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Old 1st Dec 2017, 12:29 pm   #1
Philips210
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Default Hameg HM208 Oscilloscope Timebase fault

Hi.

I have been experiencing a problem with my Hameg HM208 oscilloscope. It was working correctly then the trace suddenly went back to the left of the screen. It seems the timebase stopped. The Y amplifiers are functioning as there's vertical deflection of the dot when then probe centre is touched.
After a while of the scope being off, I once again tried it and found things were back to normal, but after about 7 minutes the timebase appeared to stop once again. This happened on a few occasions but now it's permanently faulty. It would seem the fault was initially temperature sensitive.

I was wondering if any other members have experienced timebase problems with their Hameg scopes and recommend a course of action. I do have the original hard copy manual hiding somewhere in the loft but have also just downloaded a copy from the Rohde & Schwarz website.

Regards
Symon.
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Old 2nd Dec 2017, 4:19 pm   #2
See_Mos
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Default Re: Hameg HM208 Oscilloscope Timebase fault

I haven't seen inside the HM208 but I had an intermittent fault with my HM204. One of the transistors soldered to the PCB and fixed to the chassis at the back had a poor soldered connection. It took months to find because every time I took the case off the fault disappeared until weeks after the case was put back on.

You can of course still use the scope with a probe to check if its own timebase is running, just use the shift to move the spot across a little and turn the brightness down. Assuming it has an X input you could also connect a low voltage AC source to the X input to see if the X amplifier and output stages are OK.

It could just be the X / Y switch?

Last edited by See_Mos; 2nd Dec 2017 at 4:41 pm.
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Old 2nd Dec 2017, 5:22 pm   #3
RogerEvans
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Have a look at the manual so that you can find your way around the scope safely. The standard procedure is first check all the low voltage power supplies for DC level and, if you can ripple, you could use the scope's vertical amplifier to check for ripple amplitude using AC coupling.

Is there a horizontal position control, and does that move the dot? Have you tried all the timebase ranges and also the variable control on the timebase. Is there a trigger light to show that the timebase is triggered and can you get line trigger to work?

This should begin to track the fault to the triggering, timebase generator or the X amplifier. It can be very useful to set the timebase to its slowest speed and then probe with the scope's vertical channel to see if there are any ramp voltages being generated by the timebase and where they disappear.

Roger

Last edited by RogerEvans; 2nd Dec 2017 at 5:23 pm. Reason: added a little to second sentence
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Old 2nd Dec 2017, 8:44 pm   #4
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi.

Thanks for your replies and I appreciate your input.

The power supply voltages seem to be OK on the X-Y and Timebase boards. If the timebase is set to it's lowest speed then the dot pulsates. Increasing the setting likewise increases the flash rate so it would appear the timebase is working. The X control shifts the dot across the screen but not fully.

If the timebase is set to it's highest speed then a horizontal line appears. Reducing back the speed of the timebase also reduces the width of the horizontal line. Further reducing the speed results in just the dot on the LHS of the screen.

I applied the calibration waveform and it will show this if the timebase is set to max or near max.

I can trigger the timebase as per normal so that seems fine.

I am wondering if the problem lies in the X amplifier, although it's working to some degree it seems as if it's working with reduced sensitivity. It could still be the timebase though and it looks to be a tricky problem to solve.

It's all a little annoying as everything seems to be going wrong. I was using this scope to locate a difficult problem in a Korting 82515 TV pattern generator. That's all in pieces, circuit diagrams etc taking up the bench space so I'll have to clear that away from the bench to make room for the Hameg scope. New Year's resolution, "I must build a bigger, better workshop!"

I think I'll remove the X amplifier board for an inspection and cold checks on the transistors.

I could bing my Scopex scope into use but I suspect that needs a good service. All in all I have about 10 oscilloscopes but the Hameg was the only fully function instrument.

I'll try to report back in the coming days.

Regards
Symon.

EDIT: The X-Y mode switch seems to be functioning normally.

Last edited by Philips210; 2nd Dec 2017 at 8:51 pm. Reason: Additional text.
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Old 3rd Dec 2017, 8:44 am   #5
RogerEvans
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Normally the horizontal signal should be DC coupled all the way from the ramp generator to the deflection plates. It looks like something (a switch, connecting lead, transistor ...) has gone open circuit leaving a weak capacitive coupling that passes the sweep signal only at high speeds. You should be able to track this down using the Hameg's vertical amplifier, and the length of the displayed vertical line, to show the amplitude of the timebase signal as you progress from the ramp generator through the stages of the horizontal amplifier. If the height of the signal on the scope changes when you change sweep speed then you are probing after the fault.

Begin by finding the ramp generator circuitry in the manual and checking the output signal. The amplitude should be the same at all sweep speeds, if not then the fault is in the ramp generator and you need to look closely at that. If the ramp generator is OK then work your way forwards looking especially either side of switches and plug-in connectors.

Quick check - if the timebase has a 5x or 10x magnifier then select that and see if it stretches the horizontal display as you would expect. If that works then stages after the gain switching are probably OK.

Roger
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Old 3rd Dec 2017, 9:07 am   #6
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi Roger.

Thanks for your detailed reply, I'll try to have a look at it later today.

Regards
Symon.
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Old 4th Dec 2017, 12:17 am   #7
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi.

I had another look at the scope and brought my old Scopex out of retirement to help assist with the problem. I've deduced that the amplitude of the ramp waveform is constant for all settings of the timebase. This ramp appears to remain constant in amplitude though the X amplifier stages again for all settings of the timebase control.

This oscilloscope also has digital storage capability and I noticed when in this mode the scope appears to be behaving normally so one would think the X amplifier stages should be OK. As I say, the fault was initially temperature dependent but is now permanent. It would seem likely to be a semiconductor fault.

I find the timebase circuit is quite difficult to understand, for my brain anyway.

The service information is available from here: https://cdn.rohde-schwarz.com/hameg-...08_english.pdf

Any further suggestions will be most welcome.



Regards
Symon
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Old 4th Dec 2017, 9:13 am   #8
RogerEvans
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Symon,

You may have guessed from my very generic comments that I had no idea the Hameg 208 was so sophisticated. Apologies for jumping in without finding more for myself.

Looking at the manual, the output of the timebase ramp generator is fed via D507 (on the timebase board if I read it correctly) to the diode switching matrix containing D701 - D710 which looks like it is on the horizontal amplifier board. Can you check that with the Hameg 208 in analogue mode you see the ramp voltage on both sides of D507 and that the peak to peak is essentially independent of sweep speed and the same on both sides of D507. Then repeat the measurement at the junction of R707 and R708. Is the peak to peak the same at all sweep speeds and is it roughly the same amplitude as measured at D507?

Regards,

Roger
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Old 4th Dec 2017, 12:07 pm   #9
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi Roger.

No need to apologise and I appreciate your input as it's a tricky fault to locate.
I checked the ramp waveform at D507, the junction of resistors R707/R708 and even on the X-plates at R750 and R751. The amplitude seems steady at all timebase speeds.

I wondered whether the ramp is actually OK and there's a blanking type problem. Looking at the Block Diagram for the analogue section, I see there's a section within the X-Board designated Unblank Generator. I wonder if this area is relevant and worth further investigation.

Regards
Symon
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Old 4th Dec 2017, 12:16 pm   #10
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi

Something else occured to me. Although the ramp is steady in amplitude, I wonder if the width of the ramp could be the problem. So if it was of insufficient width then could that explain there being only a dot on the left of the screen? Increasing the timebase speed eventually widens the dot to form a horizontal line. So maybe a timing problem with the ramp? I guess there should be a direct correlation between the timebase speed setting and the width of the ramp.

Regards
Symon.
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Old 4th Dec 2017, 2:38 pm   #11
RogerEvans
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Default Re: Hameg HM208 Oscilloscope Timebase fault

When you said that the spot blinked at slow sweep speeds I had a slight worry since the spot should be unblanked for almost the whole time. Anyway it looks like the unblanking is the next thing to be investigated, It looks like the ZUP signal from the timebase is what controls unblanking in analogue mode and if I have the logic the right way round a positive pulse on ZUP will unblank the beam.

ZUP comes from the logic chip K501 and with moderately slow speed speeds the beam should be unblanked nearly all the time. Check that the signals on either side of R503 and on pin 5 (Q) of K501 are all about the same and switch between a low of less than 0.5V and a high of at least 2.5V. Pin 6 (Q bar) of K501 should be the same signal but in the opposite phase. If pin 5 spends most of its time low then either K501 has failed or it is being reset too soon by the CLR input on pin 1. You probably need to use the dual trace feature on the Scopex to look at pin 5 and pin 1 at the same time - triggering on the pin 5 trace only.

Understanding the working of the end-of-sweep logic (that should reset pin 5 of K501) is beyond me at the moment but there is a 'sweep length' adjustment VR501 that you might twiddle to see if it has a bad contact and also try the different Ch1, Ch2, alt, chop options since that seems to enter into some of the unblanking circuitry.

Roger
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Old 4th Dec 2017, 4:17 pm   #12
orbanp1
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi Symon,

In your first post you indicated that the fault seemed to be heat related.
Without getting into the operation of the various circuits, and even if it is now a permanent fault, you might try using freeze spray to identify the faulty component.

You could also verify if the horizontal amplifier works in the X-Y-mode. That would eliminate partly the possible source of error and might get you closer to the faulty section.

Regards, Peter
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Old 4th Dec 2017, 4:26 pm   #13
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi Roger.

Both Channels 1 and 11 are affected in the same way. From your good advice, depressing both the Dual button and the Add/Chop button results in a normal scan. That's giving me a little more confidence to solve this issue, although it's quite complicated working out how everything works. It would have been good for Hameg to have provided a circuit description.

I suppose it's best to first check around those switches. Unfortunately the scope has been out of use for a few months and was in a damp location which couldn't have helped matters.

These scopes are quite well made and what I like about them is very little in the way of exotic components so they should generally be repairable. I did a repair on this scope about 8 years ago which was due to a faulty bridge rectifier, BR1003 (B250 C1500) located on the timebase board.

Regards
Symon

Last edited by Philips210; 4th Dec 2017 at 4:34 pm.
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Old 4th Dec 2017, 4:30 pm   #14
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Quote:
Originally Posted by orbanp1 View Post
Without getting into the operation of the various circuits, and even if it is now a permanent fault, you might try using freeze spray to identify the faulty component.
Hi Peter.

Thanks, I'll try the freezer spray on any suspect components.

Regards
Symon
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Old 5th Dec 2017, 2:38 pm   #15
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Quote:
Originally Posted by RogerEvans View Post
ZUP comes from the logic chip K501 and with moderately slow speed speeds the beam should be unblanked nearly all the time. Check that the signals on either side of R503 and on pin 5 (Q) of K501 are all about the same and switch between a low of less than 0.5V and a high of at least 2.5V. Pin 6 (Q bar) of K501 should be the same signal but in the opposite phase. If pin 5 spends most of its time low then either K501 has failed or it is being reset too soon by the CLR input on pin 1. You probably need to use the dual trace feature on the Scopex to look at pin 5 and pin 1 at the same time - triggering on the pin 5 trace only.
Hi Roger.

The pulse on both R503 and pin 5 of K501 has the trough of the pulses sitting on 0V and the peak at close to 5V so that would appear to be OK.
The pulse changes only in a subtle way when the Add/Chop and Dual buttons are depressed.
Also, the ramp waveform seems unaffected by the Add/Chop and Dual button position but as I say if these two buttons are pressed then the scope seems to be fine. Also the ZUP signal is essentially the same for all combinations of Add/Chop and Dual switch settings.

One thing I did notice is a variation between modes on the signal marked Alt. Pulse AMP. The DC level and pulse amplitude are different but the pulse width is the same.
If both Add/Chop and Dual buttons are in then the trough of the signal sits on 0V with an amplitude of 3.6V.
When Add/Chop and Dual buttons are out, the trough sits on a DC level of +1.6V and the peak is at 3.6V so an amplitude of 2V. Whether this is significant I can't tell. I can't quite make out where the Alt. Pulse AMP signal comes from and I have to admit to being perplexed at this fault. It would be great to sort this scope out as it's been a dependable old friend in the workshop.

Regards
Symon
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Old 9th Dec 2017, 5:13 pm   #16
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi.

I have take a few pics off screen from the scope showing the faulty display. The front panel including green filter was removed so the traces appear to be fuzzy but in reality they're clearer than the pics show. In most of the pics, all but the last 2 of 10, there was no input signal applied to the Y input.

Pic (a) is with the timebase control set to 100ms/cm, note there's just a dot on the left of the screen

Pic (b) timebase set to 20us/cm

Pic (c) timebase set to 5us/cm

Pic (d) timebase set to 2us/cm

Pic (e) timebase set to 1us/cm

Notice the width of the trace is increasing as the timebase rate is increased.

More pics to follow.

Regards
Symon.
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Last edited by Philips210; 9th Dec 2017 at 5:31 pm. Reason: correction
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Old 9th Dec 2017, 5:31 pm   #17
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi

Further Pics added:

Pic (f) Timebase set to 0.5us/cm

Pic (g) Timebase set to 0.2us/cm

Pic (h) The Add/Chop and Dual buttons depressed and an apparently normal display is shown even at low timebase speeds.

Pic (i) A 1kHz Calibration signal applied to CH1 input, Timebase set to 2us/cm

Pic (j) A 1MHz Calibration signal applied to CH1 input, Timebase set to 0.2us/cm.


Note how far off accuracy the trace is particularly in Pic (i) This signal is occupying approx 5.3cm so T = 10.6us, therefore f = 94.3kHz. The signal is meant to be 1kHz. I checked this calibration signal on my Scopex scope and all's well there.
The inaccuracy seems to improve as the timebase speed is increased. Its as if there's non linearity in the trace. I get the impression that the trace is being compressed at low timebase speeds.

Any further help with this will be much appreciated. Thanks.

Regards
Symon.
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Old 10th Dec 2017, 2:26 pm   #18
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi.

Would it be daft to suspect a fault with the CRT or CRT socket? If one or both X plates were not making proper contact with the X amplifier, say just a capacitive coupling rather than direct, could this account for the loss of trace at low frequencies? Roger mentioned the possibilty of a weak capacitive coupling in post #5. This would seem quite plausible.
It would be good to be able to apply a range of voltages and polarities to the X plates to rule out the CRT.
As I mentioned before, the problem occured after about 7 minutes use. Switching off for several minutes, reapplying the supply and all was well again. This repeated a few times but now it's permanently faulty.

I checked again the ramp signal at the input to the X amplifier at R707 and on one of the X plates and the waveforms closely match apart from obvious differences in amplitude.
I also thought about the possibility of a fault on one of the three boards associated with the digital storage possibly affecting the blanking of the signal.

It's a complex oscilloscope complicated further by having the digital storage. I'd really like to get this working again but the service information is very limited. Some useful waveforms and voltage test points wouldn't have gone amiss.
I don't suppose any member has the same oscilloscope and has taken various measurements around the timebase and X amplifier?

Regards
Symon
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Old 10th Dec 2017, 4:20 pm   #19
RogerEvans
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Symon,

In dual + chop mode (pic h) there should be two traces, is one just off the bottom of the screen?

Are (i) and (j) also in 'dual + chop' mode? The two traces look absolutely identical apart from a slight horizontal shift, is it correct that one is 1000Hz and the second is 1000,000Hz? Can you generate both pictures again with both traces on screen so that you can see if the 'chop' between the two channels is interfering with the displayed signals? Generally you would use 'alt' with fast signals so that each channel is drawn on alternate sweeps and 'chop' with slow signals where there is time to switch the spot back and fore many times in a single sweep.

You said in one post that the ZUP signal looked the same in single beam and 'dual+chop' mode, so for now let's assume ZUP is OK when it leaves the timebase board (you could say just where you measured it in case we come back to it later).

If the sweep speeds are correct then pictures (b), (c), (d) and (e) are consistent with the unblanking lasting for about 3usec and fading away over 2usec. It should stay active for the whole duration of the sweep.

Chop mode appears to activate the CHB signal from 'Y intermediate amplifier' board, I think CHB is short for CHop Blanking and on other scopes that I have used, this signal blanks the beam while it is moving from displaying channel 1 to channel 2 and then unblanks it in the new position. Activating this signal is probably what makes the display start working. The manual says that the chop frequency is around 500kHz so a new unblanking pulse is generated every 2usec or so which is faster than the 3-5usec we deduced from (b) to (e) pictures.

ZUP, CHB and the unblanking signal from the digital side of the scope (ZW) get combined in the circuitry around T903, T904, T905 and then drive the opto-isolator IC901. A good place to start would be to set the sweep speed to 2usec and, using a well insulated scredriver, adjust VR901 a little in each direction to see if it has an intermittent contact. BE VERY CAREFUL around VR901, it appears to be on the -2kV high voltage board and the potential for harm to yourself or the scope is not negligible. If in doubt don't touch it! If IC901 is socketed, then WAIT 30 MINUTES after turning off the scope and unplugging it , then remove and reseat IC901 to clean poor contacts– one side of the opto-isolator is at -2kV so you really need to be sure the -2kV is discharged properly.

Regards,

Roger

Last edited by RogerEvans; 10th Dec 2017 at 4:23 pm. Reason: IC901 acquired too many zeroes!
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Old 10th Dec 2017, 5:03 pm   #20
Philips210
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Default Re: Hameg HM208 Oscilloscope Timebase fault

Hi Roger.

Thanks again for your detailed reply. I'll check my notes and take a few more pics this evening and report back. I think that I've proved the CRT is OK, more about this later.

Regards
Symon.
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