1931 Marconiphone 42 oscillating problem - UK Vintage Radio Repair and Restoration Discussion Forum

Nightcruiser · 13th Nov 2015, 11:17 pm

Listening to the old marconiphone 42 this week the set suddenly lost all signals on MW/LW from the powerful sound it had. Neither is there any life or improvement from the aerial socket when probed.

It will still oscillate on both of the wavelengths but can no longer bring in the stations.

I've cleaned all the valve pins thoroughly of the MS4B and MH4 etc and given the sockets a clean at the same time but no joy.

Has anyone any ideas please

John

Skywave · 14th Nov 2015, 12:29 am

Sounds like R.F. instability to me. Given the fault conditions (i.e. not wavelength specific), the I.F. amplifier stage is a strong contender. Have a look inside and see if you can see any 'waxy' capacitors. If there are any, change them. Ditto any capacitors bearing the name 'Hunts'.

Al.

TonyDuell · 14th Nov 2015, 12:42 am

Err, what IF amplifier stage?

According to the Trader Sheet this is a TRF receiver with a regenerative detector. I assume the 'oscillation' refered to means the detector valve (MH4) can be got to oscillate by turning up the reaction control.

I would therefore start by checking round the RF amplifier, the MS4B. Make sure the heater is not open-circuit, then measure the anode voltage (on the top cap of this valve) and the screen grid voltage.

An old trick is to couple an aerial to the top cap (anode) of the RF amplifier through a small capacitor (to prevent HT+ ending up on the aerial) and see if you can receive a local (strong) station. If you do this, do not let the set oscillate as it will act as a transmitter....

Nightcruiser · 14th Nov 2015, 2:24 am

Thanks guys

The MS4B seems now to be the culprit so im gonna have to source another excellent one from somewhere to keep me fired up on these forthcoming chilly nights

John

jonnybear · 14th Nov 2015, 10:26 am

This any good shows as equivalent (NVM).http://www.ebay.co.uk/itm/New-Old-St...AAAOSwkZhWRHG4.
John

Nightcruiser · 15th Nov 2015, 2:41 pm

Thank you for the info John - I shall take a look.��
the only problem i often find with ebay valves is sometimes its a duff valve that was put back into the new valves box when a replacement was necessary.. An old habit often used by the on site repairmen over the years but as lng as it keeps me going .....��

John

13th Nov 2015, 11:17 pm	#1
Nightcruiser Pentode Join Date: May 2011 Location: Bath, Somerset, UK. Posts: 149	1931 Marconiphone 42 oscillating problem Listening to the old marconiphone 42 this week the set suddenly lost all signals on MW/LW from the powerful sound it had. Neither is there any life or improvement from the aerial socket when probed. It will still oscillate on both of the wavelengths but can no longer bring in the stations. I've cleaned all the valve pins thoroughly of the MS4B and MH4 etc and given the sockets a clean at the same time but no joy. Has anyone any ideas please John

14th Nov 2015, 12:29 am	#2
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: 1931 Marconiphone 42 oscillating problem Sounds like R.F. instability to me. Given the fault conditions (i.e. not wavelength specific), the I.F. amplifier stage is a strong contender. Have a look inside and see if you can see any 'waxy' capacitors. If there are any, change them. Ditto any capacitors bearing the name 'Hunts'. Al.

14th Nov 2015, 12:42 am	#3
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: 1931 Marconiphone 42 oscillating problem Err, what IF amplifier stage? According to the Trader Sheet this is a TRF receiver with a regenerative detector. I assume the 'oscillation' refered to means the detector valve (MH4) can be got to oscillate by turning up the reaction control. I would therefore start by checking round the RF amplifier, the MS4B. Make sure the heater is not open-circuit, then measure the anode voltage (on the top cap of this valve) and the screen grid voltage. An old trick is to couple an aerial to the top cap (anode) of the RF amplifier through a small capacitor (to prevent HT+ ending up on the aerial) and see if you can receive a local (strong) station. If you do this, do not let the set oscillate as it will act as a transmitter....

14th Nov 2015, 2:24 am	#4
Nightcruiser Pentode Join Date: May 2011 Location: Bath, Somerset, UK. Posts: 149	Re: 1931 Marconiphone 42 oscillating problem Thanks guys The MS4B seems now to be the culprit so im gonna have to source another excellent one from somewhere to keep me fired up on these forthcoming chilly nights John

14th Nov 2015, 10:26 am	#5
jonnybear Octode Join Date: Nov 2010 Location: Cullompton, Devon, UK. Posts: 1,435	Re: 1931 Marconiphone 42 oscillating problem This any good shows as equivalent (NVM).http://www.ebay.co.uk/itm/New-Old-St...AAAOSwkZhWRHG4. John

15th Nov 2015, 2:41 pm	#6
Nightcruiser Pentode Join Date: May 2011 Location: Bath, Somerset, UK. Posts: 149	Re: 1931 Marconiphone 42 oscillating problem Thank you for the info John - I shall take a look.�� the only problem i often find with ebay valves is sometimes its a duff valve that was put back into the new valves box when a replacement was necessary.. An old habit often used by the on site repairmen over the years but as lng as it keeps me going .....�� John Last edited by Nightcruiser; 15th Nov 2015 at 2:47 pm.