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Old 23rd Jan 2019, 12:48 pm   #1
Mike Noble
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Default Marconiphone record player. Loud hum.

Hello, I've been asked to look at a record player which a a hum problem.

If it has not been used for a while, when first switched on there is a loud hum. This then suddenly reduces to a low level hum after 30 seconds or so. If it is switched off and then on again the hum level is reduced and then goes to the low level hum after one or two seconds.
This is a transistorised player from around 1970. There looks like selenium or metal rectifier. The electrolytic capacitors read correct on a capacitance meter.

Should I change the rectifier to diodes? If so how are they connected?

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Old 23rd Jan 2019, 1:25 pm   #2
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Default Re: Marconiphone record player. Loud hum.

Does the player work normally when the hum settles down?

Do you have any testgear to check for ac ripple voltages in the 30sec window?
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Old 23rd Jan 2019, 2:17 pm   #3
Mike Noble
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Default Re: Marconiphone record player. Loud hum.

Yes the player works normally once it has settled down with just a low level of hum. I have an oscilloscope so can check ripple.
At present there is 80mV of ripple on the supply. I won't be able to check when in the high hum condition until the player has been left for a day.

Thank you
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Old 24th Jan 2019, 9:00 am   #4
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Default Re: Marconiphone record player. Loud hum.

Is the rectifier a half wave or full wave/bridge type?

If it is full wave or a bridge, when checking the ripple note it’s frequency as well as it’s size. It should be 100hz, if 50hz it would suggest one of the diodes in the full wave rectifier or bridge, or the circuit being O/C. Bad joint, rectifier, cable, transformer etc.

A half wave rectifier would be 50hz.
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Old 24th Jan 2019, 1:18 pm   #5
Mike Noble
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Default Re: Marconiphone record player. Loud hum.

Thanks for the reply. It's 100Hz. I now can't get the player into the high hum condition. The bridge rectifier reads ok on a multi-meter so I've ordered a replacement smoothing capacitor. It's a 2500uF 30V with 25V across it so I've ordered a 3300uF 63V which is the same size and should be more reliable.
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Old 24th Jan 2019, 4:44 pm   #6
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Default Re: Marconiphone record player. Loud hum.

Reading a few comments by the OP in the first post: "not been used for some time" and "dates from around 1970", then the last remark " I now can't get the player into the high hum condition", to me sounds like the typical scenario where the PSU filter capacitor was initially undergoing the typical 'self-heal' process, having not had voltage across it for some time - and it's quite old, too! So, yes - change it for a new one - and all should then be O.K.

Al.
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Old 24th Jan 2019, 11:12 pm   #7
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Default Re: Marconiphone record player. Loud hum.

Probably not related to the hum issue but I can see a few other electrolytic capacitors that I would check, that yellow plastic one for instance. I have replaced a few like that because of serious leakage.
Those push fit resistors with the brass end caps, I forget what they are called, are notorious as well, I would check any that are in critical locations in the circuit.

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