Ultra U930 - UK Vintage Radio Repair and Restoration Discussion Forum

dsergeant · 14th Mar 2017, 12:06 pm

This 1955 valve radio looks in reasonable condition, the previous owner seems to have been trying to fault find as all the knobs were in a bag inside and the loudspeaker leads have been unsoldered. The first fault is fairly obvious - the on/off switch, part of the volume control, is open circuit on both poles.

I have the service manual, courtesy of Paul's DVD. The volume control, 1Mohm log, is a bit unusual in that it has a centre tap which is fed to an RC filter network. Where on earth do I get a pot like that?... or shall I better fit a separate switch?

Dave

Herald1360 · 14th Mar 2017, 1:11 pm

That's a loudness control. There have been a few threads on how to get round the problem but the simplest way is to junk the network and just settle for a plain vanilla volume control.

Maybe you'll find that the switch responds to Deoxit or similar and some exercise or can be replaced by sacrificing another non tapped pot.

dsergeant · 14th Mar 2017, 5:22 pm

Thanks for the information, and having searched the site I now know how they are supposed to work. I have found a suitable pot at RS Components so will fit that and omit the loudness components.

Dave

dsergeant · 23rd Mar 2017, 11:09 am

Well the new pot sorted out that side of the problem. But it turned out there was quite a lot still wrong with it and initially apart from a low level hum nothing else worked. As is well known on these sets it was our infamous hunts capacitors, most of them cracked or low capacitance. Replacing all but one or two of those restored operation on MW/LW and it works well there with plenty of audio. But on FM although the RF and IF stages appear to work the audio is way down and only just about usable with the volume at maximum. I have checked everything around the discriminator (V4B, FM OFT3) and changed C39 2uF which was found zero capacitance but to no avail. What else should I be looking for?

Dave

ronbryan · 23rd Mar 2017, 1:42 pm

If you have a good UCC85, i would start by changing that valve.

Measuring the DC volts across C39 will give you an idea of the signal strength on FM. Does it improve with an external FM aerial?

Voltage checks on screen grids of the IF valves would be worthwhile.

Ron

dsergeant · 24th Mar 2017, 6:48 pm

Thanks Ron. I first cleaned the pins of the UCC85 and it improved FM a bit. Then found a new valve, branded Sentinel, which has improved it a bit more. Volume still down on MW/LW but quite usable - there is a bit of distortion which may or may not be the alignment but I am going to call it a day.

Dave

14th Mar 2017, 12:06 pm	#1
dsergeant Octode Join Date: Mar 2006 Location: Bracknell, Berkshire,UK. Posts: 1,172	Ultra U930 This 1955 valve radio looks in reasonable condition, the previous owner seems to have been trying to fault find as all the knobs were in a bag inside and the loudspeaker leads have been unsoldered. The first fault is fairly obvious - the on/off switch, part of the volume control, is open circuit on both poles. I have the service manual, courtesy of Paul's DVD. The volume control, 1Mohm log, is a bit unusual in that it has a centre tap which is fed to an RC filter network. Where on earth do I get a pot like that?... or shall I better fit a separate switch? Dave

14th Mar 2017, 1:11 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Ultra U930 That's a loudness control. There have been a few threads on how to get round the problem but the simplest way is to junk the network and just settle for a plain vanilla volume control. Maybe you'll find that the switch responds to Deoxit or similar and some exercise or can be replaced by sacrificing another non tapped pot. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

14th Mar 2017, 5:22 pm	#3
dsergeant Octode Join Date: Mar 2006 Location: Bracknell, Berkshire,UK. Posts: 1,172	Re: Ultra U930 Thanks for the information, and having searched the site I now know how they are supposed to work. I have found a suitable pot at RS Components so will fit that and omit the loudness components. Dave

23rd Mar 2017, 11:09 am	#4
dsergeant Octode Join Date: Mar 2006 Location: Bracknell, Berkshire,UK. Posts: 1,172	Re: Ultra U930 Well the new pot sorted out that side of the problem. But it turned out there was quite a lot still wrong with it and initially apart from a low level hum nothing else worked. As is well known on these sets it was our infamous hunts capacitors, most of them cracked or low capacitance. Replacing all but one or two of those restored operation on MW/LW and it works well there with plenty of audio. But on FM although the RF and IF stages appear to work the audio is way down and only just about usable with the volume at maximum. I have checked everything around the discriminator (V4B, FM OFT3) and changed C39 2uF which was found zero capacitance but to no avail. What else should I be looking for? Dave

23rd Mar 2017, 1:42 pm	#5
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,959	Re: Ultra U930 If you have a good UCC85, i would start by changing that valve. Measuring the DC volts across C39 will give you an idea of the signal strength on FM. Does it improve with an external FM aerial? Voltage checks on screen grids of the IF valves would be worthwhile. Ron

24th Mar 2017, 6:48 pm	#6
dsergeant Octode Join Date: Mar 2006 Location: Bracknell, Berkshire,UK. Posts: 1,172	Re: Ultra U930 Thanks Ron. I first cleaned the pins of the UCC85 and it improved FM a bit. Then found a new valve, branded Sentinel, which has improved it a bit more. Volume still down on MW/LW but quite usable - there is a bit of distortion which may or may not be the alignment but I am going to call it a day. Dave