Roberts R900 Vintage Transistor Radio With A Faulty Power Switch

I see a lot of this particular model, with the same power switch fault, so I thought I'd share the process of diagnosing, fault finding, and remediating the issue. As is the case here, it's often fouled contacts inside the switch itself. Fiddly, but easily do-able.

[@gerardcarson2779]

Hi Paul. Great video and interesting bit of fine surgery. I have done a few of these now and have developed a technique that doesn't involve removing the ferrite and switch body from the board. And I am only saying this as it might be a useful alternative. By removing the little retainer (the one that bobs around when the switch is operated) the "plunger" part of the switch can be withdrawn and this gives access to those little copper contacts which can then be either cleaned or replaced. I realise the fixed contacts in the switch can't be cleaned directly and as you have done, but have found switch cleaner down the open mouth of the switch (ie, when the plunger part is out) seems to sort that side of things. One tricky bit is to make sure a finger is held over the end of the switch when removing that little retainer - or it will act like a mini cannon and good luck to anyone trying to find those little contacts!

[@Codeeze]

Thanks! I have used that method before (very similar), it's how I actually learned to do them in situ - my workshop is likely peppered with those contacts, but can I find one of them?... When it's just dirt and grime, the switches do respond to switch cleaner, but when the carbon has built up due to arcing, it's too stubborn I find, and is best manually cleaned. With the R900, particularly the early ones, there is a power surge on power up, and over time, this can physically damage those contacts. Roberts quickly realised the fault, and issued an emergency modification to dealers back in the day, to prevent the initial inrush causing problems. I have covered this modification in another video, I know you've seen that, and done it yourself. Takes a bit of understanding. If you have time, take another look at the mod sheet, and where it says about PCB number GBP 1269D16, tell me which tracks to cut through (what the arrows are pointing at?). I swear that picture was drawn by a 9 year old... Thanks again Gerard. Best, Paul

[@gerardcarson2779]

@@Codeeze Hi Paul, and thanks. I think I must have picked up a different version of the “supplement “ (I got mine on Vintage Radio Forum) but that one was pretty unclear too. And I was more than happy to follow your expert video before I went gouging away at circuit boards! And on the later design I am not sure how successful the modification (of dual-ing the traces and changed caps) was as I have come across a couple of late R900s where the on/off switch still suffered from the burned out contacts syndrome. And seemingly as does the R800, although no modification seems to have surfaced for it. Anyway really enjoy your videos and they always do add to my understanding. So many thanks for doing them and enabling more of us to keep these nice radios going. Hacker ones too.

[@terryblackman6217]

Great repair. I love these fiddly jobs.

[@Codeeze]

Thanks Terry - I like fiddly jobs, satisfying when they result in a success!

[@AstonsVintageTechnologyWorkshp]

Nice, the service bulletin for this one goes one further and adds a short across the unused contact at the bottom to help with the inrush current. I can send it through if you would like. All the best, Ast...

[@Codeeze]

I have the service modification sheet thanks Aston, an original Gestetner version no less!. There's a mod. for serials up to 13566, one for pre 17000, and one for board number 1269D16 (this one). That's how I've always read it. It would perhaps be prudent to short those contacts anyway I guess... it could only help

[@gartmorn]

Looks like a good little chassis to remove and work on. Like you say, not the beefiest contacts to handle a high inrush current!😢

[@Codeeze]

Thanks! - It's one of my favourites. That's what the official modification was for - limiting the inrush - change a couple components, limit the current path by cutting tracks - and dividing the inrush between both copper sliders in the switch.

[@zx8401ztv]

Well done :-D I know those switches from repairing a fluke bench multimeter and old amplifiers mostly. To clean the sliders as i call them, i fold paper to make a nice smooth edge then carefully put each slider on the paper fold and slide forwards and backwards a few times. The blades, i squish a cotton bud and use drop of meths on the bud and wipe both sides of the blades. It works for me but we all have our method :-D I've also used the same way for dirty record/play mode switches, but they have lots of blades/sliders ( a bit boring lol ) Keep on tinkering :-D

[@Codeeze]

Thanks Mr zx - I've done this 'repair' so many times I thought it would be good to share - in case it helps someone else out there... I am lucky that I have around 100 of those switches, "Schadow" I think they call them, identical NOS - just in case. Most seem to respond to a bit of manual intervention, but occasionally there's one that won't play and needs a newie. When I clean the sliders, I use a sheet a wet and dry... and then drown them to clear the gunk out.

[@zx8401ztv]

@@Codeeze Schadow sounds a bit painfull lol :-D I also have a rubbing block that maplin used to sell, rubber+abrasive particles. They were great for cleaning metals. I later found a supplier, GARY BLOCKS, made for car paint cutting. Expensive but i think model train enthusiasts use something similar. Sorry im flapping my lips again, ill shut up now.

[@SweeneyTTodd]

Nice work on that switch looked "fiddly" but you made it look easy Best regards Shaun

[@Codeeze]

Thanks - It is easy Shaun. Bull by the horns, and wrestle it into submission...

[@barryfleischer6553]

You have all the skill of a surgeon. I would have drowned the switch in switch cleaner and hoped for the best. I really like the design of those Roberts radios, how old is this one? Great repair. Already looking forward to the next one.

[@Codeeze]

Thanks Barry - this one was a later version, there will be a date code inside the case (next video) but I suspect it to be 1988ish. Drowning these switches in switch cleaner does very little to slurry away the heaviest carbon deposits and damage caused by arcing, and there is an official modification to be carried out (again, next video), which shows this was a known problem over 30 years ago... surprising it's lasted this long! Surgeon, hmm, maybe don't trust me with an appendectomy... 🤣

[@ad6073]

Another great video Paul, is this switch similar to the waveband switch on the Roberts R250? Thanks Amanda

[@Radiocruncher]

Nice job Paul. I’m not sure what they were thinking with the knobs though. Are you going to do the power mod on this one? Cheers Graham

[@Codeeze]

Thanks Graham, yes! there is the official modification to be carried out (next video), which shows this was a known problem over 30 years ago...

[@GeorgeChristofi]

burnded contacts. Nicely worn out them ones innit.

[@Codeeze]

Proper sick innit bruv - safe yeah

[@SoddingaboutSi]

Interesting. Never seen a blue cutting mat before. 🙄

[@Codeeze]

😮

[@Steveuk405]

Not the best bit of design in the world was it! In the day the bench engineers tended to put loads of switch cleaner into them and hope never to see the customer again!

[@Codeeze]

Exactly Steve! - Switch cleaner won't work when the contacts are burned from arcing the inrush. Roberts realised this quite early on and issued an official modification. Next video...