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Old 2nd Jun 2019, 10:13 pm   #1
moomaloo
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Default Roberts R505 repair advice for a beginner

Good evening. I have been persuaded to try here for (substantial) help in repairing my trusty 505 rather than replacing it. I can just about handle a soldering iron but otherwise I'm totally new to this...

The only thing I can tell you is that, after several decades of service, all I get now is a loud and unchanging 'hum' when I turn it on. On all three bands and any position on the dial.

I've taken the board out and there is nothing obviously amiss to my unskilled eye. But it looks complicated in there..!

Any clues?

Thanks,

Tim
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Old 2nd Jun 2019, 10:40 pm   #2
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Default Re: Roberts R505 repair advice for a beginner

Does it have the option of batteries? If you can try batteries and the hum goes and maybe it works,, the problem is in the power supply.
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Old 3rd Jun 2019, 9:01 am   #3
David G4EBT
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Default Re: Roberts R505 repair advice for a beginner

First thing to do is to check the current being drawn by the set.

In a working set it should be about 10mA when on AM and 15mA on FM. Assuming that you're running the set from a PP9 9V battery, you can easily check the current by unclipping one of the battery connectors asnd putting a multimeter on the DC current range in series with the other battery connector and the battery. Set the meter on a high mA range to start with - say 250mA or whatever). The diagram below shows how to do this if you don't know. If more than say 20 mA is being drawn, switch the set off right away.

If excess current is being drawn, there's a strong possibility that it's likely due to C40, a 680uF electrolytic capacitor, that's gone leaky (in the electrical sense) causing the emmiter of the TR11 to draw excessive current by shorting the emitter to ground via R50 and the speaker. That can write off the output pair of transistors (TR11 & TR12) and the speaker won't like it either.

I've mentioned this problem with the Roberts R505 in more than one thread - post #8 here for example:

https://www.vintage-radio.net/forum/...t=Roberts+R505

If you don't already have it, you can download the Roberts service data from the instant download feature at the top right of the forum page for 1.99, but I've attached the relevant part of the circuit to highlight C40 (and C41&42 which can cause problems too). There are other electrolytics that I changed while I'd got the board out (C36 & C38) which - if past their best - may degrade the performance of the set, but won't be as drastic as C40 going short circuit, so I guess they are best left alone.

You just need to be careful when de-soldering the capacitors from the printed circuit boards because as was typical of the era, the substrate of the PCB is phenolic resin and the solder tracks can easily lift with too much heat from the iron.

Hope that might help Tim.

Good luck with it.
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Old 3rd Jun 2019, 9:39 am   #4
moomaloo
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Default Re: Roberts R505 repair advice for a beginner

Thanks for the info so far. I normally run it from the correct 9v mains adaptor and, as far as I know, there's nothing wrong with it. I'll need to get a 9v battery and check it with that. I'll also need to buy a multimeter (something I should probably invest in anyway - and learn how to use it properly).

I've remembered that before it died completely, it used to work for just a few seconds and then go quiet. This stage didn't last long before it resorted to the constant hum I'm experiencing now.

Whilst I still think this is beyond me, I'm interested in having a go - too much binge watching of BBCs The Repair Shop! Probably the best thing on the box...
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Old 3rd Jun 2019, 9:48 am   #5
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Default Re: Roberts R505 repair advice for a beginner

Where in Argyll are you as I will be passing there next week from Skye to Glasgow and may be able to pop in PM me


Dave
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Old 3rd Jun 2019, 9:49 am   #6
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Default Re: Roberts R505 repair advice for a beginner

Before even checking the current, clean all the switchgear and pots with Servisol. Dirty switch contacts are by far the commonest fault on radios of this age.
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Old 3rd Jun 2019, 10:00 am   #7
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Default Re: Roberts R505 repair advice for a beginner

The first step is to eliminate the possibility of there being a fault in the mains power adaptor. So try running the set on batteries with the power adaptor removed.

The Roberts and Hacker portables are very well known on this forum and their usual faults are known and documented. There is plenty of guidance available, and a battery powered portable is a good place to start learning the noble art of fixing.

The BBC repair shop programme seems OK at fixing up wooden and leather things, but it is awful when it comes to electrical/electronic things. Perhaps the produces are just scared of the possibility of being accused of inciting people to open up live equipment? Anyway, you won't learn anything on there appropriate to your radio other than the cosmetics of the case.

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Old 3rd Jun 2019, 10:22 am   #8
David G4EBT
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Default Re: Roberts R505 repair advice for a beginner

Quote:
Originally Posted by moomaloo View Post

I've remembered that before it died completely, it used to work for just a few seconds and then go quiet. This stage didn't last long before it resorted to the constant hum I'm experiencing now.
As I said in post #8 in the thread I gave a link to:

"A classic symptom [of C40 going short circuit] is that when the set is switched on, it works for maybe two seconds then goes dead".

Let's look on the bright side and hope that replacement of C40 will cure the problem, but as you've run if from a power supply for any length of time after the set became faulty, it may have written off TR11 (AC187) and possibly TR12 (AC188) too as the failure of C40 will have caused excess current to be drawn. In time, if subjected to that treatment, transistors become very agitated. (There's a link on the circuit board to enable a meter to be inserted in the supply to TR11 collector to monitor the current, but as you've yet to obtain a meter, maybe best to just focus on changing C40).

The set would still be repairable if TR11( & TR12) have failed, but life would get a bit more complicated.

The power supply won't have taken it too kindly if run for long with what amounts to a short across it.

They're nice looking sets, perform well, have a reasonable sized elliptical speaker, a tone control, good quality audio and plenty of it, so it's worth going to a bit of trouble with. You're starting off with a radio that wont work so you've nothing to lose in having a go.
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Old 10th Jun 2019, 1:41 pm   #9
moomaloo
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Default Re: Roberts R505 repair advice for a beginner

Brief update: I've bought a multimeter and a 9v battery so I'll be rolling up my sleeves as soon as I get the chance. I'll report back.
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