Roberts R800 switch. - UK Vintage Radio Repair and Restoration Discussion Forum

Viscount · 20th Jun 2022, 6:56 pm

so my old r800 has been sitting on the shelf long enough now with an intermittent on /off switch which ive heard off a few times ,its a rather flimsy design really considering the use it gets ,its a small push on /off effort,why they didnt use a switched pot i will never know,anyway back to the problem,i removed the chassis from the cabinet and tryed some switch cleaner to no avail,the prob is still there ie any slight touch of the switch and the set goes off,so looking at the underside of the circuit board i see only 2 of the switch pins are used (no suprise)so i bridged those with the opposite unused ones and hey presto perfect 1 R800 working as it should

Heatercathodeshort · 21st Jun 2022, 2:05 pm

This was a very common fault with the Roberts R800 and R900. A look at the circuit will reveal C167 1500uf connected across the DC switched side from the battery/mains unit.
Switching on produces a very heavy surge and spark at the switch contacts burning them out.
The so called cure was to replace the cap, a 470uf in the R900 with 47uf. It hummmmmed! There was no other suggestion from Roberts at the time. Reducing the value of C167 in the R800 may prolong the life of the switch but it will fail again. The switch was part of a push button waveband switch in the R900. It was horrible to change!

I had a very difficult time with my customers with the 800 and 900 that led me to letting the Roberts agency go. John.

buggies · 21st Jun 2022, 5:48 pm

Wire the capacitor to the other side of the switch??

Viscount · 21st Jun 2022, 7:50 pm

yes its an awful design really but as i said in the op linking with 2 unused contacts opposite solves the prob ,although it will prob go the same way as before but should last a while i hope

Heatercathodeshort · 21st Jun 2022, 8:03 pm

Yes George, there were a few mods but I think it suffered from very short battery life when wired before the switch. I think there were some related problems also. It is a long time ago. The R900 sounded great as a mains portable. John.

radio69 · 22nd Jun 2022, 10:00 pm

I think the R800 should receive an award as possibly the worst Roberts set ever.....
Dreadful things...

FrankB · 26th Jun 2022, 6:26 am

I might like to suggest adding a relay in place of the switch contacts. Use the switch contacts to activate the coil of the relay.
Back when I was servicing and parts were available, I would wire both switch halves in parallel on the DPST control switches or add a relay.

Philips210 · 2nd Jul 2022, 3:27 pm

Frank's suggestion looks a good solution to the problem. I am also all for doubling up on the switch contacts if possible to spread the current flow for best contact life.

On a similar note maybe a power MOSFET to carry the radios's supply current is another idea. I built a bedside timer for my late father's Roberts R701 radio which used a MOSFET to control the radio's supply. From memory the circuit was in September 2000 EPE magazine. I think it was a circuit provided by a reader.

Regards,
Symon

PaulDarzi · 25th Nov 2022, 10:47 pm

Quote:

Originally Posted by Philips210

Frank's suggestion looks a good solution to the problem. I am also all for doubling up on the switch contacts if possible to spread the current flow for best contact life.

On a similar note maybe a power MOSFET to carry the radios's supply current is another idea. I built a bedside timer for my late father's Roberts R701 radio which used a MOSFET to control the radio's supply. From memory the circuit was in September 2000 EPE magazine. I think it was a circuit provided by a reader.

Regards,
Symon

Wow, superb memory Symon!

Page 679 of EPE September 2000 indeed has a reader provided "Radio sleep timer" - with their values for about an hour, with a neat 3.5mm plug+socket arrangement that allows it to be disconnected leaving normal operation, very nice.

The actual switching uses a BUZ71A MOSFET as you accurately recalled - am going to use this idea (with different components) to help a sickly Roberts as suggested.

Philips210 · 26th Nov 2022, 9:54 pm

Hello Paul,

It's a neat little circuit that works very well and has been totally reliable.

With my dad's timer unit, I didn't make any alterations to his R701 radio. The timer circuit together with a PP9 battery was built in a diecast box which was external to the radio. The timed supply from the box simply feeding the radio's dc power jack. On the box were mounted two momentary push to make switches for starting and stopping the timed supply. I made a slight change to the circuit so it gave around thirty minutes before switching off.

The timer circuit could no doubt be built into the radio but mounting the switches without spoiling the originality of the radio could be a challenge.
I suppose the circuit could be easily adapted to give simple on/off operation without timed operation or have both as an option.

Regards,
Symon

20th Jun 2022, 6:56 pm	#1
Viscount Heptode Join Date: Jan 2018 Location: Folkestone, Kent, UK. Posts: 690	Roberts R800 switch. so my old r800 has been sitting on the shelf long enough now with an intermittent on /off switch which ive heard off a few times ,its a rather flimsy design really considering the use it gets ,its a small push on /off effort,why they didnt use a switched pot i will never know,anyway back to the problem,i removed the chassis from the cabinet and tryed some switch cleaner to no avail,the prob is still there ie any slight touch of the switch and the set goes off,so looking at the underside of the circuit board i see only 2 of the switch pins are used (no suprise)so i bridged those with the opposite unused ones and hey presto perfect 1 R800 working as it should

21st Jun 2022, 2:05 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,127	Re: roberts R800 This was a very common fault with the Roberts R800 and R900. A look at the circuit will reveal C167 1500uf connected across the DC switched side from the battery/mains unit. Switching on produces a very heavy surge and spark at the switch contacts burning them out. The so called cure was to replace the cap, a 470uf in the R900 with 47uf. It hummmmmed! There was no other suggestion from Roberts at the time. Reducing the value of C167 in the R800 may prolong the life of the switch but it will fail again. The switch was part of a push button waveband switch in the R900. It was horrible to change! I had a very difficult time with my customers with the 800 and 900 that led me to letting the Roberts agency go. John.

21st Jun 2022, 5:48 pm	#3
buggies Heptode Join Date: Jan 2009 Location: West Lothian, UK. Posts: 751	Re: roberts R800 Wire the capacitor to the other side of the switch?? __________________ George

21st Jun 2022, 7:50 pm	#4
Viscount Heptode Join Date: Jan 2018 Location: Folkestone, Kent, UK. Posts: 690	Re: roberts R800 yes its an awful design really but as i said in the op linking with 2 unused contacts opposite solves the prob ,although it will prob go the same way as before but should last a while i hope

21st Jun 2022, 8:03 pm	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,127	Re: roberts R800 Yes George, there were a few mods but I think it suffered from very short battery life when wired before the switch. I think there were some related problems also. It is a long time ago. The R900 sounded great as a mains portable. John.

22nd Jun 2022, 10:00 pm	#6
radio69 Tetrode Join Date: May 2022 Location: Hangleton, (Hove) East Sussex. UK. Posts: 61	Re: Roberts R800 switch. I think the R800 should receive an award as possibly the worst Roberts set ever..... Dreadful things...

26th Jun 2022, 6:26 am	#7
FrankB Heptode Join Date: Oct 2005 Location: Olympia, Washington, USA. Posts: 653	Re: Roberts R800 switch. I might like to suggest adding a relay in place of the switch contacts. Use the switch contacts to activate the coil of the relay. Back when I was servicing and parts were available, I would wire both switch halves in parallel on the DPST control switches or add a relay.

2nd Jul 2022, 3:27 pm	#8
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,445	Re: Roberts R800 switch. Frank's suggestion looks a good solution to the problem. I am also all for doubling up on the switch contacts if possible to spread the current flow for best contact life. On a similar note maybe a power MOSFET to carry the radios's supply current is another idea. I built a bedside timer for my late father's Roberts R701 radio which used a MOSFET to control the radio's supply. From memory the circuit was in September 2000 EPE magazine. I think it was a circuit provided by a reader. Regards, Symon

26th Nov 2022, 9:54 pm	#10
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,445	Re: Roberts R800 switch. Hello Paul, It's a neat little circuit that works very well and has been totally reliable. With my dad's timer unit, I didn't make any alterations to his R701 radio. The timer circuit together with a PP9 battery was built in a diecast box which was external to the radio. The timed supply from the box simply feeding the radio's dc power jack. On the box were mounted two momentary push to make switches for starting and stopping the timed supply. I made a slight change to the circuit so it gave around thirty minutes before switching off. The timer circuit could no doubt be built into the radio but mounting the switches without spoiling the originality of the radio could be a challenge. I suppose the circuit could be easily adapted to give simple on/off operation without timed operation or have both as an option. Regards, Symon