Marconi 4701 replacement loudspeaker. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 5th May 2018, 8:13 pm

Reference to an earlier post about this set: https://www.vintage-radio.net/forum/...d.php?t=114953

The loudspeaker employed in this set is a 20 ohm 6 X 3 inch elliptical type.
Unable to find a direct replacement I've fitted a pair of 9 ohm units removed from a scrap LCD TV.
In order that the new speakers will fit in the position of the original unit an adaptor plate will be required. The new part has now been made and fitted. Now all that is required to do is refit the chassis and find out if it works.

DFWB.

Heatercathodeshort · 6th May 2018, 8:05 am

Not another adaptor plate David. Soon we will be having adaptor plates to adapt adaptor plates.

Loudspeakers with odd speech coil impedances can be difficult to source David. We tended to chuck them out when we scrapped these old hulks years ago. John.

linescan87 · 6th May 2018, 10:20 am

It will be interesting to see (hear) how it sounds, fine no doubt. I do like your use of adaptor plates, no damage to the set should the original part become available.

Cheers, John Joe.

Graham G3ZVT · 6th May 2018, 10:44 am

Just continuing the theme in the other thread about Thorn 2000's being converted to studio monitors etc...

...Arguably the most comprehensive conversion was the one which hung on a wall bracket in the training room at Granada Rentals in Bedford.

All manner of faults could be switched in and out from the trainers rostrum, including a primary and complementary colour recognition eyesight test.
Anyone who failed got a special certificate, called a P45.

FERNSEH · 6th May 2018, 11:39 pm

The series connected twin loudspeaker system works very well delivering plenty sound as so it should considering the audio amplifier employs two BD116 power transistors. There is some intercarrier hum to attend to, can't blame the speakers for that.

Now to move on to the vision circuits. The present tests are carried out with the EHT generator panel removed.
The BRC 2000 series chassis employs slide in and out modules and was the easiest to service of all the dual standard colour sets made and sold between 1967 to 1970. All the others were not very nice to work on when it came to house visit service calls.

DFWB.

Welsh Anorak · 8th May 2018, 3:32 pm

They were nice until any intermittent fault was 'cured' by running solder all along the edge connector and the panel forced in!

FERNSEH · 9th May 2018, 3:17 pm

EHT problems in the Marconi 4701, it's too high and can't be adjusted. Tried the EHT tripler unit from the Ferguson 3700 and that one is useless, no EHT. As for the Marconi tripler, it's most likely the bleeder resistor in the tripler module has gone open circuit. Actually, the BRC 2000 circuit diagram shows the resistor as a VDR. Connects to the focus control and from the bottom end of focus pot to the base of the EHT voltage sensing transistor in the EHT generator module.

DFWB.

5th May 2018, 8:13 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Marconi 4701 replacement loudspeaker. Reference to an earlier post about this set: https://www.vintage-radio.net/forum/...d.php?t=114953 The loudspeaker employed in this set is a 20 ohm 6 X 3 inch elliptical type. Unable to find a direct replacement I've fitted a pair of 9 ohm units removed from a scrap LCD TV. In order that the new speakers will fit in the position of the original unit an adaptor plate will be required. The new part has now been made and fitted. Now all that is required to do is refit the chassis and find out if it works. DFWB. Attached Thumbnails

6th May 2018, 8:05 am	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Marconi 4701 replacement loudspeaker. Not another adaptor plate David. Soon we will be having adaptor plates to adapt adaptor plates. Loudspeakers with odd speech coil impedances can be difficult to source David. We tended to chuck them out when we scrapped these old hulks years ago. John.

6th May 2018, 10:20 am	#3
linescan87 Heptode Join Date: Mar 2014 Location: Meath, Ireland Posts: 551	Re: Marconi 4701 replacement loudspeaker. It will be interesting to see (hear) how it sounds, fine no doubt. I do like your use of adaptor plates, no damage to the set should the original part become available. Cheers, John Joe.

6th May 2018, 10:44 am	#4
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,725	Re: Marconi 4701 replacement loudspeaker. Just continuing the theme in the other thread about Thorn 2000's being converted to studio monitors etc... ...Arguably the most comprehensive conversion was the one which hung on a wall bracket in the training room at Granada Rentals in Bedford. All manner of faults could be switched in and out from the trainers rostrum, including a primary and complementary colour recognition eyesight test. Anyone who failed got a special certificate, called a P45. __________________ -- Graham. G3ZVT

6th May 2018, 11:39 pm	#5
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Marconi 4701 replacement loudspeaker. The series connected twin loudspeaker system works very well delivering plenty sound as so it should considering the audio amplifier employs two BD116 power transistors. There is some intercarrier hum to attend to, can't blame the speakers for that. Now to move on to the vision circuits. The present tests are carried out with the EHT generator panel removed. The BRC 2000 series chassis employs slide in and out modules and was the easiest to service of all the dual standard colour sets made and sold between 1967 to 1970. All the others were not very nice to work on when it came to house visit service calls. DFWB.

8th May 2018, 3:32 pm	#6
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,928	Re: Marconi 4701 replacement loudspeaker. They were nice until any intermittent fault was 'cured' by running solder all along the edge connector and the panel forced in! __________________ Glyn www.gdelectronics.wales

9th May 2018, 3:17 pm	#7
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Marconi 4701 replacement loudspeaker. EHT problems in the Marconi 4701, it's too high and can't be adjusted. Tried the EHT tripler unit from the Ferguson 3700 and that one is useless, no EHT. As for the Marconi tripler, it's most likely the bleeder resistor in the tripler module has gone open circuit. Actually, the BRC 2000 circuit diagram shows the resistor as a VDR. Connects to the focus control and from the bottom end of focus pot to the base of the EHT voltage sensing transistor in the EHT generator module. DFWB.