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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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13th Apr 2019, 6:33 am | #1 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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Philco 70 frustrating problem
Hi friends,
I have an American Philco model 70 to repair for a friend. The reported fault is that the radio will sometimes play for a while, then fade away. Sometimes it can be made to play again by switching the power off and on again. Also tuning the radio to a station can be difficult and after a brief second or so of programme material it will fade away and the whole process of turning off and turning on needs to be done again. Sometimes however the radio refuses to play and needs to be left alone for a while. I've noticed that when the radio falls silent, I can make it behave by adjusting the variable capacitor hot side of the first IF transformer. This is done with a 1/4" socket and can result in shorting to chassis which seems to jolt the radio into working. Another symptom is that when the radio falls silent the negative voltage on the grid of the 27 oscillator (measured from grid to cathode) disappears. When behaving normally, there is between 10 and 12 volts negative on the grid. So it seems that the valve is dropping out of oscillation. It puzzles me that shorting the plate voltage of the first IF valve should jolt the oscillator back into action, however. Another symptom I've noticed is that when the radio falls silent. I can faintly hear my strong local station on 1.4MHz even with the 27 oscillator valve out of the chassis. I'm guessing that at these times the radio is performing as a TRF albeit weakly. Moving my hand over the chassis forces the radio to become silent. Withdrawing my hand and the the weak station disappears. Also at these times a finger on the grid caps of the 24 valves lets me hear very faintly the morse airport beacon nearby which broadcasts on long wave on 242 KHz. The radio of course is only designed for MW. It has an IF of 260 KHz The valves used are: 24, 24, 27, 24, 24, 47, 80. I attach a portion of the schematic. Any ideas appreciated. Peter Last edited by petervk2mlg; 13th Apr 2019 at 6:34 am. Reason: typo |
13th Apr 2019, 8:05 am | #2 |
Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
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Re: Philco 70 frustrating problem
Have you a spare 27 valve?
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13th Apr 2019, 8:20 am | #3 |
Heptode
Join Date: Apr 2016
Location: Croxley Green, Hertfordshire, UK.
Posts: 979
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Re: Philco 70 frustrating problem
I have a radio like this, it has to be "re-booted" to get it going again, I've always thought it might be valve problem. I'll be watching this with interest.
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Clive |
13th Apr 2019, 9:19 am | #4 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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Re: Philco 70 frustrating problem
Boater Sam
Yeah I've tried two other 27 valves. I should add that all the paper caps and electrolytics and many of the resistors in this radio have already been changed. |
13th Apr 2019, 9:29 am | #5 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: Philco 70 frustrating problem
Try replacing the mica or ceramic oscillator coupling capacitors either on the osc grid, anode or both. I have known this to cause these symptoms. Although these types of capacitor are invariably robust and fault free, occasionally they can drift in capacity enough to affect level of oscillator injection. Cheers, Jerry
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13th Apr 2019, 9:55 am | #6 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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Re: Philco 70 frustrating problem
Thanks Jerry.
I’m also puzzled about an alignment issue. Adjusting the trimmer on the rear section of the gang doesn’t help with setting station position at the high end of the band. I guess if the oscillator isn’t running right there will be issues. |
13th Apr 2019, 11:46 am | #7 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
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Re: Philco 70 frustrating problem
The fading away symptom can be caused by a failure of the grid leak resistance to chassis.
It defeats the valve bias and can turn the valve off. Try earthing one grid at a time through a 1Meg resistor when the set had gone quiet. |
13th Apr 2019, 7:51 pm | #8 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Philco 70 frustrating problem
Check the earth wiper on the tuning condenser.
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14th Apr 2019, 4:26 am | #9 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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Re: Philco 70 frustrating problem
Thanks for all the replies. I spent ages on this finally thinking the gang has a problem.
I disconnected the three connections and just jumpered in another gang to check if this helped. As soon as I did that all the problems disappeared. Not elegant, but effective. Now I guess I'll have to remove the troublesome gang from the chassis and work out what the problem is. As PJL suggests perhaps the earth wiper. |
14th Apr 2019, 8:35 am | #10 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,737
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Re: Philco 70 frustrating problem
If it were me I would remove the earthing wiper's clean all parts where it touches re tension the wiper , re fit . Well done finding the problem I love a happy ending. Mick.
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14th Apr 2019, 8:58 am | #11 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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Re: Philco 70 frustrating problem
I've now removed the gang and tidied up all the grid and earth connections. This set has rubber grommets on the gang mounting points so one can't rely on a good earthed connection from the mounting bolts. I've checked the wiper connections and all is now tighter and cleaner.
With the rejuvenated gang back on the chassis the radio is now behaving normally and the dropping out has gone. |