# Revisiting the MHS5200A

I’ve gotten a lot of questions on the blog about the new version of the MHS5200A function generators available on eBay. Viewer Tolga was kind enough to send one in to me to review and tear down. Although some improvements have been made over the older models, there are some concerning issues with these new models too!

# Tektronix 485 Oscilloscope Repair

I got a sick Tektronix 485 oscilloscope for cheap on eBay; here is the first of several troubleshooting/repair/restoration videos, in which I attempt to troubleshoot and repair the input attenuator boards.

An easy repair of an abused HP6216A power supply, purchased for $5 as “not working”! # 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 Replacing the AD812 With the THS3092 (Be Sure to Replace the Heatsink!) Modifying the DAC Filter(s) Bessel Filter Components # HP6237B eBay Score and Repair I got a really good deal on an HP6237B power supply on eBay, but the front panel meters had taken a beating! Luckily, my cats were there to help me try to repair it. # 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 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

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.