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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 19th Nov 2018, 9:39 pm   #81
Sinewave
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Default Re: G & E Bradley CT-471

Quote:
If you can give me a list of all the things you may like me to check in one go I can, 'cause it's a right lump to get out, take apart and then put back together
Thanks, but out of ideas at the moment.

Perhaps there's still a problem in the amplifier board which isn't apparent?

DC gain perhaps?

The DC overall gain test is what I can't get past - but testing components hasn't thrown anything up yet.

On your chopper circuit do you have R79 and R80? I can't find it on mine, but there's a test procedure I've not been able to perform because I can't find it - maybe it's present on the 471 and not 471c?
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Old 20th Nov 2018, 1:49 am   #82
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Default Re: G & E Bradley CT-471

Went through as much as I can again. On the DC gain test, I need 54mV to read FSD, where it should be 40mV. Adjusting RV1 doesn't make a difference. Testing around RV1 and what it's connected to isn't showing anything.

Hmmmm.
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Old 20th Nov 2018, 10:49 pm   #83
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Default Re: G & E Bradley CT-471

Some more tests tonight from paragraph 5.6.3.

Chopper circuit test. Short the junction of R79 and R80. I couldn't find R80, so I shorted the R79 and was able to get movement using RV11 to take the scale away from zero and back to zero.

Checking the chopper waveform shows a squarewave with a sharp spike to the + peak which then slopes down to 0. Peak to peak is 4.72v, where it should be 2v.

So I need to find out why.
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Old 25th Nov 2018, 2:51 am   #84
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Default Re: G & E Bradley CT-471

R80 is next to R81, two 27K resistors on the board in between the backs of the Set Zero pot RV15 and the Ohms Zero pot. R79 on the chopper board of course. I have 27mV on the forward end of R79.

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Old 26th Nov 2018, 6:40 pm   #85
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Default Re: G & E Bradley CT-471

I've got 47mv one side and 4.5mv the other of R79.

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Old 26th Nov 2018, 7:10 pm   #86
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Default Re: G & E Bradley CT-471

I'm getting no where with this and a pile of other projects.

It may be time to shelve this one for the time being.
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Old 26th Nov 2018, 7:20 pm   #87
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Default Re: G & E Bradley CT-471

I guess that would depend on what the Set Zero pot is set to, I should have said. Shame you can't go further but understandable as these are such a complicated birds nest of wiring and switchery let alone all the different mod levels you almost need a working one alongside you to reference.

Just in case someone hasn't seem the insides of one...

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Old 26th Nov 2018, 7:39 pm   #88
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Default Re: G & E Bradley CT-471

I'm sure my one has been meddled with in the past as there is evidence that someone else has had a go.

Soldering iron marks, missing components, and a lot of the top caps for the RV pots are missing.
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Old 26th Nov 2018, 11:58 pm   #89
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Default Re: G & E Bradley CT-471

Look here http://www.richardsradios.co.uk/ct471.html
I had a similar problem a few years back the transistors were what I had to hand which had the same casing. I must admit I have not switched it on recently but hope it is still OK
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Old 27th Nov 2018, 9:32 pm   #90
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Default Re: G & E Bradley CT-471

I replaced the transistors and the meter started to settle better. I then replaced all electrolytic capacitors and the meter became functional, just reading low on all ranges.

I do have another one of these meters tucked away, I could try replacing the transistors in that one and see if it's any different, as the one open right now has probably many other faults which aren't apparent from basic testing of each component.

I could be easily missing something quite obvious though.
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Old 28th Nov 2018, 7:34 pm   #91
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Default Re: G & E Bradley CT-471

Quick addition/update.

I've been to the Avotorium and dug out my other CT471C.

It didn't do anything other than the battery check.

I replaced the three tin whiskery transistors with silicon transistors and the meter is working. A little out of spec here and there, but the readings are very very very close to what they should be.

Now, if I can probe about this one, I can maybe find out why unit 1 (the unit this thread is based on) is reading very very low, although reactive on most ranges (I've not yet tested AC ranges on any).
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Old 28th Nov 2018, 9:06 pm   #92
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Default Re: G & E Bradley CT-471

Interestingly, the Ge transistors I removed from unit 2 are not OC170s, they are CV8354's.

Testing them shows that they certainly are affected with some kind of fault, but no shorts to the can. Putting them on a Peak transistor tester shows all sorts of weird junctions, so certainly there is an issue. It's very good news indeed that I've got this working with silicon transistors.

Photos show correct waveform and voltage very close on the chopper circuit with the test probe on the 12V cal point, as per the calibration and set up procedure.
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Old 28th Nov 2018, 10:03 pm   #93
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Default Re: G & E Bradley CT-471

Good! It will useful having that worker to refer to. The CV8354 is an OC170 I think, one of mine, ex-RAF, had an AFZ12 in which is equivalent to an AF102-AF178.

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Old 1st Dec 2018, 8:36 am   #94
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Default Re: G & E Bradley CT-471

Indeed. With a good working unit I can make some direct comparisons between the two.
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Old 5th Dec 2018, 7:18 pm   #95
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Default Re: G & E Bradley CT-471

Strange.

Working unit two has gone gobbly gook today. Inconsistent readings, similar behaviour to tin whiskered transistors. Hooked up to the scope and after a while there's a lot of distortion on the chopper. Then the movement starts to oscillate quite rapidly.
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Old 10th Dec 2018, 1:05 am   #96
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Default Re: G & E Bradley CT-471

I wonder if there's a chance I made an error of judgement when trying to find differences between the two units.

The working one, which works for a short while after it's been switched off for a while misbehaves with a very fast oscillating movement.

When comparing the -4v connection to the chopper circuit I removed the -4v wire from R135 to see if I could remove a volt drop where I were previously reading -3v there on the good unit. With a probe, I may have made contact with the pin next to it which goes to RV11.

The reason I was looking at that is I had -3v on the good unit and -4v on the bad unit at R135.

It may have upset something on the amplifer board but I can't seem to work out what yet.
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Old 12th Dec 2018, 3:53 am   #97
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Default Re: G & E Bradley CT-471

well,i had another hour or two on mine today,so far ive changed all the oc 170 or oc171,forgot the number as i write this!,ive used af178's in there place,also replaced a mica cap and most of the electrolytics,now the strange thing is the movement wavers about the halfway mark on all settings,ie ohms volts dc and ac,on ac amps it sits to the left,if for example i put it on dc volts,with the range switch all the way ccw it sits about halfway up the scale,if i go cw with the range switch the meter goes more to the right untill it is full scale,that sort of tells me the range switch at least does have an affect,no the odd part,when the meter is hovering halfscale,on any mode bar ac amps,if i put my hand about 6 inches away from vt20 and 21 as i move my hand nearer the meter goes further to the right the closer i get!,its as if im radiating rf thats getting into the meter amp,any ideas??,73 m3 vuv.
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