OK, so I'm finally able to stand for reasonable periods and move about without a lot of pain, so I decide it's time to calibrate my HP428B clip on milliammeter. As an aside, it's got real live vacuum tubes in it, and is the first piece of test equipment I have purchased which is older than I am.
To cut a short story long, I got this for a reasonable price (almost nothing) because it had no probe. I managed to find a probe on ebay for a reasonable amount (more than the HP438B) and My sister in law delivered it to me, saving me the cost of freight.
The thing is, these devices were calibrated to the specific probe they were delivered with and thus require recalibration if you change probe. Well, as luck would have it, the probe I got was not the specific one that was lost by the person who sold me the meter.
It's not too bad. It reads about 10% low on all ranges. I set my power supply to 5A, 3A, 1A, 300mA, 100mA, 30mA, 10mA, and 3mA (in series with a 4 1/2 digit meter) to verify this. However, my power supply's current limiting supplies a little in excess of 1mA when set to a 1mA current limit, so I turn to my Keithley 228A current/voltage source/sink to get an accurate 1mA current.
Aaaagh! it reports Cal Err 1 and 3
So I pull it down and open it up (after checking the fuses) and do as the service manual suggests. + and - 5V rails are around 5.0x volts, but the 15V rails and the 115V rails are not there at all :-(
I also pull out my other meter just to double-check that it's not the meter, but it's not.
A bit of playing about reveals that I'm getting some sort of low frequency AC on the 15V rails.
The 15V rails are generated under a metal box covering the switchmode power supply. So now I have to remove the bottom, back, one side, the "logic" board, before finally getting to the power supply board. The manual helpfully tells me that this is how to do it without a complete dissasembly, but honestly, there's not much I haven't undone.
Pulling it out it looks unfortunately clean with the only discolouring on the board being a very slight amount under a NTC resistor designed to reduce switch on transient currents.
Having taken the board out, there is now no way to power it up. But I notice that the board uses a TL494 at it's heart and a pair of 555s(!!!!!) to drive the transformer coupled driver to the mosfet gates. Ooh, I remember that someone said they'd never use a 555 is a real design. Gotta take a photo of this.
While getting my phone to take a photo I come across another eBay purchase, a 50/500mV meter I picked up for $1. Hey, it works and it's almost accurate!
And guess what I found with it? It's the innards of a DC blocking BNC-BNC adapter that I opened up. I purchased these for my HP8590 spectrum analyser because it tolerates a maximum of 0V DC on the input!!! I was planning to replace the capacitor inside one of them with a higher value one to allow me to use it on my HP35665A DSA.
So with a few scraps of PCB I adapt it to fit the new cap and decide to compare the difference of the two of them into a 50 ohm load on the DSA. And it works just as expected. However the centre pin pulls out of the 1MHz to 3GHz one. Aaagh! Something else to fix.
Anyway, I've been soldering so I jumper the switch connections on the 228's power supply so it will be "on" and go looking for my "special" power cable.
While I'm doing that I notice that it's 4:30 in the afternoon and the 428B is no closer to being calibrated.
But I do have a photo of a pair of 555s used in a real piece of equipment and a piece of equipment in pieces.
To cut a short story long, I got this for a reasonable price (almost nothing) because it had no probe. I managed to find a probe on ebay for a reasonable amount (more than the HP438B) and My sister in law delivered it to me, saving me the cost of freight.
The thing is, these devices were calibrated to the specific probe they were delivered with and thus require recalibration if you change probe. Well, as luck would have it, the probe I got was not the specific one that was lost by the person who sold me the meter.
It's not too bad. It reads about 10% low on all ranges. I set my power supply to 5A, 3A, 1A, 300mA, 100mA, 30mA, 10mA, and 3mA (in series with a 4 1/2 digit meter) to verify this. However, my power supply's current limiting supplies a little in excess of 1mA when set to a 1mA current limit, so I turn to my Keithley 228A current/voltage source/sink to get an accurate 1mA current.
Aaaagh! it reports Cal Err 1 and 3
So I pull it down and open it up (after checking the fuses) and do as the service manual suggests. + and - 5V rails are around 5.0x volts, but the 15V rails and the 115V rails are not there at all :-(
I also pull out my other meter just to double-check that it's not the meter, but it's not.
A bit of playing about reveals that I'm getting some sort of low frequency AC on the 15V rails.
The 15V rails are generated under a metal box covering the switchmode power supply. So now I have to remove the bottom, back, one side, the "logic" board, before finally getting to the power supply board. The manual helpfully tells me that this is how to do it without a complete dissasembly, but honestly, there's not much I haven't undone.
Pulling it out it looks unfortunately clean with the only discolouring on the board being a very slight amount under a NTC resistor designed to reduce switch on transient currents.
Having taken the board out, there is now no way to power it up. But I notice that the board uses a TL494 at it's heart and a pair of 555s(!!!!!) to drive the transformer coupled driver to the mosfet gates. Ooh, I remember that someone said they'd never use a 555 is a real design. Gotta take a photo of this.
While getting my phone to take a photo I come across another eBay purchase, a 50/500mV meter I picked up for $1. Hey, it works and it's almost accurate!
And guess what I found with it? It's the innards of a DC blocking BNC-BNC adapter that I opened up. I purchased these for my HP8590 spectrum analyser because it tolerates a maximum of 0V DC on the input!!! I was planning to replace the capacitor inside one of them with a higher value one to allow me to use it on my HP35665A DSA.
So with a few scraps of PCB I adapt it to fit the new cap and decide to compare the difference of the two of them into a 50 ohm load on the DSA. And it works just as expected. However the centre pin pulls out of the 1MHz to 3GHz one. Aaagh! Something else to fix.
Anyway, I've been soldering so I jumper the switch connections on the 228's power supply so it will be "on" and go looking for my "special" power cable.
While I'm doing that I notice that it's 4:30 in the afternoon and the 428B is no closer to being calibrated.
But I do have a photo of a pair of 555s used in a real piece of equipment and a piece of equipment in pieces.