Fixing the MHS-5200A

Today we make some simple and cheap modifications in order to address the two biggest problems in the MHS5200A function generator: sine wave harmonic distortion, and square wave ringing/overshoot.

If you want to make the same modifications, here is the Digikey BOM. Note that the capacitors and inductors I used for the Bessel filter were 0402, while the pads on the PCB are 0603; either size fits, though 0402’s are obviously a bit harder to work with.

See the pictures below for more details.

Board Overview

Board Overview

Replacing the AD812 with the THS3092

Replacing the AD812 With the THS3092 (Be Sure to Replace the Heatsink!)

Modifying the DAC Filter(s)

Modifying the DAC Filter(s)

Bessel Filter Components

Bessel Filter Components

HP5385A LCD Repair

The LCD in my beloved HP 5385A frequency counter (with the OCXO option, naturally) has been gradually deteriorating, and it was high time I replaced it:

The HP5385A's failing LCD

The HP5385A’s failing LCD

Of course it’s not a standard HD44780 LCD, but a custom branded HP part (#5061-5212, revision A).

Luckily, I was able to salvage a replacement from an HP 6622A power supply front panel off ebay. The front panel cost about 1/3 of what I paid for the HP 5385A in the first place, but it’s a great frequency counter, so it was well worth the investment.

Repairing an HP3466A Multimeter

My main bench multimeter is an HP3466A that I found at a hamfest a couple years ago. Mind you it’s older than I am, but it looked to be in good functional order, and $30 seemed like a fair price for a 4.5 digit bench meter, so I picked it up.

The HP3466A

The HP3466A

After a while though, I noticed that its DC voltage readings seemed to be low when probing circuits containing larger resistances; anything around 100K ohms caused a noticeable discrepancy in the expected voltage reading, and it worsened with larger resistances.

Continue reading