Roberts R701 - UK Vintage Radio Repair and Restoration Discussion Forum

Michael Maurice · 21st May 2020, 10:09 pm

I have just finished repairing this after someone connected the wrong power unit to it.

AM works fine. but I don't get any reception on FM. I replaced both IC's.

The original socket was configured negative to centre pin, I've reconfigured it to positive on the centre pin as most modern power supplies are like that. I also fitted an 'idiot' diode to stop this happening again.

But I do need help with FM.

Nuvistor · 21st May 2020, 10:24 pm

Any circuit?

Michael Maurice · 21st May 2020, 10:41 pm

Sorry, no circuits available, from what I understand, there never has been.

simpsons · 21st May 2020, 10:53 pm

Michael

There has been an earlier posting on this radio asking for a schematic.

Whilst none was forthcoming, the issue was also NO FM.

BVWS #85086, dated Jly 2012, the problem was solved by replacing Q2 which forms a 5v regulator.

Good luck

Chris

Michael Maurice · 22nd May 2020, 4:10 pm

Hi Chris,

I've replaced Q2 and the zener (fitted 5.6V) I now get 5V on the emitter of Q2 but still no FM.

Atone stage I could pick up radio 3 but it was very faint and very very low gain.

Unless of course the diode has to be 5.1V

simpsons · 22nd May 2020, 9:24 pm

Hi Maurice.

I guess you looked at the earlier thread.

If the set uses a BA4424 FM Front End then, the Roberts R250 schematic and Mf spec for dc levels should help.

Chris

Michael Maurice · 23rd May 2020, 11:06 am

Quote:

Originally Posted by simpsons

I guess you looked at the earlier thread.

If the set uses a BA4424 FM Front End then, the Roberts R250 schematic and Mf spec for dc levels should help.

Chris

Which thread?

The radio uses a TA7358 and TA8117

This is the data sheet for TA8117N

This is the data sheet for TA7358

simpsons · 23rd May 2020, 6:09 pm

Hi Maurice.

That will be the tread on this site which read:

"Robby,

I don't know if you have solved your problem with Q2 but here is my answer.

Q2 is used as an emitter follower in a small voltage regulator cct. It's main function is to supply 4.5V to IC's 1 and 2. Q2 is a 2SC3330T/U (NPN)

When operational, voltages on Q2 should be (volume at minimum and FM selected) e=3.92v c=6.22v b=4.63 Should you need to replace the zener diode (D1) it is a RD5.1EB but any 5.1v zener of the correct wattage should do!

Best regards,
Mike"

After some research, I attach what is a typical TA 7358 front end with its voltage regulator.

You have the data sheet and typical circuit for the TA8117N

Hope this helps

Chris

21st May 2020, 10:09 pm	#1
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Roberts R701 I have just finished repairing this after someone connected the wrong power unit to it. AM works fine. but I don't get any reception on FM. I replaced both IC's. The original socket was configured negative to centre pin, I've reconfigured it to positive on the centre pin as most modern power supplies are like that. I also fitted an 'idiot' diode to stop this happening again. But I do need help with FM. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

21st May 2020, 10:24 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Roberts R701 Any circuit? __________________ Frank

21st May 2020, 10:41 pm	#3
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Roberts R701 Sorry, no circuits available, from what I understand, there never has been. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

21st May 2020, 10:53 pm	#4
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: Roberts R701 Michael There has been an earlier posting on this radio asking for a schematic. Whilst none was forthcoming, the issue was also NO FM. BVWS #85086, dated Jly 2012, the problem was solved by replacing Q2 which forms a 5v regulator. Good luck Chris

22nd May 2020, 4:10 pm	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Roberts R701 Hi Chris, I've replaced Q2 and the zener (fitted 5.6V) I now get 5V on the emitter of Q2 but still no FM. Atone stage I could pick up radio 3 but it was very faint and very very low gain. Unless of course the diode has to be 5.1V __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

22nd May 2020, 9:24 pm	#6
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: Roberts R701 Hi Maurice. I guess you looked at the earlier thread. If the set uses a BA4424 FM Front End then, the Roberts R250 schematic and Mf spec for dc levels should help. Chris

23rd May 2020, 6:09 pm	#8
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: Roberts R701 Hi Maurice. That will be the tread on this site which read: "Robby, I don't know if you have solved your problem with Q2 but here is my answer. Q2 is used as an emitter follower in a small voltage regulator cct. It's main function is to supply 4.5V to IC's 1 and 2. Q2 is a 2SC3330T/U (NPN) When operational, voltages on Q2 should be (volume at minimum and FM selected) e=3.92v c=6.22v b=4.63 Should you need to replace the zener diode (D1) it is a RD5.1EB but any 5.1v zener of the correct wattage should do! Best regards, Mike" After some research, I attach what is a typical TA 7358 front end with its voltage regulator. You have the data sheet and typical circuit for the TA8117N Hope this helps Chris Attached Thumbnails