Vertical measure problem on Hameg HM605

[Runik]

Hi,

I just got my HM605 out of storage, and I realize it has some problems measuring vertical tension : when checking 5V DC rail of a switching power supply the trace barely gets to 4.1V.
Same thing happens on both channels.
All buttons are depressed or set in calibration mode.

I checked voltages on the Y board (expected -> measured) :
+68V -> +68V
+4.65 -> +4.1V
+140V -> +139.1V
+4.3V -> -7.5V
+12V -> +12V
+5V -> +5V
-12V -> -11.9V

The only weird reading is the +4.3V one, but the manual specifies "-12V antipole" in that case, and I don't know what it's about and if it has anything to do with my problem ...

I also checked the impact of VR501 on the YF board (ie Y final amplifier gain), but it only variates of 0.2V max, which fall far short from the mark.

Do you have an idea where I should look, a direction to point to ? I'm not an oscilloscope expert, but this problem is bothering me and I would love to be able to fix it.

For reference :
English manual : http://www.hameg.com/manuals.0.html?&no_cache=1&L=0&tx_hmdownloads_pi1[mode]=download&tx_hmdownloads_pi1[uid]=839
German manual (with schematics) : http://www.hameg.com/manuals.0.html?&no_cache=1&L=0&tx_hmdownloads_pi1[mode]=download&tx_hmdownloads_pi1[uid]=1000

Thanks !

[vk6zgo]

Is the -7.5V a typo?

[Runik]

No it's not a typo, but this specific line has a special treatment in the manual as you can see :



But I don't know if it's normal or if it has anything to do with my problem ...

[PA0PBZ]

Looks like they are trying to say that pin 9 should be 4.3V above -12, and if you measure -7.5 that is close enough (-12+4.3=-7.7)

[oldway]

This is my favorite oscilloscope, I can not help but answer 
I have 3 working HM605 at home...

A first question: is the base time correct or not ?
You can test this with a signal of a well known frequency, from a function generator for example.

If the time base is ok, look at the final Y amplifier...Is there no bad solder ? Are the signals of the same amplitude (you can mesure this with 1Khz input signal and a multimeter in ac range) on both collectors of T505 and T506? (both 2N3866)

[Runik]

Thanks  for the pointers   

Unfortunately I don't have a function generator handy, I'll have to grab one before investigating further.

I'll keep you posted when I get one 

[Runik]

Just received my HPG1 from Velleman, so I could test the time base and you were right, it's a little off : using a 1KHz sinewave at 1ms, the 10 sinewaves are 9.8 cms in length instead of 10 cms.
Vertically, 3 volts are displayed as 2.5 volts using 1 V/div.

Is it a good idea to tweak VR303 on the TB board, or must I look somewhere else before attempting anything on this one ?