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1st Mar 2020, 11:01 am | #1 |
Hexode
Join Date: Aug 2014
Location: Dartford, Kent, UK.
Posts: 435
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Roberts R606 Knobs Wanted
I am on the lookout for a 'capped' volume control knob and 2x 'capped' push button knobs to complete my R606 MB. Pictures enclosed.
The pictures are AFTER this one was rescued from the Audio Jumble for a £1 and was VERY close to looking like the last chicken in the shop - which is probably what appealed to me! I have scrounged a number of parts to get it working and looking like it is now - it sounds excellent. BTW The capacitors on the mother board are looking like they have been baked by the mains transformer. I am expecting trouble from them soon especially after reading the problems paulsherwin had with VHF distortion and C18. Please contact me if you can assist me and I will try and come to some sort of arrangement with you for your help. Kind regards John |
10th Apr 2020, 10:01 am | #2 |
Heptode
Join Date: Jan 2018
Location: Folkestone, Kent, UK.
Posts: 711
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Re: Roberts R606 Knobs Wanted
hi there i have the 2 small ones for you ,pm sent
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29th May 2020, 10:30 am | #3 | |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,761
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Re: Roberts R606 Knobs Wanted
Quote:
The scorched capacitor to the left of the picture, in front of the ferrite rod coil is C30 (470 uF). If you look at the circuit you will see that it is a decoupling capacitor direct from the +ve line to ground. Over time (decades, let's face it), those blue Philips electrolytics stop behaving as capacitors and often morph into low value resistors. That particular one won't do much damage in the set, but it would drain the batteries in no time at all, will probably cause distortion, and would certainly overload the mains PSU. If you put a meter is series with the power supply (probably easier to do in series with batteries) and check the present consumption of the set, which - in a set working properly - depending on whether of AM or FM - would have a quiescent current of maybe 15 - 20mA you'll probably find is much higher. Often, when such sets are powered by batteries people will say 'When I turn the set on it works for a few seconds then goes dead'. That's because a leaky electrolytic is putting a dead short across the battery. The similar sized capacitor under the ferrite rod coil is C27 (also 470uF) in the emitter circuit of TR5. The one to the right of those two is C31 (junction of the emitters of the output pair TR5 & TR6). If that is leaky (in the electrical sense), it will put a short to ground via the speaker and may take out TR5 at least. You can check the quiescent current being drawn by the output pair in accordance with the datasheet by breaking the link 'LK' and inserting a milliammeter. In a correctly working set it should be 5mA and is adjusted on R27. Purists will say that you shouldn't change capacitors 'willy nilly' but only by diagnosis, but if you at least carry out the checks outline above, I think you'll find that as is often the case with sets such as the R505, R606 etc, the blue Philips electrolytics will at the very least degrade the performance of the set, and at worst, will take out hard to replace transistors. (Transistors are expensive 'fuses'). If you have the test equipment, (multi-meter, capacitance meter, ESR/Chinese Multi-Tester etc), you can spend time checking capacitance, leakage, ESR, but these sets are from the early - mid 1970s, so the electrolytics have had a good life and are a known source of problems. The Trader Sheet (3117) is quite comprehensive and has excellent step instructions on how to set up the audio stage. Ideally, you'd need an audio generator and 'scope but it isn't essential and it isn't complicated. It's your set not mine, but personally, I wouldn't want to use it until I'd replaced at least C27, C30 & C31, and would probably replace the smaller ones too. I can understand that if the sets are working - if not at their optimum - the focus tends to be on cosmetics, ("where can I get a new strap/badge, knob" etc) but the aim is to have a set which doesn't just look as good as when it left the factory, but works as well and as reliably as it did. It's a nice set, and worth the time and effort. I hope these notes are of some help John, and that someone can help with the volume control knob.
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David. BVWS Member. G-QRP Club member 1339. |
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29th May 2020, 12:39 pm | #4 |
Hexode
Join Date: Aug 2014
Location: Dartford, Kent, UK.
Posts: 435
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Re: Roberts R606 Knobs Wanted
Hello David,
All points noted and agreed upon! The original 'odd' AFC tuning issue was cured by simply tweaking the discriminator coil! I had not expected the discriminator circuit to give those symptoms but I am glad of a simple fix. Surprisingly when I measured the caps they were reasonably up to spec but I will replace them when I find them at a reasonable price - probably when the world gets going and the rallies start again! Thanks again for the tips. Kind regards John |
29th May 2020, 1:27 pm | #5 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
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Re: Roberts R606 Knobs Wanted
I'm now pretty sure that the distortion problems I had were down to a signal limiting diode in the VHF tuner head rather than bad caps. I still haven't plucked up the courage to dive in and fix it yet - I keep putting it off.
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29th May 2020, 6:51 pm | #6 |
Heptode
Join Date: Jan 2018
Location: Folkestone, Kent, UK.
Posts: 711
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Re: Roberts R606 Knobs Wanted
i did send you a pm ages ago about the 2 small knobs but no response ,i still have them if required
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