Roberts R760 - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 21st Jul 2019, 8:05 pm

OK, it's not really a 'vintage' Roberts but £1 got me this at a car-boot today.

Case was in nice condition - it thankfully showed no white-emulsion-splash signs of being "Painter Pete's Companion". A quick rub-over with some Ambersil "Groom" foaming car-upholstery cleaner had the black leathercloth end-cheeks and the handle looking good - some teak-oil brought up the lustre of the wood bits.

Hooked up a battery - switched on - silence!

The lack of any sort of switch-on 'pop' from the speaker led me to investigate the headphone socket and then audio amp. There's a good-sounding signal on the wiper of the volume-control but the output pin of the TDA7231 is sitting at 0.7V rather than the expected half-supply-voltage.

Cricklewood have TDA7231A at under a fiver so I've ordered a couple.

If it then works I'll power it from six NiMH AA cells (I bought loads of these when Maplin closed down) and it'll probably become my bathroom-radio for a while. At least it goes up to 108MHz.

paulsherwin · 21st Jul 2019, 11:33 pm

9V radios work quite well from a couple of LiIon cells in a holder despite the voltage being a bit low. I run my bathroom Grundig Party Boy from 2 18650s scavenged from a scrap battery pack. Ideally you would use a chip to stop them over discharging, but I just use a 500mA fuse to prevent overcurrent disasters. I charge them twice a year using a USB charge controller module (3 for £1 from China).

You could just fit a different amp chip on a bit of board, even an LM386 would probably be adequate.

G6Tanuki · 22nd Jul 2019, 2:11 pm

I hadn't thought of using Li-Ions - not that I've got any spare! (I had the Maplin rechargeables and holder so that was the path-of-least-astonishment).

Yes I did think about using an alternative chip for an amp: the LM380/LM384 or TBA810/820 series came to mind. I've never really liked the LM386 - it's a weedy little chip really (less than a Watt output) and I've experienced issues with them in the past relating to high background noise/hiss-levels.

Either way, this little Roberts will be reborn!

Mike. Watterson · 22nd Jul 2019, 5:44 pm

NiMH is safer and longer life. The only advantage of the Lithium is weight.

G6Tanuki · 22nd Jul 2019, 6:26 pm

Agreed!

And for me the biggest advantage of AA NiMH is that I've got 80 or so of them here (bought at the Maplin closeout this time last year).

Stuffing old PP7/PP9 cases with 2000mAH NiMH cells is the clear way to go. It's not as if a R760 is a big battery-drainer - I'm expecting to get quite a few weeks service from six 2000mAH NiMH cells.

[I also ordered one of Cricklewood's £15-packs of telescopic aerials, having noticed that the one on my R760 was missing the last couple of segments. Hopefully I'll be able to do something-creative]

poppydog · 22nd Jul 2019, 10:16 pm

I have one of these radios, it's probably the latest radio I own, they don't sound that wonderful but do look quite nice when cleaned up. Mine is also missing the last segments of its aerial, they sell genuine replacements on eBay at £15.00 a go.

Poppydog.

G6Tanuki · 23rd Jul 2019, 6:10 pm

Replacing the TDA7231A audio-chip has restored full functionality - as Poppydog says, it doesn't sound spectacular but it'll do fine as a cheap bathroom-radio for a few months.

It's currently running from a "Diall" branded zinc-carbon PP9 from B&Q [as mentioned in another current thread] - when this dies I'll gut it and insert some of my tag--ended Maplin NiMH cells.

Mike. Watterson · 23rd Jul 2019, 6:51 pm

Though it's a Layer Cell, which are not quite the same thing as the cylindrical Zinc Carbon and Zinc Chloride, which should be banned. Layer Cells are fine except higher internal resistance than regular cells, hence used for 9V to 90V in the past, purely for electronics, not lamps, filaments or motors. Though there were split 9V and 6V kinds.
At least the layer cells don't seem to leak.

I wonder why the IC failed?

G6Tanuki · 23rd Jul 2019, 6:59 pm

I don't know why the iC failed. It had a lot of brown gunky adhesive around it (this stuff seems to have been applied randomly to other components too) and when I came to remove it from the board - by cutting its legs off - the top of the encapsulation came away very easily.

Too easily.

Looking at it, I suspect the encapsulation had failed, because there were signs of tarnish on the parts of the lead-ins that would have been encapsulated and which I'd have expected to have remained silvery.

Manufacturing fault?

Mike. Watterson · 23rd Jul 2019, 8:08 pm

Sounds like it. Often chips are made at one facility and shipped somewhere with cheap labour for encapsulation and test.
Unless someone tried to connect the battery the wrong way round.
Major power faults in mains PSUs or lightning tend to blow out the encapsulation at the chip. I saw a HDD PCB like that from a faulty PSU. Fortunately the HDD worked with a PCB off a normally failed HDD of the same model. The owner was suitably impressed.

G6Tanuki · 24th Jul 2019, 2:44 pm

Both the old and replacement chips are marked "MAL" - looking up the manufacturing-plants operated by ST Microelectronics, "MAL" could be either Malta or Malaysia.

I've only once before experienced significant non-trauma-related encapsulation-failures of chips: in the early-80s some gear I worked on included some 74xx-series plastic-cased TTL chips with very poor sealing and high failure-rate. When deployed the equipment was in locations that were - let's say - "not amenable to quick or easy servicing" - we re-specified 5400-series ceramic-encapsulated TTL throughout, and the problem went away.

I've just done one other final fix on this Roberts: the tuning-knob was slipping slightly on the spindle. The spindle has a flat on it and there are 2 little ribs moulded along the length of the 'bore' in the plastic tuning-knob, which seemingly should deform when the knob's pressed onto the spindle. I made up a shim from an old tin-can - it's in the form of a L______ with the long part running down the side of the spindle's flat.
This packs-out the flat just enough to stop the knob rotating slightly, making tuning a much nicer and more-precise task.

21st Jul 2019, 8:05 pm	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Roberts R760 OK, it's not really a 'vintage' Roberts but £1 got me this at a car-boot today. Case was in nice condition - it thankfully showed no white-emulsion-splash signs of being "Painter Pete's Companion". A quick rub-over with some Ambersil "Groom" foaming car-upholstery cleaner had the black leathercloth end-cheeks and the handle looking good - some teak-oil brought up the lustre of the wood bits. Hooked up a battery - switched on - silence! The lack of any sort of switch-on 'pop' from the speaker led me to investigate the headphone socket and then audio amp. There's a good-sounding signal on the wiper of the volume-control but the output pin of the TDA7231 is sitting at 0.7V rather than the expected half-supply-voltage. Cricklewood have TDA7231A at under a fiver so I've ordered a couple. If it then works I'll power it from six NiMH AA cells (I bought loads of these when Maplin closed down) and it'll probably become my bathroom-radio for a while. At least it goes up to 108MHz.

21st Jul 2019, 11:33 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,929	Re: Roberts R760 9V radios work quite well from a couple of LiIon cells in a holder despite the voltage being a bit low. I run my bathroom Grundig Party Boy from 2 18650s scavenged from a scrap battery pack. Ideally you would use a chip to stop them over discharging, but I just use a 500mA fuse to prevent overcurrent disasters. I charge them twice a year using a USB charge controller module (3 for £1 from China). You could just fit a different amp chip on a bit of board, even an LM386 would probably be adequate.

22nd Jul 2019, 2:11 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: Roberts R760 I hadn't thought of using Li-Ions - not that I've got any spare! (I had the Maplin rechargeables and holder so that was the path-of-least-astonishment). Yes I did think about using an alternative chip for an amp: the LM380/LM384 or TBA810/820 series came to mind. I've never really liked the LM386 - it's a weedy little chip really (less than a Watt output) and I've experienced issues with them in the past relating to high background noise/hiss-levels. Either way, this little Roberts will be reborn!

22nd Jul 2019, 5:44 pm	#4
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Roberts R760 NiMH is safer and longer life. The only advantage of the Lithium is weight.

22nd Jul 2019, 6:26 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: Roberts R760 Agreed! And for me the biggest advantage of AA NiMH is that I've got 80 or so of them here (bought at the Maplin closeout this time last year). Stuffing old PP7/PP9 cases with 2000mAH NiMH cells is the clear way to go. It's not as if a R760 is a big battery-drainer - I'm expecting to get quite a few weeks service from six 2000mAH NiMH cells. [I also ordered one of Cricklewood's £15-packs of telescopic aerials, having noticed that the one on my R760 was missing the last couple of segments. Hopefully I'll be able to do something-creative]

22nd Jul 2019, 10:16 pm	#6
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	Re: Roberts R760 I have one of these radios, it's probably the latest radio I own, they don't sound that wonderful but do look quite nice when cleaned up. Mine is also missing the last segments of its aerial, they sell genuine replacements on eBay at £15.00 a go. Poppydog.

23rd Jul 2019, 6:10 pm	#7
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: Roberts R760 Replacing the TDA7231A audio-chip has restored full functionality - as Poppydog says, it doesn't sound spectacular but it'll do fine as a cheap bathroom-radio for a few months. It's currently running from a "Diall" branded zinc-carbon PP9 from B&Q [as mentioned in another current thread] - when this dies I'll gut it and insert some of my tag--ended Maplin NiMH cells.

23rd Jul 2019, 6:51 pm	#8
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Roberts R760 Though it's a Layer Cell, which are not quite the same thing as the cylindrical Zinc Carbon and Zinc Chloride, which should be banned. Layer Cells are fine except higher internal resistance than regular cells, hence used for 9V to 90V in the past, purely for electronics, not lamps, filaments or motors. Though there were split 9V and 6V kinds. At least the layer cells don't seem to leak. I wonder why the IC failed?

23rd Jul 2019, 6:59 pm	#9
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: Roberts R760 I don't know why the iC failed. It had a lot of brown gunky adhesive around it (this stuff seems to have been applied randomly to other components too) and when I came to remove it from the board - by cutting its legs off - the top of the encapsulation came away very easily. Too easily. Looking at it, I suspect the encapsulation had failed, because there were signs of tarnish on the parts of the lead-ins that would have been encapsulated and which I'd have expected to have remained silvery. Manufacturing fault?

23rd Jul 2019, 8:08 pm	#10
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Roberts R760 Sounds like it. Often chips are made at one facility and shipped somewhere with cheap labour for encapsulation and test. Unless someone tried to connect the battery the wrong way round. Major power faults in mains PSUs or lightning tend to blow out the encapsulation at the chip. I saw a HDD PCB like that from a faulty PSU. Fortunately the HDD worked with a PCB off a normally failed HDD of the same model. The owner was suitably impressed.

24th Jul 2019, 2:44 pm	#11
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: Roberts R760 Both the old and replacement chips are marked "MAL" - looking up the manufacturing-plants operated by ST Microelectronics, "MAL" could be either Malta or Malaysia. I've only once before experienced significant non-trauma-related encapsulation-failures of chips: in the early-80s some gear I worked on included some 74xx-series plastic-cased TTL chips with very poor sealing and high failure-rate. When deployed the equipment was in locations that were - let's say - "not amenable to quick or easy servicing" - we re-specified 5400-series ceramic-encapsulated TTL throughout, and the problem went away. I've just done one other final fix on this Roberts: the tuning-knob was slipping slightly on the spindle. The spindle has a flat on it and there are 2 little ribs moulded along the length of the 'bore' in the plastic tuning-knob, which seemingly should deform when the knob's pressed onto the spindle. I made up a shim from an old tin-can - it's in the form of a L______ with the long part running down the side of the spindle's flat. This packs-out the flat just enough to stop the knob rotating slightly, making tuning a much nicer and more-precise task.