UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 25th Sep 2015, 7:51 pm   #81
beery
Heptode
 
beery's Avatar
 
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
Default Re: Murphy V136C (1948)

Hi Mark,
I've had a look at the circuit and aligment instructions on trader sheet 1035.
I won't quote component refs as they are different on trader sheets.

However, I see from the alignment data that it was originally a double sideband set with the vision on the upper sideband. In 1956 when Crystal Palace replaced Alexandra Palace the upper sideband would not have been there anymore. So it could well be that the aim was to tune the vision circuits down to the lower sideband. This would have resulted in sound on vision problems and maybe also vision on sound. Hence modifications to try to cure it...

Anyway, most of the vision RF coils are straight tuned to 46.75MHz and also they are not the ones with the resistors across them (as is the case for the sound rf coils), so it would seem that the brass slugs are there to dampen the coil response and so have a wider bandwidth than would be normal for an rf coil.
I would not worry too much about slug length at this stage, but do try to put brass/ferrite ones where needed as who knows what response the aluminium ones will give. The important thing about core length is to make sure that a core is long enough to cover the length of the coil in question, but short enough so that when it is screwed completely free of the core it does not stick out more than a tiny bit from the coil former.

The instructions do indeed mention the use of a damping resistor (1K) and this is important.
Also, if a double is not peaking properly on the second core then it could be because the wrong peak was found in the first core, ie screwed in too far (a core screwed in too far will start to affect the other coil on the former). In this case unscrew both cores so that they begin to poke out of the top and bottom of the former respectively. Dampen the bottom coil with the 1K resistor and screw in the top core until you find the first peak. Now dampen the top coil and adjust the bottom core in the same way. Then repeat the tuning of the top and bottom cores using the damping resistor as before.

I hope this is some help.

Regards
Andy
beery is offline  
Old 25th Sep 2015, 8:22 pm   #82
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
Default Re: Murphy V136C (1948)

Quote:
However, I see from the alignment data that it was originally a double sideband set with the vision on the upper sideband. In 1956 when Crystal Palace replaced Alexandra Palace the upper sideband would not have been there anymore. So it could well be that the aim was to tune the vision circuits down to the lower sideband. This would have resulted in sound on vision problems and maybe also vision on sound. Hence modifications to try to cure it...

Thanks Andy, that may well be the reason it has been so hacked about!
I will check that the correct type of cores are fitted to all the coils before attempting a proper realignment.

I have checked all the coils for continuity, I am hoping none have been 'modified' by the bodger
I only get a vision signal from the Aurora via the preamp, going in at the receiver gives me nothing at all, although the signal generator does have an effect.

Mark
mark pirate is offline  
Old 25th Sep 2015, 9:02 pm   #83
Sean Williams
Dekatron
 
Sean Williams's Avatar
 
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,517
Default Re: Murphy V136C (1948)

Mark,

Brass slugs in a coil will tend to reduce inductance, and ferrite will increase inductance.

So Brass will cause the coil to resonate at a higher frequency, and ferrite at a lower frequency.

HTH
Sean
__________________
Engineers make things work and have spare bits when finished
Sean Williams is offline  
Old 26th Sep 2015, 2:21 am   #84
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: Murphy V136C (1948)

Quote:
Originally Posted by mark pirate View Post
When I got the set there was a capacitor inserted in L7 in place of the core
Just for fun I reinserted it and the sound came up!
I guess the metal plates of the capacitor altered the inductance of the coil just enough to tune it. The canonical non-ferrous core material is brass, but any non-magnetic, conductive material would reduce the inductance.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is offline  
Old 30th Oct 2015, 4:50 pm   #85
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
Default Re: Murphy V136C (1948)

Due to personal circumstances, the Murphy had to be sidelined for a month.
Getting back to it over the last weekend brought further progress, I found replacement cores and attempted a realignment.
This went well with the video side, I was now able to go in past the pre-amp and get a decent signal, although the sound has showed little improvement.

Then disaster, the frame output has died
I am suspecting the frame output coils, as these have always made quite a racket and never seemed quite right. A quick check with the meter does not reveal any O/C windings though, I guess I will have to remove the wiring for a proper check.

It seems 'Murphys law' has struck!

Mark
mark pirate is offline  
Old 30th Oct 2015, 6:33 pm   #86
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: Murphy V136C (1948)

Probably just a simple standard frame fault Mark. We tend to jump in the deep end and fear the worst when the answer is often a simple one. J.
Heatercathodeshort is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:08 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.