G & E Bradley CT471C

[Sinewave]

Nope . . . .
ASIDE . . . . I just now, read back, and saw that you did replace C34 and C22 . . . that's good . . .

I did do some ESR testing with a scope . . .
Was that being in the manner . . . . that you feed a 100KHZ sq wave signal into the scope . . . from a function generator and then shunt the signal with the E-cap, being tested, to see how much it attenuates that square waveform ?
That works . . . . . if you are additionally comparing against a known low ESR E-cap for reference .

Yes I used my function generator as you suggest and the calculatons suggested replacement.

 

[Sinewave]

Right, lets get to it, I'll now do some of the new tests you've suggested.

 

[Sinewave]

SIr Sinewave . . . . .

I acknowledge your tests and their results, now we need to further delve into it, to find that if any circuitry tied into that amp string, is not additionally influencing it .
That specifically would be the additional use of that same E-cap to additionally shunt . . . to ground
1 . . . . . The shared cathodes of MR14 and MR15 in the meter bridge rectifier.
That line is being a ground return and gets routed to ground thru a different series of resistances, in accordance to which range SC3 switch is being set to.

2 . . . . . The junction of VT19 and the left lead of shared R109 resistor.
That is being two incoming phase detector reference samplings.

3 . . . . . The shared and common junctions of R-145 / R107 / R83 / R135.
Which are associated with meter zero centering.

So far...

1 Not performed yet as I still can't locate MR14 and 15

2 Touching down the VT19 and R109 junction (connection to emitter?) stops the meter fluctuation.

3 Touching on the shared -4v commons (I went onto R107) stops the massive oscillation, but takes the needle to the left of the zero scale and there is an ever so slight fluctuation, really very minute movement.

Still looking for MR14 and 15, where have they got to? Maybe they're out to lunch....

Oscillation is out at the moment, so waiting for it to return before doing the rest of the tests.

 

[73's de Edd]

Referring to the schematic . . . . MR 14 & 15 are being two of the diodes in their configuration as a Full Wave Bridge rectifier that feeds their developed DC to the instruments 1 ma meter movement.
They are on the right half of it and I erroneously referred to a pair of cathodes, while it is actually being an anode and a cathode from each. They then route upwards to pass to the left to then go over to SC3 switch, terminal 3.

 

[Sinewave]

Hi Edd. Yes I see them on the schematic, but struggling to locate them in the meter. I will take another look later.

 

[Sinewave]

So, for test 1, I have some belief that I'm on the correct side of D14 and 15, touching down doesn't stop the oscillation.

They were very hard to find, buried deep down in the switch wafers.

 

[Sinewave]

Suspect diodes?

 

[Sinewave]

From the PDF page 34...

When measuring direct voltage up to 400 volts the unknown voltage is applied to TL5. It is fed via contact 3 or 5 (don't they know then? -sinewave) and the rotor of SB3 (according to polarity), the rotor and contact 10 of SC1 and the rotor of SA8 to the appropriate point along the series resistor chain R19-R23, R37-R40, R117-R121, R114, R115 and R131. The output from the resistor chain is applied directly to the input of the chopper circuit. When making measurments with the 40v, 120v and 400v ranges, voltage divider resistors R16-18 are bought into circuit by SA14. These resistors in conjunction with capacitor C20, also for the low-pass filter which minimizes any superimposed alternating voltage. On the 1200v range, the unknown voltage is applied to terminal TL3 which is connected to the chopper circuit via R154, R155, R156 and R15.

So, SC being the measuring finction range, SA being the range value and SB being the main on/off/+/- switch.

From what I can gather the numbers suffixing 'SC' on the drawings related to the wafers, with 1 being close to the front of the meter, with the numbers climbing the further to the rear of the meter the wafers go.

I'm going to see what I can spot, considering this oscillation happens only on DC, current or voltage.

 

[Sinewave]

I've given the swtich contacts a good going over. I don't think it's them. Along the amplifer all connections coupled to ground have been checked.

I'm wondering if the issue is in the chopper...

 

[Sinewave]

I have the chopper out.

I have discovered two things, which may not make much difference, however in my other unit and according to the drawings, the collectors of VT23 and VT 15 are connected to resistors R73 and R89 which then are connected to diodes MR16 and 18.

However, in this unit the diodes are connected to the collectors with the resistors before the diodes.

I wonder if this is enough to cause this issue?

 

[Sinewave]

That's a no, as the oscillation still occurs with the chopper from another unit fitted.

 

[Sinewave]

This has taken me a few hours, including make notes and taking photos, but I've removed the amplifer board to make it easier to make some checks. Also I might remove the amplifier board from the other unit and try it in this one.

 

[Sinewave]

Just a small update to say that this project hasn't been forgotten about, it's just been put far to the back of any priority. I hope to be picking things up again soon.

 

[Sinewave]

Ok, so I'm back to the G&E Bradley. I can't really remember what I actually did last. I think I'd swapped amplifier boards over from one unit to another in an attempt to see if I could move faults around. I've replaced the board in the unit that I'm working on and there's no response at all apart from 'battery check'. So I need to try and find what I've done wrong or what I haven't done'...

 

[Sinewave]

Does anyone know if there is a switch contacts drawing? I just can't find one and can't for the life of me work out which wafer is which. It sounds silly I know, but it really isn't obvious.