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Old 9th Nov 2018, 9:44 pm   #1
SeasideSteve
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Default Isolating Cartridge on Fidelity HF35

I have been working on a Fidelity HF35 with a two valve UL84 UY85 amplifier. I have replaced all of the electrolytic capacitors and fitted a new twin core mains lead.

I see from another post https://www.vintage-radio.net/forum/...t=95468&page=2 that people have suggested fitting y rated capacitors to provide mains isolation for the cartridge.

Please can I have some advice on where these would go in the circuit and what values to use.

Last edited by AC/HL; 9th Nov 2018 at 9:51 pm. Reason: Forum rules
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Old 10th Nov 2018, 12:44 am   #2
Herald1360
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Default Re: Isolating Cartridge on Fidelity HF35

If there are already isolating capacitors for the cartridge, then yes, Y rated types should be used to replace them. If there aren't any capacitors at all, fitting them may cause hum problems.

Just ensure that the amp chassis is connected to neutral and fit some insulating sleeves (heatshrink perhaps) to the cartridge connection tags to make accidental contact impossible.
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Old 13th Nov 2018, 2:05 pm   #3
SeasideSteve
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Default Re: Isolating Cartridge on Fidelity HF35

Thanks for the reply. It has already got a bit of a mains hum which is just about at an acceptable level when playing a record but very noticeable between tracks. This is probably normal from posts I have read about similar Fidelity models.
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Old 16th Nov 2018, 9:33 pm   #4
Chris55000
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Default Re: Isolating Cartridge on Fidelity HF35

Hi!

Member Harold1360 forgot to state the recommended value in his posts, 0.01uF in each lead from the cartridge, and 1000pF between the deck metalwork and amplifier chassis is normally OK.

You can earth the deck metalwork to mains–earth providing no more than 1000pF is used between deck and amplifier chassis – I have seen this used in later 1960s/early 1970s a.c./d.c. type record–player circuits at the end of the valve era.

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Old 16th Nov 2018, 11:31 pm   #5
Edward Huggins
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Default Re: Isolating Cartridge on Fidelity HF35

There is really no need to fit isolation caps between the cartridge and the amplifier if the 2 core mains lead is connected with the neutral lead going to the chassis. Yes, there may well be hum if the cartridge is failing and you are needing to use volume near to maximum. If you suspect the cartridge is still good, check out the routing of the pick up signal lead vis-a-vis the mains cabling. These kind of amplifiers, using a very crude circuit, always had a degree of hum, but some - and for no apparent reason - can be better than others.
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Old 18th Nov 2018, 6:49 pm   #6
SeasideSteve
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Default Re: Isolating Cartridge on Fidelity HF35

Thanks for the further advice.

The record player is fitted with an X5H which I understand is a crystal cartridge. Does the output of a crystal cartridge gradually reduce with age? This would then be making the hum worse as Edward suggests.
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Old 19th Nov 2018, 1:00 pm   #7
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Default Re: Isolating Cartridge on Fidelity HF35

In a word, yes. The crystal is hygroscopic and may end up just deliquescing! Luck of the draw re storage conditions, where you live etc etc. "Saltdean" sounds a bit unpromising in this respect.....

Cheap fix is one of the red'n'black Chinese cartridges (get a mounting clip (not so cheap) or DiY) and a small preamp to boost its output to crystal levels. Lots of sometimes extremely rambling threads on here about those if you search! I'd use an opamp based one.
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