UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - G & E Bradley CT-471

WME_bill · 9th Apr 2018, 9:51 pm

CT471 and CT471C.
There are two versions, one with the input stage of the amplifier using three transistors (BC108, OC202 and BSY27), up to serial BGB 2329, and the later version with a FET - 2N3819 and BC108, for the chopper input to the DC amplifier. The 471C has a centre zero position instead of battery test. Otherwise identical. CT471A is the AVO version, and quite different internally.
I will search out the manual. It is too large to post on the Forum, so send me your email by Private message.
Be aware, as you probably already are, that it uses three OC170 germanium transistors, which can suffer from the well known whisker problem. So be prepared to change some or all of them. The other Ge transistors, OC44, GET888, 2N1308, 2N1309 do not suffer from whiskers.
I have a couple of high speed ge diodes if you need those for the probes - CG91H or HG5002 or AAZ13 or GEX66.

I have a CT471 sitting under my bench awaiting this very repair. A straightforward task happily as the transistors are soldered to circuit pins and are very accessible.
I suspect a standard small signal pnp silicon will do, as the biasing is not complex apart from the chopper.

Incidentally, have you searched this forum. I seem to remember quite a lot of post some years ago, and I might have posted the circuits or something then.
wme_bill.

9th Apr 2018, 9:51 pm	#2
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: G & E Bradley CT-471 - Manual PDF CT471 and CT471C. There are two versions, one with the input stage of the amplifier using three transistors (BC108, OC202 and BSY27), up to serial BGB 2329, and the later version with a FET - 2N3819 and BC108, for the chopper input to the DC amplifier. The 471C has a centre zero position instead of battery test. Otherwise identical. CT471A is the AVO version, and quite different internally. I will search out the manual. It is too large to post on the Forum, so send me your email by Private message. Be aware, as you probably already are, that it uses three OC170 germanium transistors, which can suffer from the well known whisker problem. So be prepared to change some or all of them. The other Ge transistors, OC44, GET888, 2N1308, 2N1309 do not suffer from whiskers. I have a couple of high speed ge diodes if you need those for the probes - CG91H or HG5002 or AAZ13 or GEX66. I have a CT471 sitting under my bench awaiting this very repair. A straightforward task happily as the transistors are soldered to circuit pins and are very accessible. I suspect a standard small signal pnp silicon will do, as the biasing is not complex apart from the chopper. Incidentally, have you searched this forum. I seem to remember quite a lot of post some years ago, and I might have posted the circuits or something then. wme_bill.