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Old 20th Mar 2018, 6:49 pm   #81
'LIVEWIRE?'
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Default Re: Dansette "Monarch"

You should get a buzz if you touch any of the parts you mention. The 0.047uf is the coupling capacitor between the treble control and the control grid of V1a, whilst the 1Meg resistor is the grid leak without which V1a wouldn't function. This looks to me like a standard input circuit, similar to those in many record players. If the two components aren't included in the circuit diagram, it must be due to an error.. (Crossed with Lawrence and Edward's posts)
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Old 20th Mar 2018, 7:03 pm   #82
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Default Re: Dansette "Monarch"

I used Dansettes and other record players 50 years ago when they were new. They all hummed. You just got used to it knowing that the hum would disappear when you played a record. If you wanted less hum you saved up your pennies and bought a separate turntable, amplifier and speakers.
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Old 20th Mar 2018, 7:34 pm   #83
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Default Re: Dansette "Monarch"

Quote:
Originally Posted by DonaldStott View Post
Checked out the Westinghouse FC116 metal rectifier as this is often suspect but I'm reading 234V AC on the input and 251V DC on the output - seems OK?
Hello, that isn't the voltage on the output of the rectifier. It is the DC voltage that is reached after a few cycles when there is a reservoir capacitor after the rectifier.

You would have to disconnect the reservoir and smoothing caps to determine the voltage after the rectifier. This only conducts on alternate (positive) half cycles so the output is actually pulsed DC.


However the figures you are posting seem perfectly normal, given the role of the capacitors, so it isn't necessary to do so.
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Old 20th Mar 2018, 10:06 pm   #84
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Default Re: Dansette "Monarch"

Thanks guys for all your fascinating insights and for the really helpful information. advice and guidance.

Even at my age I can still say that "Every Day is a School Day"

I looked closely at the centre chassis tag where the wires from the cartridge terminate and noticed that the black and blue wires go to the chassis and are connected to the screens of the other cables. Looking back at my Post #1 the third picture clearly shows the existing cartridge (soon to be replaced) but with the red and blue wires attached to the connecting pins - am I misreading this or shouldn't the yellow and red wires be connected to the cartridge??

Lastly, I've got lots of things to try and reduce that pesky hum but are we all agreed that C4 0.01μF the audio coupling capacitor should be replaced?

@Edward - you mentioned "There are parts of those circuits that just don't look correct." - can you be more specific, thanks.
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Old 20th Mar 2018, 10:35 pm   #85
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Default Re: Dansette "Monarch"

Quote:
Originally Posted by DonaldStott View Post
I looked closely at the centre chassis tag where the wires from the cartridge terminate and noticed that the black and blue wires go to the chassis and are connected to the screens of the other cables. Looking back at my Post #1 the third picture clearly shows the existing cartridge (soon to be replaced) but with the red and blue wires attached to the connecting pins - am I misreading this or shouldn't the yellow and red wires be connected to the cartridge??
It's not difficult to figure out, one of the wires attached to the pickup cartridge should have continuity to chassis and the other wire that's connected to the pickup cartridge should have continuity to the top end of the volume control (via the jack socket)

Lawrence.
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Old 20th Mar 2018, 10:58 pm   #86
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Default Re: Dansette "Monarch"

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It's not difficult to figure out, one of the wires attached to the pickup cartridge should have continuity to chassis and the other wire that's connected to the pickup cartridge should have continuity to the top end of the volume control (via the jack socket)
I stand corrected - you're right if course, it's not difficult to figure out!
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Old 20th Mar 2018, 11:00 pm   #87
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Default Re: Dansette "Monarch"

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Lastly, I've got lots of things to try and reduce that pesky hum...
Hi Donald, I've never owned a Dansette but I've repaired more than a handful of them for a vintage shop owner down the road and done a fuller restoration of another. They seem eminently popular at the moment, and prices are inflated as a result.

For all that, Dansettes never were exactly high-end - from chats I've had with those who owned them first time round, they were sort of entry level sound system for teenagers at the time they were mass-produced.
As others have either stated, there was a certain level of hum native to the design from the beginning. So there is going to be a discernible hum going on unless you are actually playing a record, at which time it is masked by the signal from whatever it is that you're playing.

There is a fault condition hum, which is related to a defective or poorly fitted cartridge, but this is clearly much more intrusive than what we're talking about in normal operation.

So it's a tricky one for you to call if you have no previous experience of these sets, but nothing you can do with the original design will eliminate it entirely.


Quote:
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.... are we all agreed that C4 0.01μF the audio coupling capacitor should be replaced?
Sure thing! That's a replacement on sight for anything of this era.
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Old 21st Mar 2018, 12:17 am   #88
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Default Re: Dansette "Monarch"

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Thanks guys for all your fascinating insights and for the really helpful information. advice and guidance.

@Edward - you mentioned "There are parts of those circuits that just don't look correct." - can you be more specific, thanks.
It was the unusual way you showed the feedback loop resistor wired and earthed. No problem now though.
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Old 21st Mar 2018, 10:53 am   #89
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Default Re: Dansette "Monarch"

Yes but it is not of the type which usually needs replacing and there is 0 volts on the Grid.
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Old 21st Mar 2018, 11:32 am   #90
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Yes but it is not of the type which usually needs replacing and there is 0 volts on the Grid.
Thanks Peter - that's what I thought as these caps are reportedly very reliable - I'l swap it our for a new cap and hopefully get some voltage on the grid!

Quote:
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It was the unusual way you showed the feedback loop resistor wired and earthed. No problem now though.
Thanks Edward - is there a better way to show a modified circuit of this type - always keen to learn?
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Last edited by DonaldStott; 21st Mar 2018 at 11:35 am. Reason: Mising section.
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Old 21st Mar 2018, 11:39 am   #91
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Default Re: Dansette "Monarch"

Quote:
Originally Posted by DonaldStott View Post
Quote:
Originally Posted by peter_sol View Post
Yes but it is not of the type which usually needs replacing and there is 0 volts on the Grid.
Thanks Peter - that's what I thought as these caps are reportedly very reliable -I'll swap it our for a new cap and hopefully get some voltage on the grid!
The point is you don't want any voltage on the grid w.r.t the chassis. The capacitor is there to stop that.

You should see a voltage across the cathode resistor and between cathode and grid. This is the bias voltage.
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Old 21st Mar 2018, 11:47 am   #92
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Default Re: Dansette "Monarch"

Quote:
Originally Posted by DonaldStott View Post
Quote:
Originally Posted by Edward Huggins View Post
It was the unusual way you showed the feedback loop resistor wired and earthed. No problem now though.
Thanks Edward - is there a better way to show a modified circuit of this type - always keen to learn?
There's nothing wrong with your alterations to the schematic, the feedback resistor is shown connected the same as it is in the original schematic.

Lawrence.
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Old 21st Mar 2018, 12:56 pm   #93
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The point is you don't want any voltage on the grid w.r.t the chassis. The capacitor is there to stop that.
I'm getting 68V on one side of C4 0.01μF (audio coupling capacitor) and 0V on the other side connected to R9 (grid resistor) and then to Pin 3 (Control grid) - all voltages are w.r.t. the chassis.

Is this indicative of a problem if I don't want any voltage on the grid w.r.t. the chassis.
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Old 21st Mar 2018, 1:01 pm   #94
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Default Re: Dansette "Monarch"

There is no problem there. The 68V is on the anode of the preceding stage. The coupling cap prevents it from reaching the grid of the following stage, where it would otherwise upset the bias.

The signal, being AC, can get through the coupling capacitor.
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Old 21st Mar 2018, 2:00 pm   #95
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Default Re: Dansette "Monarch"

In other words no need to change the cap
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Old 21st Mar 2018, 2:50 pm   #96
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Default Re: Dansette "Monarch"

Thanks to everyone for their valuable input and patience!

So if there are no problems with the audio coupling capacitor and the other undocumented coupling capacitor and grid resistor then it's back to Post #70...

I can check the bias voltage between cathode and grid later tonight.
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Last edited by DonaldStott; 21st Mar 2018 at 2:51 pm. Reason: Typo
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Old 21st Mar 2018, 8:31 pm   #97
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Default Re: Dansette "Monarch"

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I can check the bias voltage between cathode and grid later tonight.
Hopefully I've measured these correctly: -

Voltage across the cathode resistor : 12.9V

Voltage between cathode and grid (Pins 2 and 3) : -12.5V (bias voltage)
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Old 21st Mar 2018, 10:48 pm   #98
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Default Re: Dansette "Monarch"

I would still change that cap. I’m using the risk vs. consequences justification. It’s old and will likely fail at some point, and if and when it does , you’ll destroy a much more expensive component. It seems a bit of a pointless leap of faith to leave it there. It costs a few pennies and you’ll likely get a long, trouble -free enjoyment of your set if you do the job properly the first time round
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Old 22nd Mar 2018, 12:37 am   #99
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Default Re: Dansette "Monarch"

I think it was said that the "That Cap" was a Mullard mustard type. These are known for their reliability and old stocks of these are highly sought after and fetch good prices (you'll have noticed chancers registering on the forum and asking for them in the 'wanted' section on occasion) and old stocks are regularly used as replacements and in new builds without problems. This is a simple old Dansette, so hardly likely that any failure is going to break the bank anyway.
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Old 22nd Mar 2018, 8:48 am   #100
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Default Re: Dansette "Monarch"

Ah I missed that somehow . Sure, absolutely agreed on thee Mustards’ reliability so leaving alone makes sense .
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