is correct. I would check the IF frequency is getting through first as it could still be one of the dreaded AF117’s playing up. Also worth testing the diodes. There is a chap I know that makes handles and also provides the knob inserts so if you need these then drop me a message and I will let you have his email address.

Good luck

Graham

[Uncle Bulgaria]

Thank you for all the pointers.

I couldn't get the wax to hold, so have put a small cable tie around the broken fibre ring for now. I wanted to fiddle with the signal generator, so have soldered the coil to the coax + plug I made up. I've put an AVO 8 across the loudspeaker as the 'AC Voltmeter on low range' suggested in the trader sheet.

I guess the markings on the dial are metres. 450 on the dial should then correspond to 667KHz.

I've turned the radio to MW and 450. The signal generator has modulation 'on', attenuator zero and RF output 10 (both maximum, or minimum for an attenuator?). There's a warble from the radio 'speaker when the signal generator's on 640KHz. This remains when modulation is turned off.

Sweeping across the dial shows that the generator can cause the radio to whistle at specific frequencies, and go silent (removal of hiss). Otherwise, the radio puts out a healthy amount of hiss and picks up some normal radio. It also goes very loud (I have a temporary 8 Ohm 'speaker in place of the 3 Ohm that remains in the case, as it's hard to remove), and plays music injected at the volume control just fine, so the AF section is OK.

I'll get the 'scope on the three RF AF117s.