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 Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 13th Apr 2019, 6:33 am   #1
petervk2mlg
Heptode
 
petervk2mlg's Avatar
 
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
Default 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
Attached Thumbnails
Click image for larger version

Name:	Philco 70 circuit.jpg
Views:	93
Size:	92.7 KB
ID:	181273  

Last edited by petervk2mlg; 13th Apr 2019 at 6:34 am. Reason: typo
petervk2mlg is offline  
Old 13th Apr 2019, 8:05 am   #2
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default Re: Philco 70 frustrating problem

Have you a spare 27 valve?
Boater Sam is offline  
Old 13th Apr 2019, 8:20 am   #3
MurphyNut
Heptode
 
MurphyNut's Avatar
 
Join Date: Apr 2016
Location: Croxley Green, Hertfordshire, UK.
Posts: 979
Default 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.
__________________
Clive
MurphyNut is offline  
Old 13th Apr 2019, 9:19 am   #4
petervk2mlg
Heptode
 
petervk2mlg's Avatar
 
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
Default 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.
petervk2mlg is offline  
Old 13th Apr 2019, 9:29 am   #5
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
Default 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
cathoderay57 is offline  
Old 13th Apr 2019, 9:55 am   #6
petervk2mlg
Heptode
 
petervk2mlg's Avatar
 
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
Default 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.
petervk2mlg is offline  
Old 13th Apr 2019, 11:46 am   #7
Silicon
Nonode
 
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
Default 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.
Silicon is offline  
Old 13th Apr 2019, 7:51 pm   #8
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Philco 70 frustrating problem

Check the earth wiper on the tuning condenser.
PJL is offline  
Old 14th Apr 2019, 4:26 am   #9
petervk2mlg
Heptode
 
petervk2mlg's Avatar
 
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
Default 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.
Attached Thumbnails
Click image for larger version

Name:	Philco 70 outboard tuning condenser.jpg
Views:	80
Size:	59.1 KB
ID:	181332  
petervk2mlg is offline  
Old 14th Apr 2019, 8:35 am   #10
vinrads
Dekatron
 
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,737
Default 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.
vinrads is offline  
Old 14th Apr 2019, 8:58 am   #11
petervk2mlg
Heptode
 
petervk2mlg's Avatar
 
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
Default 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.
petervk2mlg is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:20 am.


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.