ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
I bought a Beomaster 4400 last week and whilst pleased with its performance a slight amount of mains hum at switch on, which initially went as the unit warmed up, has now turned into something quite horrendous and I can no longer use the receiver. The hum is present regardless of volume setting and input selection so I’m suspecting the unregulated power amp power supply has fail in some way. Does this should sensible?
I ought to replace the two smoothing capacitor anyway given the units age (OC8 &9), is there any preference in part type?
Thank you for any help given.
Karl
Did you check the idle currents?An overloaded power supply can easily cause hum.
Martin
Thanks for your quick reply, I will check later today ref the current.
Well I have finally found time to look at the issue. The hum had initially gone but after a few hours running, it has returned.
Checking the power amp supply rails both +/-35V look like this –
This mains ripple is stable regardless of volume (some sagging/audio ripple at high volume levels). I do not think this is the cause of the issue because –
The ripple remains the same as the amp warms up whilst the offending hum reduces.
This ripple would be rejected by the design of the power amp.
So checking the i/p to the power amp section (TP107) shown the audio to have ripple on it which does reduce as the amp warms up. This ripple is present on both the left and right 24V rails (there is poor PPSR on the preamp stages as they are powered from a single rail). Working back, the source of all the pre/phono amp supplies is the 35V one and, yes, the ripple is present here too. See below –
I did disconnect what I could to reduce the current drawn from this supply but it made no difference. So a blast of freezer spray on the 35V smoothing cap, 5C2 220uF, restored the audio hum on a warm amp. I strapped a 470uf across 5C2 and was welcomed by a hum free amp; the noise floor is also much reduced (there is still a little hum on the phono input – normal?)
So I intend to replace all the electrolytic capacitors in the various regulated supplies, it can only be beneficial.
I did check the idle currents on the power amp, one channel was correct, the other a few mA high which was adjusted to match the other one.
One final question, should I replace the smoothing caps on the Power Amp supply (OC8/9 10000uF/50V), they are around 40 years old? This component should fit but is there a recommended preference? https://www.digikey.co.uk/product-detail/en/kemet/ALS80A103DB063/399-14324-ND/6928326?_ga=2.22754680.25739009.1569876246-1758498541.1569876246
Regards,