Roberts R707 fault - UK Vintage Radio Repair and Restoration Discussion Forum

Curriepug · 21st Aug 2018, 1:40 pm

Firstly I am new here so hello all, good to meet you.

I have a very nice Roberts R707 which seems to have developed a fault and before I started poking about to much I thought I would see if its a common/known issue.

When I turn it on I get nice click but volume is very low on all settings AM - FM etc, if I turn it right up I can hear it (just about) but get very large clicks and crackles?

I have switched cleaned the headphone section, and the same problem happens with headphones in place. Any pointers?

mark2collection · 21st Aug 2018, 2:38 pm

Welcome aboard

Have you cleaned the wavechange switch bank? Try probing the switch downwards with the set playing, to see if this has an effect.

Failing that, have a search on the forum & the rest of 'the net' for AF11* transistors, often referred to as transwhiskers ... aaaand, brace yourself!

A mighty fine set when working, they are a little tricky, but don't let that put you off, many folk here have tackled a Roberts R707 with marvelous results.

Have a look at Mark Hennessy's website, https://www.markhennessy.co.uk/artic...ransistors.htm which may shed some light.

Good luck,

Mark

crackle · 21st Aug 2018, 10:31 pm

Is this the worst radio in the world to work on, or am I thinking of another Roberts where it is murder to get at the IF module.

Mike

Nuvistor · 21st Aug 2018, 10:57 pm

It’s 40 years since I changed an IF module but they are a pain, I didn’t have many because they were relatively young but that was the job, my hands and eyes were much better at that type of work then than now.

David G4EBT · 22nd Aug 2018, 9:35 am

It's worth bearing in mind that given that these sets are now fifty years or so old, that the electrolytic capacitors are well passed their best, and may not even be functioning as capacitors. I think that whereas with valve sets, it's generally appreciated that fairly catastrophic results can arise from not changing certain capacitors, with transistor sets, the mindset is often that as long as they're working, it's best to leave things alone, given that it's so desperately easy to damage the tracks on those horrid Paxolin PCBs when de-soldering components, especially when it isn't always obvious which joint to desolder!

However, leaving the electrolytic caps in place can, in certain circumstances, not only degrade the sets performance, but can also cause problems leading to misdiagnosis and blaming transistors when the fault lies with defective electrolytic caps. Post #6 at this thread outlines my recent experiences with an R505 and R707:

https://www.vintage-radio.net/forum/...t=Roberts+R707

Hope that might be of interest.

Curriepug · 24th Aug 2018, 10:20 pm

Thanks for the pointers, I don't think it's an IF module problem given what I have read about how that manifests. All frequencies work just at low volume? and no amount of tapping the modules makes any difference/noise ??

Think I may have a go at cap replacement, which should I start with?

Radiocruncher · 25th Aug 2018, 7:40 am

Firstly welcome to the forum. Issues with the module can affect the volume but it’s more likely, as David has suggested, that the capacitors have failed in the amplifier section. Have a look at the service manual which is available here https://www.service-data.com/section.php/7346/1/r707 if you haven’t got it and check or change all the electrolytics from the volume control to the speaker. That’s the majority of the electrolytics in the radio so while you are at it you might as well do them all. I’ve repaired a lot of these recently and I did a video series of one recently here https://www.youtube.com/playlist?lis...WUefIMu96h4JBJ. I have more R707 content if you look at my other videos.

Good luck

Graham

Argus25 · 25th Aug 2018, 8:33 am

A good thing is to make an attempt at the diagnosis first before attempting a repair. So, for this all you require initially as a beginner are three things 1) a multimeter, digital and a cheap one is fine 2) A crystal earphone and 3) a circuit for the set.

For example, already you said that you can hear it just about (does that mean you can hear tuned stations ?) probably it could be something as simple as an open circuit or dried out electrolytic capacitor at the input to the amplifier in the audio system around the volume control area. The crystal earphone will act as a probe which is used to listen from the detector output and through the audio amp circuitry and if you add a series detector diode you can even use it to probe the RF circuits as then it will demodulate and RF carrier for you to hear.

With the meter you can check the DC conditions of the transistors, especially the voltages dropped across the emitter resistors is a good indication if all is well with the transistor stage.

21st Aug 2018, 1:40 pm	#1
Curriepug Retired Dormant Member Join Date: Aug 2018 Location: Portsmouth, Hampshire, UK. Posts: 2	Roberts R707 fault Firstly I am new here so hello all, good to meet you. I have a very nice Roberts R707 which seems to have developed a fault and before I started poking about to much I thought I would see if its a common/known issue. When I turn it on I get nice click but volume is very low on all settings AM - FM etc, if I turn it right up I can hear it (just about) but get very large clicks and crackles? I have switched cleaned the headphone section, and the same problem happens with headphones in place. Any pointers?

21st Aug 2018, 2:38 pm	#2
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 471	Re: Roberts R707 fault Welcome aboard Have you cleaned the wavechange switch bank? Try probing the switch downwards with the set playing, to see if this has an effect. Failing that, have a search on the forum & the rest of 'the net' for AF11* transistors, often referred to as transwhiskers ... aaaand, brace yourself! A mighty fine set when working, they are a little tricky, but don't let that put you off, many folk here have tackled a Roberts R707 with marvelous results. Have a look at Mark Hennessy's website, https://www.markhennessy.co.uk/artic...ransistors.htm which may shed some light. Good luck, Mark

21st Aug 2018, 10:31 pm	#3
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Roberts R707 fault Is this the worst radio in the world to work on, or am I thinking of another Roberts where it is murder to get at the IF module. Mike

21st Aug 2018, 10:57 pm	#4
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Roberts R707 fault It’s 40 years since I changed an IF module but they are a pain, I didn’t have many because they were relatively young but that was the job, my hands and eyes were much better at that type of work then than now. __________________ Frank

22nd Aug 2018, 9:35 am	#5
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,766	Re: Roberts R707 fault It's worth bearing in mind that given that these sets are now fifty years or so old, that the electrolytic capacitors are well passed their best, and may not even be functioning as capacitors. I think that whereas with valve sets, it's generally appreciated that fairly catastrophic results can arise from not changing certain capacitors, with transistor sets, the mindset is often that as long as they're working, it's best to leave things alone, given that it's so desperately easy to damage the tracks on those horrid Paxolin PCBs when de-soldering components, especially when it isn't always obvious which joint to desolder! However, leaving the electrolytic caps in place can, in certain circumstances, not only degrade the sets performance, but can also cause problems leading to misdiagnosis and blaming transistors when the fault lies with defective electrolytic caps. Post #6 at this thread outlines my recent experiences with an R505 and R707: https://www.vintage-radio.net/forum/...t=Roberts+R707 Hope that might be of interest. __________________ David. BVWS Member. G-QRP Club member 1339.

24th Aug 2018, 10:20 pm	#6
Curriepug Retired Dormant Member Join Date: Aug 2018 Location: Portsmouth, Hampshire, UK. Posts: 2	Re: Roberts R707 fault Thanks for the pointers, I don't think it's an IF module problem given what I have read about how that manifests. All frequencies work just at low volume? and no amount of tapping the modules makes any difference/noise ?? Think I may have a go at cap replacement, which should I start with?

25th Aug 2018, 7:40 am	#7
Radiocruncher Hexode Join Date: Dec 2013 Location: Torrington, Devon UK. Posts: 446	Re: Roberts R707 fault Firstly welcome to the forum. Issues with the module can affect the volume but it’s more likely, as David has suggested, that the capacitors have failed in the amplifier section. Have a look at the service manual which is available here https://www.service-data.com/section.php/7346/1/r707 if you haven’t got it and check or change all the electrolytics from the volume control to the speaker. That’s the majority of the electrolytics in the radio so while you are at it you might as well do them all. I’ve repaired a lot of these recently and I did a video series of one recently here https://www.youtube.com/playlist?lis...WUefIMu96h4JBJ. I have more R707 content if you look at my other videos. Good luck Graham Last edited by Radiocruncher; 25th Aug 2018 at 7:49 am.

25th Aug 2018, 8:33 am	#8
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Roberts R707 fault A good thing is to make an attempt at the diagnosis first before attempting a repair. So, for this all you require initially as a beginner are three things 1) a multimeter, digital and a cheap one is fine 2) A crystal earphone and 3) a circuit for the set. For example, already you said that you can hear it just about (does that mean you can hear tuned stations ?) probably it could be something as simple as an open circuit or dried out electrolytic capacitor at the input to the amplifier in the audio system around the volume control area. The crystal earphone will act as a probe which is used to listen from the detector output and through the audio amp circuitry and if you add a series detector diode you can even use it to probe the RF circuits as then it will demodulate and RF carrier for you to hear. With the meter you can check the DC conditions of the transistors, especially the voltages dropped across the emitter resistors is a good indication if all is well with the transistor stage.