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Old 30th Sep 2016, 8:09 pm   #101
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Default Re: Dansette Major De Luxe volume problem - 2

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Originally Posted by awc View Post
......... The amp I have in mind to try out the live chassis adaptation using the revised power supply you suggest, scandalously has no protection for the poor user, as it allows a reverse connected mains lead to put 240v on one of the cartridge pins!

As you may recall, I was proposing to mitigate this by putting a .022uf Y2 cap in the ground lead between the output from the preamp and the input to the original amp. Would this do the job, do you think?
Yes from the safety viewpoint, though from a hum problem one you might do better to use insulating sleeves on the cartridge tags to prevent accidental contact. The original shock risk was not great as long as you simply used the player as intended.

Be aware though that any shock incident involving the modified player might raise "interesting" issues over liability even if the mods apparently make it safer!
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Old 1st Oct 2016, 12:47 am   #102
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Default Re: Dansette Major De Luxe volume problem - 2

Here is my take on the circuit and its board. I added the two extra links to keep the unused half of the op-amp quiet.
Les
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Old 1st Oct 2016, 10:52 am   #103
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Default Re: Dansette Major De Luxe volume problem - 2

Hi Les

Wow, that is very impressive, I wish I had that sort of level of CAD skills!

I have inserted the links on the board, I suppose it now makes it the Mark 7!

I would ask though, about the need for the additional track breaks at points 12G and 12H, as these aren't on my Mark 6 and don't seem necessary.

Thanks again

Alex
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Old 3rd Oct 2016, 6:52 pm   #104
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Default Re: Dansette Major De Luxe volume problem - 2

Hi Alex, All my drawings I do in Paint It takes a little longer but having a library of parts helps immensely, in fact the drawings are best opened in MS Paint. I added the extra cuts to give more isolation,particularly the high impedance INPUT from the track carrying 6.3V AC. You did all the work that matters and should take all the credit .
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Old 4th Oct 2016, 10:24 am   #105
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Default Re: Dansette Major De Luxe volume problem - 2

Hi Les, I thought the answer was going to be something like that.

Well, I think we have now got the EL84 version where we want it to be(revised component location list attached), so now I'll have a look at putting together the UL84 equivalent using the resistor/zener diode power supply arrangements suggested.

This will mean getting in some further components, but in line with the original philosophy, they seem to be cheap and readily available, so it'll be a case of fitting them tidily onto a piece of stripboard. I'll try to do it in less than 7 attempts this time!

Thanks for your praise, but as my knowledge/ability on the theoretical side of things is somewhat limited, my contribution was to make up a usable device from the excellent advice and assistance given by others, such as your goodself, on this forum.

I must admit I was driven by a desire to see a project through to a satisfactory conclusion as so many of my previous attempts have fallen by the wayside.

Thanks again

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Old 5th Oct 2016, 10:04 pm   #106
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Default Re: Dansette Major De Luxe volume problem - 2

Sorry -- I think I must have forgotten to post this earlier. It's the power supply circuit for using an op-amp as a preamplifier, in a live-chassis record player. (But you can test it in a Dansette Major De Luxe, which has a mains transformer, by taking a feed from the anode of the rectifier.)
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C1 = 100 nF will allow about 7 mA. You may get away with a lower capacitance value if you are using a low-power op-amp (op-amps are pretty much either low-noise or low-power). Note that if the current is too small, the Zener diodes may be very noisy.
R2 needs to be a metal film type (or a proper fusible resistor, which are really just posh metal film types).
R1 may not be strictly necessary, especially if a double-pole power switch is fitted, as the motor ought to provide a discharge path for C1; but it does no harm including it.

Note that you will need to be careful with clearances upstream of R1 / C1. If building on copper-strip breadboard, remove as much copper as possible from the immediate vicinity of the "input" end of C1 / R1. Remove a whole track either side (heating it with a soldering iron will soften the adhesive, allowing the copper to be lifted off. If you make transverse cuts with a sharp knife beforehand, it will come away in sections), and all the copper between the two ends.
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Old 9th Oct 2016, 10:11 am   #107
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Default Re: Dansette Major De Luxe volume problem - 2

Thanks Julie for your thoughts, but as I had already ordered the parts to have a go at constructing the revised circuit suggested by Chris (Herald 1360) and Les and Martin etc. (Posts 98 and 100 etc) I'm going ahead with that version and will report my findings. (exciting stuff, isn't it!)

Just one query at this stage, insofar as is it still appropriate to connect Pin 5 of the opamp to Gnd, given that in this version this is now also the negative mains line?

Thanks again

Alex
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Old 9th Oct 2016, 1:58 pm   #108
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Default Re: Dansette Major De Luxe volume problem - 2

If pin 5 should be "grounded" then yes. Mains (neutral hopefully) is signal and power supply "ground" on live chassis equipment. The IC will only be worried about voltage differences between its pins, not whether the whole lot are floating around at mains voltage.

In the event of a problem, the magic smoke isn't very expensive anyway
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Old 13th Oct 2016, 3:22 pm   #109
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Default Re: Dansette Major De Luxe volume problem - 2

Thanks for your reply Chris. The UL84 version of the Preamp has been constructed and initially tested using my capacitor reformer which is a very basic device but gives out 350vDC with a maximum current of 5mA.

I'm pleased to say the Preamp worked fine, taking a current of about 2.5mA

Filled with confidence, I wired it in to my Regentone suitcase portable (photo attached) taking the HT line from Pin 7 of the UL84 (Screen). I am pleased to say that (so far anyway) it works just fine and I have to reign back the gain if anything, so there is plenty to spare and the hum level is quite low as the volume control doesn't now have to be turned up so high to give a louder sound.

I will soak test it, just to be sure no problems arise, but as we speak, I think we have a workable solution to the problem.

My many thanks again to all who helped me to build this device and I will now get on and build some more for the remaining players in my collection.
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Old 13th Oct 2016, 4:03 pm   #110
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Default Re: Dansette Major De Luxe volume problem - 2

Sorry, it should of course be Pin 9 of the UL84, not Pin 7
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Old 16th Oct 2016, 3:36 pm   #111
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Default Re: Dansette Major De Luxe volume problem - 2

Further news, I have constructed two further examples and everything seems ok . I also prepared (well actually my Wife did!) a components ID overlaid on a photo of the board and also a Component Location list with the enhanced feature of a suggested build sequence!

Both are attached together with a photo of the reverse side, and if Topcap would like to do one of his excellent circuit drawings (as he did per post 102) for the UL84 version, I think that will tidy up the loose ends.

Subject to any further observations from you all, I propose then to sort out the relevant posts into a new thread, so if anyone wants to build either of the preamps, it will be easier to do so.

Thanks once again to all contributors to the thread, a great collaborative effort!
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Old 18th Oct 2016, 12:46 pm   #112
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Default Re: Dansette Major De Luxe volume problem - 2

As requested Alex though I made a slight change to wire link 4, moving it over to go between 17D and 17E.
Also to discourage the HT from jumping the strip board tracks, I added track cuts at 22E, 22G, 21E, 21F and 21G.
There used to be an actual spec for Vero Board but it was a very long time ago, I think 250V was the maximum recommended between tracks but I do stand to be corrected. In today's H&S environment I would expect 50V

Best view in MS Paint.

Les
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Old 18th Oct 2016, 1:59 pm   #113
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Default Re: Dansette Major De Luxe volume problem - 2

With #111 posts to date, you have really persevered with thus. Well done.
What we need now is a Member or Assembler to turn this into a small batch, off the shelf, product.
I reckon this could be priced very attractively as the component count/cost is low. This would of course not be sold directly to the end-user, but to the repair trade.
There are far more low-gain portable record players out there than the high-output cartridges needed to serve them! Edward
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Old 18th Oct 2016, 6:28 pm   #114
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Default Re: Dansette Major De Luxe volume problem - 2

Many thanks first of all to Les (Top Cap), for his excellent work in preparing the circuit and board layout drawings. I fully understand the reason for moving wire link 4 as although I elevated the zener diode well above it on the board, moving it out of way is obviously preferable.

I have installed the first one I built in my Regentone suitcase portable which has a UL84/UY85 combination and haven’t had any problems with HT flashover between the tracks. Perhaps this is something that may happen over a period of time rather than immediately? I have modified that board in any event as recommended. I am attaching a further copy of the Component Locations list, modified to suit.

Thanks also to Edward for his kind words. I never thought when I started this thread, that it would go on for as long as it has, but I felt that this project was something I could, even with my limited knowledge and expertise, get to a satisfactory conclusion with the excellent help from more talented forum members.

I will leave things as they are for a few days to see if anyone else has any input to offer, and then do a tidier new thread to assist anyone searching in the future.

Alex
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Old 11th Dec 2016, 11:30 am   #115
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Default Re: Dansette Major De Luxe volume problem - 2

I have a amplifier that needs this boost on its input for a replacement cartridge. The problem is the grounds of signal and one side of the heater chain are connected using this preamp. This amp has a heater chain that has a hum balance pot of 100 ohm wire wound with the centre wiper connected to ground, which would be defeated as soon as plug in preamp was connected. Is there a way of overcoming this?
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Old 11th Dec 2016, 11:41 am   #116
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Default Re: Dansette Major De Luxe volume problem - 2

What make of amplifier is this please?
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Old 11th Dec 2016, 11:59 am   #117
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Default Re: Dansette Major De Luxe volume problem - 2

It's in fact a jukebox amplifier and most use a balance pot, but rebuilt cartridges or replacement ones don't have the original output. I don't want to alter the original amp, so this preamp would go in a enclosure outside of it.
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Old 11th Dec 2016, 4:17 pm   #118
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Default Re: Dansette Major De Luxe volume problem - 2

If the heater supply is humbucked and unavailable for the preamp psu, you need to rob a bit from the HT supply for the preamp. A suitable resistor and zener/capacitor should do. That way signal ground and supply ground are the same.
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