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Old 25th Feb 2020, 8:24 pm   #1
Malcolm G6ANZ
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Default Cossor Melody Maker low volume

I have a Cossor Melody Maker 524 which I have recapped and changed any out of spec resistors. With a new FM front end valve (6AQ8) it is performing quite well. However there is a 100Hz hum present. The volume of hum varies with the volume setting so it isn't the HT causing it. Now on Radio3, which I use it for, the output volume is lower than Radio 2. I put this down to lack of compression on R3, but since the volume has to be wound up the hum level increases.
All the valves are original ,apart from the front end, so would I be correct in thinking that the output valve (EL84, 6BQ5) is on its way out?
Medium and long wave seem OK with plenty volume.
Malcolm
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Old 25th Feb 2020, 9:37 pm   #2
PJL
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Default Re: Cossor Melody Maker low volume

At this stage I would check the DC voltages on each valve as it will give a rough indication of valve condition. Have you got a replacement 6AK8?
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Old 25th Feb 2020, 10:22 pm   #3
Cobaltblue
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Default Re: Cossor Melody Maker low volume

Have to agree with PJL

The 6AK8 or components around it is favourite.

This stage has a number of high value resistors and as we all know high value resistors have a habit of going even higher

I know you have checked all the resistors but I would certainly recheck the 6.8M grid bias resistor on the triode of the 6AK8.

These Diode, Double Diode, Triodes are a bit prone it's a lot to get into a single bottle.

So if you have a sub and its not the 6.8M that would be my next step.

I would be surprised if this is a fault in the 6BQ5 (but I have had surprises before).

Cheers

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Last edited by Cobaltblue; 25th Feb 2020 at 10:23 pm. Reason: spacing
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Old 26th Feb 2020, 8:08 am   #4
Malcolm G6ANZ
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Default Re: Cossor Melody Maker low volume

Thanks for the insight. I don't have a spare 6AK8. When I get a chance I'll go through the high value resistors. They were OK when I first looked at the set but its possible they have gone out of spec with use.
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Old 26th Feb 2020, 2:57 pm   #5
cathoderay57
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Default Re: Cossor Melody Maker low volume

Hi Malcolm, you should also find that the 6AK8 (=EABC80) has a low voltage electrolytic capacitor across 2 of its diode cathodes.This is part of the FM discriminator circuit and if it has gone leaky or lost capacitance it can cause problems similar to yours on FM. A.M. will be unaffected. Remember if you replace it that the positive lead usually goes to chassis but check the circuit diagram first. Cheers, Jerry
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Old 26th Feb 2020, 3:21 pm   #6
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Default Re: Cossor Melody Maker low volume

Quote:
Originally Posted by Malcolm G6ANZ View Post
Medium and long wave seem OK with plenty volume.
Malcolm
So there is no apparent hum on MW/LW? If so that could indicate a faulty diode within the EABC80. These valves can cause some strange effects so it's possible that you have a faulty valve.

Check the electrolytic as suggested by Cathoderay.
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Old 27th Feb 2020, 6:13 pm   #7
Malcolm G6ANZ
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Default Re: Cossor Melody Maker low volume

The Cossor is now back to 'normal'. There were two resistors around the FM discriminator that had gone high. A 6M8 that was nearer 10M and a 470K that was up to 500K. From memory I had already replaced the EABC 80 when I first started looking at the set along with the electrolytics. So the volume is now back to a usable level and the hum has all but gone.
There is still a slight problem the tuning is very critical otherwise there is distortion. I have a sig gen that will go to 100MHz but no wobbulator so I can't check/set up the discriminator as outlined in the service sheet.

At present I'll live with it as its much better than it was. Thanks all.
Malcolm
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