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Old 14th Jun 2020, 12:55 pm   #21
westcliff
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Default Re: EKCO AD75 restore

According to Trader aheet 784, L13 is connected to pin 8 (cathode) of the rectifier.

Gus.
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Old 14th Jun 2020, 1:10 pm   #22
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Default Re: EKCO AD75 restore

Indeed. Unless it's connected there'll be no HT to the other valves and the set will never work. I am aware that the rectifier has been replaced with an MR, but this still applies although there may be a series resistor.

I'm really surprised that a check hasn't been made for HT across the smoothers C23 and C24. It should be one of the first things to check. You don't even need a circuit diagram to do it as it's easy to recognise the smoothers.
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Old 14th Jun 2020, 3:06 pm   #23
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Default Re: EKCO AD75 restore

The dual electrolytic may have been replaced with a single so there may be no C24 to check. Given the number of changes I would want to know what has been done before trying to get it working. The HF choke may also have been taken out of circuit but wires floating around says to me that the modifications need to be tidied up!

I suggest the new circuit should be drawn up next. The wiring in the pictures is unfortunately too difficult to follow.
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Old 14th Jun 2020, 3:59 pm   #24
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Default Re: EKCO AD75 restore

Thanks all. I don't think the dual electrolytic has been replaced- there is still both C23 and C24. However they are both 24mF not as stated in the spec sheet 8 and 24mF.

I've connected up the loose wire.

The voltages at C23 and C24 are:
C24: 190V on start up, drops to 160V after 2 mins
C23: 187V on start up, drops to 150V after 2 mins
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Old 14th Jun 2020, 4:36 pm   #25
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Default Re: EKCO AD75 restore

Are you confident it is wired as in the trader sheet? If so, the next stage is to measure the voltage on the valves starting at V3.
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Old 14th Jun 2020, 8:05 pm   #26
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Default Re: EKCO AD75 restore

Thanks PJL

I have measured the voltages on V3.

Pins 1, 4 and 5 are zero V
Pin 6 measures -0.8V initiallr rapidly diminishing towards zero. Some crackling is heard from the speaker during measurment.
Pin 7 is 150V
Pin 8 is 146V


(If it is a CBL1 then Cathode appears to be zero, a(d2) is -0.8V and annode is 146V.
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Old 14th Jun 2020, 9:08 pm   #27
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Default Re: EKCO AD75 restore

For CBL1.

1. 0V Metallisation.
2. Heater.
3. Heater.
4. 0V Cathode.
5. 0V a d1
6. -0.8V a d2
7. No connection. 150V. Tag probably used as a termination point.
8. 146V Anode.

What's the reading on the top cap which is the control grid?

There should be some voltage on the cathode, as the anode current will be flowing through R14 and R14. This would be negated if C18 were short-circuit though.
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Old 14th Jun 2020, 9:36 pm   #28
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Default Re: EKCO AD75 restore

r-type.org has the pin layout wrong. Pin 7 is g2 so the voltage is OK but the HT is very low. As Graham says, the cathode should not be 0V so replace C18 or disconnect it if you do not have a 50uF electrolytic.

Are you running with the lamp limiter?
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Old 14th Jun 2020, 9:50 pm   #29
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Default Re: EKCO AD75 restore

Quote:
Originally Posted by PJL View Post
r-type.org has the pin layout wrong.
Quite right I just checked another source.

I don't honestly think the OP is going to get far with this without replacing a lot of capacitors. The chassis does look restorable though, so long as no wound components are faulty.
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Old 14th Jun 2020, 11:17 pm   #30
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Default Re: EKCO AD75 restore

Graham, I agree as this set is quite a challenge given the condition and the changes that have been made. I would just have dived in and replaced the capacitors and out of tolerance resistors but it would probably be best to persevere and see if we can get it sort of working so that the parts can be change one by one.
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Old 15th Jun 2020, 12:39 pm   #31
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Default Re: EKCO AD75 restore

Thanks Station X and PJL.
I would prefer to go one step (component) at a time as I believe this is the best way to learn- especially as the spec sheet and other information is not always correct.

C18 has already been replaced at some point with a 25mF item which measures as leaky on my peak meter. I've replaced it with a 47mF one- see photos.

The top cap (control grid) measures 3.0V
The Cathode now rises to 6.2V and I now get signs of life from the speaker, some hum/white noise and crackling which increases via the volume control and appears to be affected by the tuning being adjusted.
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Old 15th Jun 2020, 12:46 pm   #32
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Default Re: EKCO AD75 restore

here are the pics
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Old 15th Jun 2020, 12:56 pm   #33
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Default Re: EKCO AD75 restore

The cathode resistor is 330+150. I=V/R so the cathode current is 6.2/480 = 12.9mA and it should be 40mA according to the trader sheet.

What is the V3 anode voltage now? If the HT is too low the oscillator may not run. Low HT can be due to excess current from leaky electrolytics and capacitors, or a weak metal rectifier. We can estimate the current by measuring the DC voltage between C24 and C23.
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Old 15th Jun 2020, 1:52 pm   #34
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Default Re: EKCO AD75 restore

Given that the rectifier valve has been replaced with a metal rectifier, it should be possible to check the HT with all the valves removed. This will give a clue as to whether the low HT is down to a faulty MR or excess HT current being drawn by the valves.

Of course low HT could also be down to any number of capacitors being leaky, including the smoothers.
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Old 16th Jun 2020, 10:01 am   #35
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Default Re: EKCO AD75 restore

Apologies, I should have been clear that the measurements were with the lamp limiter in place.
With it bypassed annode voltage starts over 200 then gradually reduces to 190V DC

V3 heater voltage is 38V AC
V3 cathode voltage is 13.8V so around 29mA and closer to the expected 40 but still a little low.

During the tests one of the paper caps (C12 I think) started bubbling so I have replaced it.
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Old 16th Jun 2020, 10:23 am   #36
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Default Re: EKCO AD75 restore

If you touch an insulated screwdriver on the volume control wiper (centre) connection does it click/hum?

To avoid more bubbling, it is time to replace a few more capacitors: C7, C4 and C10 all of which have fairly high voltages across them. Ultimately, the rest will need replacing but this should be enough to leave it running without the lamp limiter and hopefully get it going.

Once you have done this take some DC measurements of each of the valves, anode, cathode, grid, screen grid.
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Old 18th Jun 2020, 3:05 pm   #37
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Default Re: EKCO AD75 restore

Thanks PJL
I'm not sure what part of the volume control you mean- there does not appear to be a center pivot on the inside. If you mean the metal shaft of the volume control then this does not result in click or hum from radio. However touching the caps of V1 and V2 does.

I've replaced C4 and C10- see photo. However C7 appears to be missing. I've attached a photo of the radio as it was when I got it. I replaced a paper cap that also started bubbling which I think was C22, bus as far as I can see C7 was never present.

I will do some voltage measurements and post them shortly
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Old 18th Jun 2020, 3:10 pm   #38
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Default Re: EKCO AD75 restore

And here are the voltage measurement.
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Old 18th Jun 2020, 6:02 pm   #39
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Default Re: EKCO AD75 restore

There are a few odd -ve voltages on grids that may imply a problem with the AGC resistors R1, R16 and R17. Can you check them, 25% out is still OK. What type of meter did you use? The EF9 may also be weak but don't do anything until all the capacitors have been replaced.

What we want to do now is to see if the oscillator running. Measure the voltage on V1 triode anode (pin 4) and grid (pin 5).
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Old 19th Jun 2020, 1:22 pm   #40
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Default Re: EKCO AD75 restore

Thanks PJL.

The voltage measurements are:
on V1 triode anode (pin 4) =0.8V
and grid (pin 5)=85.5V

The AGC resistors measuremnts are more tricky. Is the ARG present on this radio? I don't see any of the R1, R16 or R17 resistors to measure. I've tried to trace a resistor from the L3 coil but nothing obvious is there, similarly as the valve V3 is replaced with the metal rectifier I can't trace the R16 & R17 from the V3 pin-outs.

I've attached a photo of where the resistors should be according to my spec sheet, but as the spec sheet has many differences (all 3 valves are different just for a start) it may be of limitied use.
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