R1155 Performance. - UK Vintage Radio Repair and Restoration Discussion Forum

JHGibson · 2nd Aug 2006, 1:33 pm

I finished restoring my R1155A and it appears to work fine in general but there are two aspects of it which may need work. One is that in the AVC mode, I cannot reduce the audio to zero with the volume control. The other is that the overall gain seems very high and the noise level is high though I can hear a 1 uV am signal fine. Is this the performance to be expected?
John.

GMB · 2nd Aug 2006, 1:57 pm

You have got the power supply corectly wired haven't you?

The HT- is not GROUND on the R1155, one of it's curious design features. By grounding the HT- you totally mess up the AGC.

JHGibson · 2nd Aug 2006, 5:12 pm

My power supply is correctly wired, I was warned about the HT- and earth problem. With no signal input, the standing bias on all the avc controlled valves is about -3 volts.

XTC · 3rd Aug 2006, 1:24 pm

The AF gain control is in series with a 100K resistor which connects to HT -ve. The resistor is easy to get at and it's on the tagboard behind the waveband control. I'm not quite sure what the purpose of the resistor was when the set was used in aircraft, but if it's been modded for use with a speaker, it's a nuisance. Short it out. As I recall, the volume control also changes the tone quite a lot and goes from bassy to tinny as the control is advanced. I recall there's a mod involving an electrolytic cap to stop that.

The sensitivity seems a bit high, but is that anything to worry about? I assume the set has the original valves and hasn't been modified to use B7G valves or something.

I've found that for some reason, 1155s are particularly good at picking up interference.

AGC problems can be caused by the resistors changing value and that's worth checking for.

There's also a 2K carbon stick resistor from HT -ve to the chassis. That may have been changed or have the wrong value.

I can't recall offhand what the AGC voltage range is, but if you are stuck I can measure it.

Is this an original set or one that's been modified in the past?

The other thing you may need to do is to set the IF traps to tune out any AM broadcast station too close to the 560KHz IF.

Pete.

JHGibson · 3rd Aug 2006, 4:31 pm

Pete,
Thank you for confirming that my R1155A is indeed working normally. I had thought that the inability to reduce the audio volume to zero was a design feature to prevent the radio operator from accidentally turning the volume knob to zero and hence missing incoming messages.

Looking at the circuit diagram, there seems to be a half-hearted attempt to bootstrap the junction of the bottom of the volume control and the 100K resistor, via .1u cap (C105) from the cathode of the audio amp. Possibly to increase the impedance loading the 2nd det and so improving selectivity somewhat. Anyway as you noted, to do a real bootstrapping job here would require an electrolytic in place of C105.

My set is completely original and the IF traps are working fine, A good job too as I have a "Radio Athlone" on 560 kcs only 5 miles away.

Others too have found the R1155 noisy. There was an article in Practical Wireless www.vintageradio.me.uk/military/ef50mods1155.htm on improving the noise performance. The article suggests replacing the RF and IF valves but replacing only the RF valve should do the job. One day when I get a round tuit, I might try a 6BA6 with an octal to B7G adaptor socket.
John.

XTC · 3rd Aug 2006, 5:45 pm

Quote:

Originally Posted by JHGibson

I had thought that the inability to reduce the audio volume to zero was a design feature to prevent the radio operator from accidentally turning the volume knob to zero and hence missing incoming messages.

That's a good point. Also, in a bomber with powerful piston engines, there'd be so much background noise that the minimum useful volume would be pretty high anyway, even with headphones.

Quote:

Originally Posted by JHGibson

My set is completely original and the IF traps are working fine, A good job too as I have a "Radio Athlone" on 560 kcs only 5 miles away.

Others too have found the R1155 noisy. There was an article in Practical Wireless www.vintageradio.me.uk/military/ef50mods1155.htm on improving the noise performance. The article suggests replacing the RF and IF valves but replacing only the RF valve should do the job. One day when I get a round tuit, I might try a 6BA6 with an octal to B7G adaptor socket.
John.

It's easy to short out the resistor discretely, but if it's a completely original set, it would be a shame to mess with it at all. There are plenty of codged ones about which have been used for every conceivable experiment and you feel less responsibilty for.

Resistors suddenly going O/C or behaving strangely as they warm up was a problem I noticed.

Pete.

JHGibson · 6th Aug 2006, 9:42 pm

A further note about my receiver restoration. Around the outside rim of the R1155 dial is a metal half-moon metal strip which sandwiches the plexiglas dial cover to the case. Between the two is a oversize gasket of matte white plastic, which has three sixteenths of width exposed on the inside. The material was badly cracked so I replaced it.

I thought it was only a stress relief device to prevent the mounting bolts from cracking the plexiglas. I have since heard that it is a logging system. The radio operator would make a pencil mark on the plastic by the dial pointer's position so that he could return to that station later.
John.

2nd Aug 2006, 1:33 pm	#1
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	R1155 Performance. I finished restoring my R1155A and it appears to work fine in general but there are two aspects of it which may need work. One is that in the AVC mode, I cannot reduce the audio to zero with the volume control. The other is that the overall gain seems very high and the noise level is high though I can hear a 1 uV am signal fine. Is this the performance to be expected? John.

2nd Aug 2006, 1:57 pm	#2
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,095	Re: R1155 Performance. You have got the power supply corectly wired haven't you? The HT- is not GROUND on the R1155, one of it's curious design features. By grounding the HT- you totally mess up the AGC.

2nd Aug 2006, 5:12 pm	#3
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: R1155 Performance. My power supply is correctly wired, I was warned about the HT- and earth problem. With no signal input, the standing bias on all the avc controlled valves is about -3 volts.

3rd Aug 2006, 1:24 pm	#4
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: R1155 Performance. The AF gain control is in series with a 100K resistor which connects to HT -ve. The resistor is easy to get at and it's on the tagboard behind the waveband control. I'm not quite sure what the purpose of the resistor was when the set was used in aircraft, but if it's been modded for use with a speaker, it's a nuisance. Short it out. As I recall, the volume control also changes the tone quite a lot and goes from bassy to tinny as the control is advanced. I recall there's a mod involving an electrolytic cap to stop that. The sensitivity seems a bit high, but is that anything to worry about? I assume the set has the original valves and hasn't been modified to use B7G valves or something. I've found that for some reason, 1155s are particularly good at picking up interference. AGC problems can be caused by the resistors changing value and that's worth checking for. There's also a 2K carbon stick resistor from HT -ve to the chassis. That may have been changed or have the wrong value. I can't recall offhand what the AGC voltage range is, but if you are stuck I can measure it. Is this an original set or one that's been modified in the past? The other thing you may need to do is to set the IF traps to tune out any AM broadcast station too close to the 560KHz IF. Pete.

3rd Aug 2006, 4:31 pm	#5
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: R1155 Performance. Pete, Thank you for confirming that my R1155A is indeed working normally. I had thought that the inability to reduce the audio volume to zero was a design feature to prevent the radio operator from accidentally turning the volume knob to zero and hence missing incoming messages. Looking at the circuit diagram, there seems to be a half-hearted attempt to bootstrap the junction of the bottom of the volume control and the 100K resistor, via .1u cap (C105) from the cathode of the audio amp. Possibly to increase the impedance loading the 2nd det and so improving selectivity somewhat. Anyway as you noted, to do a real bootstrapping job here would require an electrolytic in place of C105. My set is completely original and the IF traps are working fine, A good job too as I have a "Radio Athlone" on 560 kcs only 5 miles away. Others too have found the R1155 noisy. There was an article in Practical Wireless www.vintageradio.me.uk/military/ef50mods1155.htm on improving the noise performance. The article suggests replacing the RF and IF valves but replacing only the RF valve should do the job. One day when I get a round tuit, I might try a 6BA6 with an octal to B7G adaptor socket. John. Last edited by JHGibson; 3rd Aug 2006 at 4:41 pm.

6th Aug 2006, 9:42 pm	#7
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: R1155 Performance. A further note about my receiver restoration. Around the outside rim of the R1155 dial is a metal half-moon metal strip which sandwiches the plexiglas dial cover to the case. Between the two is a oversize gasket of matte white plastic, which has three sixteenths of width exposed on the inside. The material was badly cracked so I replaced it. I thought it was only a stress relief device to prevent the mounting bolts from cracking the plexiglas. I have since heard that it is a logging system. The radio operator would make a pencil mark on the plastic by the dial pointer's position so that he could return to that station later. John.