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Old 7th Jul 2020, 7:23 pm   #81
PJL
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Default Re: EKCO AD75 restore

Cx is C25 as it connects the aerial earth to chassis. So what did you determine was C25?
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Old 7th Jul 2020, 8:16 pm   #82
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Default Re: EKCO AD75 restore

In the 1st circuit you have drawn aerial earth connected to HT+ (C24), hopefully this is wrong!

In the 2nd circuit Cx is C25 as it connects the aerial earth to chassis. So what did you determine was C25?
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Old 8th Jul 2020, 8:34 pm   #83
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Default Re: EKCO AD75 restore

Thanks. Starting to make sense. Yes, of course Cx is C25! I have now replaced it with a Y class type. I've re-drawn the circuit to match the layout of the one in the data sheet. The capacitor that I had labeled as C25 under the chassis is not shown on the data sheet as far as I can see, I've labelled it CZ and you can see where it fits from the schematic below.

I've rechecked R1, R16 and R17- both resistances when not powered and voltages when powered. I've added a few additional voltages in to try to establish why there is -4.2V on V1 grid.
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Old 8th Jul 2020, 9:52 pm   #84
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Default Re: EKCO AD75 restore

You're showing different voltages on the top ends of R1 and C3, but these points are connected?

The voltage on V1 and V2 grids is the AGC voltage derived from one of the diodes in V3.
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Old 8th Jul 2020, 10:10 pm   #85
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Default Re: EKCO AD75 restore

Cz is an additional suppression capacitor.

The voltages suggest that you still have instability. The wayward oscillation gets rectified on the EF9 grid where the lowest voltage is detected.

Can you confirm all of the paper capacitors are now replaced?

Did you ever find C7?

Also, the trader indicates the earlier model did not have R7/C12, can you confirm this is true on your chassis?

The EF9 valve should have a red conductive coating on it. Can you confirm you can measure a low resistance between the coating and V2 pin 1. Failure of the conductive coating is a common problem with the EF39.
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Old 9th Jul 2020, 5:57 pm   #86
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Default Re: EKCO AD75 restore

Thanks PJL
Yes all paper capacitors are now replaced.
C7 is present- I've indicated it on the attached photo with R2 and R6
I think R7/C12 are missing.
I measured the resistance between V2 and pin 1- It appears OC. Also did the same on V1 between Pin 1 and conductive paint- same result. There is a reading on the wire that is wound between the base and the glass, but nothing from the paint, even if I scrape it a bit.
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Old 9th Jul 2020, 8:39 pm   #87
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Default Re: EKCO AD75 restore

If it has a wire wrapped around then someone has already tried to repair the broken connection to the conductive paint. This suggests instability may be a long standing problem.

A method to see if this is the cause is to wrap silver baking foil around the valve making sure it does not touch the top cap and put another few turns of tinned copper wire over the top to hold it in place and make good contact with the wire already wrapped around the valve. I suggest you do this to both the EF9 and ECH3.

If that still does not fix it I would be tempted to add R7/C12.

Post some pictures of the valves so we can see.
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Old 10th Jul 2020, 10:13 am   #88
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Default Re: EKCO AD75 restore

Here are a couple of pics of the 2 valves, showing the wire I referred to.Note they are ECH3 and EF9 and not CCH35 and EF39 as in the spec sheet.
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Old 10th Jul 2020, 10:36 am   #89
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Bit of conductive paint run around with the wire to bridge to the red conductive coating would be best but the silver foil trick should tell you if this is your issue. Just make sure the foil does not touch the top cap or chassis. This is a common problem as I imagine having the screening isolated is worse then none at all.
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Old 10th Jul 2020, 9:24 pm   #90
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Default Re: EKCO AD75 restore

Here is a short video of the radio wit the foil covered valves. Still no stations located

https://mega.nz/file/adZ31QaR#nkwPTx...ZVz2NYP9BJGd-A
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Old 10th Jul 2020, 11:20 pm   #91
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What are the grid voltages now?
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Old 11th Jul 2020, 10:24 am   #92
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Here is an updated voltages table. Voltages as measured this morning with foil in place.
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Old 11th Jul 2020, 11:21 am   #93
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Default Re: EKCO AD75 restore

I would measure the oscillators grid voltage with a DMM using a resistor in series with the meter probe (say 100k) to reduce the effect of the meter/probe capacitance on the oscillator, should be a few volts -ve if all's ok.

The mixer g1 volts shown in your table shouldn't be that high, should be less than the IF amp g1 voltage.

HT's a bit low, might be worth sorting that out as well.

Lawrence.

Last edited by ms660; 11th Jul 2020 at 11:30 am. Reason: addition
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Old 11th Jul 2020, 11:37 am   #94
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You are getting some very odd readings. -1.9V on V3 G1 makes no sense, it should be +4.5V. Are you measuring from chassis, you must not use the aerial earth as it is floating. What type of meter are you using?
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Old 11th Jul 2020, 11:40 am   #95
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Lawrence, I have been chasing -ve voltages on high impedance grids that thejazzageuk has reported but I am not convinced they are 'real' as it is also showing on V3 grid. Any ideas as we need some reliable voltage measurements?
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Old 11th Jul 2020, 11:44 am   #96
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Default Re: EKCO AD75 restore

V3 g1 is TC, I think the -ve 1.9 volts is on one of the diodes anode, ignore the OP's electrode designations in that table and just go by the pin numbers.

According to the CBL1 cathode and a1 (diode) measurements the meter is a 10 meg. job

Lawrence.

Last edited by ms660; 11th Jul 2020 at 12:08 pm. Reason: additional info
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Old 11th Jul 2020, 12:09 pm   #97
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Well spotted, but that doesn't explain the -7.5V on V1 G1 that strongly suggests instability and would kill the hexode gain.
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Old 12th Jul 2020, 4:42 pm   #98
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The isolating power supply I bought has now arrived. Shall I try to take a scope trace of v1 g1?
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Old 12th Jul 2020, 7:09 pm   #99
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V1 triode anode (pin 5) and V2 anode (pin 8). Scope needs to be on a x10 probe or the capacitance of the probe might kill the signal.
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Old 13th Jul 2020, 12:42 pm   #100
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Thanks pjl. Is it sensible to use an isolation power supply to take the scope traces, I see lots of different views on when to use one. The scope probe has a 'max voltage 600v dc/ac' sticker on it so should be well in its range.
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