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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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23rd Sep 2020, 2:33 pm | #1 |
Dekatron
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Motorola 808
I have been asked to look at a non-working Mororola 818T. I expect that AF117's might have something to do with the fault.
However, I have fallen at the first hurdle as I cannot get it out of its case. There aren't any instructions n the service sheet, presumably because they assume that anyone wanting to take it apart would have the nous to do that. I have removed lots of screws but it doesn't want to shift. It looks as though it should just pull out but I don't want to pull too much on it. Can anyone help? Thank you
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23rd Sep 2020, 9:26 pm | #2 |
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Re: Motorola 818T
It's ok, I applied more force and the chassis slid out after I disconnected the plug to the power amp.
Mods - this thread can be removed as it is a bit of a non-starter. Thanks
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3rd Oct 2020, 9:57 am | #3 |
Triode
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Re: Motorola 818T
It sounds like its an 808 to me as you mention a plug to the amplifier. The 717/818 models were wired direct to the amplifier - no plug, Cheers Tony
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3rd Oct 2020, 10:02 am | #4 |
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Re: Motorola 818T
Right - that will be why it doesn't match the circuit diagram I got! I think that replacing the AF117s and the detector diode will get it working again. Unfortunately I bought AF127Ns and the pinout is different which means they don't fit quite as easily
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3rd Oct 2020, 7:36 pm | #5 |
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Re: Motorola 818T
Short lengths of PTFE sleeving will keep the crossed legs watertight.....
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4th Oct 2020, 2:41 pm | #6 |
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Re: Motorola 808
I have only tacked the transistors in at the moment but I will take your advice about insulating the leads. I tend to be a bit "cheap" over this and rescue insulation from other bits of wire!
Foolishly I didn't do anything about the detector diode despite there being no sound emanating from the tuner at all so I have sent off for a germanium diode having found it to be not working.
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5th Oct 2020, 7:21 pm | #7 |
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Re: Motorola 808
Chris's suggestion about insulating the legs of the transistors gave me the brilliant but I am sure far from original idea of colour coding them. In the restricted space available it has made them much easier to identify.
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5th Oct 2020, 10:03 pm | #8 |
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Re: Motorola 808
Coloured sleeving featured on quite a few early sets with long leaded devices. Later on, PCB's were designed with the device footprint in mind and leads pretty much vanished. I was lucky enough to have access to "free" PTFE sleeving at work and its soldering iron proof properties came in very handy for tight substitution "home office" jobs. PVC sleeving recovered from old wire is still a lot better than nothing as long as you're quick with the soldering....
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6th Oct 2020, 9:24 am | #9 |
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Re: Motorola 808
Colour coding the legs is a good idea ,make a note what colour to which leg and stick to it ,I have done it that way for years, Mick.
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6th Oct 2020, 7:07 pm | #10 |
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Re: Motorola 808
I had hoped that this would be a simple repair based around iffy AF117s but I am struggling with it. I have replaced the AF117s by AF127Ns (being careful of the different pinout) I can get a signal out of it if I connect the signal generator to the base of the second IF transistor. It isn't as loud as I would hope for but it is coming through. If I connect it to either the collector or base of the first IF transistor nothing comes through. I am actually using the circuit diagram for the 818 as that is the one I downloaded by mistake and it seems close enough for the moment.
The voltages on that transistor are: (Circuit diagram figures in brackets) C 10.02 (8.5) B 1.63 (1.73) E 0.002 (1.68) If it was a valve I would say that it isn't conducting but I am not sure whether that applies to transistors. I don't know whether that would account for no signal out when one is applied to the collector. Can anyone point the way forward please? Thank you
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6th Oct 2020, 8:32 pm | #11 |
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Re: Motorola 808
My first reaction was O/C device too.
Can you replace it? Any old device will do to see if the dc conditions come back to normal. Are you certain its legs are crossed "just so"?
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6th Oct 2020, 8:59 pm | #12 |
Hexode
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Re: Motorola 808
It's really easy to get AF117 replacements wired wrong as alternate devices have different pin outs. That would be my first port of call. Is the screen (can) actually connected to chassis? I ask as your emitter voltage is Zero (to all intents and purposes)
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7th Oct 2020, 8:49 am | #13 |
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Re: Motorola 808
Thanks both for the replies. We are going to do a bit of shopping this morning but I will look later in the day.
When thinking about valves I find electron flow easier to envisage but with transistors it is current flow. Odd!
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7th Oct 2020, 1:11 pm | #14 |
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Re: Motorola 808
If you have any 'Kynar' insulated wire (commonly surplus, used for aviation), it strips more easily than PTFE and gives you insulating sleeves that don't melt at normal soldering temperature.
David
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7th Oct 2020, 3:28 pm | #15 |
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Re: Motorola 808
I will have a look for Kynar wire, it sounds useful.
Blushes here. I looked up the pinout for the AF127N on the computer and was convinced I had it right. I then realised I had gone onto another page and was looking at the AF127. Fortunately the others are ok. I have rewired the transistor and the voltages are now correct. However there is still nothing is getting through the first IF transformer. I hope the coils are not o/c. I will investigate further.
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7th Oct 2020, 5:44 pm | #16 |
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Re: Motorola 808
Kynar is a bit dear, but once you learn to recognise it you sometimes spot remainders on reels at radio rallies. It's good for point-point wiring under chassis because a wrong poke with a soldering iron doesn't spoil things. It's designed to handle high temperatures and be less nasty than burning PVC in an aircraft disaster.
Beware of some RF transistors and MMICs that are made in both normal and mirror image pinouts to make board layout easy. THry can drive you up the wall. David
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7th Oct 2020, 6:30 pm | #17 |
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Re: Motorola 808
Thanks David
To be honest I was so convinced I had that transistor right that I put it in the transistor tester. When it showed my colour coding to be wrong I checked the transistor to make sure I had the right one before I saw that the coulrs did not match the other two. I am wondering about the 400pF capacitors in the i.f. transformers. The exact value isn't one of the preferred ones of course but I have some 330pF ones. The coils themselves look ok with no green spots but I have yet to remove them from the set to check the continuity of them. Edit - The coils seem ok so I will change the capacitors tomorrow. I hope I can still peak them
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8th Oct 2020, 1:17 pm | #18 |
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Re: Motorola 808
I tacked a couple of new 330pF caps into the first i.f. and I can now get a 470kHz signal through it. The oscillator is now running fairly accurately as far as I can tell on the oscilloscope and I can get a signal through if I run the signal generator at full blast into the aerial socket. It is all very quiet and there are no actual stations apparent though.
It is progress, though. I think I will change the caps in the second i.f. and do a general tidy up and an alignment.
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