Electronics > Metrology

Solartron 7061 noise

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grizewald:

--- Quote from: branadic on June 12, 2021, 02:20:16 pm ---I'm quite curious if you grizwald still have the LTC1052 in the meter without any issues. Have  you changed the zener diodes? I saw the

--- End quote ---

Sorry for the late reply, I haven't been on the forum for ages! (Pressure of work, life etc.)

I had the LTC1052 in the meter for about a month and although it did result in about 30% less noise, it changed the meter's calibration.
Seeing as I'd actually paid to have the meter calibrated, sacrificing my calibration for 30% less noise seemed like a poor deal, so I put the original ICL7650 back in.

Defpom's problem looks like an unlucky infant failure to me, maybe precipitated by the slightly high 15V rails as Kleinstein suggests.

grizewald:

--- Quote from: branadic on June 13, 2021, 08:23:38 pm ---Played a bit with Solatron7061 this weekend and monitored some voltages. I spotted large hum in the readings, so I looked into the oscillator of the PLL and bingo, it was way off. So I brought it back into the range where the PLL was able to lock and readings are much better now. Figure 1 and 2 (100 samples) were taken with filter turned off. Figure 3 shows the effect with filter turned on, reducing the repitition rate in 7 digit mode to about 7.4 s.

-branadic-

--- End quote ---

It would be very interesting to see what you get if you repeat the burst mode capture that both Dek and I tried in this thread: https://www.eevblog.com/forum/repair/solartron7061/msg3047176/#msg3047176

branadic:
Why should I use the burst mode instead of taking continuous readings?

-branad-

grizewald:
To see if you get the same 50Hz noise as we did when the chopper is running at 1.5KHz.  As you can see in the linked thread, the discussion kind of ran out into the sand, but even so, another data point would be interesting.

TheDefpom:
I had a poke  around my 7061 (original version with PCB3 and 5) yesterday and scoped the oscillator and PLL etc, basically the same results everyone else is seeing, a PLL that never really seems to lock properly, just constantly adjusting its control voltage and the frequency jumping around by huge amounts.

As I have played with PLL's and VCO's in the past in my designs for CB radio conversion boards, I played around with it a bit and even tried tweaking the tantalum capacitor values to see if I could improve the phase detectors control voltage filter characteristics, but with no real luck, I could influence it and maybe improve it very slightly, but nothing fixed the response and control accuracy.

One thing I did do was replace the Philips 47uF 10V electrolytic capacitor at C422, this capacitor was completely dead (I have found that the same type of Philips capacitors in the Datron multimeters are also always dead, so I always have them as a suspect), anyway, replacing that capacitor appears to have improved the noise on the display slightly, it doesn't appear to be  jumping around quite as much as it was, so that might be something worth checking.

I was hoping to dig right into this PLL/VCO system and try to find a resolution to its instability, I will try again in the near future as I am sure it should be possible to get it locked properly, I have some other ideas for things I could try but after spending a few hours on it yesterday I have put the meter back together and moved onto my other projects.

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