G & E Bradley CT-471
 

Hello all.

I've had a look on the web, but can't find anything I'm after.

Does anyone have a PDF (or something downloadable) manual for the G & E Bradley CT-471?

Thanks.

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.

Re: G & E Bradley CT-471 - Manual PDF
 

Mine is the CT471C

It has the battery check as well as the centre zero.

Re: G & E Bradley CT-471 - Manual PDF
 

A couple of earlier threads:
https://www.vintage-radio.net/forum/...ad.php?t=55832
https://www.vintage-radio.net/forum/...ad.php?t=80219

Re: G & E Bradley CT-471 - Manual PDF
 

Further to Bills post, these are the three OC170 to change, the ones in a vertical row. I have the 471/471C manual but the circuits are printed folded into large sheets so can't really scan.

Andrew

Re: G & E Bradley CT-471 - Manual PDF
 

Another member's thread there has made me dig this out and take a look.

So, battery check appears to do something, + - and centre zero do not appear to be working in the way the manual suggests they should.

First image, battery check.

Second image, centre zero. I can't get the zero set to change that. The + or - functions either do the same as pictured, or oscillate around that setting or sit somewhere else on the scale at times.

Re: G & E Bradley CT-471 - Manual PDF
 

Might be worth asking the mods to put this the test gear section for further Q&As. I like these meters!

Andrew

Re: G & E Bradley CT-471 - Manual PDF
 

That's where it was, I hadn't realised it were moved, as I revisited it via searching my created threads.

But it's open and in bits. Just need to find a replacement for the OC171s and then discover what else may be wrong with it.

Re: G & E Bradley CT-471
 

Thread moved, title amended.

Re: G & E Bradley CT-471
 

Thanks Bill. Sinewave it was in the 'Wanted' section earlier, anyway RV20 sets the 'centre zero' zero but I guess there may be an issue down stream if that doesn't work. I stuck AF125 in one meter and GT332 (the Russian Ge ones) in the other.

Andrew

Re: G & E Bradley CT-471
 

Hi Andrew could you not photograph the relevant parts of the manual and post?

Re: G & E Bradley CT-471
 

Thank you.

Re: G & E Bradley CT-471
 

It doesn't appear to operate at all when I connect it up to a current or voltage source. I removed one of the OC170's (OC171 on the tin). All leads appear to be shorted, so I expect to find the rest like that. I'm looking at the three you mentioned in post 5, but it's taken so long for me to get the opportunity to take a look.

Re: G & E Bradley CT-471
 

Seems the calibration data for mod 5 onwards is like impossible to get, seem to be going in circles!!!

Re: G & E Bradley CT-471
 

What's mod 5?

Re: G & E Bradley CT-471
 

Mod 5 so I was told is the model with the FET in and no RV12.

Re: G & E Bradley CT-471
 

I was wondering whether mod meant modification, model or modulation.

Re: G & E Bradley CT-471
 

Testing another one of 'those' tin whiskery transistor shows the following on my DMM

Negative to base, testing to

• Emitter = 153kOhms
• Collector = 9.6kOhms

Positive to base, testing to

• Emitter = 13MOhms
• Collector = 47KOhms

Negative to Emitter, testing to

• Collector = 9MOhms

Positive to Emitter, testing to

• Collector = 150KOhms

Screen shows OC between BCE.

Interestingly my Peak Atlast component tester does indeed say there's a shunt between base and emitter (but doesn't suggest that's the case for the other pins). Another transistor was so much worse that the Peak wouldn't read it

Re: G & E Bradley CT-471
 

So, does anyone know of a replacement PNP transistor for this meter which has proved to work ok?

Also, whilst ordering parts, what other likely issues should I be looking for on the circuits? What else do you think I may need to look at to replace? I know some caps were mentioned in another thread, but no mention of which were the ones replaced.

I don't have an ESR meter, but I could fashion up an ESR tester with a scope and signal generator though.

Re: G & E Bradley CT-471
 

The spec of the OC117 is here, I hope it's correct

https://alltransistors.com/transisto...ansistor=44466

So I'm trying to find a replacement by matching as close as I can some of the parameters. I'm wondering if a silicon replacement would work, given that some of them require a slightly higher forward bias voltage drop.

Searching here and the rest of the internet for an equivalent brings up results for transistors no longer in production.