Bradley CT471 AC surge - UK Vintage Radio Repair and Restoration Discussion Forum

WME_bill · 8th Jan 2014, 12:52 pm

CT471. As we are talking about these meters, can anyone suggest the cause of a huge surge of the meter pointer off scale when switching from DC to AC or vice-versa. After a few seconds the meter comes back and works correctly . The problem is that there are two closed AC feedback loops. The DC level is changed slightly when you switch from DC to AC (as para 5.6.2) and the feedback altered. I have tried breaking each loop but that does not identify the problem.
The main meter feedback loop to SC3/3 has no capacitors in it, and sets the DC conditions for DC and AC gain separately. I have changed all the capacitors in the AC amplifier feedback loop around C15 ,C29 and C16, C26 without solving it.
I note that amplifier loop gain is lower on the 12mv range and then increased for the other DC ranges, and I can switch between the DC ranges without problems.
So now I am stuck. Suggestions welcomed. wme_bill m0wpn

glowinganode · 8th Jan 2014, 9:33 pm

Hi Bill, I had a similar fault which I put down to excessive offset current in the chopper input stage. I never did sort it out, as it had been stored in the damp and had all sorts of leakage around the bottom divider board which were compounding problems.
It's still under my bed drying out...

Rob.

8th Jan 2014, 12:52 pm	#1
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Bradley CT471 AC surge CT471. As we are talking about these meters, can anyone suggest the cause of a huge surge of the meter pointer off scale when switching from DC to AC or vice-versa. After a few seconds the meter comes back and works correctly . The problem is that there are two closed AC feedback loops. The DC level is changed slightly when you switch from DC to AC (as para 5.6.2) and the feedback altered. I have tried breaking each loop but that does not identify the problem. The main meter feedback loop to SC3/3 has no capacitors in it, and sets the DC conditions for DC and AC gain separately. I have changed all the capacitors in the AC amplifier feedback loop around C15 ,C29 and C16, C26 without solving it. I note that amplifier loop gain is lower on the 12mv range and then increased for the other DC ranges, and I can switch between the DC ranges without problems. So now I am stuck. Suggestions welcomed. wme_bill m0wpn

8th Jan 2014, 9:33 pm	#2
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: Bradley CT471 AC surge Hi Bill, I had a similar fault which I put down to excessive offset current in the chopper input stage. I never did sort it out, as it had been stored in the damp and had all sorts of leakage around the bottom divider board which were compounding problems. It's still under my bed drying out... Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.