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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 18th May 2021, 9:14 pm   #1
MisterKrupa
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Default Thorn 6204 hum after fitting new plug wire

Hi, this is my first post here. I found this site before rescuing a lovely Thorn 6204 from a a local charity shop yesterday; the glowing testimonials of this and i’s brethren made me go back in the shop and get it. As the cable had been cut short I got it going quite quickly, testing it with a terminal block attached to an old but mostly unused length of 13 amp extension, and after a thorough play through, record and playback thought I would do the right thing and replace the cable with the length I’d already used... finding my way in, and replacing red with brown, black with blue and green with earth, I put it back together and plugged it in. But this time there was a massive amount of hum, the volume pot no longer seems to work and playback is extremely quiet underneath that hum.

I’m not entirely sure that I replaced the two earth collars in the right place, but the metal case shows connection to the earth pin of the plug...

Does anyone have any ideas?

Many thanks

Chris
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Old 18th May 2021, 9:39 pm   #2
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Default Re: Thorn 6204 hum after fitting new plug wire

Hi Chris and welcome to the forum.

It's difficult to see in the picture but is that earth wire connected to the back of the switch?

It looks as if it's connected to the tag next to the blue.

Cheers

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Old 18th May 2021, 9:45 pm   #3
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Default Re: Thorn 6204 hum after fitting new plug wire

Cheers Mike, that one is connected to the volume pot yes, I think that’s how it came - I actually took it off today and cleaned it all in case the connection was bad - is it likely not meant to be there? I did initially put both the collars on that casing screw but I’ve now moved the longer one that comes from the front speaker to another screw on the other side, to no great effect...
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Old 18th May 2021, 10:03 pm   #4
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Default Re: Thorn 6204 hum after fitting new plug wire

Ah sorry I see what you mean, this photo shows it better (I’ve since improved the joint too)
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Old 18th May 2021, 10:31 pm   #5
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Default Re: Thorn 6204 hum after fitting new plug wire

OK it looks as if the on/off switch is on the tone control not the volume control.

I can't see anything immediately wrong.

The tone control would have little effect on the hum.

I am not familiar with this recorder so I am assuming it's valve but confirmation would be good.

Cheers

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Old 18th May 2021, 10:37 pm   #6
MisterKrupa
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Default Re: Thorn 6204 hum after fitting new plug wire

Yeah it’s got three, the thing that’s just confusing is that it was working pretty much perfectly with the original cable extending with the block, I wish I’d videoed myself doing the job so I could see what I did wrong, or inadvertently changed...
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Old 18th May 2021, 10:45 pm   #7
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Default Re: Thorn 6204 hum after fitting new plug wire

One clue might be that the hum appears gradually, as the tubes warm up, then it quiets for a moment, then returns, fades and then returns, settles in as it were...
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Old 19th May 2021, 7:15 am   #8
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Default Re: Thorn 6204 hum after fitting new plug wire

I uploaded a short clip of the hum and the nil effect of the volume control, it’s definitely not a normal valve background hum...

https://youtu.be/zSoxFkh6eG8
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Old 19th May 2021, 8:47 am   #9
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Default Re: Thorn 6204 hum after fitting new plug wire

To my ears not a massive amount of hum as per your Post 1 and I agree maybe not typical mains hum.

You say the volume control no longer works, does that also apply when playing a tape ?

David
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Old 19th May 2021, 8:54 am   #10
MisterKrupa
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Default Re: Thorn 6204 hum after fitting new plug wire

Yes, after sleeping on it, I’m wondering if I’ve tucked some wires that carry signal too close to either the transformer or the fan motor; the original was tucked down so I mimicked that when I replaced it, but there were others underneath that I didn’t consider, maybe I’ll try picking through those and seeing if that affects anything.

I guess the volume of the hum doesn’t come through on the video, it really is relatively loud, thanks for looking!
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Old 19th May 2021, 8:56 am   #11
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Default Re: Thorn 6204 hum after fitting new plug wire

Check the continuity of the earth pin in the mains plug to the chassis to make sure there is not a break in the earth wire. Your second photo in Post 1 shows some indentations to the outer insulation, unlikely in this case but if the cable previously has had some flexing at the indentations area then the earth wire could have got broken, I would cut off the damaged area and remake the cable connections.

David
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Old 19th May 2021, 9:00 am   #12
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Default Re: Thorn 6204 hum after fitting new plug wire

I did get positive result checking that with the meter, chassis to Earth pin beeps
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Old 19th May 2021, 9:01 am   #13
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Default Re: Thorn 6204 hum after fitting new plug wire

Quote:
Originally Posted by MisterKrupa View Post
Yes, after sleeping on it, I’m wondering if I’ve tucked some wires that carry signal too close to either the transformer or the fan motor; the original was tucked down so I mimicked that when I replaced it, but there were others underneath that I didn’t consider, maybe I’ll try picking through those and seeing if that affects anything.

I guess the volume of the hum doesn’t come through on the video, it really is relatively loud, thanks for looking!
Chris, what about the volume control operation when playing a tape question ?

David
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Old 19th May 2021, 9:05 am   #14
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Default Re: Thorn 6204 hum after fitting new plug wire

It seems likely if the hum wasn't there before something has become dislodged or disconnected when it was taken out of its case.

There is probably a Humdinger make sure that hasn't been misadjusted or gone open circuit. Check the connections to the main smoothing capacitor (this will most likely be a large aluminium can about 30 to 40mm in diameter 60 to 80mm tall) and clean the contacts (carefully) of the valves. You need to use a proper contact cleaner like Servisol, don't use WD40.

On the face of it it sounds like Smoothing isn't working or something has gone open circuit.

As David has far more experience of tape recorders happy to hand over the Baton!

Cheers

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Last edited by Cobaltblue; 19th May 2021 at 9:11 am. Reason: David more experience of Tape recorders
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Old 19th May 2021, 9:10 am   #15
MisterKrupa
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Default Re: Thorn 6204 hum after fitting new plug wire

Quote:
Originally Posted by DMcMahon View Post
Quote:
Originally Posted by MisterKrupa View Post
Yes, after sleeping on it, I’m wondering if I’ve tucked some wires that carry signal too close to either the transformer or the fan motor; the original was tucked down so I mimicked that when I replaced it, but there were others underneath that I didn’t consider, maybe I’ll try picking through those and seeing if that affects anything.

I guess the volume of the hum doesn’t come through on the video, it really is relatively loud, thanks for looking!
Chris, what about the volume control operation when playing a tape question ?

David

Sorry David, I should have separated my answer - yes, even when the tape is playing it’s still not functioning, I haven’t dared to connect anything else to test if it happens in amp mode yet, but I suspect it would be the same...
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Old 19th May 2021, 9:12 am   #16
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Default Re: Thorn 6204 hum after fitting new plug wire

Is there any possibility that when first tested with the terminal block connections you could have got live and neutral swapped over at that stage ?

In rare situations sometimes hum can be cured by swapping over live and neutral at the recorder. Live and neutral connect to the primary winding of the mains transformer (via on/off switch and sometimes mains fuse) so generally it is not technically important which way round live and neutral are connected, although I prefer to have live directly going through the on/off switch.

David
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Old 19th May 2021, 9:12 am   #17
MisterKrupa
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Default Re: Thorn 6204 hum after fitting new plug wire

Quote:
Originally Posted by Cobaltblue View Post
It seems likely if the hum wasn't there before something has become dislodged or disconnected when it was taken out of its case.

There is probably a Humdinger make sure that hasn't been misadjusted or gone open circuit. Check the connections to the main smoothing capacitor (this will most likely be a large aluminium can about 30 to 40mm in diameter 60 to 80mm tall) and clean the contacts (carefully) of the valves. You need to use a proper contact cleaner like Servisol, don't use WD40.

On the face of it it sounds like Smoothing isn't working or something has gone open circuit.

Cheers

Mike T
Thanks Mike, good things to try there, I’ll give it a look, bit nervous of the high voltage but I’ll be careful; I’ve been experimenting with low voltage valve oscillators lately so used to handling them, but it’s all at 12v so far
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Old 19th May 2021, 9:13 am   #18
MisterKrupa
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Default Re: Thorn 6204 hum after fitting new plug wire

Quote:
Originally Posted by DMcMahon View Post
Is there any possibility that when first tested with the terminal block connections you could have got live and neutral swapped over at that stage ?

In rare situations sometimes hum can be cured by swapping over live and neutral at the recorder. Live and neutral connect to the primary winding of the mains transformer (via on/off switch and sometimes mains fuse) so generally it is not technically important which way round live and neutral are connected, although I prefer to have live directly going through the on/off switch.

David
I did actually try that late last night as I thought it could be the case, I just swapped them in the plug temporarily, but alas no effect... thanks 🙏
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Old 19th May 2021, 9:16 am   #19
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Default Re: Thorn 6204 hum after fitting new plug wire

Quote:
the volume pot no longer seems to work and playback is extremely quiet underneath that hum.
This suggests that something in the signal path has become open-circuit or high-resistance, allowing only a small amount of signal to leak through. The lack of a defined source of signal allows the point downstream of the break to pick up hum readily. An example would be a break at the wiper connection on the volume control, disconnecting the last stage of the amplifier, rendering the volume control ineffective and allowing it to pick up hum from the nearby mains wiring and a small amount of signal from the proximity of the feed to the control. Inspect everything in this area carefully. With the volume control turned up, touch a screwdriver tip on the wiper tag (middle one of the three.) Does it crackle and make the hum louder?

A common problem that breaks the signal chain in small tape recorders is the record/replay switching system, that re-configures the circuit extensively between the two modes. Before getting too deeply stuck into tracing the fault, work the switch back and forth a few dozen times to clean the contacts, in case the disturbance of opening the machine has moved it for the first time in years and the contacts have started spreading the accumulated gunk over their active surfaces.
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Old 19th May 2021, 9:20 am   #20
MisterKrupa
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Default Re: Thorn 6204 hum after fitting new plug wire

Cheers Lucien, that sounds like a good starting point, I’ll check those
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