Yet Another Roberts R600 - UK Vintage Radio Repair and Restoration Discussion Forum

cathoderay57 · 11th Nov 2022, 1:58 pm

Hi folks, I have been gifted a decent looking Roberts R600, newer style. It works on FM but not AM (totally silent). I have checked the obvious things and at the moment I am guessing that the AM oscillator transistor within the Mullard LP1171 IF module is kaputt. I have successfully removed the LP1171 and opened it. It contains 3 x AF114 transistors (albeit their soldering indicates that they have been removed or replaced before). I have ordered 3 x AF124 from Langrex and will replace them then see what happens. The question I have got has been addressed before but I don't think answered definitively: this is, has anybody actually used a 9v wall-wart for one of these (like this https://www.ebay.co.uk/itm/154776431163 ) and did it cause background noise/interference? Clearly I could build my own stabilised PSU but it is likely to cost more since I don't have all the bits in my spares box. Jerry

simpsons · 11th Nov 2022, 4:54 pm

I would use a linear power supply in preference to a switched mode supply.

That is a transfomer with a regulated output which will not interfere with your am reception or those on other receivers near by.

Such supplies can be sourced new from Radio Spares Part No 139-1769 or second hand Mnf. by other makes on eBay.

On eBay, you may well find that the supplier doesn't tell you how the PSU works but as a rule of thumb, SMPS are physically small and can operate between 110 and 240 whilst transformers are big and only work at 240 volts ac.

Alternatively, ask members if they have one spare or for sale in the thread on this site.

Unfortunately I am unable to help in this instance.

Chris

paulsherwin · 11th Nov 2022, 6:34 pm

I've found switchers are generally fine, though it's certainly a lottery if you're going to buy one.

Don't you have the usual banana box full of old wall warts sitting somewhere? Radios like this aren't very fussy. A 6V unregulated linear will probably be fine.

cathoderay57 · 11th Nov 2022, 7:10 pm

Thanks both. My wall wart collection has nothing below 12v. However, eventually I did find a 9v 6VA RS mains transformer, some Si bridge rectifiers and a small aluminium enclosure and so I am in process of knocking up a PSU and will probably use an L7909 -9v regulator. Jerry

paulsherwin · 11th Nov 2022, 7:26 pm

You can of course use a 12V wall wart with a separate regulator to adjust the voltage. You could mount the reg inside the R600 if you don't care about keeping it stock.

cathoderay57 · 12th Nov 2022, 7:29 pm

Thanks Paul. My replacement transistors arrived from Langrex today so I launched into the LP1171 IF module. The circuit that I downloaded was of so low a definition that it was more or less illegible and no help. I found Radiocruncher's You Tube videos and from one of those identified the transistor base connections on the LP1171 PCB. For ease of reference for those who have not yet worked it out I have attached an annotated image of the reverse side of the PCB. One of the AF114 transistors had short circuits to case. The other 2 measured OK but rather than put up with the fandangle of opening it up again, I replaced all 3 using the Langrex AF124 devices. I stuck a label on the outside of the module case stating that AF124 replacements had been fitted and the date. To cut a long story short, following reassembly, all 3 bands worked perfectly. Jerry

paulsherwin · 12th Nov 2022, 7:41 pm

It's the old AF11x story I'm afraid.

Station X · 17th Nov 2022, 12:42 pm

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=196089

cathoderay57 · 17th Nov 2022, 7:20 pm

L7909CV -9.0v regulator arrived today so I'll finish off the homebrew PSU tomorrow and let you know if all works well. If it does, I shan't bother with a wall wart but thanks for the ideas. Jerry

cathoderay57 · 20th Nov 2022, 10:08 am

Homebrew PSU completed and working well. 9-0-9v transformer parallel connected to a bridge rectifier with 2200uF cap produces -15v offload. Rectifier output fed to L7909CV input, with output decoupled by 1uF tantalum cap. Produces -9.2v on load. Works well on all bands, no buzzing. I left the supply leads floating since connecting one side to earth made no difference. Jerry

simpsons · 20th Nov 2022, 11:13 am

Jerry, excellent news.

For the record, I had serious interference on AM when listening to my Roberts
R 606. So bad that even Radio4 LW was drowned out.

Turned out to be my coffee maker. The type where you put a pod into a space made only for one make of pod, a Christmas present favoured by those who bought it for me and only used by them. My nearest and dearest has left it switched on at the mains.

Why it uses a SMPS to run the contraption I'll never know but who ever designed it, suitably modified, it would make a good broad band AM "minipod" transmitter!

Chris
PS Coffee pod - minipod, get the joke? Ah well they say humour doesn't travel!

PaulDarzi · 20th Nov 2022, 11:19 am

Quote:

Originally Posted by cathoderay57

Homebrew PSU completed and working well. 9-0-9v transformer parallel connected to a bridge rectifier with 2200uF cap produces -15v offload. Rectifier output fed to L7909CV input, with output decoupled by 1uF tantalum cap. Produces -9.2v on load. Works well on all bands, no buzzing. I left the supply leads floating since connecting one side to earth made no difference. Jerry

Well done, thanks for the details - all noted for future use

Joxtinkerer · 21st Nov 2022, 5:57 pm

Hi,

I tested this PSU with the following 9V 'wall wart' and I've been using them for a few months now. They work fine with the following models:

R250 Revival
R760
R761
R505
RT22
RFM3
R701

Should work fine with your R600 too and they're 5.99 each

https://www.ebay.co.uk/itm/174511657744

Regards,

Craig.

cathoderay57 · 21st Nov 2022, 10:05 pm

Hi Craig, thanks for the info. I note that the wall wart is unregulated. What does the offload voltage measure at? Cheers, Jerry

Joxtinkerer · 22nd Nov 2022, 4:04 pm

Hi Jerry,

I haven't checked it yet but Paul Darzi has ordered a couple, so either I'll find my multimeter (missing) or I can ask him. I was worried these would introduce a lot of hum as they were unregulated but they work fine.

Regards,

Craig.

paulsherwin · 22nd Nov 2022, 4:11 pm

Lack of regulation doesn't lead to hum, it just means that the supply voltage will vary depending on the current being drawn. This is undesirable, but usually acceptable with a typical transistor radio. You just need to ensure that the off load voltage doesn't rise high enough to fry something.

11th Nov 2022, 1:58 pm	#1
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Yet Another Roberts R600 Hi folks, I have been gifted a decent looking Roberts R600, newer style. It works on FM but not AM (totally silent). I have checked the obvious things and at the moment I am guessing that the AM oscillator transistor within the Mullard LP1171 IF module is kaputt. I have successfully removed the LP1171 and opened it. It contains 3 x AF114 transistors (albeit their soldering indicates that they have been removed or replaced before). I have ordered 3 x AF124 from Langrex and will replace them then see what happens. The question I have got has been addressed before but I don't think answered definitively: this is, has anybody actually used a 9v wall-wart for one of these (like this https://www.ebay.co.uk/itm/154776431163 ) and did it cause background noise/interference? Clearly I could build my own stabilised PSU but it is likely to cost more since I don't have all the bits in my spares box. Jerry

11th Nov 2022, 4:54 pm	#2
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: Yet Another Roberts R600 I would use a linear power supply in preference to a switched mode supply. That is a transfomer with a regulated output which will not interfere with your am reception or those on other receivers near by. Such supplies can be sourced new from Radio Spares Part No 139-1769 or second hand Mnf. by other makes on eBay. On eBay, you may well find that the supplier doesn't tell you how the PSU works but as a rule of thumb, SMPS are physically small and can operate between 110 and 240 whilst transformers are big and only work at 240 volts ac. Alternatively, ask members if they have one spare or for sale in the thread on this site. Unfortunately I am unable to help in this instance. Chris Last edited by simpsons; 11th Nov 2022 at 5:03 pm.

11th Nov 2022, 6:34 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,945	Re: Yet Another Roberts R600 I've found switchers are generally fine, though it's certainly a lottery if you're going to buy one. Don't you have the usual banana box full of old wall warts sitting somewhere? Radios like this aren't very fussy. A 6V unregulated linear will probably be fine.

11th Nov 2022, 7:10 pm	#4
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Yet Another Roberts R600 Thanks both. My wall wart collection has nothing below 12v. However, eventually I did find a 9v 6VA RS mains transformer, some Si bridge rectifiers and a small aluminium enclosure and so I am in process of knocking up a PSU and will probably use an L7909 -9v regulator. Jerry

11th Nov 2022, 7:26 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,945	Re: Yet Another Roberts R600 You can of course use a 12V wall wart with a separate regulator to adjust the voltage. You could mount the reg inside the R600 if you don't care about keeping it stock.

12th Nov 2022, 7:29 pm	#6
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Yet Another Roberts R600 Thanks Paul. My replacement transistors arrived from Langrex today so I launched into the LP1171 IF module. The circuit that I downloaded was of so low a definition that it was more or less illegible and no help. I found Radiocruncher's You Tube videos and from one of those identified the transistor base connections on the LP1171 PCB. For ease of reference for those who have not yet worked it out I have attached an annotated image of the reverse side of the PCB. One of the AF114 transistors had short circuits to case. The other 2 measured OK but rather than put up with the fandangle of opening it up again, I replaced all 3 using the Langrex AF124 devices. I stuck a label on the outside of the module case stating that AF124 replacements had been fitted and the date. To cut a long story short, following reassembly, all 3 bands worked perfectly. Jerry Attached Thumbnails

12th Nov 2022, 7:41 pm	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,945	Re: Yet Another Roberts R600 It's the old AF11x story I'm afraid.

17th Nov 2022, 12:42 pm	#8
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Yet Another Roberts R600 One post moved to a new thread here:- https://www.vintage-radio.net/forum/...d.php?t=196089 __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

17th Nov 2022, 7:20 pm	#9
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Yet Another Roberts R600 L7909CV -9.0v regulator arrived today so I'll finish off the homebrew PSU tomorrow and let you know if all works well. If it does, I shan't bother with a wall wart but thanks for the ideas. Jerry

20th Nov 2022, 10:08 am	#10
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Yet Another Roberts R600 Homebrew PSU completed and working well. 9-0-9v transformer parallel connected to a bridge rectifier with 2200uF cap produces -15v offload. Rectifier output fed to L7909CV input, with output decoupled by 1uF tantalum cap. Produces -9.2v on load. Works well on all bands, no buzzing. I left the supply leads floating since connecting one side to earth made no difference. Jerry

20th Nov 2022, 11:13 am	#11
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: Yet Another Roberts R600 Jerry, excellent news. For the record, I had serious interference on AM when listening to my Roberts R 606. So bad that even Radio4 LW was drowned out. Turned out to be my coffee maker. The type where you put a pod into a space made only for one make of pod, a Christmas present favoured by those who bought it for me and only used by them. My nearest and dearest has left it switched on at the mains. Why it uses a SMPS to run the contraption I'll never know but who ever designed it, suitably modified, it would make a good broad band AM "minipod" transmitter! Chris PS Coffee pod - minipod, get the joke? Ah well they say humour doesn't travel!

21st Nov 2022, 5:57 pm	#13
Joxtinkerer Diode Join Date: Nov 2022 Location: London, UK. Posts: 9	Re: Yet Another Roberts R600 Hi, I tested this PSU with the following 9V 'wall wart' and I've been using them for a few months now. They work fine with the following models: R250 Revival R760 R761 R505 RT22 RFM3 R701 Should work fine with your R600 too and they're 5.99 each https://www.ebay.co.uk/itm/174511657744 Regards, Craig.

21st Nov 2022, 10:05 pm	#14
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Yet Another Roberts R600 Hi Craig, thanks for the info. I note that the wall wart is unregulated. What does the offload voltage measure at? Cheers, Jerry

22nd Nov 2022, 4:04 pm	#15
Joxtinkerer Diode Join Date: Nov 2022 Location: London, UK. Posts: 9	Re: Yet Another Roberts R600 Hi Jerry, I haven't checked it yet but Paul Darzi has ordered a couple, so either I'll find my multimeter (missing) or I can ask him. I was worried these would introduce a lot of hum as they were unregulated but they work fine. Regards, Craig.

22nd Nov 2022, 4:11 pm	#16
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,945	Re: Yet Another Roberts R600 Lack of regulation doesn't lead to hum, it just means that the supply voltage will vary depending on the current being drawn. This is undesirable, but usually acceptable with a typical transistor radio. You just need to ensure that the off load voltage doesn't rise high enough to fry something.