Help finding a replacement crystal for an HP3478A - Page 1
Forum section: electronics / repair

author: Bluespeed
author_rank: Newbie
timestamp: March 28, 2018, 08:19:26 pm
content: Let me preface this. I am a programmer by trade and have only recently started getting into electronics out of a desire to know how things work at a lower level.I recently got an HP 3478A from e-bay for a good price but it is not fully working. The biggest problem I am facing is with no leads plugged in the DC volts range is unstable going from around +2.8v and dropping to about -2v. But when testing a 1.5v and 9v battery I am getting good values within tolerance of my BM235. Ohms reading also appears find when testing a 100ohm and 100kohm 5% resistors. With my initial inspection I found all the power rails are good and there is no obviously bad components. Going through the service manual I found the ALE line at U501 pin 11 is not quite as shown in the manual. Checking pins 2 and 3 of U501 show no sign of the 5.856MHz clock. Now the service manual is very helpful saying that I should replace the crystal Y501 which is a 5.856Mhz crystal.And now my problem. I have no clue where to get a 5.856Mhz crystal. Digikey has a 5.5Mhz and 5.9904Mhz crystal but I don't know if those will work. Can someone tell me if those replacement crystals will work or if I should look for an exact match? Third option is I am doing this all wrong and the weird reading with no leads plugged in isn't a problem at all.Here are some photos of the waveforms I am getting from Pin 11 and pin 3https://imgur.com/a/nQMBt

author: guido
author_rank: Regular Contributor
timestamp: March 28, 2018, 08:56:01 pm
content: I have the opposite problem with a 3456A. I managed to find a cheap source for the 4.875 Xtal for running on 50Hz mains. Mentioned here before:http://www.krystaly.cz.But i also have a spare 5.85MHz, type HC-50U with thicker leads. Part no 0410-1226. It says 5.85 in the manual, don't know the real value in the machine. Mabye it would fit your meter. Otherwise try the link (note, i'v just ordered, don't have it yet).

author: ModemHead
author_rank: Frequent Contributor
timestamp: March 28, 2018, 11:44:44 pm
content: Do I understand correctly that the only problem you see is that the unit displays random readings with no input? If so, this is perfectly normal. The 300mV and 3V ranges on this meter are high-impedance, much higher than the usual 10Meg. The input will just accumulate charge from the environment and show readings until it reaches 3V, at which point the unit will auto-range up, which will dissipate the charge, and the process will repeat.If you short the inputs and it reads zero plus/minus a few counts, then it is working normally. If the random readings bother you, obtain or make a shorting plug and leave it in the jacks until you use the meter.As for your scope waveforms, no they don't exactly look normal. Using a X1 probe maybe? Hanging loads on crystals can kill or alter their oscillation. At least the ALE signal is about the right amplitude and frequency. You're probably seeing the limitations of the pocket scope.

author: Bluespeed
author_rank: Newbie
timestamp: March 28, 2018, 11:49:44 pm
content: Thanks for clearing up my confusion. I am glad that there was nothing wrong with this meter. Now to try to replace that aging battery.