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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 20th Jun 2022, 5:56 pm   #1
Viscount
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Default 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
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Old 21st Jun 2022, 1:05 pm   #2
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Default 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.
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Old 21st Jun 2022, 4:48 pm   #3
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Default Re: roberts R800

Wire the capacitor to the other side of the switch??
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Old 21st Jun 2022, 6:50 pm   #4
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Default 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
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Old 21st Jun 2022, 7:03 pm   #5
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Default 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.
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Old 22nd Jun 2022, 9:00 pm   #6
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Default Re: Roberts R800 switch.

I think the R800 should receive an award as possibly the worst Roberts set ever.....
Dreadful things...
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Old 26th Jun 2022, 5:26 am   #7
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Default 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.
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Old 2nd Jul 2022, 2:27 pm   #8
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Default 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
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