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Old 17th Jun 2020, 12:10 am   #1
paul1962
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Default Dansette Monarch Amplifier Issues

Hello everyone,

I have a Dansette Monarch with a major mains hum issue (I know, they all do that etc etc), but I had one of these as a child and it lasted me nearly 10 years, so I know just how much hum they put out normally. Also it's good to note there is no audio output from this one, even with a signal injector attached to the cartridge wires..

I have been looking through the threads here and I found this one. https://www.vintage-radio.net/forum/...d.php?t=114851 which has long since been closed. In the thread, a helpful chap called David G4EBT laid out a rational and logical way of testing this chassis, which I have followed.

Everything was fine until I checked the voltage on pins 7 and 9 on the valve base. According to both the posting and the schematic I have, they should be reading at around 150v on pin 7 and 100v on pin 9. Mine read 322v on pin 7 and 262v on pin 9

Now, this may be because I didn't have the valve in place and took readings from the base, but unless the valve is going to drop voltage by at least half, then I have another issue at hand. Just for the record, I have changed the 32/32 Mfd mains caps and also the 25Mfd 25v cap which was also part of the "triple can". I have also sourced a new unused ECL82, none of which have reduced the hum or output.

Looking at the schematic, possibly R10 could be at fault. Has anyone any input as to whether they agree or disagree with my thoughts. This resister seems to be running the output of the rectifier and appears to be the voltage drop resistor to the 150v supply that is haywire. On my schematic, I can't see what the resistance should be as it reads "1blob.k where it looks like the ink ran

Over to you good people
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Old 17th Jun 2020, 1:58 am   #2
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Default Re: Dansette Monarch Amplifier Issues

R10 is likely 18k. Don't have the specific 64-65 RTVS book, but found another Dansette circuit with ECL82 and 18k for the screen and triode anode feed circuit. The blob could only be 2, 5 or 8 anyway and 8 looks most promising.

The high voltages with no valve present seem reasonable- with no HT current being taken the HT voltage will rise and there will be no resistive voltage drops either except for meter loading current produced ones.
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Old 17th Jun 2020, 6:18 am   #3
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Default Re: Dansette Monarch Amplifier Issues

This was one of the better sounding Dansettes and the only UK portable record player ever made using two 8" x 5" speakers. Hum is problematic given its liable induction around the seperated edge mounted controls sub-assembly. These could be screened using a fabricated casing. Also there can be induction from those long, very seldom used, trailing cables to the rear sockets. Are you sure that triple can is 32/32/25uF? They are usually 32/16/25uF. Replacing this with seperate 50uF 250V and 25uf 25V components might improve overall hum level.
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Old 17th Jun 2020, 8:16 am   #4
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Default Re: Dansette Monarch Amplifier Issues

I forgot in my haste to add the part of the circuit affected. Not everyone has the circuit diagram for Dansettes and I guess it would have been helpful.
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Old 17th Jun 2020, 8:38 am   #5
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Default Re: Dansette Monarch Amplifier Issues

Hi
First, with the valve unplugged then the voltage readings will be meaningless.
R10 is dropping the voltage for the V1a anode and V1B screen, to me it looks like 10k or 18k. What are the colours on your resistor.
If you do not have anything connected to the pickup then you will probably notice more hum.
This record player only has half wave rectification and a single smoothing capacitor on the output valve anode so it is going to hum.
So put the valve back in, check the voltages again.
Does it still hum with the volume control at minimum?
Try shorting the pickup wires.

You don't mention if you have changed C4 the 0.01uF cap, the audio coupling cap to R9 R8 and then the grid of V1B. !!

Mike

Last edited by crackle; 17th Jun 2020 at 8:43 am.
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Old 17th Jun 2020, 9:36 am   #6
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Default Re: Dansette Monarch Amplifier Issues

Have you tried measuring R10 using your meter's resistance ranges? If it were open circuit there'd be no voltage on pin 7, valve or no valve.
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Old 17th Jun 2020, 10:31 am   #7
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Default Re: Dansette Monarch Amplifier Issues

Details of my own Dansette Monarch restoration can be found here:-

https://www.vintage-radio.net/forum/...d.php?t=144404

It does tend to meander a but had helpful contributions from some of the Forum members above - you might find something useful there?

Notwithstanding the excellent advice so far I'd recommend taking the detailed and structured approach set out in this extremely helpful Thread by Darren-UK:-

https://www.vintage-radio.net/forum/...ad.php?t=75401

Good luck and keep us informed but before you do anything else, including switching it back on, please change that audio-coupling capacitor (C4) as mentioned by Crackle.
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Old 17th Jun 2020, 4:21 pm   #8
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Default Re: Dansette Monarch Amplifier Issues

Hello all .. many thanks for all of your replies and input in this matter.

I take note of everything said this far and to answer those who question which Caps I have replaced, I can confirm it's the 2 X 32Mfd and the 25Mfd contained as part of the main electrolytic can only this far.

I realise that to change every component on the PCB is these days is only pennies and it's not down to being tight, but rather that I don't hold the components in stock. But I can confirm that according to the schematic and also the can removed (which is dated 1975 btw) that the 32/32/25 seems to be correct for this model.

I will replace the valve and take readings again, as it seems I was wrong to leave it out during readings, mainly because I wanted to make sure that a NOS valve didn't get destroyed by incorrect voltages etc.

I have to say also, that according to the old mains plug that the fuse has been blown at some stage in its past and badly too. It's the old MK type and has the tell tale black burnmarks where the fuse sits, maybe this was why the can was changed back in 75, who knows ?

C4 looks as though it has got hot at some stage based on the yellow sleeving, but is cold to the touch at present, but again, I agree it's prudent to do away with it for a new one.

Kind Regards to you all,

Paul
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Old 17th Jun 2020, 4:49 pm   #9
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Default Re: Dansette Monarch Amplifier Issues

Quote:
Originally Posted by paul1962 View Post
C4 looks as though it has got hot at some stage based on the yellow sleeving, but is cold to the touch at present, but again, I agree it's prudent to do away with it for a new one.
Wise to replace C4 before trying the new valve.

Alan
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Old 17th Jun 2020, 6:05 pm   #10
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Default Re: Dansette Monarch Amplifier Issues

Quote:
Originally Posted by paul1962 View Post
But I can confirm that according to the schematic and also the can removed (which is dated 1975 btw) that the 32/32/25 seems to be correct for this model.
C5 (32μF) C6 (25μF) C7 (32μF) is indeed the correct line-up for the Monarch but I'm intrigued by the 1975 date on the can?

Quote:
C4 looks as though it has got hot at some stage based on the yellow sleeving, but is cold to the touch at present, but again, I agree it's prudent to do away with it for a new one.
Some pictures would be great ... the original C4 would have been a Mullard mustard type known for their reliability!
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Old 17th Jun 2020, 7:07 pm   #11
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Default Re: Dansette Monarch Amplifier Issues

I am assuming that this Monarch had been looked at or repaired sometime in it's history Donald and putting that as an explanation for the 75 dating on the mains can. My mistake concerning C4, it was actually the 64uF C3 cap that looks previously hot, but C4 and the 0.047uF caps also look like they have also been replaced maybe in 75 ? they are rated at 400v, but me being me, I shall replace them with modern 630v types
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Old 17th Jun 2020, 7:42 pm   #12
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Default Re: Dansette Monarch Amplifier Issues

Quote:
Originally Posted by paul1962 View Post
My mistake concerning C4, it was actually the 64uF C3 cap that looks previously hot, but C4 and the 0.047uF caps also look like they have also been replaced maybe in 75 ? they are rated at 400v, but me being me, I shall replace them with modern 630v types
Apologies if I'm stating the obvious but you do appreciate that C3 (64uF) is an electrolytic capacitor?

For the other two, modern 630v types are suitable.
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Old 17th Jun 2020, 7:58 pm   #13
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Default Re: Dansette Monarch Amplifier Issues

Yes Donald, but thanks for pointing that out. I could have been one of those who didn't know with it being an axial electrolytic and mistaken it for a normal cap. I am due to replace it with a 68uF modern type of 16v as the only 64uF I can find would be an old stock phillips unit.
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Old 17th Jun 2020, 10:11 pm   #14
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Default Re: Dansette Monarch Amplifier Issues

Hi!

I am familiar with the lettering on the circuit diagram extract posted and confirm the black blob after the "1" for the value of R10 is correctly a figure "8" – the "zero" is quite noticeably wider than the "8", so the correct value of R10 should be 18k.

The voltage figure shown for the smoothed h.t. at the primary of the o/p transformer is 240V and for reliable operation of the tetrode section of the ECL82, this figure should not be exceeded, if you have fitted a replacement rectifier, the smoothing resistor should be chosen such that the h.t. to the o/p transformer is 240–245V and no more, once the ECL82 has warmed up and operational.

The correct negative bias for the tetrode of an ECL82 under the circuit conditions given is –12.5 to –13.5V.

Hope this helps a bit!

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Old 18th Jun 2020, 12:14 pm   #15
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Default Re: Dansette Monarch Amplifier Issues

Hi Chris,

Many thanks for your input (and to you all). I must say, I bow to your knowledge of these matters. It is good to see not only an active forum, but one with so many knowledgeable people. It's been over 40 years since I last played around with circuits and to be honest, I have forgotten more than I remember.

I have now replaced C4 and are awaiting arrival of C3 and the additional 0.047 uF cap before I power up the Dansette and will keep you all updated with progress. Since components don't break the bank, I decided to replace all caps in circuit as a matter of precaution and longevity apart from the tone control ones as they are not subject to breaking down as far as I know. I'm hoping at this stage, with NOS ECL82 valve, all caps replaced, NOS BSR X5M and stylus, this baby will live again. But your input has also assisted this, and for that, I thank you all !

Last edited by paul1962; 18th Jun 2020 at 12:19 pm.
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Old 4th Jul 2020, 2:06 pm   #16
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Default Re: Dansette Monarch Amplifier Issues

Hello again everyone,

Many thanks for all your help once again and I can now confirm the Monarch is living again

The lack of any output was down to C3 as suspected. This has now been changed and everything is working. It still has the usual Dansette mains hum, but when the needle is on a record, it's about the same level as the spaces between tracks, so I put this down to just the valve being a valve

Many thanks for all of your assistance and input, it has been greatly appreciated !!

Kind Regards,
Paul
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Old 4th Jul 2020, 2:36 pm   #17
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Default Re: Dansette Monarch Amplifier Issues

Glad it's now working well. You were lucky to source a BSR X5M. This model does sound good. As to that residual hum, I did suggest how this can be improved. On my "Monarch" there was only an audible hum at the very end of the maximum travel of the volume control and cannot be heard when about 6' away. Those controls just need an earthed, isolated, screen. Of all the very many Dansettes using that very common ECL82 amplifier chassis that I have owned or serviced, I never found a hum issue that could not be minimised........Mind you, maybe "that typical Dansette hum" is all part of the aura of owning a Dansette?!
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Old 4th Jul 2020, 4:07 pm   #18
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Default Re: Dansette Monarch Amplifier Issues

Many thanks for your input Edward,

I have to agree that it would be an ideal to remove some of the mains hum. To the "modern ear" I would imagine it would frighten them
I may try and reduce some of the hum, the mains cable is earthed to the control unit and the hum does increase with the volume, but I wouldn't say excessively so. I have tried a jumper cable to the chassis but it didn't make much difference (if any). The deck doesn't make any difference either, so maybe a screening box around the controls may just make the difference.

Again ... Many thanks for your thoughts on the matter
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Old 10th Jul 2020, 2:42 pm   #19
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Default Re: Dansette Monarch Amplifier Issues

Hello again everyone.

Just as an update, I have made a shielding box as Edward suggested and during this, I have realised what the issue is with these players.

The control panel is situated right above the speaker and rather than an earth issue, the problem seems to be that the controls are highly microphonic. Just for the future, if anyone gets one of these players .. the shielding box is a must !

Once again everyone, many thanks for all the help and advice, it is really appreciated.
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Old 10th Jul 2020, 3:54 pm   #20
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Default Re: Dansette Monarch Amplifier Issues

Thanks for this update, it's appreciated. Does this now means that the hum level is reduced? I don't quite understand what you mean by "microphonic" though. That term is usually applied to active components rather than those passive control components.
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