It is currently Wed Dec 13, 2017 12:07 pm

Marconi 4701. 2000 series colour. My greatest restoration p

 
Posts: 1553
Joined: Thu Aug 30, 2012 6:43 pm
Location: Wakefield, West Yorkshire.

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by marc » Fri Dec 25, 2015 5:24 pm

Till Eulenspiegel wrote:Hi Everyone,
I'm sorry to only offer up bad news, the set has let the side down. .

Till Eulenspiegel.

Hi Till,

After all that hard work too, what a shame.
Maybe you'll catch the repeat this evening.

Marc.

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Fri Dec 25, 2015 6:15 pm

I set about at once to find what was causing the no luma fault. Instead of volt meter tests the scope will be used. No video present at the base of emitter follower VT3. My attention was drawn to the luma delay line transistor VT1 but as soon as the probe was connected up the picture reappeared. However, another fault has manifested itself, no colour, almost certainly the sub-carrier frequency needs resetting.
What is most likely to be causing all the new problems will be those carbon composition resistors are changing in value. more tests called for.
The brightness control sets the sync tip level at the anode of video DC restorer diode W1. Under the fault condition the voltage was 3 to 4volts. After the fault was cleared it became the correct figure of 8 to 9 volts.

Till Eulenspiegel.

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Fri Dec 25, 2015 8:06 pm

Minutes after submitting my last post how about this? Only two hours twenty minutes after the 3pm deadline. From BBC News: An excellent picture of Her Majesty the Queen.

Till Eulenspiegel.
Attachments
4701_2000-1.jpg

 
Posts: 1553
Joined: Thu Aug 30, 2012 6:43 pm
Location: Wakefield, West Yorkshire.

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by marc » Fri Dec 25, 2015 8:18 pm

As the saying goes....better late than never ! :qq1
Well done Till :aad

Marc.

 
Posts: 2986
Joined: Thu Jun 28, 2012 12:20 pm
Location: The Vale Of York

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Katie Bush » Fri Dec 25, 2015 8:31 pm

Electrical wrote:The Queens speech is repeated on BBC2 at 18:20, so you may have time to sort it out.
Regards Stan.


That's actually a fair suggestion, and since the Queen's Christmas Message is pre-recorded anyway, it's not quite as bad as missing a live event.

It is nonetheless a disappointment for you, and given the time and effort you've expended on the set, you deserved better.. I'll bet you could have left that telly running all night without a hiccup, but because you turned it off, and gave it cold start, it was too much stress for the old circuits.

I recall having similar 'fun' with several G6, a Bush CTV25 and funnily enough, a GEC 2028.. Not all at the same time, but scattered over the space of a couple of years, but you could guarantee if you left them to soak, they'd never fail - start them up from cold? :aao

How about a fall back position? Could I suggest the BBC's New Year annual review? At least there will be footage of all the major news events of 2015.

Marion

 
Posts: 79
Joined: Wed Apr 08, 2015 2:52 pm
Location: Newcastle Upon Tyne

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Derren » Fri Dec 25, 2015 9:22 pm

Well done! All in the name of Queen and country. (and quite a few interested eccentrics perhaps) :)

 
Posts: 324
Joined: Fri Aug 24, 2012 7:32 pm
Location: Swords, Dublin

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by jayceebee » Sat Dec 26, 2015 12:29 pm

A commendable and valiant effort Till. Lets not forget that the Queen in the late 60's would have been watching TV on a set fitted with the same chassis, in fact probably the same model as in the Christmas banner at the top of the page.

John.

EDIT

The banner has just changed but the set is still there, second in from the left.

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Sat Dec 26, 2015 1:12 pm

The cause of the no picture fault was a broken resistor on the video board. The resistor was R13, it is connected across the DC restorer diode W1. It is a special type with tags fitted at the ends which can function as meter or scope test points. One end is TP1 which is used to measure the DC level from the brightness control. The other end of R13 can be used to check the video waveform present at the base of the emitter follower transistor VT3.
The fault was an easy fix. The oscilloscope probe had been connected to R13 and what had happened the end cap had been pulled off the resistor. This most likely happened on Christmas eve when the set was repositioned for the photos.
It was a simple matter to refit the end cap to the resistor. This will not be a reliable repair so later today the resistor will be replaced.
R13 is 22Kohms in early boards and 39K in the later 235 boards.

Till Eulenspiegel.

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Sun Dec 27, 2015 1:36 pm

The attachment shows the damaged resistor R13. It has been replaced, the test point tags have been retained by forming the lead out wires.

Till Eulenspiegel.
Attachments
4701_2003-1.jpg

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Sat Jan 09, 2016 1:04 pm

Terrykc wrote:
Till Eulenspiegel wrote:The BF167/BF196 has special properties as it is operated in a "forward" AGC system. Max gain is set by an emitter current of 3.9 to 4mA. An increase of emitter current reduces the gain of the amplifier.
No, Til, these are not special properties! They are standard properties of any RF transistor used in gain controlled stages!

We tended to grow up with transistor radios which used reverse AGC but it just won't work in a TV because reverse biassing a transistor pushes it into the non-linear part of its characteristic. Fine for the single signal narrow bandwidth signals found in a radio but useless where several signals are involved as it causes cross modulation which makes the signal unusable - just consider how many individual signals there might be at any one time in the wide bandwidth of a vision signal - even more in a tuner, where the sound carrier is also present.

With forward AGC , the transistor operates in the linear portion of its characteristic at all times - only the gain changes. Any RF transistor used in a gain controlled stage - including those hidden inside Integrated Circuits - must work in this fashion.

Re: vision AGC. I'm trying to sort out a Pye TT1 all transistor TV set which was made in 1961. Got problems with the line output stage to sort out.
I will confirm later on if this set employs reverse AGC. The vision IF has four stages using OC171 transistors, it's likely the AGC follows normal transistor radio practise.
Read bout the restoration of two Pye TT1 TVs here:
http://www.vintage-radio.net/forum/show ... hp?t=85972

Till Eulenspiegel.

 
Posts: 19
Joined: Mon Feb 08, 2016 8:38 am
Location: Neath, South Wales.

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Tech12 » Thu Feb 11, 2016 6:06 pm

Brilliant thread, great to read & a fantastic restoration :thumb

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Fri Feb 12, 2016 10:31 am

A certain number of jobs are still needed to be done before the restoration is considered complete. The original back cover is well degraded. A replacement could be made, however, there is a chance that a much better one might be available. Also, although not essential it might be a good idea to introduce pin cushion raster correction, the PCB artwork allows this to be done. Even more desirable in 25" models.
Finally, a stand should be made for the Marconi. That's not going to be an easy task because of the front legs are shaped to follow the profile of the front corners of the cabinet. The HMV cabinet is similar.

Two spare 2000 series colour decoders need repair. Both have missing ident coils.

Till Eulenspiegel.

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Sun Apr 10, 2016 3:54 pm

The Mullard book PAL COLOUR TV describes how to make the ident coil. Coil L4 is wound on a 1/4" S.R.B.P. tube with a ferroxcube core type FX1068 mounted in an adjustable sleeve.
No mention the method of winding the coil whether if it is wavewound or just pie wound.

Till Eulenspiegel.

 
Posts: 57
Joined: Sun Mar 30, 2014 6:23 pm
Location: Eindhoven, NL

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Jac Janssen » Sun Apr 10, 2016 4:18 pm

Hi Till,

I don't think that the winding method matters, if I assume the correct coil you refer to.
The coil is parallel to 23.5 nF of capacitors, and the working frequency is 7.8 kHz, so the inter-winding capacitance is of little influence here.
So I think that wild winding or layer winding is fine for this.
I'm not sure though what the winding gears mean....
S.W.G. 42 equals 0.1 mm if I am correct.

What is S.R.B.P. ?

Jac
Last edited by Jac Janssen on Sun Apr 10, 2016 4:24 pm, edited 1 time in total.

 
Posts: 5844
Joined: Sat Jul 23, 2011 8:19 pm

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by crustytv » Sun Apr 10, 2016 4:23 pm

Jac Janssen wrote:What is S.R.B.P. ?

Jac

An acronym for Paxolin

 
Posts: 57
Joined: Sun Mar 30, 2014 6:23 pm
Location: Eindhoven, NL

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Jac Janssen » Sun Apr 10, 2016 4:25 pm

CrustyTV wrote:
Jac Janssen wrote:What is S.R.B.P. ?

Jac

An acronym for Paxolin


Thanks!
Jac

 
Posts: 2665
Joined: Sun Sep 01, 2013 6:19 pm
Location: Behind the sofa

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Cathovisor » Sun Apr 10, 2016 4:34 pm

Jac Janssen wrote:What is S.R.B.P. ?

Synthetic Resin Bonded Paper.

See http://www.tufnol.com/materials-full/pa ... -full.aspx

 
Posts: 57
Joined: Sun Mar 30, 2014 6:23 pm
Location: Eindhoven, NL

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Jac Janssen » Sun Apr 10, 2016 5:00 pm

Thanks Cathovisor!
Jac

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Sun Apr 10, 2016 5:22 pm

Jac Janssen wrote:Hi Till,

I don't think that the winding method matters, if I assume the correct coil you refer to.
The coil is parallel to 23.5 nF of capacitors, and the working frequency is 7.8 kHz, so the inter-winding capacitance is of little influence here.
So I think that wild winding or layer winding is fine for this.
I'm not sure though what the winding gears mean....
S.W.G. 42 equals 0.1 mm if I am correct

Jac

Hello Jac,
It is fortunate that the 7.8Khz coil in the BRC is also tuned by a 23.5nF capacitor, same the Mullard circuit, actually two series connect 47nF capacitors. The BRC ident circuit differs from the Mullard design is some respects. In the BRC the colour killer rectifier is supplied from the junction of the two tuning capacitors and the PAL switch ident pulse comes from the collector of the 7.8Khz amplifier. In the Mullard circuit the junction of the capacitors supplies the ident pulse and the killer rectifier is supplied from the collector of the amplifier.

According to my calculations it will have to be 42 meters of wire for the coil. Seems a lot!

Till Eulenspiegel.

 
Posts: 57
Joined: Sun Mar 30, 2014 6:23 pm
Location: Eindhoven, NL

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Jac Janssen » Sun Apr 10, 2016 5:38 pm

Hello Till,

I can imagine that "any" other coil of c. 17.7 mH will do the job as well (if my calculation is correct).
Or something in that region and adapt / parallel some capacitors to get the resonance right.
Or two coils in series to get the value right... When one is adjustable, it makes things easier.

Still, if you have a suitable coil former with an adjustable core, winding might be the faster job.
If I am looking at the right L4, the # of turns is 1700.
Do you have a coil winder?

Jac

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Sun Apr 10, 2016 5:45 pm

The GEC 2040 series single standard colour TVs has an ident circuit which is save for a few component value differences is similar to the Mullard circuit. Two series connected 0.047mfd capacitors tune the 7.8Kc/s coil. The Decca 10 series "Bradford" uses a single 0.1mfd capacitor, this will result in considerably less turns on the ident coil.

Till Eulenspiegel.

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Sun Apr 10, 2016 5:54 pm

Jac Janssen wrote:Hello Till,

Still, if you have a suitable coil former with an adjustable core, winding might be the faster job.
If I am looking at the right L4, the # of turns is 1700.
Do you have a coil winder?

Jac

Hello Jac,
It's likely there will be a 1.7mH or thereabouts "off the shelf" inductor available. It is essential that the ident coil is peaked up at 7.8Khz because off set tuning will result in mistiming of the PAL switch. High Q tuned circuit.
It is L4 in the Mullard circuit.
I think it's about time I acquired a coil winder. ebay sellers have such items.

Till Eulenspiegel.

 
Posts: 209
Joined: Mon Jun 04, 2012 11:40 am
Location: Newcastle on Tyne

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Ed Dinning » Sun Apr 10, 2016 7:05 pm

Hi David, plenty of wire, SRBP tube and a Q meter over here if you need it.

Ed

 
Posts: 2839
Joined: Tue Jul 26, 2011 5:30 pm
Location: Gateshead

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by Till Eulenspiegel » Mon Apr 11, 2016 12:40 am

Hi Ed,
The Q meter will save a lot of time setting up the precise resonant frequency of the L C tuned circuit. The Decoder can be tested outside of the set. Actually there is a number of other damaged coils in the decoders, again the Mullard book will assist the construction of these. The BRC 2000 doesn't differ that much from the Mullard design.

Till Eulenspiegel.

 
Posts: 5844
Joined: Sat Jul 23, 2011 8:19 pm

Re: Marconi 4701. 2000 series colour. My greatest restorati

Post by crustytv » Mon Apr 11, 2016 9:29 pm

Hi David,

I really hate to spoil your fun but I have found what you need (see below).

Of course if you wish to continue making one as I know you're the equivalent of McGyver to the vintage TV world and get a kick out of making things, then I will return it to stock.

But its yours if you want it and save yourself the hassle.

2KL4.jpg

PreviousNext

Return to Colour Television



Who is online

Users browsing this forum: No registered users and 1 guest