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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 7th Oct 2018, 12:33 pm   #1
dannyboy45
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Default 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
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Old 7th Oct 2018, 1:33 pm   #2
Edward Huggins
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Default 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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Old 7th Oct 2018, 6:26 pm   #3
dannyboy45
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Default 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
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Old 7th Oct 2018, 10:40 pm   #4
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Default 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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Last edited by ben; 7th Oct 2018 at 10:48 pm. Reason: extra info
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Old 8th Oct 2018, 10:55 am   #5
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Default 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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Old 8th Oct 2018, 5:20 pm   #6
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Default 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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Old 8th Oct 2018, 5:39 pm   #7
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Default 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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Old 8th Oct 2018, 7:16 pm   #8
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Default 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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Old 8th Oct 2018, 10:43 pm   #9
dannyboy45
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Default 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
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Old 9th Oct 2018, 4:24 am   #10
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Default 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

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Old 9th Oct 2018, 3:15 pm   #11
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Default 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.
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Old 12th Oct 2018, 5:14 pm   #12
dannyboy45
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Default 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?
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Old 12th Oct 2018, 9:22 pm   #13
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Default 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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Old 12th Oct 2018, 10:30 pm   #14
dannyboy45
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Default 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
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Old 12th Oct 2018, 10:59 pm   #15
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Default 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.
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Old 13th Oct 2018, 12:45 pm   #16
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Default 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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Old 13th Oct 2018, 1:49 pm   #17
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Default Re: Fidelity HF42 help please with loud Hum

Quote:
Originally Posted by dannyboy45 View Post
I'll resolder the pickup to check but I don't think that will help...
I can't really tell from the photos but it looks to me like two-part connectors on the PU wires so they should just unplug.
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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Old 13th Oct 2018, 4:12 pm   #18
dannyboy45
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Default 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
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Old 13th Oct 2018, 5:21 pm   #19
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Default Re: Fidelity HF42 help please with loud Hum

The circuit.
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Old 14th Oct 2018, 1:31 pm   #20
dannyboy45
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Default 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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