Roberts R800 Dead - UK Vintage Radio Repair and Restoration Discussion Forum

martinhog · 12th Aug 2020, 1:21 pm

Afternoon all, after getting the case looking good again, I tried to turn the radio on for checking, it had worked before dismantling !.

Anyway, you hear a faint click when you switch on with a current draw of around 30mA for a fraction of a second which then quickly falls to a few uA.

Looking at the service sheet from this forum, my voltages around the output stage are incorrect IE T306 base is at approx 8 volts and the emitter of T307
is just under 8 volts, it should be about 4.8 volts. The output transistors check out ok on diode test on the DVM. C365 was way out of specification and was replaced, but it did not help.

All help /advice is appreciated.

Many thanks Martin.

paulsherwin · 12th Aug 2020, 2:31 pm

It sounds as if there is something wrong with the -ve power rail. The output capacitor is charging through one of the output transistors, but nothing else is getting -ve power.

martinhog · 15th Aug 2020, 10:00 pm

Many thanks for your reply. It looks like the radio was dropped at some time so maybe a crack on the 0 V line ? . I will check further, thanks.

martinhog · 23rd Oct 2020, 3:09 pm

Update: I got a different pair of eyes to look at it ! The Roberts now has the correct voltages. I had replaced L308 as it was physically damaged, however it seems this either had shorted turns or even the wrong value. Time to get a replacement. Thanks for your comments and advice.

martinhog · 27th Oct 2020, 12:02 pm

Good Morning, I now have this Roberts working on FM.
However, can anyone advise on the specification for the coil L308?. It was damaged when I got the Radio, so I fitted what I thought was a correct replacement (Red Core), this however causes the power rails to collapse. All answers appreciated, thank you

Herald1360 · 27th Oct 2020, 12:43 pm

L308 is some sort of RF tuned transformer. A random coil with a red core is unlikely to match it!

There's some comprehensive coil information in the TBA570A datasheet but whether this would be of any practical help here.........?

martinhog · 27th Oct 2020, 1:01 pm

I have purchased the service sheet from this site but the spec's for the coils are not mentioned. Thanks

FIXITNOW · 27th Oct 2020, 1:51 pm

Coil specs are rarely mentioned just a manufactures part number , i guess you will need to ask if anyone has a spares set

martinhog · 28th Oct 2020, 9:34 pm

Thanks for your replies, the data sheet on the TBA570 chip was interesting. I have put an “ad” in the wanted section.

simpsons · 29th Oct 2020, 2:47 pm

May I suggest you post the schematic.

This will help identify the part and function following which further advice can be given. A photo of the faulty component will confirm if this part has equivalents or used in other models.

Chris

AC/HL · 29th Oct 2020, 5:27 pm

The schematics are available on the DVD, so only a small excerpt can be posted.

martinhog · 16th Nov 2020, 5:19 pm

Thanks for all your replies. I have attached a small section of the Service Sheet
showing L308. I have obtained a replacement coil from a donor set but would like to revive the donor set at some time for the workshop. The original R800 now works very well on FM and MW but just white noise on LW.

12th Aug 2020, 1:21 pm	#1
martinhog Pentode Join Date: Aug 2005 Location: Ashford, Kent, UK. Posts: 204	Roberts R800 Dead Afternoon all, after getting the case looking good again, I tried to turn the radio on for checking, it had worked before dismantling !. Anyway, you hear a faint click when you switch on with a current draw of around 30mA for a fraction of a second which then quickly falls to a few uA. Looking at the service sheet from this forum, my voltages around the output stage are incorrect IE T306 base is at approx 8 volts and the emitter of T307 is just under 8 volts, it should be about 4.8 volts. The output transistors check out ok on diode test on the DVM. C365 was way out of specification and was replaced, but it did not help. All help /advice is appreciated. Many thanks Martin.

12th Aug 2020, 2:31 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Roberts R800 Dead It sounds as if there is something wrong with the -ve power rail. The output capacitor is charging through one of the output transistors, but nothing else is getting -ve power.

15th Aug 2020, 10:00 pm	#3
martinhog Pentode Join Date: Aug 2005 Location: Ashford, Kent, UK. Posts: 204	Re: Roberts R800 Dead Many thanks for your reply. It looks like the radio was dropped at some time so maybe a crack on the 0 V line ? . I will check further, thanks.

23rd Oct 2020, 3:09 pm	#4
martinhog Pentode Join Date: Aug 2005 Location: Ashford, Kent, UK. Posts: 204	Re: Roberts R800 Dead Update: I got a different pair of eyes to look at it ! The Roberts now has the correct voltages. I had replaced L308 as it was physically damaged, however it seems this either had shorted turns or even the wrong value. Time to get a replacement. Thanks for your comments and advice.

27th Oct 2020, 12:02 pm	#5
martinhog Pentode Join Date: Aug 2005 Location: Ashford, Kent, UK. Posts: 204	Re: Roberts R800 Dead Good Morning, I now have this Roberts working on FM. However, can anyone advise on the specification for the coil L308?. It was damaged when I got the Radio, so I fitted what I thought was a correct replacement (Red Core), this however causes the power rails to collapse. All answers appreciated, thank you

27th Oct 2020, 1:01 pm	#7
martinhog Pentode Join Date: Aug 2005 Location: Ashford, Kent, UK. Posts: 204	Re: Roberts R800 Dead I have purchased the service sheet from this site but the spec's for the coils are not mentioned. Thanks

27th Oct 2020, 1:51 pm	#8
FIXITNOW Octode Join Date: Feb 2007 Location: Ely, Cambridgeshire, UK. Posts: 1,975	Re: Roberts R800 Dead Coil specs are rarely mentioned just a manufactures part number , i guess you will need to ask if anyone has a spares set

28th Oct 2020, 9:34 pm	#9
martinhog Pentode Join Date: Aug 2005 Location: Ashford, Kent, UK. Posts: 204	Re: Roberts R800 Dead Thanks for your replies, the data sheet on the TBA570 chip was interesting. I have put an “ad” in the wanted section.

29th Oct 2020, 2:47 pm	#10
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: Roberts R800 Dead May I suggest you post the schematic. This will help identify the part and function following which further advice can be given. A photo of the faulty component will confirm if this part has equivalents or used in other models. Chris

29th Oct 2020, 5:27 pm	#11
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Roberts R800 Dead The schematics are available on the DVD, so only a small excerpt can be posted.

16th Nov 2020, 5:19 pm	#12
martinhog Pentode Join Date: Aug 2005 Location: Ashford, Kent, UK. Posts: 204	Re: Roberts R800 Dead Thanks for all your replies. I have attached a small section of the Service Sheet showing L308. I have obtained a replacement coil from a donor set but would like to revive the donor set at some time for the workshop. The original R800 now works very well on FM and MW but just white noise on LW. Attached Thumbnails