HP 428B Clip On DC Milliameter Repair and Adjustments

[@andymouse]

Thoroughly enjoyed this walkthrough and repair and I learned a great deal, have a great new year....cheers.

[@robnaulty1750]

Thanks for the thorough repairs.

[@KhunThomat]

Double-layer PCB with tubes... I didn't know that such designs exist! And also gold-plated? What a decently crafted instrument - surely made to last for an engineer's lifetime...!

[@karinjohnson4354]

This was a timely video for me. As it turns out I have a HP428B, very late serial number. First off thank you for unraveling the manual. I have three different versions of the manual, two of them for serial numbers above 0995. Your comments were most helpful. Now the main reason I am leaving a comment. I have a problem with my unit on the 10 Amp range. All other ranges calibrate successfully, but when I switch to the 10 Amp range the meter pegs off scale. It is sort of random, but most of the time it pegs positive, but there are times when it pegs negative. I've not replaced any of the electrolytic capacitors, so it could be that, but I am not going order capacitors just yet as it may turn out to be another component. Best I can assume is that the tubes are all sort of good. Oscilloscope and meter readings look correct and the fact that the lower ranges calibrate OK suggest good tubes. BTW, have you looked at prices for 6DJ8 on ebay recently??? I do hope you have an opportunity to read my comments, any suggestions would be most helpful. I can post email address if it makes any sense, but I don't know if youtube allows that. Regards, Karin Anne Johnson P.E. K3UU

[@FreeElectron]

The 6DJ8 is a popular audio tube so it will tend to demand a higher price. From what I remember about this unit there isn't anything special about the 10 amp range as compared to the other ranges. Check the divider resistors and switch for poor contact. The filter caps may (or may not) need replacement, you'll have to check ripple on the power supply. Good luck and thanks for the comment.

[@karinjohnson4354]

Thank you for the response. Both resistor chains attached to the range switch have been verified as correct values. Yes, capacitors could be an issue, I will have to go down that route eventually. I can not be sure that the probe was the original shipped with this unit. There is no serial number etched on the body of the probe but there is a number on the probe connector. I did have to repair a broken wire in the probe head itself. All coils in the probe measure DC ohms correctly. I suspect some interaction with the probe inductance and the feedback circuits causing a very large 40 kHz oscillation when measured at the junction of L5,R1, and R20. The top of the 66.5kohm resistor on the chain. This with zero current in the probe pass thru wire. All lower ranges show a quiet signal at that point. Onward and upward I guess. Thanks again.

[@karinjohnson4354]

I wanted to thank you again for posting this repair and adjustment. I've had an HP428B for a couple of years now and had relegated it to the closet, as I could not get it to calibrate and operate as it should. I posted before about the symptoms. I was reluctant to check all of the electrolytic capacitors, as it appeared they looked OK with my basic ohmmeter. I invested in a better DMM that could check capacitors and lo and behold I found two bad electrolytic capacitors. C12 and C21. After replacing them the calibration went very smoothly and now it works as it should. Many thanks again for the video. It gave me the inspiration I needed.

[@FreeElectron]

Good to hear. Now you’ve got a little more knowledge, a little more experience and some new gear. That’s how it goes in this hobby. Thanks for the comment.