Help requested for HAMEG HM 203-6 scope

[Postmanpop]

Hello
My Hameg scope has been working for some time but suddenly a fault developed which has me foxed.
With no inputs the display shows on both traces a sine wave of about 100Hz at an apparent amplitude of 150 volts. As it shows on both traces (and because of the frequency) I suspected a power supply issue. I have checked the + and -12 volt rails on the XY board and found that the -12 rail is actually measuring -16 volts. I have changed the following components IC 705 (741 op amp), T717 (BD237), T716 (BC237) and C716 (10uF all to no avail.

Does anybody have any suggestions as to what else I can try? Working on the boards is very difficult as it is necessary to do a major stripdown to get at the solder side of the boards so I would like to be sure that I have exhausted all non powered tests before reassembling again.

Thanks

Alan

[Marconi_MPT4]

Might be worth checking capacitors C711, C712 (1000uF) and C716 (10uF). Resistors R758 and R759 (10k1) sets the -12V supply which also relies on the +12V being correct.

Rich

[frsimen]

In addition to the components listed by Rich, I would check the components feeding the negative supply to IC705, the 741 op amp. They are R766, D717 and C714 which decouples the supply. If the negative supply has failed, the BD237 won't be controlled properly.

The Zener diode D718 should be checked in case it is short circuit. Test it like a diode, if it has a voltage drop of about 0.6V one way (assuming you are testing using a normal diode test feature on a meter) and open circuit the other way around it's probably fine. The same test will work for D717.

Check that R764 is correct too, as that is needed to ensure that the BD237 can be cut off.

If all the components test fine, check for dry joints or cracks in the PCB. If none are found, the remaining component which may cause this sort of trouble is C718, but it's right at the bottom of the list of suspects.

Paula

[Postmanpop]

Hi,
Thanks for the suggestions. I have now checked every component in the +/- 12 volt section of the power supply and the only out of spec measurement is the -16 volts rail. R758 and R759 should set pin 3 of IC705 to about zero but the measured voltage is about -2.3 volts which is roughly consistent with +12 and -16 rails. C716 was giving an iffy esr so I changed it but both 1000uF electros are well within expected values.
I am now reaching the “clutching at straws” stage by following the -ve rail around the XY and TB boards and trying to see if there might be some faulty component which is forcing the -16 rail down ( probably unlikely but I can’t think of what else to try)

Thanks

Alan

[David G4EBT]

Do you have the full manual with all the circuits and board layouts Alan?

The 31-page English manual excludes those pages, and you need pages 32-43 of the Spanish Manual (which are annotated in both English and Spanish). Not sure why Hameg did that. (In the 203-7, pages 32 - 46 of the German 203-7 manual contain that information, which is excluded from the English manual).

If you don't have the required pages, drop me a PM with your email address and I'll forward the Spanish manual to you.

Good luck in sorting the fault.

[Chrispy57]

Hi Alan,
if that 741 pin 3 voltage should be zero, I wonder what would happen if you temporarily force it to be zero? 2.3V is a significant offset. May guide your next steps to find out.

Good luck
Chris

[Marconi_MPT4]

Hi Alan,
That input error of -2.3V possibly suggests the 741 is operating with limited or no negative supply on pin 4.

I just had a look at a data sheet and the input pins can not be biassed within 2 to 3 volts of the supply voltage. What is the measured voltage on V- pin 4?
Rich

[Postmanpop]

Well as a result of the suggestions you good folk have given me I decided to repeat all of my measurements lo and behold I measured a few mV on the 741 pin 4 , clearly nowhere near what it should be. So I repeated all the checks on the fil caps and found that C714 was shorted (don’t know how I missed that!) Anyway I am currently reassembling the board when the eyesight went haywire ������ so I decided to continue tomorrow. I will report back if and when normal service is resumed!

On the assumption that things will go to plan I take this opportunity to thank you all for you very helpful pointers.


Alan

[Chris55000]

Hi!

I've had a chance to study the layout & circuit diagrams!

The –ve rectified supply for pin 4 of IC705 is about –21.5V across C712, developed by the negative half of the small round bridge D738 (W02G if you need a new one!), so if this is missing, either the transformer is faulty (not the case if your +ve o/p is ok, obviously!), or the unmarked reservoir electrolytic C712 is s/c, altho' that would blow a fuse I suspect!

Allowing for a small drop across R726, the zener diode D717 and the small current taken by IC705 from the negative supply, you can expect –15V at pin 4 of IC705!

If the voltage at pin 4 is completely missing, either there's a deat short to earth in IC705, or elsewhere across that part of the circuit, or either R726/D717 is going o/c when power is applied, in which case the best cure is to fling both R726 and D717 in the bin and fit new ones!

(Use a 1.3W type for D717)

Don't forget to check the print carefully for the inevitable nasty hidden cracks tho!

Chris Williams

[Postmanpop]

Hi Rich Paula David and the two Chris's
I am very glad to report that the scope is working again after a further hiccup. I changed C714. I also changed the 10 uF electros and also the 741 IC and the BC237 and BD237 regulator transistors resulting in +12 and 0 volt negative rails. After a bit of head scratching I found that the mica insulating washer under BD237 was in a poor state and not insulating the collector from ground. Replacing the offending washer has cured the problem and now I can get back to my ongoing projects. I also spent an hour or so checking all boards for dry joints and cracks (Any work on boards seems to require extensive strip downs!!!)

Once again thanks again folks


Alan

[Chrispy57]

Hi Alan - well done, I was waiting for an update on your progress and it sounds like a great result after a difficult fault finding exercise. I still like my Hameg 203-7 and use it as my daily 'scope leaving "better" scopes on the shelf for specific purposes. Good luck with the on-going projects (and the eyesight issues!).

Cheers
Chris