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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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7th Oct 2018, 12:33 pm | #1 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Fidelity HF42 help please with loud Hum
Hello all,
I have just joined and must immediately admit to being an electronics novice but I do have the fixit gene and would like to do more of this especially on old hifi. So would appreciate your help starting with a Fidelity HF 42 I have had knocking around for a while. When the start and volume wheel is turned on it makes a loud buzz/hum which does not change with any adjustments. I have taken off the two connectors to the cartridge and that makes no difference. Any pointers please? couple of photos included. Also on this model is the red plastic version the earlier model? cheers, Dan |
7th Oct 2018, 1:33 pm | #2 |
Dekatron
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,302
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Re: Fidelity HF42 help please with loud Hum
I have always thought that both the red, and the optional wood-effect versions, were available from Fidelity at the same time. You may possibly need to change a component (capacitor) - what skills do you have in electronic measuring, fault finding and soldering?
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Edward. |
7th Oct 2018, 6:26 pm | #3 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
Edward,
I am good at soldering though not so great at fault finding, have a very basic multi meter though and open to any guidance, Dan |
7th Oct 2018, 10:40 pm | #4 |
Dekatron
Join Date: Aug 2006
Location: Madrid, Spain / Wirral, UK
Posts: 7,484
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Re: Fidelity HF42 help please with loud Hum
The mains wires go across that switch/vol pot, so you must tread very carefully as there will be no second chances if you slip and touch them if you work on this when plugged in.
Initially, I would do as much as possible with the unit unplugged from the mains. The hum suggests there may be a problem with the rectifier (the diode bridge is the round thing near that 330UF cap in the corner of the circuit board) which in turn may have damaged the output chip. But lets not jump the gun. To avoid removing that circuit board and disturbing the fine tonearm wires, you could initially solder a new capacitor onto the legs of the the large black capacitor in the centre of the 1st pic, piggy back style. Ideally you need an axial type 1000uF and 25v or a bit higher. Then power up and test. I suspect that 330uF cap may be the reservoir cap as it is so near the diode bridge. However, it's been decades since I last went inside one of these players. I am sure others will put me right.
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Regards, Ben. Last edited by ben; 7th Oct 2018 at 10:48 pm. Reason: extra info |
8th Oct 2018, 10:55 am | #5 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
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Re: Fidelity HF42 help please with loud Hum
If those cartridge wires are connected via push-on tags to the board, board removal may be quite straightforward. Just be careful as you prise them off or you may need those soldering skills!
Pics of the quality of those already posted of both sides of the board would make identifying which component does what much easier. What is the type number on the IC- the black multilegged thing near the middle of the board?
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8th Oct 2018, 5:20 pm | #6 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
OK thanks for advice so far, what I'll do next is try and remove the board and post some more pictures.
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8th Oct 2018, 5:39 pm | #7 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
Have removed the cartridge connections which were soldered and now have the PCB out so hence some more pictures.
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8th Oct 2018, 7:16 pm | #8 |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 760
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Re: Fidelity HF42 help please with loud Hum
As Ben said in post #4, I would try another cap across the reservoir 1000uF 25V as a first bet. If that does not settle it then the WO4 rectifier will need checking.
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George |
8th Oct 2018, 10:43 pm | #9 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
Thanks, any recommendation where best to buy the components?
Any chance you could explain why putting that capacitor across the reservoir may help in layman's terms? Dan |
9th Oct 2018, 4:24 am | #10 |
Moderator
Join Date: Dec 2010
Location: Exeter, Devon and Poole, Dorset UK.
Posts: 6,823
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Re: Fidelity HF42 help please with loud Hum
Electrolytic Capacitors can go internally high resistance and low capacitance it's a quite common failure mode.
Putting the new capacitor across the suspect part is a quick simple way to see if it's failed. If it makes no difference then the fault may be elsewhere. If the cap is showin signs of bulging or leaking "Goo" its better to replace it rather do this since the Cap may be drawing excessive current and be at risk of becoming a party popper chucking aluminium foil everywhere. This failure mode risks destroying the rectifier and or mains transformer as well as other parts. Cheers Mike T
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9th Oct 2018, 3:15 pm | #11 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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Re: Fidelity HF42 help please with loud Hum
If you can make sure the mains wires going to the on/off switch are routed well away from the printed circuit board and the thin wires to the pickup cartridge.
Mike |
12th Oct 2018, 5:14 pm | #12 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
I have replaced the 1000uF 25v capacitor but still the loud buzz from the speaker. What can I try next?
Dan |
12th Oct 2018, 9:22 pm | #13 |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 760
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Re: Fidelity HF42 help please with loud Hum
My suggestion only...
Assuming you have some sort of multimeter with a diode test function - unsolder one of the wires from the transformer to the pcb and test each of the four diodes in the WO4 bridge rectifier.
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George |
12th Oct 2018, 10:30 pm | #14 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
George,thanks for the suggestion but I might need a bit more detail to be able to do what you say, certainly happy to try but will need simple instruction please?
Dan |
12th Oct 2018, 10:59 pm | #15 |
Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
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Re: Fidelity HF42 help please with loud Hum
Is there an open input on the amp now that the pickup leads have been unsoldered?
Without a working pickup connected the amp will hum though I have noted that no adjustments to controls affects the hum. |
13th Oct 2018, 12:45 pm | #16 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
I'll resolder the pickup to check but I don't think that will help...
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13th Oct 2018, 1:49 pm | #17 | |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 760
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Re: Fidelity HF42 help please with loud Hum
Quote:
If re-connecting does not cure the hum then you could check the voltage across the large capacitor. It should be a fairly steady DC voltage in the region of 12 to 15 I would guess. If the reading is very noisy or you get a reading on the multimeter AC range of more than (say) 200mV then the rectifier is suspect. Testing it - suggest viewing any number of youtube videos e.g. https://www.youtube.com/watch?v=11JPy4t36oo
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George |
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13th Oct 2018, 4:12 pm | #18 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
Resoldered the cartridge connectors with no change, George by the large capacitor do you mean the 1000mf one that I have already swapped or another?
dan |
13th Oct 2018, 5:21 pm | #19 |
Rest in Peace
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Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Fidelity HF42 help please with loud Hum
The circuit.
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14th Oct 2018, 1:31 pm | #20 |
Triode
Join Date: Oct 2018
Location: Guildford, Surrey, UK.
Posts: 31
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Re: Fidelity HF42 help please with loud Hum
Thanks for all suggestions so far.. followed the youtube clip to test the rectifier and all readings come out just over 600 so presume that is normal?
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