bradley ct471 repair - Page 1
Forum section: electronics / repair

author: m3vuv
author_rank: Super Contributor
timestamp: November 09, 2021, 04:18:37 am
content: Hi all im trying to get this bradley ct 471c multimeter working,has anyone had any luck in fixing these?,i have changed the oc170 transistors for af178 units and recaped it,i had a post going on uk antique radio forum but banned me for moaning about the mods closing my thread about my tek 2215 ongoing repair,can post the service manual in 3 parts if needed,tia.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 09, 2021, 01:15:45 pm
content: HiYou do not say what is the fault with your CT471C, the C means that it is a centre zero meter.I have a CT471 and the full user manual if I can help.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 11, 2021, 01:48:26 am
content: Thanks for the offer george,the meter only works on the bat test,any other rage/mode it just oscillates to different degrees,i do have all the manuals,ive found a diode on the psu bard with one leg unsoldered,+ one is in two pieces,its an 0a47,i have same 1n5711 mat try one.

author: m3vuv
author_rank: Super Contributor
timestamp: November 11, 2021, 02:26:44 am
content: just replaced that diode,heres a rundown on what i get,on ac current range it oscillates from half scale to full hard over,it does that on all the acv ranges apart from 0.4 and 1.2 ranges then it goes hard over to the right,on dc volst on 0.012,its sits off scale to the left as in no feed to movement,rotating the range knob does nothing until i get to 0.4 then it moves up to zero,4v scale it returns to left just off scale,and stays like that on all other ranges,acvolsoscilates like on the dc range( about 2x per second),0.4 scale its hard over to the right sort of quivers about 2mm very fast,same on 1.2v,all the other ranges,back to a slow oscillation from 3/4 to full scale,dcv,reads low ie 9v connected to the probes measures 1 on the topmost scale,rf range same as dcv range with a 9vdc input,ohms pos,probes shorted 1 division movement to the right on top scale,barley moves,zero ohms pot does more or less nothing.dc current range does nothing,hope this helps,cheers paul.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 11, 2021, 04:36:44 am
content: HiAn OA47 is a germanium diode, is the +12VDC calibration voltage on the front panel present and correct/stable?To give you a laugh, many years ago now I worked for a large company as their northern hemisphere product line specialist, my manager who was ex RAF insisted that I used a CT471 and carried it on scheduled aircraft. I must have possibly the only retail Bradley CT741 in existence as when I left the company they, surprisingly, did not want the CT471 back, I have had it ever since.George.

author: m3vuv
author_rank: Super Contributor
timestamp: November 11, 2021, 11:16:22 am
content: yes george the 12v cal on the front panel varies from 12.9v to 11.8v ,that on the current I SETTINGS, on the acv setting its 12.86,dc volts setting=11.9v ,rf setting=11.89v,ohms setting=11.92v,hope that helps?,cheers Paul.m3vuv 73

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 14, 2021, 05:18:00 am
content: HiUnless the 12VDC ref is correct and stable the main meter will not work as the 12VDC ref is simply resistively derived from the main instruments stable +20VDC supply. You need to sort out the instruments power supplies first, they start with a 2.5VDC regulator and then a switch mode DC to DC converter.It looks like you are trying to sort out a previously failed repair, good luck with that.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 11:32:02 am
content: cheers george,should the 2.5v be present on all modes and ranges do you know,ive just finished cobbling together an octopuss to use with my scope+spent 6hrs clearing my workbench,hope to get back to it tomorow,i had a sort of rubberised non slip beer mat on my benchtop,it was filthy so decided to bin it,trouble is the rubber/neoprine backing had welded itsself to the worktop,had to use a stanley blade to scrape it then finish with celly thinners,took forever,i had forgoten what the bencktop looked like!!!

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 11:33:45 am
content: does the dc-dc converter supply 20v from 2.5v?.

author: silverback
author_rank: Contributor
timestamp: November 14, 2021, 01:28:40 pm
content: yes

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 05:25:46 pm
content: Well just having a poke about in this meter,firstly ive replaced the 0a47 diode with a good one from a spare parts meter,strange thing,i was just measuring the voltage drop over various diodes,mr8 zenner reads .7v in both directions,is that normal,on the octopuss it looks like a zenner,could it be my fluke 83 is causing reverse breakdown of the zenner ,i just measured the voltage across the fluke probes on diode test and its 9v !

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 07:35:06 pm
content: well ive been working thru the manual,im on page 49 where it tels me to set the collector voltage of vt13 to 13.6v using rv 13,the most i get is about 1 volt tops,any ideas?http://www.vmarsmanuals.co.uk/archive/1248_CT-471_Multimeter.pdf

author: silverback
author_rank: Contributor
timestamp: November 14, 2021, 08:41:02 pm
content: RTFM,section 5.6.2 R96 can be shorted or in circuit. Also check the -4 volt rail.

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 08:48:02 pm
content: r96 is a wire link on this

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 08:53:08 pm
content: i assume set meter switch to set zero should say center zero?

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 08:55:53 pm
content: -4v rail is at 3.96v.

author: silverback
author_rank: Contributor
timestamp: November 14, 2021, 09:02:05 pm
content: Section 5.6.2 from issue TP2 of manual.CT471 has a set zero switch position for zeroing the metre.CT471C has a centre zero function so that the metre can display positive or negative voltages without reversing test probes.

author: silverback
author_rank: Contributor
timestamp: November 14, 2021, 09:11:47 pm
content: You should shorting the input as you do not have the check zero position.

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 09:15:30 pm
content: can you explain that?,is that not the correct manual then for the ct471"c"?

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 09:24:01 pm
content: this has a set zero pot,what inputs do i short then,all of them the common and 400v top right terminal?

author: m3vuv
author_rank: Super Contributor
timestamp: November 14, 2021, 09:28:44 pm
content: I just noticed i had it set to 4v not 0.12,vt 13 collector still adjusts to 1v max.

author: silverback
author_rank: Contributor
timestamp: November 14, 2021, 10:15:21 pm
content: The manual you have has some of the changes to implement the centre zero, CT471C, metre.You really want the later TP2 issue.Check the terminal voltages around the FET VT11 and VT13.

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 05:28:54 am
content: heres the voltage readings on what you asked,they both fluctuate in line with the meter needle oscillating first vt13 collector=0.98v min/1.051 v max,base= min -2.69v ,max=-2.61v ,emitter=min 3.2v/max= 3.3vvt11 drain=14.8v,source=1.1v min/ max=1.2v,gate=0.25v steady.thats with the meter 400v and common inputs shorted and the mode switch on dcv range at 0.012 and the center zero at +.

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 05:31:55 am
content: what is the tp2 issue,ie what does tp stand for?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 15, 2021, 06:08:18 am
content: HiWhat is the serial number of your CT471C, there appears to be more than one version from Bradley. TP2 is the version of the manual.George G6HIG Dover UK

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 15, 2021, 06:22:43 am
content: HiI have just checked, the VMARS manual appears to be version TP1 (although it does not say so)If your serial number is BGB2730 and onwards you need version TP2 of the manual, suspect TP stands for Technical Publication. Version TP2 fully covers both the CT471 and CT471C.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 08:22:59 am
content: hi george my serial number is bgb 6111,any idea where to get the needed manual?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 15, 2021, 08:57:15 am
content: HiI have a paper copy, but cannot scan it easily as I only have an A4 single sided scanner at present, later today I will post copies of the power supply section schematics and the power supply adjustment procedures.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 11:39:19 am
content: thanks george,have a look at this folder on my google drive,this is the info i have on the meter,guess there the wrong manuals tho?,cheers Paul.https://drive.google.com/drive/folders/1sJILKOMX3hN2dddbRe1WnFDkG6rQtyBr

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 15, 2021, 12:23:56 pm
content: HiI have attached some scans of the TP2 manual, hope you can make use of them, If you need more just yellGeorge G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 12:59:21 pm
content: cheers george,did you look at the ones i have on my google drive?

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 04:00:25 pm
content: can i use an avo 3 mk2 or a fluke 83v to take measurements on the psu sections of this as the manual is stating a meter with 100k input inpeadance,seems high to me,do they mean use a valve volt meter?

author: silverback
author_rank: Contributor
timestamp: November 15, 2021, 04:12:51 pm
content: Yes they mean a valve voltmeter, the CT471 is a valve voltmeter, on DC it has an input impedance of 10Mohm per volt up to 120Mohms.I am having trouble down loading the TP2 issue manual, if you can send me your email address I can directly send it to you.

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 05:02:48 pm
content: cheers pm sent.

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 05:03:54 pm
content: should of said avo 8 not 3.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 15, 2021, 05:59:11 pm
content: HiI cannot look at your Google drive, it does not like my Firefox Browser and add ons setup as I block Googles undesirable data collection activities.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 15, 2021, 10:47:58 pm
content: sb did you get my pm,as nothing in my email yet.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 16, 2021, 07:47:35 am
content: HiHave I lost the plot with this CT471C. Is the power supply section now working correctly and producing the reference +12VDC correctly and by far more importantly the correct outputs from the 550Hz multivibrator switch. These outputs, highlighted in yellow on page B, control both the input chopper amplifier and the phase detector, if they are not correct the instrument cannot possibly work correctly.Needles to say that both the stable +2.5VDC and the regulated +20VDC must also be correct, I doubt if an AVO 8 is capable of meeting the reading accuracy required by the setup section of the service manual.Let me know how you are getting on.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 16, 2021, 01:24:02 pm
content: hi george,would a fluke 83v be ok for the psu measurements?,havent done much wth it for a day as my pc hard drive died so been sorting that,73 m3vuv.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 16, 2021, 03:51:34 pm
content: HiYour Fluke will be fine for the DC voltages, but you will need a scope to check the 550Hz signals.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 21, 2021, 07:38:15 am
content: well i found one of the 3 amp transistors tested as a back to back diode and a missing ground on the psu board,think it was vt 13 but not sure as i took all 3 out and didnt note what was what,still not working,need to check psu voltages now.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 21, 2021, 10:01:59 am
content: HiI cannot see a VT13 on the schematic that I have, as a general note the 2N1304 transistors have probably all gone leaky by now. There were problems with all early Ge NPN transistors.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 21, 2021, 12:02:58 pm
content: dont you mean the oc170 germainium george ?,a 2n-3904 is a pnp silicontheres 3x af-178 /1x bc-108,1x get 888,1x 2n-1308 and 1x oc-44 on my amp board,im trying to upload a vid of its issues onto youtube but its taking forever with my crap phone network,will post a link when it finaly uploads,its been nearly 3hrs to reach 43% for a 380 meg vid!!

author: silverback
author_rank: Contributor
timestamp: November 21, 2021, 01:01:08 pm
content: VT13 is a BC108, VT16/17 & 18 are OC170's known for tin whisker growth these can be replaced by 2N2907's without problems.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 21, 2021, 01:45:01 pm
content: HiI was under the impression that you were doing the logical thing and getting the power supply and chopper/detector drivers working first, clearly I was wrong.With the TP2 manual power supply circuits I have VT3 and VT6 are marked as being 2N1304, these are defiantly Ge NPN’s, I changed thousands at one time in the 1960’s which were leaky.George G6HIG Dover UK

author: m3vuv
author_rank: Super Contributor
timestamp: November 21, 2021, 05:12:48 pm
content: finaly got a vid on youtube of it not working,was going to sort the psu george but had my pc hd go down,so been busy on that,gonna get back on the bradley tomorow,cheers Paul.

author: silverback
author_rank: Contributor
timestamp: November 21, 2021, 06:15:22 pm
content: I am not to sure what you are trying to show, in the check battery position you have 5.1k resistor in series with the meter movement. If you have any fluctuating meter movement you must have a lose connection. The other switches or the meter zero knob should not have any effect on the meter when check battery is selected.

author: m3vuv
author_rank: Super Contributor
timestamp: November 22, 2021, 06:39:29 am
content: it wasnt clear and done with my phone,ive just got windows movie maker so will do another vid with that .,well the moviemaker program didnt like my video card so done another on my phone

author: m3vuv
author_rank: Super Contributor
timestamp: November 23, 2021, 07:30:07 pm
content: going to pull vt5 +6 tomorow and test them for leakage.

author: silverback
author_rank: Contributor
timestamp: November 23, 2021, 07:47:22 pm
content: First of all you are in AC mode, the meter is only driven by the amplifier. But you are seeing low frequency movement of the needle. You must check the power supply, check the 2.5V regulator make sure it has a stable voltage. Then check the output voltages of the converter again checking there stability. It looks like a power supply fault somewhere which is causing it to be oscillating at low frequency. What are you using to power the meter? is it current limiting?

author: m3vuv
author_rank: Super Contributor
timestamp: November 23, 2021, 08:39:42 pm
content: Im using a modified 30amp 2-18v current limiting linear psu.