DAC90A dropper...

[Nick]

Yet another DAC90A in the in-tray - this one has a burnt-out dropper - its a white dropper (now rather dark brown in places...)...

Anyone know a source for these?

Thanks

[Ed_Dinning]

Hi Nick, plenty of scrap chassis around so should not be too difficult to find. I have done auto transformers for these sets especially if it is the white case version, as it prevents cabinet heat cracking.
The chassis was used in many different sets, with different build standards and on removal of the dropper you may well see additional holes to fit the transformer onto.

Ed

[Retired]

Hi,

I don't know of a source for these droppers Nick but I've used Capacitor Droppers previously with great success.

I had a good selection of X2 caps and would start by adding say 3uF increasing the mains voltage using a Variac whilst taking voltage readings on the set and adjusting the caps until I got the voltage just right. I could never get the maths to work correctly so took the more practical approach.

http://www.vintage-radio.com/repair-rest...calcs.html

Kind regards, Col.

[Joe]

I don't mean this in a sarcastic way but given your skills and knowledge, Nick, wouldn't it be interesting to come up with a more sophisticated method of getting rid of the excess voltages?

- Joe

[Nick]

I don't mean this in a sarcastic way but given your skills and knowledge, Nick, wouldn't it be interesting to come up with a more sophisticated method of getting rid of the excess voltages?

That wouldn't be a problem - either a capacitive dropper, auto- or isolated (far safer) transformer etc. but is not the point - at the very least, I'd like something physically similar in place of the current smoking ruin!

[Radio Fixer]

Quite agree Nick. Only go other routes if you cant find an original, or close to, part. Actually the heat from a dropper doesnt matter as they dont get played for long periods anymore.

Anyway, to cut to the chase as they say.

Looked in my droppers box and I have two that look likely. See DAC90A is given as 1250 on Trader Sheet. One of mine is 1240 Ohms with two moveable tap bands. Never been used.

The other is 1380 Ohms (again never been used) with no tap bands. But one could be taken off another to tap it down to 1250 Ohms.

Both are nominally 20 mm diam. and 95mm tall and on white ceramic formers.

The one with the tap bands, the first, is a coaser wire, less turns. The other is much finer shinny wire and lots of turns.

Can do pics if interested.

Gary

[Yorkie]

Gary's offer of the 1240R dropper sound just the ticket.

Failing that, what about this – a 30W Watt 1K Ohm 'Wire Wound Variable Resistor Ceramic Rheostat', just £2.93 post free (on a slow boat from China!), which - if not on this occasion - may be worth thinking about in other applications:

http://www.ebay.co.uk/itm/30W-Watt-1K-Oh...upplies_ET&hash=item51a5257868

What’s not to like about that?

The dropper on the DAC90A is 950 + 150 +150 = 1250R total. If run on the 250V setting, (kindest to the valves), the HT tapping point is at 300R, so if this 1,000R dropper was used, and the tapping set at 300R for the HT, the HT would be fine, but there would be a shortfall of 250R for the heater chain, so an additional series resistor would be needed. The heater chain draws 0.1Amp, so if the nearest preferred value resistor to 250R was selected – 270R, then 27 Volts would be dropped across that resistor, and with 0.1 Amp flowing, the wattage would be 27 x 0.1 = 2.7 Watts. Hence, a ceramic wirewound 5 Watt 270R resistor, which is only 10mm square x 23mm long, should do the trick. Or to be on the safe side, a 10Watt one is 48mm x 10mm square. EG:

http://www.esr.co.uk/components/products...istors.htm

Just a thought.

[Radio Fixer]

Here's a pic of the 1240 Ohm one. I have loosend the moveable bands for the pic. Guess they can be set in the correct places from what David has said. The wire being uninsulated its clever how the strands are kept apart. Thats what makes them hard to repair if a wire break as the tension goes.

   

Gary