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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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9th May 2022, 5:17 pm | #1 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Murphy V410
Hi all, please can I have some advice. I have two Murphy V410 televisions, that I intend to make into one working set. So far I have removed the mains filter capacitor, replaced the boost capacitor and re-stuffed the HT electrolytic. I checked the other HT de-coupling electrolytics and powered up the set via a variac with little results. I do have a little bit of activity around the top cap of the 30P4, but not much. Removing the eht rectifier makes no difference, neither does changing the 30P4 and U101. The HT is low, but nothing seems to be loading it excessively. I have taken the following readings. For 245Volts AC, I read 228 volts at the AC side of the metal rectifier and only 152 volts on the DC side with an HT current of 200ma measured at the HT fuse. Could the metal rectifier be at fault? I have checked the resistors around the LOP stage and they are all a bit on the high side, but not by a huge amount (max 15%).
Dave |
9th May 2022, 6:11 pm | #2 |
Octode
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
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Re: Murphy V410
You can fit a 1N4007 diode onto the existing metal rectifier, this will increase your HT but not excessively because the metal rectifier has internal resistance.
Your next objective is to see if the oscillator is giving you drive to the grid of the line output valve, this should be about -30 volts.
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9th May 2022, 7:34 pm | #3 |
Hexode
Join Date: Feb 2018
Location: Durham, County Durham, UK.
Posts: 324
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Re: Murphy V410
best to disconnect the metal rectifier and fit a 25ohm surge limiter [5watts][on the ac side] in series with an 1n4007 fit them on a bit of tag strip . then the ht will increase
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9th May 2022, 9:12 pm | #4 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
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Re: Murphy V410
Hi.
Agree in replacing the HT rectifier with a silicon type, having been working with Murphy TV sets since an apprentice the recommended limiter was a 39 ohm of 10 watts, the HT is a little too high with 25 to 27 ohms when fed from 240v on the 240 setting on the carousel. The fault could be a couple of things, firstly did you replace the 0.01uf coupling cap to the 30P4 G1? This can leak and effect the biasing of the valve, this results in low EHT and width. The line osc also could be well off frequency and of too fast will result in low drive and EHT. I'd expect around -20 to 30v drive on the 30P4 grid if low try adjusting the line hold. The other thing is the LOPT itself. Can you draw a spark from the U26 top cap?
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10th May 2022, 10:39 am | #5 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Re: Murphy V410
Hi There. Thank you all for the advice. To answer Trevor, yes, the 0.01uf capacitor to the 30P4 has been replaced. The free running line frequency is correct with the hold control centered. I'm reading -16 volts at the 30P4 grid, but considering the low HT, I think that's probably Ok. And yes, I can draw a spark from U26 top cap. I think the next step is to get the HT right, I'll order a 39ohm resistor today and see how that goes.
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14th May 2022, 4:34 pm | #6 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Re: Murphy V410
Good afternoon all. I've fitted a 39 ohm resistor and a 1N4007 and the HT is now correct. I've sorted out a few minor problems and changed several wax capacitors but I still have a major problem with the frame linearity (see picture). I have changed all the wax capacitors in the frame stage with the exception of C88 (arrowed in picture 2). The two linearity controls have an effect but both are at minimum resistance. Could the problem be C88? It's not an easy component to change which is why it's been left up to now. Any help is welcome
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14th May 2022, 5:35 pm | #7 |
Nonode
Join Date: Jun 2008
Location: Coningsby, Lincolnshire, UK.
Posts: 2,814
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Re: Murphy V410
I’d probably be looking at C85a, C85, c86, and C87. C88 looks like it could be to do with blanking?
Regards Lloyd |
14th May 2022, 9:12 pm | #8 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Murphy V410
Make sure you have identified each capacitor to the circuit. Some of these capacitors in Murphy receivers look like resistors with similar coloured banding and rounded ends.
As Lloyd has suggested, C85a the cathode bias electrolytic could be O/C. I presume you have changed the others. 100% linearity should be easily obtained with a V410. John. |
15th May 2022, 5:19 pm | #9 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Re: Murphy V410
I must pay more attention to what I’m doing.
I must pay more attention to what I’m doing. I must… sorry, the fault was mine. While re-capping the frame stage I fitted a .1uf capacitor (C86) instead of a .01uf in the linearity feedback circuit. Next job, the video and IF circuits. Thanks for all the comments and help. Dave ... |
15th May 2022, 5:53 pm | #10 |
Dekatron
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Location: Warnham, West Sussex. 10 miles south of DORKING.
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Re: Murphy V410
The only other component that might catch you out is the decoupling capacitor to pin 8 of the sound IF amplifier, .001uf C62 R&TV servicing but I think the Murphy manual show them as the same number. It used to go O/C upsetting the sound and causing havoc to the picture due to oscillation. It has no doubt been replaced years ago but worth a check. The 17" Mazda CME1702 looks good and improving. It was one of Mazda's better tubes. John.
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15th May 2022, 9:06 pm | #11 |
Octode
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Re: Murphy V410
Dave, how great it is to see one of these sets woken up! I'm really pleased to see your fantastic progress so far. Both LOPT and CRT ok then. Great stuff!
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Now where on earth did I remove that from? |
15th May 2022, 9:36 pm | #12 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Re: Murphy V410
Hi Andy (af024), yes, LOPT and CRT both Ok, I chose the cleanest of the two sets to work with. The CRT of the second set also seems to be Ok but the line transformer looks like it has been leaking because the chassis is very oily. I'll finish changing the wax capacitors in the first set and see how it runs. I've stripped back the varnish from the best cabinet, but not re-varnished it yet - watch this space.
Dave |
16th May 2022, 11:19 am | #13 |
Octode
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Re: Murphy V410
I shall be regularly tuning in with much interest Dave. Thank you for sharing your progress.
Kind regards, Andy
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Now where on earth did I remove that from? |
22nd May 2022, 8:44 pm | #14 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Re: Murphy V410
I think I've just had a major setback. I had almost finished the re-cap job and switched on to for a test and was met with a blank screen. After spending some time examining my recent work I started to look elsewhere and I think I have a duff line transformer. The drive to the 30P4 is -30 volts but I can only draw a quarter inch spark from the top cap of the EHT rectifier, and this is with the rectifier removed from the circuit. I have checked the boost capacitor and the 'S' correction capacitor. The HT is 210volt and the HT current draw is 240mA. Is there anything else I should check before condemning the transformer?
Dave |
22nd May 2022, 9:08 pm | #15 |
Dekatron
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Location: Kilmarnock, Ayrshire, UK.
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Re: Murphy V410
Make sure you haven't disturbed a connection to the scan coils or the width control. The LOPT doesn't usually fail in that way.
Re the frame fault check R120 the cathode resistor on the frame output valve.
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Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member |
23rd May 2022, 11:02 am | #16 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Re: Murphy V410
Thanks for the reply Trevor, I have checked the connections to the width, lin and scan coils and they are intact. I was working on the IF and video circuits when the fault occurred, so I don't think anything I've done (this time). The boost volts have always been low (380v), but the width was about right so I ignored it. Also, thinking about it now, when I first woke the set up from it's sleep, I had picture 'ballooning' with increased brightness which just... got better. The other odd thing I noticed is that I have never been able to draw a spark from the top cap of the 30P4. The frame problem was down to me. See post 9.
Dave |
23rd May 2022, 11:27 am | #17 |
Dekatron
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Location: Warnham, West Sussex. 10 miles south of DORKING.
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Re: Murphy V410
Just a thought. Check the heater leads to the U26 EHT rectifier. They have been known to short together and damp the line output stage.
As Trevor has mentioned, It's very unusual for the LOPT to fail in this manner but it is 65 years old! If you have any spare 405 lopts, disconnect the 30P4 and U191 top caps and connect the valves to the test transformer. Do not disconnect anything else. If the LOPT is faulty you should get EHT from the test transformer and if it has an EHT rectifier attached it will probably light up. John. |
23rd May 2022, 11:58 am | #18 |
Dekatron
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Location: Warnham, West Sussex. 10 miles south of DORKING.
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Re: Murphy V410
As an example I am using a Ferguson 991T, top caps of PL81 and PY81 disconnected and with croc clips the valves have been connected to the respective connections on the test transformer. Some EHT is produced, the EHT rectifier lights up.
You could use your other V410 LOPT. No need to disconnect it, just jump the top cap leads over. John. |
24th May 2022, 10:24 am | #19 |
Pentode
Join Date: Jul 2010
Location: Nottingham, Nottinghamshire, UK.
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Re: Murphy V410
John, that's just brilliant. I take my hat of your ingenuity. I checked for a short in the rectifier heater winding - no short. I then fitted the LOPT from the second V410 with no results. The windings were all within a ohm of the published values but I couldn't draw anything from the EHT output. I'll have a second go with the first transformer, but I'm almost sure it's faulty.
Dave. |
24th May 2022, 11:00 am | #20 |
Dekatron
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Location: Warnham, West Sussex. 10 miles south of DORKING.
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Re: Murphy V410
An odd one. If you have -drive on pin 5 grid of 30P4 and voltage on the screen grid pin 4 I would have thought you would have had a degree of timebase operation, either overheating 30P4 [red anode] or a strained line whistle.
Try with the top cap of the U191 disconnected. You may have a new boost cap faulty, unlikely but you never know. If it is S/C the line timebase will liven up. Make sure you have not moved the voltage select carousel on the back. These used to get accidently moved or fiddled by children giving all sorts of odd symptoms. I can't believe both transformers give the same odd result. John. PS The test transformer used is from a mid 50s Ultra 1750 but any post 1953 transformer will do as test. |