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Monday, 9 April 2018

Dansette Junior Deluxe repairs & modifications.

Will turned up for an emergency curry. There was something ominous lurking in the boot of his car ....


It's a small Dansette Junior Deluxe record player...

"Can you take a look?"

Yeah, why not ....

It's in good cosmetic condition, and dates from around 1961.

Further inspection reveals some issues...

The valve (yes, just one!) is rattling around in the bottom, but is thankfully undamaged. The motor suspension has a few bits missing and the platter won't seat. These are really quite minor issues and are soon sorted.
Once the thing is spinning round reasonably, We try playback .... It's awful. Distorted and really low in volume...

The amplifier is simplicity itself. It has a nice mains transformer so it's reasonably safe, even has a 3 core mains lead :)

The output from the transformer is rectifier by a metal rectifier, and smoothed by the 16 and 32uF capacitor block, along with a 47K resistor. The smoothing cap is in really good condition!

The cartridge is a BSR TC8 medium output crystal cartridge,  producing an output of a few hundred mV or so. This feeds the top of the volume control, and straight onto the grid of the EL84 pentode. There's no cathode bypass capacitor fitted to this unit, nor an indicator bulb, but this is the nearest schematic I had ...


The distortion is partially caused by the 0.05uF tone control capacitor (a dreaded hunts), and is changed ...

The rest of the distortion, and the woefully low output is being caused by a poor cartridge. It's terminals have got chronic verdigris, and this means the crystal inside will have turned to goo. It's a very common failure. (It's not all bad though, as this mono heavyweight cartridge will have chewed Will's records to pieces.)
So we dig around in the box of phono carts to find a suitable donor. The big issue here is, we have nothing to mount to, except the original "flip over" mount...

There's a couple of Acos M7 moving magnet carts that may well do ...











... and after a bit of filing, it fits !

Tracking weight is set for about 2.5g, and it manages to track the test record.... but we can't hear it ....
The output from the Acos M7 is around 1mV .... not enough to drive the EL84's grid much at all... we will need to make an amplifier. Also the output from the crystal cartridge approximately followed the  RIAA equalisation curve, so no correction in the amplifier was needed. We'll have to add this, otherwise the output will sound very thin and tinny.


OK, so the above was quickly dreamed up. It's a (very) basic phono stage, and has some simple EQ in the feedback loop, to approximate the RIAA curve. 
  
I've stolen some power from the 6.3V heater winding on the mains transformer inside the record player. You'll notice there's only a half wave rectifier here, as one side of our heater winding is connected to ground. It's imperative to get this the right was round or you'll short out the heater winding on the transformer, and it will burn out pretty quickly... 

C1, R2 and C3 form a filter for this simple power supply. The cartridge is connected to JP2, and it's loaded by R1. C1 couples the audio though, which is biased to half-rail by R3 and R5. This is then fed to the op-amp, which does it's amplifying and RIAA correcting duties, and then passes the audio out via C8 to our existing volume control. C4 provides DC stability, and a little sub-sonic filtration.

A board is etched...

And duly fitted....

You'll notice the cable coming from the cartridge is screened. It was originally a twisted pair, but the hum pick-up was ghastly, as the arm is all plastic and has no shielding, lives over the mains transformer, by the mains input!
... and, because nothing in life is every easy, the mains switch failed open circuit during testing ! It was stripped and repaired....

How's it sound? ... honestly? Pretty damn awful, but at least Will's records are safe (r) 

here's some more pictures, including the obligatory arty valve shot ;)


... the big pre-amp project is coming ...

1 comment:

  1. Gosh, I was given one when we moved to Tewkesbury mid 1960s. I've no idea as to what happened to it. Classic!

    ReplyDelete