A while back, I noticed the voltage on my Instek GPS-3030DD power supply wandering over a wide range. Crap, something's wrong with it. I turned it off and left repairs for another day.
Yesterday was the day to repair it. My first thought was bum filter caps, with another likely possibility being noisy pots. I opened it up and had a peak around. There's just one large filter cap and it looked fine - none of the bulging of a bad electrolytic cap. Nothing else on the main board looked bad.
I hadn't actually checked to output voltage; I had only noted the voltage fluctuations on the meter display. Checking the output voltage with a DVM showed it to be perfectly stable. The power supply's voltmeter was all over the place and wrong by a few volts, but the output voltage was fine.
Must be a meter problem. I turned my attention to the meter. On the back of the meter, there's a small board with two 5 volt supplies, and a very bulging electrolytic cap. I think the problem has been located!
These two 5 volt supplies are fed by separate windings from the power transformer. I suspect that the metering circuits for voltage and current are completely separate, thus the need for isolated 5 volt supplies.
This cap is on the output of a 7805 regulator. 1000uF at 16 volts. I have a few caps of that rating, but the lead spacing is too small. The 1000uF at 25v caps I had were a perfect fit. I replaced the caps on both supplies and I was back in business. Steady meter readings at the correct voltage.
A quick repair returned my power supply to service. Not bad for a power supply with a calibration sticker dated 2009 from a previous life before I bought it at a pawn shop.
Yesterday was the day to repair it. My first thought was bum filter caps, with another likely possibility being noisy pots. I opened it up and had a peak around. There's just one large filter cap and it looked fine - none of the bulging of a bad electrolytic cap. Nothing else on the main board looked bad.
I hadn't actually checked to output voltage; I had only noted the voltage fluctuations on the meter display. Checking the output voltage with a DVM showed it to be perfectly stable. The power supply's voltmeter was all over the place and wrong by a few volts, but the output voltage was fine.
Must be a meter problem. I turned my attention to the meter. On the back of the meter, there's a small board with two 5 volt supplies, and a very bulging electrolytic cap. I think the problem has been located!
These two 5 volt supplies are fed by separate windings from the power transformer. I suspect that the metering circuits for voltage and current are completely separate, thus the need for isolated 5 volt supplies.
This cap is on the output of a 7805 regulator. 1000uF at 16 volts. I have a few caps of that rating, but the lead spacing is too small. The 1000uF at 25v caps I had were a perfect fit. I replaced the caps on both supplies and I was back in business. Steady meter readings at the correct voltage.
A quick repair returned my power supply to service. Not bad for a power supply with a calibration sticker dated 2009 from a previous life before I bought it at a pawn shop.