EKCO AD75 restore

[thejazzageuk]

Thanks Station X and PJL
Good to get to the bottom of the mystery! I will order some Y class caps and replace C25.
I think the one in the photo is C26 and will also need replacing with a similar one.

Unfortunately I only have trader sheet 784. I will replace the remaining paper caps that are straightforward. The one under V1 looks like it may be more tricky.

If there is no positive result from this then I will do as PJL suggests and start to draw out the circuit. Apart from anything else it will help me understand what is going on.

[PJL]

I started but it is impossible to follow the wiring. This is using PowerPoint which makes it easier to add text.

[thejazzageuk]

Thanks PJL

I have orded some Y2 caps and will fit these as soon as they arrive. I've started the process of labelling the caps- see attached photos.

It's difficult tracing some of the wires as I don't want to disturb and damage the radio. C8 and C9 seem very delicate. I tried to measure their value (in-circuit with the fluke 83III) but not surprisingly this was not very successfull.

According the the spec sheet, there are only two 0.01mf cpapcitors on the radio- C19 and C11. One of the C? I've marked is 0.01mf so it could be C11 if the other 0.01mf is C19.

The radio does sound like the oscillator is working. There is a constant high piched tone audible over the speakers and different hums and white nosie come and go as you search through the tuning dial.

I have noticed that V3 does appear a bit loose- the glass can rotate about 1 or 2 degrees with the base fixed in place.

[PJL]

Without an accurate circuit diagram and layout we will have real trouble trying to locate any faults and this set does not match the trader sheet in many areas.

Your C19? appears to be connected to pin 8 of the EBL1 which is the anode and would be either C22 or C21? There are some odd components next to the potentiometer that look like resistors and have a red body and one with a blue and another with a green band - not sure what these are!

You have proved that the audio is doing something so it will be possible to make the set work but it will be a long haul hampered by your limited experience and the difficulty in trying to provide remote help for this.

[Station X]

If it's difficult to trace a wire physically or visually, use you meter on ohms range to find points to which it shows very low resistance.

[PJL]

I have had a shot at the output stage and the circuit seems to be the same as the trader. You need to replace C19. You need to also replace C13 and try to identify the capacitors in the RF end.

[thejazzageuk]

Thank you PJL

That really helps. Now I think I have enough information to start tracing the circuit. I've made a start and have kept track of progress both on the powerpoint format you started for me and by annotating the circuit that we all have from the data sheet. The two pics show my progress so far.

I have now replaced C5 and C13. I've also oredered some replacement ceramic caps for the 470pf and 220pf C8 and C9. They are 50V so should be OK?

Is the 'loose' glass/base V3 likely to cause a problem?

[HamishBoxer]

50V should be fine.

[PJL]

C8 and C9 look like silver-mica covered with wax and should be OK although I have not seen them with sticky plaster before! C19 is the one that does need doing.

Have you cleaned the wavechange switch and the valve sockets and bases?
Does it make a noise when you plug in the aerial wire?

[PJL]

C8 and C9 look like silver-mica covered with wax and should be OK although I have not seen them with sticky plaster before! C19 is the one that does need doing.

Have you cleaned the wavechange switch and the valve sockets and bases?
Don't touch the earth or aerial socket/wire until C25 and C26 have been replaced with class Y caps.
What are you using for an aerial wire? Does it make a noise when you plug it in?
We could do with a new set of voltage measurements, anode, cathode, grid and screen grid for each of the valves (V1 triode too) now you have replaced most of the paper capacitors.

[thejazzageuk]

Thanks PJL
I've continued to trace through the circuit and have now replaced all the paper caps. C25 and C26 are now Y2 class. There appears to be an additional capacitor on my radio that is not shown in the circuit. It sits next to C3 and is connected to the valve side of L2 and the other side to gnd.
I will re-check the pin voltages and post the results shortly. Meanwhile, here is what it sounding like

https://www.dropbox.com/s/nq76tu51vu...01802.mp4?dl=0

[Lloyd 1985]

Sounds like a lot of switch mode power supply noise, maybe try going round turning things off, like any lights, computers or anything else that may be making noise. I find it’s the LED lights in my workshop are the worst offenders, closely followed by the VCR that feeds signals to my old TV’s.

Regards
Lloyd

[ms660]

I might be wrong but it sounds like the oscillator's packed up to me.

[PJL]

On MW you are getting some peaks that could be stations but you are rushing through them. Tune slowly around the peak and see if it resolves to a station.
On LW just a lot of hash that as others have said is probably interference.

You were mentioning that you had one disconnected twisted wire capacitor - have you wired this back?

Most likely the set needs alignment. Do you have a signal generator?

[thejazzageuk]

Thanks all

The voltage measurements are the same as in the earlier table (re-attached in this post) except:
V2 cathode is 1.2V not 0.6V, G2 (pin7) is 127V not 72.7V, G1 is -0.2V not -1.2V
V1 G1 (top cap) is -4.2V not -2.2V

Yes PJL, I have an advance signal generator (type E model 2) but I don't have an isolating power supply. I've seen some comments around the alignment process but would apreciate some guidance to do it safely.

[PJL]

What is the value of CX? It should be a very low value.

Did you try slowly tuning through the MW band?

I am still not sure where the -4.2V on V1 grid is coming from. Can you confirm that C3 is wired correctly. Did you clean the valve base and socket carefully?

And yes, you need an isolating transformer to use the signal generator.

[thejazzageuk]

Thanks. CX is a 0.1mf capacitor. It was originally one of the larger red capacitor types used for safety in C25 C26- see photos of radio prior to c3/cx replacement and the other pic of the 2 caps removed. There is also a small silver-mica covered with wax above them which I will trace later.
I did try going through the MW very slowly, but no stations could be tuned into.

Will check and re-clean the voltages/base and socket.

I have looked for a suitable isolation transformer. Most of the ones on eBay seem to be step down, but this one I thought would be OK?

https://www.ebay.co.uk/itm/164263425830

[HamishBoxer]

I have restored 2 of this model and one has a noise like florescent light interference when not on station.

Must investigate further in due course.

[PJL]

Check Cx connections as it will block all reception as it is shown in the circuit diagram. I thought we had decided the one on the top was C25?

[thejazzageuk]

Thanks PJL
I am a bit unclear about C3 and CX so have re-traced the circuit from the live side of each (other side of both is earthed)- see pics.
I've also tried to re-trace the circuit from mains in. There appears to be some differences to the published circuit. In particular point A appears to host multiple connections including the aerial earth. Also C24 and C25 don't appear to be connected wheras they are in the published circuit. Does any of this make sense? (Maybe HamishBoxer if he sees this could indicate if this matches the AD75s he saw?)