G & E Bradley CT-471C Calibration Problem - UK Vintage Radio Repair and Restoration Discussion Forum

Jeffreyabb · 8th Jan 2014, 12:00 am

Hi
Sorry, but thus is a bit of a long-winded request for help.
Whilst I was waiting for parts to make up the R F probe leads, I decided to check the meter to see if all was OK. Put batteries in, probe to Cal check, only 6V instead of 12V. I left it swiched on for a good while & came back to it, the reading was getting moving up the scale. A set of (cheap) batteries later, it was 12V. I switched off & left it overnight, switched on, cal only 8V, so I opened it up. The 2.5V was a steady 2.51V, so OK there. Checked the resistors in the Cal voltage divider, all pretty close, so I thought that the two caps in the 20V supply were tha likely problem, so I changed them. I didn't have 25uF to hand, so I used 33uF. The spec on the caps was +100%, so Iwasn't worried about that.) That made a considerable improvement. Adjusted RV14 using my AVO DA116 to give 12V at the Cal point. Then I checked the meter and got around 11.5V. Very confusing for me. I could easily adjust the meter to read 12V, but it would it be correct? Time to do it properly, so I got the manual out and followed the procedure. The equipment spec calls for an AVO8, a CT471, or equivalent to be used to set the voltages. Well, I tried an old AVO8 of dubious accuracy, the DA116 and in desperation a Solarton 7060. They all ended up at the same place, with the CT471 being about .5V low comapared to the others. Now what is really bugging me is that the manual is pretty particular about that calibration point. It says that the cal point will only give an accurate reading with a meter which has an input resistance of greater than 100M. So why does the DA116 & the 7060 give a higher reading than the CT471 when they have only 10M input resistance?
I'm at a loss where to go now. Does anyone have any suggestions, in particular how to check the calibration of the meters on Volts? I think I have an old Weston Standard Cell somewhere in the attic which might be useful.
Regards
Jeff
G6CQH

glowinganode · 8th Jan 2014, 12:54 am

Hi Jeff, I've been round in circles like this too.
Firstly, I found my movement to be slightly off balance, so if you intend to use it vertically, it's best calibrated vertically. Also note the preset pots benefit from a good dose of contact cleaner and a good waggle. If you find it has a dead spot right where it needs to be set, add a suitable padder resistor between slider and one end to move the set point.
The 12 V t/p is fed from the 20 V supply via a voltage divider with quite a high source resistance.
Ultimately what matters is that the meter reads true, decide which meter you want to use as your reference and connect both it and the CT471 to the 12 V test point. Your reference meter will probably read around 11.85 V if it's 10 M. Adjust cal to get same reading on CT471 scale. Now disconnect reference meter and adjust 20 V supply to get FSD on 12 V range. Re-check a couple of times.

73's, Rob.

Jeffreyabb · 8th Jan 2014, 7:48 pm

Hi Rob
Thanks for the info & encouragement. I have spent some time on it this afternoon, with a little progress. I tend to use it horizontally and have worked on it like that. The presets aren't too bad, but I gave them a squirt of contact cleaner and exercised them as you said. Those two zero set pots are another thing entirely. Try to adjust it & it's either no change, slight change either way or big jumps either way. Almost but not entirely useless. The ohms pot isn't as bad as the set zero. And they're right down in the middle of the meter. tried putting some cleaner down the spindles. It made a bit of difference on the ohms one and slightly on the set zero, but they ended up behaving badly again. This is nothing new, it's been like this since I got the meter a few years ago. I need to change them to get any sort of reliability with this meter. If the zero jumps around then its a bit difficult to take a reading.
I decided to go through the cal procedure with the D116 as it agreed very closely with the 7060 and is much handier to use. After much faffing around, including one of the voltage divider resistors which was very slightly low, I get 2.5V on the 2.5 circuit, 22.7V into the 20V regulator (should be 23) and 12.1V on the 12V cal point. The meter itself is just about on 12V. I need a reliable set zero pot to get any better. The strange point about all this is that the 10Meg ohm test meters I have used consistently show higher voltages than the meter itself when they should be lower. Before I go any further I need to change the two zero pots,to eliminating the intermittently wandering zero. Something I've been trying to avoid since I got it. Thanks again for the help.
Jeff
G6CQH

glowinganode · 8th Jan 2014, 9:03 pm

Hi Jeff, it is possible to drill in situ a small (1/16") hole at the edge of the rear cover of the two "zero" pots to inject contact cleaner directly onto the wirewound track. Tedious and fiddly but achievable with care and totally cures the twitchiness.
Rob.

Jeffreyabb · 8th Jan 2014, 10:08 pm

Hi Rob
I did consider that. But decided against it because I didn't know just where to drill. I was considering the centre but thought that the spring might bear there. Thanks for that info. I'll have a go tomorrow.
Jeff

glowinganode · 8th Jan 2014, 11:51 pm

See attached.
Go gently and use a sharp drill by hand, held in a pin-chuck.

Good luck,
Rob.

RobinBirch · 9th Jan 2014, 10:59 am

Rob,
You mentioned getting some cleaner into the presets. Most of them appear to be completely enclosed. How would you recommend getting in to them?

Robin

glowinganode · 9th Jan 2014, 11:46 pm

Hi Robin,
I assume you mean the sort pictured below.
There is a small hole between the lug on the black top and the clear portion.
For the good of mankind, I have popped off the black top to show where to direct spray.

Cheers,
Rob.

RobinBirch · 11th Jan 2014, 7:53 pm

Exactly what I was after

Many thanks

Robin

Jeffreyabb · 13th Jan 2014, 1:01 am

Rob
I drilled holes in the two pots and got the cleaner in. The ohms pot responded well, but the zero pot didn't after several attempts. I decided to change both pots to get a stable zero. When I got them out, there was what looks like araldite sealing the terminals and between the front and back of the body. It didn't look like a factory job but the inside didn't look tampered with.
Several times I wished I hadn't started. Still in the end it was worth it. Both pots now operate smoothly & I have calibrated the DC volts as you suggested. Now I can get back where I started - to making the RF cable.
Thanks for all the help & encouragement.
Jeff

8th Jan 2014, 12:00 am	#1
Jeffreyabb Pentode Join Date: Apr 2009 Location: Durham, UK. Posts: 165	G & E Bradley CT-471C Calibration Problem Hi Sorry, but thus is a bit of a long-winded request for help. Whilst I was waiting for parts to make up the R F probe leads, I decided to check the meter to see if all was OK. Put batteries in, probe to Cal check, only 6V instead of 12V. I left it swiched on for a good while & came back to it, the reading was getting moving up the scale. A set of (cheap) batteries later, it was 12V. I switched off & left it overnight, switched on, cal only 8V, so I opened it up. The 2.5V was a steady 2.51V, so OK there. Checked the resistors in the Cal voltage divider, all pretty close, so I thought that the two caps in the 20V supply were tha likely problem, so I changed them. I didn't have 25uF to hand, so I used 33uF. The spec on the caps was +100%, so Iwasn't worried about that.) That made a considerable improvement. Adjusted RV14 using my AVO DA116 to give 12V at the Cal point. Then I checked the meter and got around 11.5V. Very confusing for me. I could easily adjust the meter to read 12V, but it would it be correct? Time to do it properly, so I got the manual out and followed the procedure. The equipment spec calls for an AVO8, a CT471, or equivalent to be used to set the voltages. Well, I tried an old AVO8 of dubious accuracy, the DA116 and in desperation a Solarton 7060. They all ended up at the same place, with the CT471 being about .5V low comapared to the others. Now what is really bugging me is that the manual is pretty particular about that calibration point. It says that the cal point will only give an accurate reading with a meter which has an input resistance of greater than 100M. So why does the DA116 & the 7060 give a higher reading than the CT471 when they have only 10M input resistance? I'm at a loss where to go now. Does anyone have any suggestions, in particular how to check the calibration of the meters on Volts? I think I have an old Weston Standard Cell somewhere in the attic which might be useful. Regards Jeff G6CQH

8th Jan 2014, 12:54 am	#2
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: G & E Bradley CT-471C Calibration Problem Hi Jeff, I've been round in circles like this too. Firstly, I found my movement to be slightly off balance, so if you intend to use it vertically, it's best calibrated vertically. Also note the preset pots benefit from a good dose of contact cleaner and a good waggle. If you find it has a dead spot right where it needs to be set, add a suitable padder resistor between slider and one end to move the set point. The 12 V t/p is fed from the 20 V supply via a voltage divider with quite a high source resistance. Ultimately what matters is that the meter reads true, decide which meter you want to use as your reference and connect both it and the CT471 to the 12 V test point. Your reference meter will probably read around 11.85 V if it's 10 M. Adjust cal to get same reading on CT471 scale. Now disconnect reference meter and adjust 20 V supply to get FSD on 12 V range. Re-check a couple of times. 73's, Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

8th Jan 2014, 7:48 pm	#3
Jeffreyabb Pentode Join Date: Apr 2009 Location: Durham, UK. Posts: 165	Re: G & E Bradley CT-471C Calibration Problem Hi Rob Thanks for the info & encouragement. I have spent some time on it this afternoon, with a little progress. I tend to use it horizontally and have worked on it like that. The presets aren't too bad, but I gave them a squirt of contact cleaner and exercised them as you said. Those two zero set pots are another thing entirely. Try to adjust it & it's either no change, slight change either way or big jumps either way. Almost but not entirely useless. The ohms pot isn't as bad as the set zero. And they're right down in the middle of the meter. tried putting some cleaner down the spindles. It made a bit of difference on the ohms one and slightly on the set zero, but they ended up behaving badly again. This is nothing new, it's been like this since I got the meter a few years ago. I need to change them to get any sort of reliability with this meter. If the zero jumps around then its a bit difficult to take a reading. I decided to go through the cal procedure with the D116 as it agreed very closely with the 7060 and is much handier to use. After much faffing around, including one of the voltage divider resistors which was very slightly low, I get 2.5V on the 2.5 circuit, 22.7V into the 20V regulator (should be 23) and 12.1V on the 12V cal point. The meter itself is just about on 12V. I need a reliable set zero pot to get any better. The strange point about all this is that the 10Meg ohm test meters I have used consistently show higher voltages than the meter itself when they should be lower. Before I go any further I need to change the two zero pots,to eliminating the intermittently wandering zero. Something I've been trying to avoid since I got it. Thanks again for the help. Jeff G6CQH

8th Jan 2014, 9:03 pm	#4
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: G & E Bradley CT-471C Calibration Problem Hi Jeff, it is possible to drill in situ a small (1/16") hole at the edge of the rear cover of the two "zero" pots to inject contact cleaner directly onto the wirewound track. Tedious and fiddly but achievable with care and totally cures the twitchiness. Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

8th Jan 2014, 10:08 pm	#5
Jeffreyabb Pentode Join Date: Apr 2009 Location: Durham, UK. Posts: 165	Re: G & E Bradley CT-471C Calibration Problem Hi Rob I did consider that. But decided against it because I didn't know just where to drill. I was considering the centre but thought that the spring might bear there. Thanks for that info. I'll have a go tomorrow. Jeff

8th Jan 2014, 11:51 pm	#6
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: G & E Bradley CT-471C Calibration Problem See attached. Go gently and use a sharp drill by hand, held in a pin-chuck. Good luck, Rob. Attached Thumbnails __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

9th Jan 2014, 10:59 am	#7
RobinBirch Hexode Join Date: Apr 2003 Location: Cotswolds, UK. Posts: 465	Re: G & E Bradley CT-471C Calibration Problem Rob, You mentioned getting some cleaner into the presets. Most of them appear to be completely enclosed. How would you recommend getting in to them? Robin

9th Jan 2014, 11:46 pm	#8
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: G & E Bradley CT-471C Calibration Problem Hi Robin, I assume you mean the sort pictured below. There is a small hole between the lug on the black top and the clear portion. For the good of mankind, I have popped off the black top to show where to direct spray. Cheers, Rob. Attached Thumbnails __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

11th Jan 2014, 7:53 pm	#9
RobinBirch Hexode Join Date: Apr 2003 Location: Cotswolds, UK. Posts: 465	Re: G & E Bradley CT-471C Calibration Problem Exactly what I was after Many thanks Robin

13th Jan 2014, 1:01 am	#10
Jeffreyabb Pentode Join Date: Apr 2009 Location: Durham, UK. Posts: 165	Re: G & E Bradley CT-471C Calibration Problem Rob I drilled holes in the two pots and got the cleaner in. The ohms pot responded well, but the zero pot didn't after several attempts. I decided to change both pots to get a stable zero. When I got them out, there was what looks like araldite sealing the terminals and between the front and back of the body. It didn't look like a factory job but the inside didn't look tampered with. Several times I wished I hadn't started. Still in the end it was worth it. Both pots now operate smoothly & I have calibrated the DC volts as you suggested. Now I can get back where I started - to making the RF cable. Thanks for all the help & encouragement. Jeff