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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 26th Nov 2020, 1:48 pm   #1
AD360 Rob
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Default Roberts R300 ressurection

Hi all,

This success story was going to start out as a help request as the final fault that was present had really strange symptoms.
But first, I'll tell you the journey that lead to that final fault.

During lockdown I'd been attacking the "roundtuit" pile which was comprised mainly of cosmetically challenged Roberts R200 and 300 sets, this one was typical, no handle, no knob brights, filthy faded covering and a horrible battery leak at some point in its past. So, it was stripped down, I gave the case to a very good friend to re cover (I'd tried cleaning to no avail sadly) and made a start on servicing the chassis. It was completely original as most of these appear to be so I replaced on sight the 3 AF117s with those Russian GT322B transistors which brought the chassis to some sort of life. I could receive 1 Station on MW and LW was dead. Right, I thought, let's get it receiving properly on MW first. A quick test of the audio stages revealed that there wasn't much gain happening there so the usual suspects were checked and replaced (by this, I mean all the black & red plessey electrolytic, all were way off value with a couple almost open circuit.) this brought up the output quite well and I could now hear the normal amount of stations that I can usually get here albeit not quite as loud as I'd like but certainly acceptable for now. I then had a look at long wave, still nothing, not even interference, I confirmed that the oscillator was working on LW and was at roughly the right frequency so my next checks involved the LW aerial coil. Upon close examination I found that the previous battery leakage has eaten away at the coil which was a green corroded mess.. I didn't have any spares so a quick message to the same friend who was recovering the case resulted in a freshly re wound coil which I fitted. This sorted the LW out and with a slight tweak of the LW osc and aerial trimmers, stations were on the right parts of the dial.
I then noticed that after the set had been left off for a while, on switch on there was a lot of amp hiss that gradually faded away. This turned out to be a noisy OC81D, a replacement solved that problem (never had one do that before, I've had them low gain, noisy, short and open circuit but never noisy then get better). It was then I noticed a strange effect. Reception was adequate on both bands but I felt it could be better especially as touching the aerial coil improved things so I performed an RF alignment which did make a slight improvement but there was still something amiss as the same symptoms were still present. It was then I noticed that it wasn't just touching the aerial coil that improved things, any bit of exposed metal that was touched brought about a similar boost in reception. Careful scrutiny of the board revealed nothing, no broken tracks, bad earth's, damaged resistors, nothing to indicate why the whole thing was behaving as an aerial. Next I double checked my work fitting the LW coil just in case I'd wired it backwards or something but no it all checked out.
In desparation I looked at the decoupling caps which in this set were the grey plastic dubillier jobs and not the usual brown hunts which I tend to change on sight as they're always leaky. I removed a random one and tested it on the Chinese tester I have, it gave all sorts of odd readings, way over value and an esr of 47 ohms!! Now I know that these small value caps generally don't have an esr reading so that was highly suspicious so I double checked using my fluke dvm, even with the relatively low voltage from the fluke on resistance range I was getting leakage of a few megohms whereas a new cap showed o/c so I thought "I've nothing to lose by changing them all" so that's what I did. Testing the set afterwards I was rewarded with bags more sensitivity, no wierd hand capacitance effects and all stations still in the proper places. A new reproduction handle and a set of brights from another friend were fitted (he manufactures these) and a cleaned and painted scroll badge completed the job.

On to the next one in the pile!

Rob
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Last edited by AD360 Rob; 26th Nov 2020 at 1:54 pm.
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Old 4th Dec 2020, 9:09 pm   #2
tim.norris
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Default Re: Roberts R300 ressurection

Rob, well done , great looking radios, Roberts must have thought so too as the basic design is still used in their modern day equivalents , regards, Tim.
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Old 4th Dec 2020, 11:35 pm   #3
AD360 Rob
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Default Re: Roberts R300 ressurection

Thanks Tim, This one did lead me on a merry dance but is fine now. I'm using it daily just to be sure and yes, they still use the basic shape and look today, a testament to the design I think.
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Old 10th Dec 2020, 1:09 pm   #4
MurphyNut
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Default Re: Roberts R300 ressurection

I have a green one of these, used as my bedroom radio. I don't think it's ever been touched and still performs quite happily.
Incidentally I use a 6 AA battery holder with a home made lead adaptor instead of the usual large single 9 volt battery. I use rechargeable AA's so it's not as wasteful or expensive to run.
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