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Old 11th Dec 2018, 5:08 pm   #1
Clarky-R300
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Default Roberts R300 OC81's getting hot

Hello, a newbie here.

I have been reading a lot of good info on here for vintage radio sets, my primary reason for joining is that I have inherited a couple of vintage radios from my Dad.

My first project is hopefully to get working again a Roberts R300, however on installation of a new PP9 battery I get a whiff of something getting hot and using my finger discover it is the two OC81's that are too hot to touch.

Above the two OC81 (when viewed normal installation) there is a resistor (5.6ohms?) looks like it has blistering of the surface or is this the way the resistor is finished?

Thanking you in anticipation of some pointers/help.

Cheers

Clarky
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Old 11th Dec 2018, 6:25 pm   #2
60 oldjohn
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Default Re: Roberts R300 OC81's getting hot

Hi Clarky, welcome to the forum. Have you the circuit diagram ? It is available top right of this page for £1.99. Makes any advice given a lot easer for you to follow. .


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Old 11th Dec 2018, 7:22 pm   #3
Al (astral highway)
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Default Re: Roberts R300 OC81's getting hot

Quote:
Originally Posted by Clarky-R300 View Post
Above the two OC81 (when viewed normal installation) there is a resistor (5.6ohms?) looks like it has blistering of the surface or is this the way the resistor is finished?

Thanking you in anticipation of some pointers/help.
Hey Clarky, welcome to the form.

There shouldn't be blistering on the surface of the resistor. The transistors you mention are apparently passing too much current and so is the resistor.

It's best to leave the set unpowered or you might cause catastrophic damage to some components.

As mentioned above, first step is to get that circuit diagram. Fragments of it are already on the forum, but the resistors and capacitors on it are marked in sequential numbers, not values, so you need the full trader sheet.

Also, have a little search for this radio on the forum - it's popped up before.
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Old 11th Dec 2018, 8:34 pm   #4
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Default Re: Roberts R300 OC81's getting hot

There's only the 'ERT' data available - not Roberts datasheet, but the ERT sheet is helpful.

Clearly something is causing the output pair to draw excess current.

The datasheet explains that RV2 - the bias control - should be set so that output stage current drawn is 4mA. Hence, a good starting point would, I think, be to disconnect the link 'LK1' which is from the centre tap of the primary of the output transformer, and to insert a meter in place of the link, on say the 100mA range and measure the current being drawn. Once the bias has been set it shouldn't need adjusting again unless the transistors have been changed. If the current is excessive (well above 4mA), adjusting RV2 is unlikely to reduce it to the desired level, so it's best left alone.

Ordinarily, the three OC81s (one OC81D, TR4 - two OC81, TR5 & 6) in the output stage are reliable - not so the three AF117s TR1/2/3, but they won't have a bearing on the output stage. It's possible that TR5 or TR6 may have developed a fault, but I think it more likely that something else is distressing them and causing them to draw excess current. I don't know what the effect of a faulty MR3 (AA129) stabilising diode would be, but you could check if its OC/SC.

In my experience, with Roberts and other transistor sets of the 60s, the electrolytic caps often become near enough short circuit, lose their capacitance and behave more like low value resistors. R17 - which is from the slider of the volume control to the base of TR4 - is only 2 µF and will be physically small. If that is acting like a low value resistor - not a capacitor, whether that could cause excess current to be drawn by the output pair, I wouldn't like to say, but I'd certainly want to check it for capacitance and leakage. I don't think the other three electrolytics in the output stage (C18/19/21, all 100µF) could be the source of the problem, even if - as may be likely - they're well past their best before date, remembering that the set dates from 1964.

I'm reluctant to suggest that you start yanking out components from a Paxolin PCB risking damage to the board without testing them in situ where possible, or just unsoldering one end of R17 for example, to test its capacitance and leakage. The PCB has a high component density, with the electrolytic caps mounted vertically and it's not very easy to know which solder joints arse which from above the board.

I've just highlighted the checks that I'd make - others may take a different view.

I'd certainly suggest that you check the current being drawn by the set as a whole by inserting a meter on the 100mA scale in series with the + VE battery lead. With the volume control turned to a minimum and no stations tuned in, it should be 12mA or so, and with the volume turned up to normal listening levels, it should be about 20mA. Very much more than that, confirms that there are problems that need to be diagnosed - not guessed at by making assumptions.

Hope that helps a bit.

Good luck with it.
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Old 12th Dec 2018, 11:36 am   #5
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Default Re: Roberts R300 OC81's getting hot

Quote:
Originally Posted by Clarky-R300 View Post
Above the two OC81 (when viewed normal installation) there is a resistor (5.6ohms?) looks like it has blistering of the surface or is this the way the resistor is finished?
Clarky
I forgot to mention that yes, the data shows that it is 5.6 Ohms. Despite the blistering it may still read 5.6 Ohms on an Ohmmeter, in which case it can be left in place till you get to the bottom of the cause ofv the excess current, but it would be worth replacing it in due course because like so many carbon composition resistors of that era, it could be 'noisy' and degrade the audio quality.

There are two threads here on my own experiences with Roberts, citing an R505 in which excess current was being drawn and the output pair getting hot. Inevitably it was due to an electrolytic capacitor which on test, came up as a 0.53 Ohm resistor - basically, a dead short:

Post #8:

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

Post #3:

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

If you do get around to removing any electrolytic capacitors to test them, remember of course that as with transistors and diodes, they're 'polarised' so must be fitted the correct way around. It's well worth taking a picture before you disturb anything so you can be sure you don't make a mistake when either re-soldering it in place, or fitting a replacement.

I don't know whether you just want to 'get your Roberts R300 going again' or are interested in getting into restoring and repairing vintage radios. If so, I guess that at the moment you won't have much beyond a multi-meter, soldering iron and a few other tools. If that's so, in the threads above you'll see an example of a Chinese 'Multi-tester' that I built into a little box. They came to prominence some years ago as unboxed modules to be used either as they were, or fitted into some sort of project box.

They test transistors including JFETs and MOSfets, resistors, inductors, diodes, capacitors, including ESR ('effective series resistance'). There are three things we need to know about capacitors - the capacitance, the ESR (of electrolytic capacitors), and any leakage, as was the culprit in my Roberts R5050 and other Roberts I've repaired, and may well be the case with your R300. You could use such a tester to test the capacitors, transistors, diodes and resistors in your R300.

Time has moved on and the multi-testers are now even more versatile and come ready to use in a nice case.

They also now test Infra Red remote controls showing the waveform, have built-in li-ion batteries, and a mini USB socket and lead so the batteries can be recharged from a USB port. They have a 'ZIF' socket (Zero Insertion Force), but also come with three test-clip leads to plug into the ZIF socket for better convenience. If you're testing a transistor it doesn't matter in which order you connect the three leads - it will identify which one is the collector, emitter and base and will show that on the display, along with parameters including the gain of the transistor and whether it's PNP or NPN.. If you're testing a capacitor, resistor, diode or inductor, you use any two of the three test leads and again, it will show up on the display.

They typically cost £10 - £15 post free from several seller in China, and maybe a few pounds more from UK suppliers. Here is an example, where you can read the full specification and can see the readouts that it provides. Please note that the link below is provided to help you find an example of the multi-tester, where you can see the specification and displays of the range of tests that it will perform. Forum rules do not permit further discussion on the e-bay listing:

https://www.ebay.co.uk/itm/8C99-Test....c100005.m1851

Hope that helps to get you on your way Clarky.
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Old 12th Dec 2018, 12:01 pm   #6
g4wim_tim
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Question Re: Roberts R300 OC81's getting hot

I recently had the same problem on a Fidelity RAD21 made in 1976.

I replaced the output transistors as they had gone short with a pair of AC127 / 128.

I think I have some spares stashed away if they are of any use - bias would need adjusting though.

It also had a suflex 3300pF capacitor which had ggone o/c rendering MW inoperative - dreaded suflex capacitors....
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Old 12th Dec 2018, 12:40 pm   #7
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Default Re: Roberts R300 OC81's getting hot

The component tester linked to in post#5 is the same as the one I have got.
It also does remotes and comes with a USB lead for charging.
If the battery gets tired there is always a spare in last years mobile phone waiting to go into it.
I got mine from Banggood.
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Old 12th Dec 2018, 2:03 pm   #8
60 oldjohn
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Default Re: Roberts R300 OC81's getting hot

Quote:
Originally Posted by g4wim_tim View Post

I replaced the output transistors as they had gone short with a pair of AC127 / 128.

Just a reminder, the AC127 is now known to suffer from Tin whisker syndrome, same as the AFxxx series.


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Old 12th Dec 2018, 3:18 pm   #9
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Default Re: Roberts R300 OC81's getting hot

It is also possible that the idle current of the final stage is out of adjustment.
The manual calls for 4mA, measured at the "LK" link.
Adjust RV2 for the correct idle current.

Regards, Peter
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Old 12th Dec 2018, 3:51 pm   #10
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Default Re: Roberts R300 OC81's getting hot

I'd also check that the diode MR3 has not gone open-circuit as this would mess up the bias.
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Old 12th Dec 2018, 5:46 pm   #11
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Default Re: Roberts R300 OC81's getting hot

Quote:
Originally Posted by 60 oldjohn View Post
Just a reminder, the AC127 is now known to suffer from Tin whisker syndrome, same as the AFxxx series.
The OC81 is a PNP transistor, it can be replaced with an AC128 transistor.

The two OC81's have overheated excessively and will need replacing, along with the

5.6 Ohm resistor, and as stated in #10 above, check the diode MR3 and the

intactness of RV2 across it, especially the slider connection, it may need

replacing. Chances are that it is just the output transistors that have failed,

but capacitors C17 C18 C19 would be worth changing, one at a time, after

the audio stage is working. Then any faults such as faulty AF*** transistors in

the RF IF stages can be checked.
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Old 12th Dec 2018, 11:28 pm   #12
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Default Re: Roberts R300 OC81's getting hot

Add C21 to the capacitor list too.
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Old 15th Dec 2018, 9:14 pm   #13
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Default Re: Roberts R300 OC81's getting hot

The O.P. hasn't been on since his first post - maybe we've scared him off?

The R300 is a commonplace radio, so the info imparted may help others.
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Old 16th Dec 2018, 8:14 am   #14
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Default Re: Roberts R300 OC81's getting hot

If the bias adjust trimmer is of this type
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then I advise checking it, I have found that everyone I have come across is way out in value, they can be as high as 4 times their marked value. One 50 ohm example I came across was close to 200 ohms.
https://www.vintage-radio.net/forum/...6&postcount=30

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Old 16th Dec 2018, 9:47 am   #15
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Default Re: Roberts R300 OC81's getting hot

As I've noted in the past, I usually replace skeleton pre-set pots. with Piher or similar enclosed types. These are available in vertical and horizontal types from the usual suppliers. They come in two sizes and all the usual values, and are usually a 'drop-in' replacement.
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Old 17th Dec 2018, 2:20 pm   #16
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Default Re: Roberts R300 OC81's getting hot

Fantastic response. Thank you all. Certainly some tinkering to keep me occupied for a little while, will get the circuit diagram first up.

Thank you again and cheers.
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