Thorn 6204 hum after fitting new plug wire

[MisterKrupa]

Originally Posted by DMcMahon

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 🙏

[MisterKrupa]

Interesting result, wiggling/pressing the amp switch repeatedly didn’t have much effect, but I tried the same thing on the record button, and that did fix the passive / non playing hum! So I played my tape, and that plays fine, but at a fixed volume, no matter the position of the pot... hmm

[DMcMahon]

Originally Posted by DMcMahon

I prefer to have live directly going through the on/off switch.

David

My reference above was for the many machines where only one side of the main supply is switched, from your photo it looks like both live and neutral are switched.

Can you confirm if it is the tone control or volume control that has the on/off switch. If volume control then in line with Lucien's info check for any break in the signal wire connections at the control.

Yes potentially dangerous AC/DC voltages present in valve recorders, so be very careful and work safely. Capacitors such as the main reservoir and smoothing capacitors also can retain a charge for quite some time after unit is switched off.

David

[MisterKrupa]

Cheers David, the on off switch is on the tone control, and yeah I’m being well careful around the caps, they’re massive compared to the 100uF Max I’ve been used to lately...

[majoconz]

Let's not over-think this! It did work on the extension cable, you changed the extension and now the fault is apparent!

Is that a double-pole power switch on the back of the tone control. If only a single pole, which is more usual, why does it have the phase and the neutral connected to it? Unless, of course, the tag where the neutral is soldered is just being used as a tie point and another wire goes off to the transformer primary.

Incidentally, I don't know if it appears in the UK regs, but here you're not allowed the main earth-to-chassis connection to be soldered, it has to be a crimped terminal bolted to the chassis and it has to be near where the cable enters the appliance via a firm cable clamp.

[MisterKrupa]

That’s an interesting notion, you’re thinking I might have either a dodgy joint, or that I’ve interfered with something there I guess?

I was waylaid by work stuff yesterday, so didn’t get chance to open it up, but I’ll try today.

I was thinking the ideal solution would be a kettle style plug at some point, and I think that it would be better coming out of the case as directly as possible, seems mad to me that they’d have strung it between the transformer and the fan motor in the first place, but then again, it lasted half a century like that

Originally Posted by majoconz

Let's not over-think this! It did work on the extension cable, you changed the extension and now the fault is apparent!

Is that a double-pole power switch on the back of the tone control. If only a single pole, which is more usual, why does it have the phase and the neutral connected to it? Unless, of course, the tag where the neutral is soldered is just being used as a tie point and another wire goes off to the transformer primary.

Incidentally, I don't know if it appears in the UK regs, but here you're not allowed the main earth-to-chassis connection to be soldered, it has to be a crimped terminal bolted to the chassis and it has to be near where the cable enters the appliance via a firm cable clamp.

[MisterKrupa]

I’m not sure if this is a clue, I can’t tell if this is supposed to be this way or not, but I suspect there shouldn’t be loose wires in something like this... this is next to the volume pot.


I’ve attached photos showing where it goes to on the board, I guess it’s earth as there’s loads going to the same strip? Also attached a photo of the volume pot arrangement...

[ms660]

That braid looks as though it should be connected to the ground end of the pot, the same pot tag that the 2.2meg resistor is connected to, it might also be that
the tag should be connected to the metal body of the pot, it's hard to say for sure from the photo about the latter.

Lawrence.

[MisterKrupa]

Originally Posted by ms660

That braid looks as though it should be connected to the ground end of the pot, the same pot tag that the 2.2meg resistor is connected to, it might also be that
the tag should be connected to the metal body of the pot, it's hard to say for sure from the photo about the latter.

Lawrence.

Cheers Lawrence, that was my instinct looking at it, it’s possible that I detached it while either removing the chassis from the case, or while working on the other end... I’ll have a think and a bit more of a trace of the circuit to see if it makes sense to try that out or not,

Thanks 🙏

[MisterKrupa]

As far as I can tell, this is the right service manual for what I have...

https://www.service-data.com/product...70/8113/m16870


I guess it'll have enough info on it to determine what should go where?


[EDIT} or is this one what I need?

https://www.service-data.com/product...99/8113/a16199

[DMcMahon]

Believe these are just the BRC/Thorn Service Manuals for the Tape Decks, they will not show the rest of the tape recorder parts/connections.

David

[MisterKrupa]

Ah cheers David, you may have saved me a disappointing purchase, or two

I have in the meantime got curious and let that bit of wire touch the terminal in question and it seemed to dissipate the hum, and most promisingly, the lights etc stayed on - I'm tempted to give it a solder and see...

[DMcMahon]

Originally Posted by majoconz

Is that a double-pole power switch on the back of the tone control. If only a single pole, which is more usual, why does it have the phase and the neutral connected to it? Unless, of course, the tag where the neutral is soldered is just being used as a tie point and another wire goes off to the transformer primary.

I cannot find a schematic for the Thorn/Ultra 6204 but on similar BRC/Ferguson/Marconiphone/Thorn/Ultra most of them have the mains going through a double pole on/off switch.

David

[DMcMahon]

Yes a similar model Ferguson schematic confirms that Lawrence's input that the cable screen/braid connects to the end of the volume pot where the 2.2MOhm resistor connects to is correct.

Can see this fixing hum but unsure if it would fix the volume pot not working issue, hope it does though.

David

Update - thinking about it more the screen/braid on the schematic connects to ground which in turn grounds the bottom end of the 2.2M resistor, with the screen disconnected then the resistor is effectively floating which might result in the audio signal getting through the pot but not be adjustable.

[MisterKrupa]

Originally Posted by DMcMahon

I cannot find a schematic for the Thorn/Ultra 6204 but on similar BRC/Ferguson/Marconiphone/Thorn/Ultra most of them have the mains going through a double pole on/off switch.

David

That's good to know, it's how it was when I found it - the loose bit is on the other side, I do now think it's extremely likely that I snagged it on first removing the chassis from the case as that bundle of wires is attached to another, which did hook under the lid catch mechanism. If it was a bit loose / dry, the I reckon even a tiny amount of force in the wrong direction would have pulled it free...

[MisterKrupa]

Originally Posted by DMcMahon

Yes a similar model Ferguson schematic confirms that Lawrence's input that the cable screen/braid connects to the end of the volume pot where the 2.2MOhm resistor connects to.

Can see this fixing hum but unsure if it would fix the volume pot not working issue, hope it does though.

David

Swweeeet, that's my prompt to get the iron hot!

nice one, I'll report back

[ms660]

Careful you don't melt the inner insulation of the coax so as to cause a short when soldering.

Lawrence.

[MisterKrupa]

Originally Posted by ms660

Careful you don't melt the inner insulation of the coax so as to cause a short when soldering.

Lawrence.

It was pretty quick and easy, seems to have done the trick, all back together and functioning as well as it was on Monday!

I'll still maybe try and fit a better socket solution at some point, and maybe try and figure out either a DIN cable breakout thing for the back, or some way of shutting off the speaker.

I love this unit, especially the strange greenish level meter that's driven by the valve on the top somehow...

Many thanks all!

Chris

[Lucien Nunes]

Good. So it sounds like the low level was caused by poor contact in the record/play switch, and the ineffective volume control was due to the anticlockwise end of its track not being grounded via the coax.

Now, once you are sure it is entirely back to where it was when you started, consider changing at least a few capacitors even though it is working. The output valve grid coupling cap can damage the valve and transformer if it fails, and if it is one of those Hunts Moldseals I can see in the pic, it will fail in due course. Someone with a circuit diagram will be able to see which, if any, others are immediate cause for concern.

[MisterKrupa]

Cheers Lucien, I’ll consider doing that, maybe with more expert help though. I’ll have some fun recording stems with it for a bit in the meantime though, and I think fitting some sort of socket and maybe an isolator for the supply would be very sensible as well,

Cheers