The HP 3437A is a “voltmeter”, but it’s not what you think of when you hear the word. This is a device which only measures voltage, only up to 30V, and only at 2000 counts! So what use is it?
This voltmeter’s purpose is computerized test and measurement. It has two important connectors on the rear: GPIB, and a trigger input. When triggered by a pulse on the trigger input, it waits for a programmable delay, then quickly samples the signal using a fast sample-and-hold circuit and reads it out. Because of the trigger and programmable delay, the HP 3437A can work as a sort of low bandwidth, low noise, high resolution, sampling oscilloscope. I intend to use it to measure aspects of the output waveform from my function generator kit, because the 8-bit ADC found in the average digital oscilloscope does not have the necessary resolution to measure, for example, ramp linearity.
I picked up a broken 3437A for peanuts on eBay. In the photo taken by the seller, the voltage readout was “stuck” at 9999; when I received it, the voltage readout was blank and the time readout was zero, and it was unresponsive. My suspicion is that it’s as simple as stuck keys – most of them were gunked up with filth and could barely move.
The input on the back panel is actually a triaxial BNC. I have no idea why – the input on the front panel is not (the two connectors are directly paralleled), there appears to be nothing connected to the guard contact, and the specs of this device are nowhere near what requires triax.
It’s a filthy son of a bitch…. Time for a bath!
This appears to be the microprocessor. He looks irreplaceable. I think I’ll remove him before bathtime.
Looks like I broke something while cleaning. Oops! It’s just a 7812 voltage regulator, though, I’ve got a couple of those. And what is that disgusting film on it? The microprocessor had it too, prior to being cleaned. Probably whatever horrid dust was all over this, baked on through years of heat.
Here’s a field of old-style discrete logic:
Those thin “fences” spanning the width of the board appear to be distributed decoupling capacitors.
Here’s that ceramic microprocessor in its natural habitat:
This ROM looks toasty…
Some analog stuff on the digital board – this is part of the trigger and delay circuit:
On to the analog board. This has some pretty heavy shielding: not only is it sandwiched between heavy metal plates, but one of those metal plates extends through a gap in the mains transformer, electrostatically isolating its transformer winding from the others. I didn’t yet take a picture of the shielding – I am still in the process of cleaning the filthy chassis – but I will. It has a very nice block diagram of the ADC printed on top, with testpoints labeled.
That’s quite the star ground:
We’ve got a couple more of the beautiful ceramic chips. These are actually both precision resistor networks. The left network is an R/2R (or similar, I can’t read the sloppy schematic scan that I have) network being used as a DAC (the ADC is the SAR type – successive approximation register – which employs a DAC and a comparator to perform what essentially amounts to an analog “binary search”). The right contains miscellaneous matched resistors used in feedback networks.
WTF do they need point to point-style turrets for on a PCB? They’ve even jammed that IC’s leg into one! Edit: I didn’t notice the plastic (Teflon?) rings around the turrets – thanks to “sync” on the EEVblog forum. These are insulation turrets, because these signals are too high impedance to even run them along the PCB!
Unless I missed a cutout in the shield, these LEDs are completely in the dark…
Here is the front panel board, with all removable parts taken off:
And a nice view of a primitive seven-segment LED display. It’s constructed on a separate PCB with press-fit connectors, with individual display elements glued to the PCB and connected with bond wires.
As a matter of course for refurbishing ancient devices, I will replace all of the aging aluminum electrolytic capacitors (I didn’t see any tantalum – did you?). This isn’t always necessary, especially if they are good quality, but they do age, and if I want to resell this at some point I want it to be trustworthy. I will add a heat sink to the 12V regulator when I replace it, and perhaps find a sturdier way to mount it.