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Old 1st Jan 2019, 11:06 pm   #1
lesparapluies
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Default Fidelity HF43 hum and capacitor problem

Just restoring a Fidelity record player - no model ID anywhere on the unit but the PCB says HF43 Mk111 - anyway, got the deck working beautifully despite virtually everything being seized..so to the amp. It works, but has a very loud and harsh hum / buzz for the first 2 or 3 seconds after switch-on, then settles down to what I assume to be normal. Have replaced the electrolytics and a couple of resistors which were well out of value and that has reduced the noise a bit. What intrigues me are the grey capacitors which look like resistors and which I haven't come across before (see pic) - I wonder if someone could fully explain the value letters and numbers - there are 200U, 1KM, 4K7N - how does this system work and what type of cap should they be replaced with? Also, if someone could point me towards a schematic, that would be great, many thanks.
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Old 2nd Jan 2019, 7:38 am   #2
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Default Re: Fidelity HF43 hum and capacitor problem

Welcome t' forum. No idea what the 200u denotes, obviously not 200 micro farads, they could be 0.02, 1000p and 4.7n but then they could be resistors, 200 something, 1k and 4k7. if you look there aren't enough R's on the PCB, so that makes sense. also they are right near the tone pot. Try measuring them.

As for schematic, it should be very easy to back engineer ( 20 components) or they might have lifted the whole lot off the IC's datasheet/application notes.

BTW, I wouldn't leave those big electrolytics off the board, they'd be better flush fitted. If the holes don't match, drill new ones if the trace underneath will fit, if not put a dogleg in one lead. If it were me I'd put sleeving on the big electro's leads and neaten the lead dressing. It is only a gash old record player, but still, it's nice to do the best job you can and to have pride in your work.

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Old 2nd Jan 2019, 1:18 pm   #3
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Default Re: Fidelity HF43 hum and capacitor problem

Quote:
Originally Posted by lesparapluies View Post
...What intrigues me are the grey capacitors which look like resistors and which I haven't come across before (see pic) - I wonder if someone could fully explain the value letters and numbers - there are 200U, 1KM, 4K7N - how does this system work and what type of cap should they be replaced with? Also, if someone could point me towards a schematic, that would be great, many thanks.
They are capacitors. They are tubular ceramics. (They died out because disc ceramics are cheaper to make.)
I wish I could give a code for the lettering. May be it is manufacturer specific? Most commonly they were banded like resistors, try working with that!
I think you have to assume they are as D.A says, 0.02uF, 4700pF and 1000pF.
Only way to be sure is measure them. Being ceramic they are likely to be fine anyway.
Circuit diagram? Your chip is a Texas SN76013ND and I cannot find a data sheet, other than the Radio Museum listing. You should be able to work the circuit from the tracks though. (The Radio Museum listing for SN76023ND gives a general use scheme that looks close though?)

Actually 'Fidelity 43' finds several posts here too, if you search for them.
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Old 2nd Jan 2019, 1:21 pm   #4
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Default Re: Fidelity HF43 hum and capacitor problem

200pF, 1000pF, 4,700pF ?

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Old 2nd Jan 2019, 3:00 pm   #5
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Default Re: Fidelity HF43 hum and capacitor problem

For a comparison to the values suggested in Post#4 look at the values of C6, C7 & C8 in the IC's test circuit shown in the link in Post#7 in this thread:

https://www.vintage-radio.net/forum/...ad.php?t=28174

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Old 2nd Jan 2019, 3:30 pm   #6
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Default Re: Fidelity HF43 hum and capacitor problem

Yes,
You are correct, 200pF.
Now I look closer at the actual picture the clue may be that it is marked 200UU not 200U.

Looks very close to the RCA CA3131 chip circuit too. Just the feedback loop added. Alan
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Old 3rd Jan 2019, 12:47 pm   #7
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Default Re: Fidelity HF43 hum and capacitor problem

Many thanks to you all, quite a lot to take on board (pun not intended). I had suspected they might be ceramics, in fact I seem to remember similar ones were used in 1960s Japanese pocket radios..The circuit diagram for the SN76013 is very helpful and I will indeed try to create a rough schematic of my own. I suspect the buzz / hum at start up may be a failing rectifier? I'm kind of coming back to the electronics thing after a career in music - the last major construction project I did was as a teenager in 1973 when I built a complete stereo hi-fi amp based on Mullard circuits, so the wheels are starting to turn again...will keep you posted, thanks again for the advice
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Old 4th Jan 2019, 8:17 pm   #8
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Default Re: Fidelity HF43 hum and capacitor problem

I have an HF43 and there is no hum like you describe when I switch mine on so there is definitely a problem some place. There is a slight hum which gets louder the higher I turn the volume up but remembering my past record players of the 1970s this was normal. Not much help I know but I hope you get it sorted. I love my orange HF43 and would never part with it.
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Old 13th Jan 2019, 3:35 pm   #9
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Default Re: Fidelity HF43 hum and capacitor problem

Once again thank you to all who have commented so far. Did a bit more investigation today and found both volume and tone pots are faulty (volume pot open circuit on the track ends and tone pot no connection to the wiper) Tried dismantling and cleaning but they're more or less falling apart, lots of rust on the interior etc, so need to replace both - Quick question - the tone pot is a 50K Log; I've never come across any circuit where the tone control was a log pot and I'm just wondering if they simply ran out of linear ones during manufacturing and used any ones they could find as stand-ins. Wondering whether to go with my instincts and replace with a 50K Lin, any ideas please?
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Old 13th Jan 2019, 3:51 pm   #10
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Default Re: Fidelity HF43 hum and capacitor problem

We often get suggestions that Manufacturers ran out of parts, or used up a windfall found in a dusty corner of the warehouse. Unlikely, I'd think.
Unless you already have a lin pot you wish to use, I'd be inclined to replace like for like.
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Old 13th Jan 2019, 4:09 pm   #11
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Default Re: Fidelity HF43 hum and capacitor problem

I agree.

1st photo post number 10:

https://www.vintage-radio.net/forum/...d.php?t=147560

Lawrence.
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Old 13th Feb 2019, 12:25 pm   #12
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Default Re: Fidelity HF43 hum and capacitor problem

Just come back to the Fidelity after a little break - once again thanks to all replies so far. OK, I still have the unpleasant buzz at start up. (I tried to attach a small video file but not possible) I'm getting a little frustrated at this one as I don't seem to be getting anywhere and maybe it would have been easier just to drop in a ready-made audio amp module... but I don't like things to defeat me! So far I've replaced both pots (old ones beyond saving), replaced the 4 electrolytics plus 2 of the ceramics which were slightly out of tolerance, and one resistor. I was sure that the fault was power-related, so also swapped the metal rectifier for a silicon one. Just some more info: I've tried running the board from a separate 20V AC source, so by-passing the turntable motor winding - no difference. I've tried connecting it to a different speaker - no difference. I've monitored the DC supply voltage on start-up, and it takes about a second to achieve the 24V, and it's during this second that the buzz happens. Also, if I switch off the supply and watch the DC voltage fall, if I then turn the supply back on before it's fallen below about 10V, the buzz doesn't happen; anything less than that, it does. I'm beginning to think it may be the IC - but do ICs deteriorate? I'd always thought they either worked or failed.... any more thoughts anyone? Thanks, MD

Here is a link to a short video to demonstrate the fault https://www.youtube.com/watch?v=TtzI...ature=youtu.be
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Old 13th Feb 2019, 3:33 pm   #13
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Default Re: Fidelity HF43 hum and capacitor problem

That hum does sound nasty. Does it play loudly afterwards? Maybe the output IC is going leaky.
The fact that it acts up when the voltage/operating conditions aren't at optimum is ringing a bell. Will give it some thought!
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Old 13th Feb 2019, 8:47 pm   #14
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Default Re: Fidelity HF43 hum and capacitor problem

Thanks Ben, yes, once it's gone past that nasty half-second or so, things are absolutely fine!
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Old 13th Feb 2019, 10:27 pm   #15
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Default Re: Fidelity HF43 hum and capacitor problem

Hi, this sounds similar symptoms to a Fidelity UA3 I have except that makes a nasty loud crackling sound for a second or two on power on/off that fades in the same manner. Sounds similar to a dirty volume pot but only on startup/shutdown and not related to moving any pots. (which are clean now).

Haven't looked awfully close yet but I have to wonder if the two could be related. If I delve deeper into this one and find a cause I'll chip back in.
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Old 18th Feb 2019, 2:58 pm   #16
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Default Re: Fidelity HF43 hum and capacitor problem

Once again thanks to all who have commented. I think it must be the IC which is going bad, general view is that it's running very close to its max voltage and over time this takes its toll..have located a replacement and will fit it this week...looking for a simple way to regulate the DC at at about 18V, will post again soon
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Old 18th Feb 2019, 6:18 pm   #17
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Default Re: Fidelity HF43 hum and capacitor problem

What voltage do you have at the moment?
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Old 18th Feb 2019, 10:07 pm   #18
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Default Re: Fidelity HF43 hum and capacitor problem

It's reading at about 24.5V
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Old 19th Feb 2019, 8:50 am   #19
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Default Re: Fidelity HF43 hum and capacitor problem

What about an LM317 based adjustable power supply?
You can build your own, but the Chinese modules are small, cheap and would fit between the transformer and the board. You can set the voltage to what you want.
Less than a fiver, UK stock even off ebay - try searching for 'LM317 Adjustable Regulated Rectifier Filter Power Supply Board' as an example.
Make sure you get one that is AC to DC, not DC to DC.
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Old 24th Feb 2019, 8:36 pm   #20
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Default Re: Fidelity HF43 hum and capacitor problem

Again, thanks for replies - I have indeed got an LM317 based supply... but the saga continues...the general feeling was that the chip was faulty. I ordered a NOS one which arrived this week. Took the old one off the board, then the fun and games started. If anyone else has worked on this PCB, you'll know that the pins on the IC have to be slightly bent one way or the other to fit the holes on the board - they are not in a straight line, some of them are offset...God knows how they did it in the original assembly...anyway the upshot is I spent about an hour trying to get 14 pins to line up with the holes, and of course after a while one pin objected to such manipulation and broke off. I really feel I've reached the end of the line with this, and my intention now is to keep the front-end of the circuit as is, as far as after the volume and tone circuit, but drop in a ready-made 3W audio amp board to drive the speaker - there are a few online, anyone recommend a particular one? I appreciate that I'm ditching the authenticity etc but I can't afford to waste any more time on what to be honest wasn't that good an amp in the first place! (turntable works a treat though, and cabinet and hinges have come up beautifully!)
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