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Old 16th Jan 2020, 5:16 pm   #1
ferryman
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Default Roberts R707 not working

I have a Roberts R707 which was purchased in1973 but has only been used occasionally for many years. The radio is in good general condition and used to sound nice. Until recently it has always worked but now it is not working.

When it is switched on there is a hum from the speakers and I have noticed that a lead from TR7 is overheating. TR7 is shown on the circuit diagram as an AC188.

I suspected the capacitors and have replaced C35,C39,C41,C42 but the fault remains and I am wondering whether TR6 and TR7 need to be replaced?

I have no real knowledge of radio electronics and am very much an amateur. I am new to this Forum and have joined in order to request assistance / advice which would be much appreciated.

Julian
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Old 16th Jan 2020, 6:30 pm   #2
'LIVEWIRE?'
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Default Re: Roberts R707 not working

If Tr7 is overheating there's a 99% chance that it's faulty, but as the amplifier is DC coupled, there could be, and almost certainly are, other faults, probably in one or more of the other transistors in the AF Amp. Both Tr6 and 7 should, IMHO, be replaced, but only after other components, particularly transistors, have been checked and replaced as needed.
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Old 16th Jan 2020, 6:46 pm   #3
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Default Re: Roberts R707 not working

The Germanium transistors used in these radios are prone to forming 'tin whiskers' with age, which causes an internal short-circuit.

This can be between the active electrodes of the transistor and the outer case. In your radio the outer case of the two output transistors is connected to earth via the copper heatsink-clips the transistors are fitted into - an internal 'tin whisker' short can therefore cause the transistors to pass an excessive current.

Try removing the copper heatsink-clip from its mounting screw and pulling it off the two transistors, so they are then left in free air. Then see if the problem continues.

[Note: if the radio then works, don't play it loud without the heatsink fitted, as the output transistors could overheat in other harmful ways].
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Old 16th Jan 2020, 7:06 pm   #4
ferryman
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Default Re: Roberts R707 not working

Thank you for your quick responses. I have removed both the transistors from the heatsink clip but the problem continues.

I don't think I am able to properly test the transistors as I only have a basic multimeter but can try replacing TR6 and TR7
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Old 16th Jan 2020, 8:45 pm   #5
stevehertz
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Default Re: Roberts R707 not working

If the problem is tin whiskers, attempts to remove them can be made using these methods: https://www.vintage-radio.net/forum/...ad.php?t=76440
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Old 16th Jan 2020, 9:35 pm   #6
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Default Re: Roberts R707 not working

As long as your multimeter has resistance ranges, as most do, you can check the transistors (out of circuit) as described here:

https://www.vintage-radio.com/repair...or-faults.html

The link also provides further information about 'tin whiskers' amongst other things.

Alan
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Old 17th Jan 2020, 12:18 pm   #7
ferryman
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Default Re: Roberts R707 not working

OK thanks. I will try testing the transistors. If any need replacing I was wondering about whether using modern alternatives was possible, just to find out if I can get the radio to work? If so … any ideas on suitable equivalents?

Julian
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Old 17th Jan 2020, 5:25 pm   #8
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Default Re: Roberts R707 not working

From what I understand not all AC187/AC188 transistors are vulnerable to tin whiskers depending on the manufacturer and whether or not the cases are sealed with resin. For example the Tesla equivalents on sale here should be ok:

https://www.ebay.co.uk/itm/GC511-GC5...sAAOSwWKpcbIG5

By the way I would recommend replacing them both if one is faulty once you are satisfied that the rest of the audio amplifier circuit is fault free.

Alan
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Old 18th Jan 2020, 6:10 pm   #9
ferryman
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Default Re: Roberts R707 not working

Thank you for all your responses. I will have a go at checking /replacing the transistors and see how I get on. Many thanks.

Julian
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