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Old 30th Jul 2020, 5:07 pm   #121
thejazzageuk
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Default Re: EKCO AD75 restore

Thank you PJL. I have cleaned the wavebands switch again and measured L4-7 resistances.
They are
L4 8.5R
L5 3.8R
L6+L7 3.3R
Here are 3 traces again (taken using ac coupling).
Pic 1. V2 pin 8 anode
Pic2. V1 pin 5 triode anode
Pic 3 V1 pin 4 cathode
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Old 30th Jul 2020, 5:51 pm   #122
thejazzageuk
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Default Re: EKCO AD75 restore

Here are the traces..

The voltage readings were
PIN 4 1.1V
Pin 5 68V
Pin 6 1.1V
Pin 7 63V
Pin 8 176V
Cap 0.65V
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Old 30th Jul 2020, 9:46 pm   #123
PJL
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Default Re: EKCO AD75 restore

The oscillator coils all appear intact.
The voltage measurements make much more sense than some of your earlier ones.
The hexode anode (Pin 8) is low so the HT must also be low. Are you using the lamp limiter, if so remove it? The low HT will affect the valve currents.
The triode anode (pin 5) voltage is reasonable and it calculates as an anode current of about 2.3mA which is a little low.
The cathode (pin 4) corresponds to 5.6mA which is a little low. This suggests the hexode is taking around 3.3mA.

There does not seem to be any obvious reason why the oscillator is not running. It could be the valve or maybe because you have accidentally soldered one of the replacement caps to the wrong place. Check your work carefully against the circuit diagram, particularly C5 and C10. Make sure you are testing on the MW waveband.
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Old 5th Aug 2020, 8:56 am   #124
thejazzageuk
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Default Re: EKCO AD75 restore

Thanks PJL
I'm not using the limiting lamp, but the isolation transformer only has 220V output so that is why the readings are a little low.
I have re checked the wiring. Caps 5 and 10 in particular. I did replace C10 as the peak meter showed it to be leaky but it measured OK once removed.
Here are 3 traces again (taken using ac coupling).
Pic 1. V2 pin 8 anode
Pic2. V1 pin 5 triode anode
Pic 3 V1 pin 4 cathode

There did not seem to be much happening on V1
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Old 5th Aug 2020, 11:05 am   #125
PJL
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Default Re: EKCO AD75 restore

Still not oscillating. You need to double check all of the wiring that forms the oscillator.
1. Check the wiring and value of these components C5, C7, C8, C10, C30, C31, R5. I think R5 was wrong, did you replace it?
2. Switch to MW and measure the resistance from C6/C31 junction to chassis. It should be open circuit, if it is not then it is most likely the tuning condensor is shorting out.
3. Measure the resistance from C6/31 junction to C8/L4 junction. It should be less than 10 ohms, if it is a higher value then the switch S3 probably needs cleaning.
4. Check the earth connection to the tuning condensor and clean the contacts that make the connection to the shaft with the moving vanes.
5. Switch your meter to AC volts and measure the voltage on V1 heater (pin 2 to pin 3) it should be around 6.3V. If it is low then you will need to move the connection on the dropper to account for 220V input but as the dropper is a replacement I suggest you measure the resistance of each section first.

Last edited by PJL; 5th Aug 2020 at 11:11 am.
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Old 18th Aug 2020, 6:33 pm   #126
thejazzageuk
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Default Re: EKCO AD75 restore

Thanks PJL
By an odd coincidence I have been offered a damaged but functioning AD75 so I think it's a good idea if i hold of more checks until it arrives then I can do voltage and component checks back to back with the working radio.
Will keep this forum updated -should be here in a couple of weeks.
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