Hameg HM 203-6 scope needs a simple calibration (SOLVED!) - Page 1
Forum section: electronics / testgear

author: Calambres
author_rank: Frequent Contributor
timestamp: October 01, 2025, 05:46:40 pm
content: Hi there!My old trusty Hameg HM 203-6 scope needs calibration for the vertical gain adjustment in channel 2. The readings are a bit on the low side when the "CAL" knob is set to the "CAL" position (full counterclockwise with a "click"). Channel 1 is OK.I have some technical manuals for this scope but none of them with the calibration instructions. Anyone?Reading the schematics it seems that the adjusting trimmer should be VR102, but I'm not sure. See this from theSchematics (Page D3):The red circle is the CAL potentiometer itself and I presume that the calibration trimer is the one in the green circle: VR102 tagged "Gain Adj". See the next image with a close up:As you can see, the second image represents VR102 with a green square in the board itself. Providing I'm right, I see a problem: that trimmer is in a vertical position that is almost impossible to adjust because of its position and the lateral metallic tabs surrounding the board. Besides, it is flanked with an IC above and a variable capacitor below... almost no place to get a trimming tool there!Have a look at this:The last image is from a youtube file:https://youtu.be/goZZCXTjEYs?t=677Am I right?Is this the adjusting trimmer for the channel's input gain?Anyone have the calibration instructions for this scope?Thanks a lot in advance.

author: Calambres
author_rank: Frequent Contributor
timestamp: October 02, 2025, 12:09:18 pm
content: Well, problem solved. It was indeed VR102 but, boy... is it a pain in the butt to tweak that trimmer?It is absolutely impossible to tweak that VR with a standard trimmer tool and I have a lot of them in many shapes and flavours. None of them worked as it is impossible to access the center slot of that 2K5 vertical PIHER trimmer. One side is obstructed by a variable capacitor trimmer and the other side by an adjacent IC (UA733CN).The only viable solution was to slightly bend the end of a small flat screwdriver...to get into the left-hand side of VR102, close to the UA733CN:A small tweak and now the second channel of my 203-6 is measuring correctly in the vertical axis.Still... if someone has the calibration instructions for this scope, make'em available here, please!

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 04, 2025, 09:41:08 am
content: Quote from: Calambres on October 02, 2025, 12:09:18 pmThey should have simply used a horizontal trimmer potentiometer. There would certainly have been enough space on the board.My solution:I made a tool. I filed the tip of a 3 mm nail flat, like a screwdriver, and then bent the front 3 mm of it 90° in a vise. The tip can now be inserted into the trimmer from the side and adjusted using the resulting lever.

author: Zenith
author_rank: Frequent Contributor
timestamp: October 04, 2025, 03:25:24 pm
content: Hameg manuals are not up to the standard of HP, Tek etc. I have an HM1005, which has no access holes in the shields for the presets on the pre amplifier boards. It was as if they expected it never to need adjustment after it left the factory. After the scope was released there were revisions to a couple of the PCBs, not covered by the manuals available. Mine is an early model. It needed adjustment. A very kind gentleman at SKY Messtechnik GmbH sent me circuit diagrams which were very helpful, but there was nothing like a factory set up procedure. I had to do it using hints on the diagrams showing adjustment points and experience of setting up other scopes.Like Greybeard I've often had to make adjustment tools. Something I've found useful is the tip of a scope probe compensation tool fixed into a tube from a ball point pen. It has long reach, and a combination of stiffness and flexibility.

author: Calambres
author_rank: Frequent Contributor
timestamp: October 04, 2025, 04:16:46 pm
content: Quote from: Zenith on October 04, 2025, 03:25:24 pmWhat bugs me is that there are calibration instructions for the 203-4, the 203-5 and for the 203-7 but not for the 203-6...at least not at my reach!Similar scopes but not the same.

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 04, 2025, 05:57:34 pm
content: Quote from: Zenith on October 04, 2025, 03:25:24 pmI know what you mean, but there is no such "HP or Tek standard."HP manuals from the 60's, 70's often contained complete functional descriptions of the circuit diagrams. However, the newer the devices, the fewer descriptions and circuit diagrams were available.The same was true of the Hameg manuals. They didn't contain functional descriptions, but often complete circuit diagrams. Hameg often had many different versions of the same series (e.g. HM512-1, -2, -3, -4, -5, -6, -7, -8), and sometimes some parts of the circuit diagrams or adjustment precedures were assigned incorrectly. But here, too, the newer the devices, the fewer circuit diagrams or other info were available.As far as I know, after the end of the CRT era, no circuit diagrams were available at all.

author: F64098
author_rank: Regular Contributor
timestamp: October 04, 2025, 10:37:56 pm
content: Have fun with it and also success!The 203-6 was my first scope, bought for 1023,- DM (~550€) sometimes in the 80's.Best regardsFrank

author: Zenith
author_rank: Frequent Contributor
timestamp: October 04, 2025, 11:11:13 pm
content: I haven't looked at Tek/HP/LeCroy/R&S manuals from anything they produced in the last 25 years, but I guess it's gone the same way as electronics products in general; ASICS, µcontrollers, specialised diagnostic rigs and and so on. The best their dealers might do is a board level swap. At one time there was a healthy TV repair industry, which included such things as specialists regunning CRTs. That's long gone.As for this discussion, comparing manuals from Tek 400 series and HP1700 series with Hameg, well, they were operating in different markets. I have two 203-5s, a 203-6, a 203-7, a 60 MHz and an HM1005 - also a 20MHz Hameg storage scope which seems to work. I can't take it seriously, but it only cost £5 and the seller threw in a Tek 465B manual.What I don't like about Hameg scope manuals I've seen:No parts list.No proper diagnostic or set up scheme.No theory of operation description.Only OK schematics.Undocumented changes - at least to the higher end ones. I did come across a 203 sold cheap on ebay because it had an SPMS which had failed and the seller had been unable to find anything about it.

author: Calambres
author_rank: Frequent Contributor
timestamp: October 05, 2025, 08:18:33 am
content: Quote from: F64098 on October 04, 2025, 10:37:56 pmfrank, thanks a lot but I already have that manual. It has a "chack plan" and "Maintenance instructons" but they're quite vague and there's not a proper "calibration guide" which is what i'm looking for... unlike the service manuals for the 203-4, 203-5 and 203-7. The service manuals for those do have calibration instructions.

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 05, 2025, 05:25:37 pm
content: What exact calibration problem do you have?

author: F64098
author_rank: Regular Contributor
timestamp: October 05, 2025, 05:50:15 pm
content: Are the differences in the schematics so big, that you can't use the manual for the -5 for adjusting your -6?I would give it a try, comparing the -5 manual with the -6 reality.Best regardsFrank

author: Calambres
author_rank: Frequent Contributor
timestamp: October 06, 2025, 05:38:21 am
content: Several:- Astigmatism out of whack -> easy to correct. The adjusting trimmer is clearly labeled- Zero reference for invert/non invert channels VERY out of whack- Proportionality of x measures not... proportional- x scale measures a bit too high.- XY mode unadjusted- Square wave flanks and corners not sharp 90º with some overshoot in the rising edge and verticals slightly slanted.- The power supply surely needs adjustment. Not sure how to deal with those -1900V- Some others I haven't found yet...Quote from: F64098 on October 05, 2025, 05:50:15 pmYep!... that's my only chance providing there's no way to get hold of the instructions for the 203-6. The 203-5 is pretty much the same scope.

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 06, 2025, 08:36:47 am
content: QuoteOK, done.Before turning any trimmer, mark all the positions of the trimmers you want to adjust with a pencil or permanent marker.Then let the oscilloscope warm up for about 30 minutes or more.QuoteYou should check and adjust that first.Check VDC and hum.QuoteThe HM203-6 has no -1900V adjustment, so you can skip the following part.The following method is risky! Only use it if you are absolutely sure. If you're a beginner leave your hands off.High voltage may be hold for some minutes after switching the oscilloscope off!If you havetwoidenticaldigital multimeters, each capable of handling 1000 VDC, you can connect them in series.Use the manual voltage range of 1000 V or higher. Set the same voltage range on both digital multimeters.Check the input resistance of each digital multimeter by measuring it with the other.Both input resistances must be identical.If one of the meters changes range while measuring 2000 V (autorange possibly change input resistance), one or both meters may be damaged.Therefore, usemanualranges (1000V).Connect the 'minus' of the series circuit to chassis ground.Use probes and cables rated for 2000 V or more!Hold the probe with one hand only (put the other hand in your pocket)! Or better: switch off the oscilloscope, connect all connections to the DMMs by clamps, then switch the oscilloscope on, read the voltages and switch it off again! Work while another person is watching to immediately turn off the circuit breaker if a problem occurs.QuoteAre you referring to the INV balance? Set the input switch to GND. Adjust the horizontal line to grid zero.Try adjustingVR104[VR12@HM203-5]. Repeatedly toggle between Y-INV on and off. The horizontal line should not move up or down.If adjustingVR104[VR12@HM203-5] makes no change, try adjustingVR101[VR10@HM203-5]. If this is OK, adjust the other trimmer of the two while repeatedly turning the Y-VAR potentiometer clockwise and counterclockwise.The horizontal line should not move up or down.Repeat the entire process several times until neither the Y-VAR potentiometer nor the Y-Inv switch moves the line up or down.HM203-5 /HM203-6:HM203-5 /HM203-6:The circuits are nearly identical but component numbering is different.QuoteDo you mean linearity?Check the sawtooth linearity with another oscilloscope at the sawtooth checkpoint.QuoteHM203-5: Adjust the horizontal line length to about 10 cm (10 divs) using VR51. The manual states 10.2 cm, but I set it to 10 cm (checking at several x-deflection rates).HM203-6: I could not find a trimmer to adjust the horizontal line length in the timebase circuit. Probably you have to adjust x-gain instead (VR708).Set the x-deflection rate to 50 µs/cm. You can adjust it with aVR503[VR52@HM203-5].Set the x-deflection rate to 0.5 µs/cm. You can adjust it with aVC501[VC52@HM203-5].Check that the horizontal line length is 10 cm..Repeat the whole procedure if necessary.HM203-5 /HM203-6:Finally you should checkx-Magn. x10by applying time marker signal and adjusting VR709 (VR72@HM203-5) on the XY board.QuoteWhat exactly is wrong?QuoteAre you sure your square wave generator signal and probe adjustment are correct?Please show pictures.By the way:I have to say, the adjustment plan for the HM203-5 is almost self-explanatory, whereas for the HM203-6 there are only a few notes on the PCB printing.EDIT: Added some pics, adapted text.

author: Calambres
author_rank: Frequent Contributor
timestamp: October 06, 2025, 04:31:18 pm
content: Wow!... thanks mate!

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 07, 2025, 11:27:13 am
content: Quote from: Calambres on October 06, 2025, 04:31:18 pmVery nice guitar website!

author: Calambres
author_rank: Frequent Contributor
timestamp: October 08, 2025, 07:11:59 am
content: Thanks, mate!

author: TaylorD93
author_rank: Regular Contributor
timestamp: October 08, 2025, 08:31:47 pm
content: Just a heads up, being someone who has played around alot with the Hameg gear in recent years, ive tried to download as many service manuals etc as possible.The 205-2 uses the same front end as the 203-6, likewise the 205-3 has the same front end as the 203-7. The main difference being the 205 models have the digital memory.If you look at the 205-2 service manual, you will find afull adjustment planwhich covers the input attenuators and i would put good money on covering most if not all of the calibration points of the 203-6. Yes there will be some differences in PSU as the 205-2 has a large 5V supply for the logic. however if youre concerned about the input attentuator adjustments, it will have your answershope that helps!

author: Calambres
author_rank: Frequent Contributor
timestamp: October 09, 2025, 05:04:59 am
content: Thanks!!!The 205-2 service manual has very detailed calibration instructions.This topic is very useful to me. Thanks to all!

author: TaylorD93
author_rank: Regular Contributor
timestamp: October 09, 2025, 06:25:44 pm
content: some of the VR VC numbers vary between the 205-2 and 203-6 but when you look at the schematic, they are located in the same places so you can cross reference the component references to the test plan quite quickly.I should have addded that to my last message.thanks,

author: Calambres
author_rank: Frequent Contributor
timestamp: October 10, 2025, 06:15:56 am
content: Yep... I've seen that.Thanks again.

author: Calambres
author_rank: Frequent Contributor
timestamp: October 10, 2025, 05:29:28 pm
content: Greybeard, I have sent you a private message