Dansette Major Deluxe Amplifier - UK Vintage Radio Repair and Restoration Discussion Forum

Ian Russell · 26th Apr 2017, 5:30 pm

I'm rebuilding an amp for an early Major Deluxe. The amp board is marked 14.3. The schematic for the 14.3 shows an 1800pf capacitor but there isn't one on mine. It looks more like a MC/1961 and I have rebuilt it as such. The problem is that the 4.7k resistor burns out on power up. I'm using a 1/4 watt resistor, should I be using one with a higher wattage?
I'd welcome some advice.

Edward Huggins · 26th Apr 2017, 5:44 pm

I would definately fit a 5 watt type as belt and braces. That 4.7 K smoothing resistor seems a high value, the circuit shows a 3.3K. Hav you checked the contact cooled rectifier? The 1800pf is for additional tone correction. I would not fit one as you already have a tone control, unless there is some HF instability, which seems unlikely.

ms660 · 26th Apr 2017, 5:47 pm

Does it burn up with the valve removed?

Lawrence.

Station X · 26th Apr 2017, 5:53 pm

Edward.

Please read posts carefully and check the facts before posting a reply.

The OP states that he has rebuilt the circuit to the MC/1961 diagram. I have looked at that diagram and the resistor is 4.7k. There is no 1800pF capacitor.

Quote:

Originally Posted by Edward Huggins

I would definately fit a 5 watt type as belt and braces. That 4.7 K smoothing resistor seems a high value, the circuit shows a 3.3K. Hav you checked the contact cooled rectifier? The 1800pf is for additional tone correction. I would not fit one as you already have a tone control, unless there is some HF instability, which seems unlikely.

Ian Russell · 26th Apr 2017, 6:20 pm

Quote:

Originally Posted by ms660

Does it burn up with the valve removed?

Yes it does Lawrence.

ms660 · 26th Apr 2017, 6:23 pm

Re: Post#6: That suggests a short on the screen grid side of the resistor, maybe the electrolytic or something else shorting, check over the work you have done.

Lawrence.

Edward Huggins · 26th Apr 2017, 6:24 pm

Quote:

Originally Posted by Station X

Edward.

Please read posts carefully and check the facts before posting a reply.

The OP states that he has rebuilt the circuit to the MC/1961 diagram. I have looked at that diagram and the resistor is 4.7k. There is no 1800pF capacitor.

Graham, you are absolutely right! I made the mistake of going straight to a Dansette 14/3 curcuit as that was on the PC board. It's interesting though that such a very similar circuit should have such a change in smoother resistor value.

Ian Russell · 26th Apr 2017, 6:30 pm

Quote:

Originally Posted by ms660

Re: Post#6: That suggests a short on the screen grid side of the resistor, maybe the electrolytic or something else shorting, check over the work you have done.

Lawrence.

Thanks Lawrence, will do.
Ian.

Station X · 26th Apr 2017, 6:30 pm

Two posts moved to a new thread here:-

http://www.vintage-radio.net/forum/s...d.php?t=136030

David G4EBT · 26th Apr 2017, 6:45 pm

Looking at the MC/1961 circuit, the 4K7 resistor doesn't look as though it has to work hard for a living, (it isn't a resistor that has to pass the full HT current as in a R/C filter network across a smoothing & reservoir capacitor), so I'd concur with Lawrence and wonder if there's a wiring fault, (valveholder tags touching for example or the 16uF capacitor is duff, or even the EL84 Valve), but that's just speculation on my part, but it looks to me as though summat's up.

You first need to check the Voltage drop across the resistor to determine the current flowing, then multiply the current by the Voltage drop to ascertain the wattage being dissipated in the resistor. Obviously too much current is being drawn for a 1/4Watt to cope with so it's best to work out the correct Wattage needed by using Ohm's Law. ( V/I x R).

If you clip your Voltmeter across the resistor to check what Voltage is being dropped, then divide the Voltage by 4,700, it will give you the current passing through the resistor in Amps (In your case, a fraction of an Amp, hopefully!). If you then multiply the current by the Voltage drop, that will tell you what Wattage is being dissipated by the resistor. Choose a resistor with a higher Wattage to that to help it run cooler. So for example, if by calculation 1/2 Watt is being dissipated, used a 1 Watt resistor, or if 1 Watt is being dissipated, use a 2 Watt resistor.

To use a simple example, let's say (hypothetically) that 47 Volts was being dropped across a 4,700 Ohm resistor. 47/4700 = 0.01 Amps (10 mA). Thus, 0.01A x 47V = 0.5Watts. (I'm not suggesting that 47 Volts is being dropped across the resistor - that's something you'll only discover with your Voltmeter).

Hope that helps a bit.

Ian Russell · 26th Apr 2017, 7:09 pm

Thank you very much for that David, very informative.

David G4EBT · 26th Apr 2017, 7:46 pm

Just a small point worth mentioning Ian - the cathode bypass cap, 25uF, (probably 25V) which connects from pin 3 of the valveholder to chassis, will almost certainly be duff, so if you haven't already done so, I'd suggest that you change it. If you get any pronounced hum when you've got the amp working, that will almost certainly be the 32uF/16uF smoothing caps assuming that they're original. (I guess they'll be in one can).

Good luck with it - let us know how you get on.

26th Apr 2017, 5:30 pm	#1
Ian Russell Tetrode Join Date: Aug 2016 Location: Wakefield, West Yorkshire, UK. Posts: 75	Dansette Major Deluxe Amplifier I'm rebuilding an amp for an early Major Deluxe. The amp board is marked 14.3. The schematic for the 14.3 shows an 1800pf capacitor but there isn't one on mine. It looks more like a MC/1961 and I have rebuilt it as such. The problem is that the 4.7k resistor burns out on power up. I'm using a 1/4 watt resistor, should I be using one with a higher wattage? I'd welcome some advice.

26th Apr 2017, 5:44 pm	#2
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,336	Re: Dansette Major Deluxe Amplifier I would definately fit a 5 watt type as belt and braces. That 4.7 K smoothing resistor seems a high value, the circuit shows a 3.3K. Hav you checked the contact cooled rectifier? The 1800pf is for additional tone correction. I would not fit one as you already have a tone control, unless there is some HF instability, which seems unlikely. __________________ Edward.

26th Apr 2017, 5:47 pm	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Dansette Major Deluxe Amplifier Does it burn up with the valve removed? Lawrence.

26th Apr 2017, 6:30 pm	#9
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Dansette Major Deluxe Amplifier Two posts moved to a new thread here:- http://www.vintage-radio.net/forum/s...d.php?t=136030 __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

26th Apr 2017, 6:45 pm	#10
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,763	Re: Dansette Major Deluxe Amplifier Looking at the MC/1961 circuit, the 4K7 resistor doesn't look as though it has to work hard for a living, (it isn't a resistor that has to pass the full HT current as in a R/C filter network across a smoothing & reservoir capacitor), so I'd concur with Lawrence and wonder if there's a wiring fault, (valveholder tags touching for example or the 16uF capacitor is duff, or even the EL84 Valve), but that's just speculation on my part, but it looks to me as though summat's up. You first need to check the Voltage drop across the resistor to determine the current flowing, then multiply the current by the Voltage drop to ascertain the wattage being dissipated in the resistor. Obviously too much current is being drawn for a 1/4Watt to cope with so it's best to work out the correct Wattage needed by using Ohm's Law. ( V/I x R). If you clip your Voltmeter across the resistor to check what Voltage is being dropped, then divide the Voltage by 4,700, it will give you the current passing through the resistor in Amps (In your case, a fraction of an Amp, hopefully!). If you then multiply the current by the Voltage drop, that will tell you what Wattage is being dissipated by the resistor. Choose a resistor with a higher Wattage to that to help it run cooler. So for example, if by calculation 1/2 Watt is being dissipated, used a 1 Watt resistor, or if 1 Watt is being dissipated, use a 2 Watt resistor. To use a simple example, let's say (hypothetically) that 47 Volts was being dropped across a 4,700 Ohm resistor. 47/4700 = 0.01 Amps (10 mA). Thus, 0.01A x 47V = 0.5Watts. (I'm not suggesting that 47 Volts is being dropped across the resistor - that's something you'll only discover with your Voltmeter). Hope that helps a bit. __________________ David. BVWS Member. G-QRP Club member 1339.

26th Apr 2017, 6:23 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Dansette Major Deluxe Amplifier Re: Post#6: That suggests a short on the screen grid side of the resistor, maybe the electrolytic or something else shorting, check over the work you have done. Lawrence.

26th Apr 2017, 7:09 pm	#11
Ian Russell Tetrode Join Date: Aug 2016 Location: Wakefield, West Yorkshire, UK. Posts: 75	Re: Dansette Major Deluxe Amplifier Thank you very much for that David, very informative.

26th Apr 2017, 7:46 pm	#12
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,763	Re: Dansette Major Deluxe Amplifier Just a small point worth mentioning Ian - the cathode bypass cap, 25uF, (probably 25V) which connects from pin 3 of the valveholder to chassis, will almost certainly be duff, so if you haven't already done so, I'd suggest that you change it. If you get any pronounced hum when you've got the amp working, that will almost certainly be the 32uF/16uF smoothing caps assuming that they're original. (I guess they'll be in one can). Good luck with it - let us know how you get on. __________________ David. BVWS Member. G-QRP Club member 1339.