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Fluke 5220A Problems

We have a Fluke 5220A Transconductance Amplifier with a problem that's been getting progressively worse over the years. It's supposed to put out at least 20 amps, ac or dc, with light inductive loads. The overcompliance circuit is tripping the unit out at around 19A DC, and much less for AC, around 13A. These are both with loads connected with minimal voltage drop across them. The loads, a shunt and coil of wire, have been used for years with no problems.All supplies are clean and stable. After troubleshooting for hours and getting nowhere, we disabled the overcompliance circuit to see if it could indeed put out full current. R114 and R115 on A7 burnt up (page 129,

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). We replaced all the transistors on A7 although none were blown, re-enabled the protection, and we're back where we were.. I'm curious why those resistors burned. I'm not familiar with this type of amplifier configuration. We checked, and the amplifier output is stable and not oscillating. At this point, we'll take any input. Next up I'm planning on replacing all the active parts on the driver board, I'm tired of troubleshooting and wasting time, it's shotgun time.

Thanks, Steve

Reply to
Avionics Tech
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I'm not at all familiar with that piece, but I've found that when a problem gets progressively worse as you say, that usually it's one or more electrolytic caps going high ESR. I'd start by replacing them.

Reply to
JW

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Unfortunately, Fluke puts a lot of thought into their products. The only electrolytics on this unit are the large ones in the power supply busses. There's 8 of them, 50,000uF. The rest are ceramic, tantalum, mylar, or polyester.

Also, I forgot to mention, the unit works fine functionally besides the overcompliance problem i.e. for 5V input, it puts out 5A. It's actually in tolerance. Also, the overrvoltage circuit adjustment works properly, we are able to set it at 3.3V.

Thanks, Steve

Reply to
Avionics Tech

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Also, we ran through both troubleshooting procedures in the manual, they both passed just fine. What I'd really like to know is what would cause those two resistors to burn up in that type of amplifier configuration.

Steve

Reply to
Avionics Tech

Hi Steve,

this is a massive long shot - I found this old post on google groups regard ing a problem you had with a Fluke 5220A Transconductance Amp. I've got a similar problem with one here - it can drive resistive loads at

20 A DC and AC without an issue, but is very hit and miss with the 50-turn coil (used to calibrate current clamp meters up to 1000 A). Sometimes it wi ll drive 1 A or 2 A through the coil, other times it will just go straight into an overcompliance fault.

Sometimes I can slowly ramp up the input voltage and get up to 5 A or 6 A t hrough the coil but it depends on the day. Ramping up in ~100 mV steps is u sually okay but very, very tedious, and sometimes even this causes it to cu t out.

I did some reading and it seems that there is meant to be a feature in the

5220A where the current will slowly ramp up if the input voltage is applied first, then the 5220A set to 'Operate' mode, rather than having the 5220A in ' Operate' mode and changing the input voltage. I suspect this is what i s causing the problem in both your case and in my case, but don't even know where to start in terms of fault-finding.

If you did ever end up identifying a faulty component or an alternative sol ution for this problem I would be eternally grateful for some assistance!

Thanks, - Alex

Reply to
alex.h.rodger

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Actually, there ARE electrolytics, on the regulator board, other than those big 'uns. I'm not sure why R114 and R115 would fail, but they supply base current to the final transistors (Q103...Q106). So, could you monitor test points TP3, TP4 and TP5, TP6, to see if the emitter currents are in balance? If a final goes bad (low beta) the good one paired with it would overheat, and then the opposite pair would have to drive the leakage current of the hottie as well as the load.

Reply to
whit3rd

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