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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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20th Nov 2018, 8:22 pm | #1 |
Nonode
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Another Dansette Major - hum this time!
Quite a bit of hum independent of volume. Needless to say the caps are in a can. The 32 uF is the easiest to get at and bypass. Question is - can I use a 47uF cap 350V there (just because I have one of those to hand - HT is 240V)? And is it OK to leave the dead 32 in the can in circuit?
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20th Nov 2018, 8:49 pm | #2 |
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Re: Another Dansette Major - hum this time!
If the 32uF is the smoother, it's fine. If it's the reservoir cap ensure that 47uF is not too big for the rectifier.
Inadvisable to leave any old faulty cap in circuit. Fixing by just substituting parts on a whim is a poor way to repair IMO. |
20th Nov 2018, 10:42 pm | #3 |
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Re: Another Dansette Major - hum this time!
Which is why I'm not too happy when 'temporarily connecting a new capacitor is parallel with a suspect one to see if the hum disappears" is suggested as a fault finding method.
It's fine if the suspect capacitor has simply lost capacitance or gone open circuit. It's more likely to be leaky though and connecting a new cap in parallel will have little or no affect on the hum.
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21st Nov 2018, 12:12 am | #4 |
Nonode
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Re: Another Dansette Major - hum this time!
Thanks - I don't think I have any way of testing the cap - only a simple analogue multimeter. Also I can't see a way of disconnecting the ones in the can as they are soldered into a PCB....help! And - how can I work out which is the smoother and which is the reservoir cap from the circuit diagram...and what is the difference between them?
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'....don't go mistaking Paradise for that home across the road!' (Bob Dylan) Last edited by RojDW48; 21st Nov 2018 at 12:35 am. |
21st Nov 2018, 12:58 am | #5 |
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Re: Another Dansette Major - hum this time!
Reservoir is the one closest electrically to the rectifier- either connected direct to the cathode(s) or via a lowish value resistor. Smoother is next in line with either another resistor or in posher sets a choke connecting it to the reservoir.
The reservoir has to cope with a much higher ripple current and associated power dissipation than the smoother, so it will physically be the outermost capacitor in a multicap can. If appearances matter, you can restuff a can type with modern smaller capacitors or if function is the only requirement you can junk it and wire the replacement caps directly to the PCB. Beware that some layouts rather trickily use two of the can tags to bridge a gap in the 0V PCB trace so you'll need to provide a suitable wire link instead.
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21st Nov 2018, 7:06 am | #6 |
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Re: Another Dansette Major - hum this time!
These use a compact triple capacitor too small to reshuffle. Best to remove and replace with separate caps. The can forms part of the circuit so you will need to use a wire link.
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21st Nov 2018, 9:29 am | #7 |
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Re: Another Dansette Major - hum this time!
As I've often mentioned, modern dual can electrolytics, which are often 'drop-in' replacements for the originals are available from various sources, including Cricklewood Electronics, the BVWS, etc. The third cap. in the triple cap. can is almost always the o/p valve cathode bypass one, having a value of 25uf@25v. I replace these with a modern small axial type wired directly between the cathode pin on the valveholder and chassis.
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21st Nov 2018, 12:41 pm | #8 |
Nonode
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Re: Another Dansette Major - hum this time!
Much wisdom as always gents - thanks. That means the 32 must be the reservoir cap, so I shall have to extract the whole thing - I get the point about the can being the ground path. So there are (at least) 4 joints to the caps and can in the PCB - any tips about how to melt the solder on all of those at the same time - so I can pull it off?
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21st Nov 2018, 2:02 pm | #9 |
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Re: Another Dansette Major - hum this time!
Remove the solder with solder wick or vacuum pump.
For example, there will be other available. https://www.amazon.co.uk/TABIGER-Des...SIN=B07G9T1THB
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21st Nov 2018, 2:19 pm | #10 | |
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Re: Another Dansette Major - hum this time!
Quote:
Didn't fix the hum though!
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21st Nov 2018, 3:17 pm | #11 |
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Re: Another Dansette Major - hum this time!
Some hum is endemic in basic old valve record players. To get rid of it, you may well have actually to improve the design and wiring layout from the original. Knowing what's original sin (and therefore presumably acceptable) and what isn't is where it gets interesting.
A simplistic (if perhaps historically accurate) approach is just to buy a better quality machine.
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21st Nov 2018, 3:56 pm | #12 |
Nonode
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Re: Another Dansette Major - hum this time!
Thanks Frank - I have both - forgot about that!
Checked Cricklewood but they only seem to have value + same value cans. Currently the reservoir and smoothing caps are 32 and 16uF respectively. I know these old bangers hum a bit - but this one's well out of order!
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21st Nov 2018, 5:01 pm | #13 |
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Re: Another Dansette Major - hum this time!
Are you sure the 32uf is the reservoir? Usually the smaller one is the reservoir cap, usually is not always the case though.
If the 32uf is indeed the reservoir, then you could fit a 32+32 if there is enough room, the extra smoothing would do no harm.
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Frank |
21st Nov 2018, 6:22 pm | #14 |
Nonode
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Re: Another Dansette Major - hum this time!
Are you sure the 32uf is the reservoir? Usually the smaller one is the reservoir cap, usually is not always the case though.
I think so Frank. It appears to be the first one after the rectifier - see detail.
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21st Nov 2018, 6:33 pm | #15 |
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Re: Another Dansette Major - hum this time!
Yes that’s the reservoir. Is the amplifier just an EL84, if so the 16uf is probably just decoupling the screen grid.
A 32+32 would work but depends on how much room there is.
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21st Nov 2018, 6:47 pm | #16 |
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Re: Another Dansette Major - hum this time!
Roj, increasing the smoothing cap. to 32uf won't do any harm - in fact it may well reduce the hum level. The reservoir cap and smoother can both be the same value.
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21st Nov 2018, 7:45 pm | #17 |
Nonode
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Re: Another Dansette Major - hum this time!
Yes - 1 lonely EL84. That looks right for the 16uF as well. And Livewire's suggestion for the 25uF looks good. Cheers.
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26th Nov 2018, 10:23 am | #18 | |
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Re: Another Dansette Major - hum this time!
Quote:
The power supply looks very similar to the Decca with just a half wave rectifier.
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Paul Last edited by PaulR; 26th Nov 2018 at 10:28 am. |
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26th Nov 2018, 11:25 am | #19 |
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Re: Another Dansette Major - hum this time!
A semiconductor diode can stand a higher value reservoir capacitor than a valve rectifier. There will be enough resistance in the transformer to limit the peak current to a reasonable level.
The hum could be reduced by changing from a half wave rectifier to a bridge. You can get bridges (WO8) not much bigger than a diode. |
26th Nov 2018, 1:37 pm | #20 |
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Re: Another Dansette Major - hum this time!
I have one of these and they are pretty hum free after the electrolytics are replaced (mine was VERY loud before I fixed it). Mechanical hum from the transformer is the main contributor now.
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