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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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19th Jun 2020, 3:06 pm | #41 |
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Re: EKCO AD75 restore
The layout is different because of the side contact valves. You can measure R1 from V1 top cap to the junction of R16/R17.
It is not obviously oscillating as that would result in a -ve voltage on the triode grid. What sort of meter are you using? I suggest you replace the cathode decoupling capacitors C5 and C14 next. Last edited by PJL; 19th Jun 2020 at 3:17 pm. |
20th Jun 2020, 10:38 am | #42 |
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Re: EKCO AD75 restore
Thanks PJL
The resistance measurements are: R1 760Kohms R16 1.1M ohms R17 510K ohms All were measured in circuit. I used a standard digital multimeter (Fluke 83 III). However I have Hantek DSO5102P scope that we can use which could be quite useful now. I've replaced C5 and C14 (see photos). C14 should be 20nF but I only had 10nF closeset. I can add another in parallel but would prefer to replace with correct value if that is necessary. I re-measured the DC voltages that I posted in the table in an earlier post. All pretty similar except V1 top cap was -4.4V instead of -2.2V previous. |
20th Jun 2020, 10:46 am | #43 | |
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Re: EKCO AD75 restore
Quote:
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20th Jun 2020, 10:56 am | #44 |
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Re: EKCO AD75 restore
OK, it is oscillating but not in the right way! The -4.4V occurs because the grid has a large AC voltage on it which is being rectified. You will need to replace the remaining paper capacitors and in particular C3 and C11 as these decouple the AGC feedback.
Have you cleaned the waveband switch and all valve pins and sockets? |
21st Jun 2020, 9:59 pm | #45 |
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Re: EKCO AD75 restore
Thanks PJL and Station X
I've changed C3 and an unknown capacitor next to it. Unfortunately the schematic is different to my radio. The schematic shows C3 on its own but there are two 0.1uF capacitors side to side on mine. Also I could not find C11. I tried tracing the circuit from L9 and the junction of R17/17 but still not sure which it is. However after replacing C3 and its adjacent capacitor the V1 top cap is now 3.6V The noise from the speaker when searching for stations now sounds much more like what I'd expect to hear (but still no stations found). I don't really want to just go and change all the remaining paper caps as I'd prefer to do it one by one to ensure I don't add any faults. I have cleaned the waveband switch and all valve pins and sockets. Last edited by Station X; 22nd Jun 2020 at 9:02 am. |
21st Jun 2020, 10:27 pm | #46 |
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Re: EKCO AD75 restore
OK, so the audio stage is working.
It definitely has some major differences to the later version. Some differences are described in the trader sheet but I am not sure it mentions the capacitor you have replaced. What was the value? |
21st Jun 2020, 11:04 pm | #47 |
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Re: EKCO AD75 restore
The copper wire with thinner turns over the top are mentioned in the trader sheet being fixed capacitors of about 15pF.
We need some good pictures of the complete underside, you might need to do this in two parts. |
22nd Jun 2020, 9:13 am | #48 |
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Re: EKCO AD75 restore
As far as C3?? and the unknown capacitor go, the thing to is to trace the wiring a see where they're connected to in the circuit. For example is there continuity from the unearthed side of C3?? to the control grid of V1 via L3 and L2?
The unknown capacitor would appear to be connected between earth and somewhere. The question is where.
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22nd Jun 2020, 9:37 am | #49 |
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Re: EKCO AD75 restore
C3 is on my two early AD75's I have restored and goes from V1 grid to earth (chassis).
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22nd Jun 2020, 10:59 am | #50 |
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Re: EKCO AD75 restore
Thanks PJL, Station X and HamishBoxer
I've re-investigated C3 and 'unknown'. They both have a common ground then the near one (C3?) goes to the L1/2/3 coil. The 'unknown' cap goes to the adjacent pin on the L1/3/3 coil. See photo. |
22nd Jun 2020, 11:03 am | #51 |
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Re: EKCO AD75 restore
C25? L1 to chassis.
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22nd Jun 2020, 11:06 am | #52 |
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Re: EKCO AD75 restore
In response to PJL's Q regarding the 15pF caps. I see 4 of these. 3 appear OK but A has the thin wire hanging loose. I've taken some photos of these and a photo of the underside in 2 halves to improve resolution.
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22nd Jun 2020, 12:39 pm | #53 |
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Re: EKCO AD75 restore
Do you have a different service sheet? We are using trader 784. If you have something for the earlier version it could be very useful. The trader has a capacitor C2 that is switched across L1 (aerial winding) but that is a Hunts capacitor visible under the chassis.
This is going to be quite a challenge as there seem to be many differences to the trader. You really need to draw out the circuit and layout. I would print out a copy of the circuit diagram, parts list, and pictures of the underside and top for the layout so you can mark up changes and put component designations on the layouts. (You can do this electronically with PowerPoint). Work methodically across the chassis and start marking up the pictures with the component ID, check the value in the parts list and record differences, and add any new components to the parts list and circuit giving them a new ID. The only alternative is to replace all remaining paper capacitors and see if it works but if you make a mistake it will be next to impossible to work out how it was wired before. |
22nd Jun 2020, 12:43 pm | #54 |
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Re: EKCO AD75 restore
There is a variation on service sheets due to the two release dates of that model.Though the difference should be mentioned on the Trader sheet or mine does.
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22nd Jun 2020, 12:55 pm | #55 | |
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Re: EKCO AD75 restore
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On the right hand side of the diagram is a capacitor C25 which is electrically connected between L1 and chassis. Where it's physically located I wouldn't like to say. The under chassis layout shows where C25 is physically located, but the location may have been changed. Of course we don't know whether the unknown capacitor is actually connected to L1. To confirm this the tags on L1, L2 and L3 would need to be idenitified.
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22nd Jun 2020, 1:44 pm | #56 |
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Re: EKCO AD75 restore
L14? These are the inductors attached to the rear panel. C25 and C26 are part of the mains suppression and should be class Y types.
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22nd Jun 2020, 1:51 pm | #57 |
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Re: EKCO AD75 restore
C25 is electrically connected between L1 and L14. One end of L14 goes to chassis as does one end of the mystery capacitor. Whether the mystery capacitor is C25 I don't know. If C25 is to be found physically elsewhere then it isn't the mystery capacitor and further investigation is needed.
If the set differs greatly from sheet 784, then it will have to be reverse engineered.
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22nd Jun 2020, 3:56 pm | #58 | |
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Re: EKCO AD75 restore
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There are 4 of the wire capacitors but only 3 mentioned in the trader, it does mention C9 which would be the mica cap on top but nothing about C25 or any other paper capacitor. C26 seems to be larger than the other types and I assume C25 should be the same so a picture of the part that has been removed would be good. |
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22nd Jun 2020, 7:13 pm | #59 |
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Re: EKCO AD75 restore
Here is a picture of the 2 caps that were removed. Both 0.1mF but the the inner one is bigger.
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22nd Jun 2020, 9:55 pm | #60 |
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Re: EKCO AD75 restore
Graham was right, that is C25. It should be replaced with a class Y cap.
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