Dansette Major De Luxe volume problem - 2

[Hartley118]

Congratulations on getting it working Alex.

I just have a nagging doubt as to why you need that capacitor in the ground connection to the main amp.

It's worth checking whether you've followed the instructions 100% as follows:

"1. Disconnect pin4 of the op amp (the -V pin) from GND by cutting that bit of PCB track so that pin 4 is isolated.

2. Connect pin 4 of the op amp directly to -V on Michael's power supply. That will stop the short-circuit and the sizzling transformer.

3. Remove R3, which is biasing the op amp to work from a single rail supply. Michael's circuit gives you a split rail (+ and -) supply, so you now only need R4 to tie the op amp input to GND."

In particular, did you fully isolate the -V pin of the op amp (pin 4) from ground? I wonder about it because that 0.1 uF capacitor seems to be like an emergency band-aid to overcome a short circuit.

Martin

[awc]

Hi Martin

I must confess that having discovered the cause of the reduced heater voltage and remedied that, that I haven't carried out your further suggested modifications, especially as the thing appears to be working quite nicely now.

I have with your goodself and other members, now put in a lot of time on this, driven only by a desire to find a fairly straightforward solution to the problem and I can now sort out the other players in my collection which have been "in the pile" waiting for this day!

It certainly, for what seems like a simple problem to solve, been anything but. I was proposing the put a .1uf cap in the ground lead anyway when using the opamp with an AC/DC player to fully isolate it, will use a Y2 type in that instance.

It is a little difficult to track through all the posts and members suggestions to summarise the end result, but I will have a go if other members feel it will be useful.

Alex

[Hartley118]

OK Alex. Well it seems to work with your op amp ground isolated with the 0.1 uF capacitor. It just goes to show that there's usually more than one solution to a circuit problem!

I think that your op amp ground is now sitting at -8V relative to the main amp ground, and your 0.1 uF isolating capacitor is 'maintaining the peace'. That probably also means that the record deck cartridge ground, and possibly the deck itself, is also sitting at -8V, depending on how it's wired. So be sure to maintain that separation between deck and main amplifier, or you'll end up with a hot transformer once again. Or you could put in another 0.1 uF in the cartridge ground connection.

Best of luck with it.

Martin

[Tim]

I would have been tempted to fit an ECL82 and use the triode stage as a pre amp. It should only require a few components. More difficult if player has a PCB of course. Any way of doing it that works increases one's experience.

[Edward Huggins]

Yes, a cathode, an anode and a grid resistor and a 0.02uF coupling cap is all you need. But an ECL86 is more appropriate as it's Anode loading is a better match to the primary of the (albeit a midget) O/P transformer than the (lower output) ECL82. It might put a bit of a strain on that modest half-wave power supply though. Edward

[awc]

Hi Tim

I would have indeed been tempted to fit a ECL86 (apparently better suited to this particular output transformer) but for two reasons:

1. This amp does indeed use a printed circuit board and it would have meant making some non-reversible changes to it to wire it in.

2. Really the same as 1, insofar as fitting the opamp is easily reversible, requiring only the unsoldering of a few wires and reconnecting the cartridge wires to the tagboard and volume control.

I am hoping to have a go at pulling together the various suggestions in this thread to make up a "kit" with construction instructions for others to have a go with, but frankly my talents (such as they may be!) do not really lay in that direction.

Alex

[awc]

Right, here we go. I am attaching an amended version of Chris, Herald1360's variation of the Maplin Kit with further amendments by Martin, Hartley118 and using the power supply suggested by Michael Maurice.

The idea is that in players with a mains transformer, the heater supply will be used to power our collaborative effort, with a separate small mains transformer, possibly from a scrap power supply, being used instead for live chassis AC/DC players.

Would the knowledgeable ones please check this out and advise of any errors or omissions.

When this has been done, I will try (!) to prepare a narrative of the procedure involved in the conversion etc.

Thanks

Alex

[Tim]

Or an EL86. If a PCB I might see if I could make some sort of plug in adaptor.

[Hartley118]

Hi Alex,

Well done for writing it up.

I think the power supply part of your circuit should show Michael's split rail (V+ 0V V+) diagram with V- connected to IC pin 4 and V+ connected to the op amp IC pin 8.

Nothing else, other than the power supply V+ and V-, should connect to pin 4 and pin 8.

0V on the power supply should connect to GND on the op amp circuit.

[Edward Huggins]

EL86? Edward

[Herald1360]

Like this?

[af024]

It looks like this keeps on coming around.
https://www.vintage-radio.net/forum/...ad.php?t=65007
This is an interesting thread all the same.

[af024]

There's also this of course ...
http://www.epemag3.com/lib/free_proj...20Part%202.pdf

[Hartley118]

Originally Posted by Herald1360

Like this?

Yes Chris, that's a very tidy drawing of what I was describing.

Alex - Is this how you've got it wired?

Martin

[awc]

Yes, except that C3 and the .1uf cap I put in the ground output seem to have disappeared, presumably they are not required, even when connecting to a "live chassis" AC/DC player.

Also I can't make out the values of C1 and C2 in the power supply drawing nor their polarity. I have 1000uf in mine, as in Michaels drawing.

Alex

[Hartley118]

No harm in leaving C3 in there - it gives a minor improvement in stability by maintaining the DC gain at unity irrespective of what AC (Signal) gain you've set with R9.

I think that at one stage in your work you needed the 0.1 uF in the output ground because you hadn't isolated the V- connection from ground and there was a short causing transformer overload. Check that pin 4 ONLY goes to V-. If so, you can remove the 0.1 uF.

Chris has shown your 1000uF smoothing caps as 1mF = 1millifarad = 1000uF.

Polarity for V+ cap is + on the cap to V+. Polarity for V- cap is + on the cap to GND.

Martin

[Herald1360]

I just copied the psu circuit from a simulator schematic capture window- easiest way to draw it quickly. It doesn't differentiate between different sorts of capacitor, just does perfect ones unless you add the ESR, leakage R etc as discrete components.

The TX "winding" is just an inductor with its "L" reference deleted and a value of 6.3 Henries .

[awc]

Hi Martin

Thanks for your further reply. I have just one more question. If the .1uf (to be a Y2 type in this instance) in the ground output wire is omitted when, say, connecting up the opamp to a Dansette Popular amp (UL84/UY85 combo) where one side of the mains is connected to chassis and could therefore become live, could not a potential 240v reach the cartridge terminals?

Alex

[Edward Huggins]

Well yes, if the isolating caps (and not always fitted) have failed and the live/neutral wires are transposed. Edward

[Hartley118]

Well, Alex, you raise some tricky issues here.

First, let me reveal my prejudice that I'm uncomfortable hanging live mains voltage on to a low power op amp.

An 'isolating cap' does stop DC, but, particularly one as big as 100nF, passes around 7mA with 230V AC across it - certainly enough to give you a healthy (or more likely unhealthy) tingle and maybe trip an RCD. Also the capacitor has the opportunity to charge up to somewhere near mains voltage, so anyone working on the circuit, even with mains switched off, might accidentally discharge the cap into the op amp output - bad news!

Also, I'm not sure what we're isolating from what here. If the 6.3V AC feeding the valve heater and your rectifier diodes isn't an isolated winding, but an autotransformer, then Ground on the op amp circuit could be potentially live. So isolating capacitors - probably no more than 10nF - should be present in the connections from the cartridge (both signal and shield). That also means that the metal parts of the turntable aren't earthed, but are potentially connected to mains via that 10nF capacitor of around 300k ohms reactance at 50Hz. Unless you then ensure that the amplifier chassis ground is connected to mains neutral, it all gets rather hummy. But I did warn you of my prejudice against a live chassis in these circumstances!

If your op amp power supply connections are correct, there's no point in that 100nF capacitor in the output GND line. If removing it causes that transformer overload again, then recheck your wiring.

Martin

Martin