Hameg HM705 Y-Pos. Problem - Page 1
Forum section: electronics / repair

author: marcchep
author_rank: Newbie
timestamp: April 11, 2018, 02:46:24 pm
content: Hello people,very recently I bought my first real oscilloscope, a Hameg HM705, but I ran into a problem. When I turn the scope on I get a nice crisp green line, but the problem is that, on the y-axis, the line is not in the middle. Even with the y-pos. dial I cannot adjust the line to be in the middle. For channel 1 and channel 2 I can get the line to about 1cm to the middle line of the oscilloscope. Then I took a 1,2V battery and hooked it up to the oscilloscope, the positive measurement seems about right, but when switching the battery leads, the line will bottom out just bellow the middle line, no matter what the channel sensitivity is set to. I have no idea what the problem is, but any help would be greatly appreciated.

author: oldway
author_rank: Super Contributor
timestamp: April 11, 2018, 04:36:42 pm
content: Welcome to the forum and congratulations because this oscilloscope is an excellent choice.You can find the complete manual in German with schematics and service informations here:https://elektrotanya.com/hameg_hm705-1_oscilloscope_um.pdf/download.htmlWait until "processing" becomes "get manual".Since the problem is the same on both channels, it is probably a fault of the final Y amplifier.First see the solders around the components that are getting very hot like R592 and R593 (560R 4W) and the transistors T521 and T522 (2N3866)Also check these two transistors and resistors.

author: marcchep
author_rank: Newbie
timestamp: April 11, 2018, 08:47:24 pm
content: Thank you for the warm welcome! I wanted to pick up a working oscilloscope, but I got a great deal on this one.I checked the solders around the components and they all seem fine, but resistor R592 looks like there is a part chipped of and it does not measure 560but only 360. Could that be the problem I am looking for?Regards, Marc

author: oldway
author_rank: Super Contributor
timestamp: April 12, 2018, 06:52:55 am
content: I am an Hameg analog oscilloscopes fanboy, I bought a new one (HM605) in 1982 and I am still using it in on a daily base.It has been repaired several times, but it has thousands of hours of use and still working as new.Replace R592 and T521 (2N3866)EDIT: NB: I guess you measured the resistors "in circuit" and that R592 is still good but you have measured a lower resistance because T521 is shorted or have a huge leakage.If R592 is still measuring 560R, you don't need to replace it, only T521 has to be replaced.

author: marcchep
author_rank: Newbie
timestamp: April 12, 2018, 05:16:29 pm
content: I understand now why you like Hameg analog oscilloscopes so much.I checked T521 and it is not shorted. R592 in itself measures at 360R. So I guess only R592 is bad?Regards, Marc

author: oldway
author_rank: Super Contributor
timestamp: April 12, 2018, 07:45:23 pm
content: As I said, replace R592 AND T521....This final Y amplifier dissipate a lot of power and with lower value of R592, it is still worst....So, don't loose your time, replace also T521 even if it is not shorted....

author: marcchep
author_rank: Newbie
timestamp: April 12, 2018, 10:33:28 pm
content: Will do!Thank you so much for your help! I really appreciate it.I will report back when the parts arrive.Regards, Marc

author: oldway
author_rank: Super Contributor
timestamp: April 13, 2018, 08:22:14 am
content: Do not be too enthusiastic .... we follow a repair tactic that consists first and foremost of replacing what is usually the cause of this kind of breakdown.I avoided asking you to take live measurements knowing that you do not have much experience and that there are dangerous voltages in a crt oscilloscope.If that does not solve the problem, we will continue to make the diagnosis more thoroughly.But in any case, we will have already eliminated a defect that has been clearly identified.

author: marcchep
author_rank: Newbie
timestamp: April 14, 2018, 10:17:04 am
content: Yes, I know. But I am used to setbacks, so if that is not the only issue I am not surprised.I don't really want to mess with these dangerous voltages, but if I need to I will. I just have to be super careful when doing so.Regards, Marc