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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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14th Nov 2017, 12:24 am | #1 |
Diode
Join Date: Aug 2014
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 5
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Murphy V240C.
Hi everyone this is my first post on here. I am interested in any information or experiences anyone may have had with this particular TV. I have recently acquired one and have restored it to a reasonable working condition. I am hoping to post some photos on here soon.
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14th Nov 2017, 10:35 am | #2 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: Murphy V240C.
V240/250. Murphy's first purpose built Band 3 model.
Not one of the easiest receivers to overhaul but if it is dusty and dry you are 50% of the way to success. Weak points. The tube is a Mazda CRM141/142 circular 14" that was not one of Mazda's best. The EHT rectifier a U25 is sealed within the oil filled line output transformer. It can be replaced by opening the sealed can but it is messy and needs a bit of ingenuity to seal it up again. The chassis has many capacitors that will need replacement including some 'that look like resistors'. In spite of the above the receiver may have just been set aside in working order when it became simply obsolete. All the faults now present simply due to age related deterioration. The model like all Murphys, does give a very good picture. Due to the cost of replacing the CRT and LOPT [that usually failed after replacing the tube] these receivers were shunned by guys that overhauled for reconditioned sales. It was not until the arrival of the V310 that Murphy were once again loved. The spares lady at Murphy Radio was fantastic! She knew every part number from memory down to the tiny screws in the V310 lid assembly. I can still hear her voice. Good luck with this one. It will be interesting to watch it's resurrection. John. |
18th Nov 2017, 11:14 pm | #3 |
Diode
Join Date: Aug 2014
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 5
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Re: Murphy V240C.
Hi John you are quite right about it not being one of the easiest sets to work on. From the start I had a problem with the CRM141 CRT. I thought I was in luck when I noticed it was a rebuilt one, but on checking the emission found it had none at all The next 6 weeks were spent trying to coax some life from it The problem was the cathode connection was open circuit and this took a great deal of time to sort out. I tried to weld the internal open circuit cathode connection by applying the EHT overwind voltage from another TV to grid 1 of the CRT and connecting the cathode to chassis. This failed to work and after many attempts using different combinations of heater voltages and tapping the neck of the CRT I gave up on this method. I took the decision to try and short the heater to cathode to get a cathode connection this way and then run the heater from an isolation transformer.
With nothing to lose I connected the overwind to grid 1 with both heater connections grounded. On powering up a fireworks display was seen in the CRT neck for a few seconds which then abruptly ceased followed by a distressed sound from the other TV supplying the high voltage. I deduced it was time to switch off. , Testing the CRT this time showed good emission. Unfortunately the heater cathode weld cannot be maintained and keeps going open circuit even after repeating the "welding procedure" many times. I can't get a permanent short However on the last attempt I have a situation where the short forms after the set has been on between 5 and 10 minutes. Once the picture is on it stays on and this happens every time the set is used so I have settled for this as I don't want to keep trying and eventually blow the heater open circuit. So that's just the CRT saga more to come. Simon |
19th Nov 2017, 12:55 pm | #4 |
Octode
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,990
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Re: Murphy V240C.
Use the line output valve top cap to create the short, it works better if the heater is powered, use an external power supply for this.
I used the same method to create a short on a GEC 6505A that an open cathode. Do not make a permanent connection with the anode power if using the powered heater method, just touch and hold for the seconds it takes. you may need an isolation transformer if using an external psu.
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Stephen _________"It`s only an old telly" ___ |
19th Nov 2017, 2:17 pm | #5 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: Murphy V240C.
This is an interesting problem. Is it known if the break to the cathode connection is definitely inside the confines of the tube ? Could it possibly in the base wiring outside the tube? If it is in the tube, why was that, were the welds hopeless?
Also, it would only require a tiny bit of heater cathode leakage for the now floating cathode to assume the average heater voltage, so it probably doesn't have to be a very solid connection once the heater is moved to an isolated supply. I once had a valuable CRT with an open heater, I could see the break in it looking through the glass, a horrible sight. |
19th Nov 2017, 5:36 pm | #6 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: Murphy V240C.
The problem with the Mazda tubes of the CRM141 type is a faulty spot weld. It went on for years! The 19" and 23" 'Gold Star' tubes from the mid 60's suffered O/C cathode connections to the disgrace of their makers.
I do have a Mazda 'New Life' CRM142 going spare with reasonable emission. You are welcome to it FOC but the problem is getting it to you. It cannot be posted or couriered. Maybe something can be worked out. Regards, John. |
21st Nov 2017, 11:41 pm | #7 |
Diode
Join Date: Aug 2014
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 5
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Re: Murphy V240C.
Might go for broke and try the line output valve anode as the voltage source to break down the heater cathode insulation now that the line output stage is working, will have a go at the weekend time permitting Thanks everyone for you thoughts on this Simon
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18th Jan 2018, 10:07 pm | #8 |
Diode
Join Date: Aug 2014
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 5
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Re: Murphy V240C.
Hi everyone i regret that i have not had any time to sort out the problem with the crt due to many other issues . however i took some photos tonight to share with you
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19th Jan 2018, 11:29 am | #9 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,724
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Re: Murphy V240C.
What does the "picture quality" pot do, is it a white spot limiter?
What does the "balancing capacitor" do? I've not seen that before and I'm supposed to know about this stuff.
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-- Graham. G3ZVT |
19th Jan 2018, 11:41 am | #10 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Murphy V240C.
As I remember, sharpness, often a variable C in the video stage cathode.
I don't know what the balancing capacitor does. Lawrence. |
19th Jan 2018, 12:09 pm | #11 | |
Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Murphy V240C.
Quote:
I haven't got a definitive answer to this but I suspect it is a reference to C51 which is in the circuit across one half of the line scan coils. Adjusting it changes the balance of the flyback between the two halves of the line scan coils? The Trader sheet advises to adjust it for minimum striations on the LHS of the screen. |
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