Hi Evryone,
On my workbench I have an old Ming-Da MC3008-AB brought to me for restoration after being in storage for 10+ years.
It had a broken 805 tube and all the capacitors were bulging.
So I replaced all the caps and tubes but unfortunately there is an annoying hum coming from the speakers and audible physically.
As you can see on the schematic the A+ filter caps are only 330uF instead of 1000uF. I added 470uF the the first set of four caps but this didn't improve anything.
Also, lowering or removing the NFB makes the hum louder. I've also added balacepots to the filaments of the 300B and 805 tubes but no effect. I adjusted the bias-current for the 805 at around 100mA.
Could it be that the transformers are causing the hum? Or any other suggestions?
On my workbench I have an old Ming-Da MC3008-AB brought to me for restoration after being in storage for 10+ years.
It had a broken 805 tube and all the capacitors were bulging.
So I replaced all the caps and tubes but unfortunately there is an annoying hum coming from the speakers and audible physically.
As you can see on the schematic the A+ filter caps are only 330uF instead of 1000uF. I added 470uF the the first set of four caps but this didn't improve anything.
Also, lowering or removing the NFB makes the hum louder. I've also added balacepots to the filaments of the 300B and 805 tubes but no effect. I adjusted the bias-current for the 805 at around 100mA.
Could it be that the transformers are causing the hum? Or any other suggestions?
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Hi,
I don’t know this exact model. Did you ever hear this amplifier before restoration? Why are the capacitors only 330 uF? Do you hear any difference when adding the 470 uF cap? Do the balance pots actually work? Does the hum level change? What hum do you hear, is it 50 Hz. or is it 100 Hz.?
I know hum is a serious nuisance, I’m still trying to get rid of the last 2 mV of hum I measure with my current tube project.
Regards, Gerrit
I don’t know this exact model. Did you ever hear this amplifier before restoration? Why are the capacitors only 330 uF? Do you hear any difference when adding the 470 uF cap? Do the balance pots actually work? Does the hum level change? What hum do you hear, is it 50 Hz. or is it 100 Hz.?
I know hum is a serious nuisance, I’m still trying to get rid of the last 2 mV of hum I measure with my current tube project.
Regards, Gerrit
I would start with measuring ripple on the A+ and B+
Possibly the 330uF ( each ?) caps is too small for this voltage doubler.
Measuring will tell.
No
This is a very old model so I guess when they first made it they used 330uF. I contacted Mei-xing (the current Ming-da builders) and they said 330uF, 470uF and 1000uF are all ok.[/QUOTE]
Nope
Not really, turning them only adds a different hum after a while
I think 50Hz, but i'll check it on the scope
Yes. To make matters worse.. The owner first told me (through a friend) that as long as it's working again he's ok with it, but suddenly he tells me it was used for mastering and needs to buy as new again.. pfftt
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Do you know why they were bulging? Could be the cap type is no suitable in this application as it has high ESR increase heat and reducing capacitance. You might want to double check the polarity of every e-cap is correct.
Hi,
It look as if the output transformer gets some induced signal from the power transformer. But it’s weird this would not have been the case years ago...
Try to check with a scope what frequency hum you have. Perhaps this can be useful:
VinylSavor: Technical guidelines
Regards, Gerrit
It look as if the output transformer gets some induced signal from the power transformer. But it’s weird this would not have been the case years ago...
Try to check with a scope what frequency hum you have. Perhaps this can be useful:
VinylSavor: Technical guidelines
Regards, Gerrit
I don't know but they could be fake Epcos caps? Also the old ones were 85 degree versions and this amp get hot. Also all the date codes point to 1996 so they were pretty old.
Check (OFF state) if any big cap get hot after switch on for sometimes 15mins to 30mins. They may need to re-film (one get hot) due to long storage. Also the diode rectifiers in PSU.
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The ripple freq is 50Hz, my interpretation is it should be 100Hz as it's voltage double circuit. It' detected 50Hz because one half is not doing the filtering as shown by ripple peak while the other is flat (not visible).
So A+ripple is higher with tubes (loads), what is it like on output again when it's loaded?
That should be alright.
You said that it was huming without tubes ? hat indicated magnetic
leaking between mains and output transformer.
I don't know. Have the transformers been unmounted ? Are all shields
and lids mounted ?
If shielded by mü-metal the metal might have been "hardened" by mechanical
disturbing.