HAMEG 203-7 Calibration issues - UK Vintage Radio Repair and Restoration Discussion Forum

LancsRick · 13th Nov 2016, 5:01 pm

Evening all!

I've picked up a HAMEG 203-7 scope which is (nearly) in good working order - frequency cal seems spot on, but voltage is an issue, off by about 10%.

I've found both user and service manuals, but there's a total lack of information on the calibration procedure. Plenty of pots to go out on the PCB's it would appear. Can anyone advise please on the procedure as I'm hoping this can be repaired!

Cheers.

David G4EBT · 13th Nov 2016, 7:13 pm

Have you checked the V/div using the internal 2V & 0.2V square wave calibrator and if so, does that show incorrect voltages?

Wen you say 'out by about 10%' you don't say whether that's high or low. Without wishing to state the obvious, if it's 10% low, have you got the variable gain controls - the little round knobs in the centre of the Volts/Div controls fully rotated clockwise? If not, the voltages will read low - if turned fully ccw, probably 10% low. Those knobs, by the way, shouldn't continuously rotate, they should come to a stop at the cw and ccw positions. If they don't, it means that the pot is jiggered. They're special pots with the shaft fixed on with a small Allen grub screw and the plastic of the pot becomes brittle with age, so the front panel control which is concentric with the V/Div switches on each channel just twiddles round and round. I'm not saying that's the problem, just that it's a known issue with the 203-7, and maybe others in the same family.

When you say you've found the service manual, have you also downloaded the 13 pages from the German manual which has all the circuits and PCB layouts which aren't in the English language one? If you don't have that, you may still be able to download it FOC from the Hameg legacy site at Rhode-Schwarz website. However, though at the link below you'll see that the manual is still (allegedly) downloaded in Spanish and German, I was only able to open the English version.

https://www.rohde-schwarz.com/uk/sea...sort=relevance

If you don't have the German one and can't get it, drop me a PM with you email if you'd like it and I'll wing it to you. (5MB).

Excellent scopes - I've got two - one of which has defective variable gain pots, which I intend at some point to repair by making brass sleeves to hold the cracked plastic part at the rear of the pots together, but it's quite a palaver to strip the scope down to get at the pots. (front panel off etc). In other respect the scope is fine, and I've managed to rotate the pots to the 'cal' position so that the voltages read accurately. I don't often find that I need to use the scope as a 'voltmeter' - more interested in observing waveforms.

Hope that might help a bit.

Good luck with it.

LancsRick · 13th Nov 2016, 7:19 pm

Thanks but I've gone past that. The front panel pots function correctly, and the display is consistently low on cal, a known voltage source, and with different probes. The cal adjusters affect the display correctly, they just don't go 'high' enough to correct it.

I've got the full service manual so I'm sorted there thanks

.

Having cracked open the unit, I found vr201 which gives the 12v reference voltage, and that is at 11.43v, with adjustment of the pot only making. 01v difference. I've stopped for the night now but I guess I'm into gradually tracing the fault back unless this sounds like a familiar/common issue to anyone?

LancsRick · 13th Nov 2016, 11:48 pm

Right, on to the next problem.

Through the manual I managed to tweak the Y-Gain adjusters directly to compensate for the 11.43v at the 12v reference point. Returning to check the cal waveform now, I'm pleased to say that the vertical div measurement is correct. Hurray! However...

The duty cycle for the cal waveform seems to be wrong. it's approx 1:1.5 on off. I don't have a square wave generator so I'll have to borrow one, but any ideas on what might be causing that? If it's just confined to the cal signal then I'm not fussed, but I need to ascertain that.

WaveyDipole · 14th Nov 2016, 5:48 pm

LancsRick, have you checked that you are actually getting the correct voltages on all of the DC power supply lines? You might have a leaky capacitor dragging the line voltage down. I would suggest that there is no point rushing to make changes to the calibration settings until you establish why you are not getting the correct 12v reference voltage.

LancsRick · 14th Nov 2016, 6:43 pm

Good counsel. I won't have chance to work on it again until the weekend but I'll start again with a clear head and be a bit more methodical

. Thanks.

LancsRick · 20th Nov 2016, 11:11 am

I thought it would be rude not to post back here as I've now for the scope up and running.

Long story short, the calibration signal generator on the scope was the issue, not the scope itself. Having made a trip to my dad's place and borrowed some of his kit, I managed to do a 3-way verification of both the frequency and voltage gain accuracy of the scope, so I'm really happy now.

Wavey's absolutely right in that I might be compensating for a fault elsewhere, but given my usage is typically very crude (e.g. tracking down the 100hz output buzz I have on my first valve amp restoration), this isn't being used as a precision instrument by any means. I know, I'm a heathen!

Thanks for all the advice, and if anyone sees this and wants the manuals for a 203-7 then feel free to drop me a message and I'll email them over.

MotorBikeLes · 20th Nov 2016, 12:07 pm

If you have questions about internal calibrator accuracy, but don't have proper calibration gear, there is a simple check you can do:-
Assume calibrator is correct. Set Y-gain to suit. Set to DC coupled if not already set, and connect a 1.5v cell across scope input, with your best AVO/DVM also in parallel across scope input. Avo/DVM should read same as scope, if not, it is fair to assume calibration waveform is incorrect. If internal calibration needs a different attenuator setting for the battery test (maybe 10mV/div), then a simple divider resistor across the battery will get the input level suitable, and any decent DVM will still give good check.
Les.

13th Nov 2016, 5:01 pm	#1
LancsRick Retired Dormant Member Join Date: May 2015 Location: Preston, Lancashire, UK. Posts: 10	HAMEG 203-7 Calibration issues Evening all! I've picked up a HAMEG 203-7 scope which is (nearly) in good working order - frequency cal seems spot on, but voltage is an issue, off by about 10%. I've found both user and service manuals, but there's a total lack of information on the calibration procedure. Plenty of pots to go out on the PCB's it would appear. Can anyone advise please on the procedure as I'm hoping this can be repaired! Cheers.

13th Nov 2016, 7:13 pm	#2
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,766	Re: HAMEG 203-7 Voltage cal isue Have you checked the V/div using the internal 2V & 0.2V square wave calibrator and if so, does that show incorrect voltages? Wen you say 'out by about 10%' you don't say whether that's high or low. Without wishing to state the obvious, if it's 10% low, have you got the variable gain controls - the little round knobs in the centre of the Volts/Div controls fully rotated clockwise? If not, the voltages will read low - if turned fully ccw, probably 10% low. Those knobs, by the way, shouldn't continuously rotate, they should come to a stop at the cw and ccw positions. If they don't, it means that the pot is jiggered. They're special pots with the shaft fixed on with a small Allen grub screw and the plastic of the pot becomes brittle with age, so the front panel control which is concentric with the V/Div switches on each channel just twiddles round and round. I'm not saying that's the problem, just that it's a known issue with the 203-7, and maybe others in the same family. When you say you've found the service manual, have you also downloaded the 13 pages from the German manual which has all the circuits and PCB layouts which aren't in the English language one? If you don't have that, you may still be able to download it FOC from the Hameg legacy site at Rhode-Schwarz website. However, though at the link below you'll see that the manual is still (allegedly) downloaded in Spanish and German, I was only able to open the English version. https://www.rohde-schwarz.com/uk/sea...sort=relevance If you don't have the German one and can't get it, drop me a PM with you email if you'd like it and I'll wing it to you. (5MB). Excellent scopes - I've got two - one of which has defective variable gain pots, which I intend at some point to repair by making brass sleeves to hold the cracked plastic part at the rear of the pots together, but it's quite a palaver to strip the scope down to get at the pots. (front panel off etc). In other respect the scope is fine, and I've managed to rotate the pots to the 'cal' position so that the voltages read accurately. I don't often find that I need to use the scope as a 'voltmeter' - more interested in observing waveforms. Hope that might help a bit. Good luck with it. __________________ David. BVWS Member. G-QRP Club member 1339.

13th Nov 2016, 7:19 pm	#3
LancsRick Retired Dormant Member Join Date: May 2015 Location: Preston, Lancashire, UK. Posts: 10	Re: HAMEG 203-7 Voltage cal isue Thanks but I've gone past that. The front panel pots function correctly, and the display is consistently low on cal, a known voltage source, and with different probes. The cal adjusters affect the display correctly, they just don't go 'high' enough to correct it. I've got the full service manual so I'm sorted there thanks . Having cracked open the unit, I found vr201 which gives the 12v reference voltage, and that is at 11.43v, with adjustment of the pot only making. 01v difference. I've stopped for the night now but I guess I'm into gradually tracing the fault back unless this sounds like a familiar/common issue to anyone?

13th Nov 2016, 11:48 pm	#4
LancsRick Retired Dormant Member Join Date: May 2015 Location: Preston, Lancashire, UK. Posts: 10	Re: HAMEG 203-7 Calibration issues Right, on to the next problem. Through the manual I managed to tweak the Y-Gain adjusters directly to compensate for the 11.43v at the 12v reference point. Returning to check the cal waveform now, I'm pleased to say that the vertical div measurement is correct. Hurray! However... The duty cycle for the cal waveform seems to be wrong. it's approx 1:1.5 on off. I don't have a square wave generator so I'll have to borrow one, but any ideas on what might be causing that? If it's just confined to the cal signal then I'm not fussed, but I need to ascertain that.

14th Nov 2016, 5:48 pm	#5
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: HAMEG 203-7 Calibration issues LancsRick, have you checked that you are actually getting the correct voltages on all of the DC power supply lines? You might have a leaky capacitor dragging the line voltage down. I would suggest that there is no point rushing to make changes to the calibration settings until you establish why you are not getting the correct 12v reference voltage.

14th Nov 2016, 6:43 pm	#6
LancsRick Retired Dormant Member Join Date: May 2015 Location: Preston, Lancashire, UK. Posts: 10	Re: HAMEG 203-7 Calibration issues Good counsel. I won't have chance to work on it again until the weekend but I'll start again with a clear head and be a bit more methodical . Thanks.

20th Nov 2016, 11:11 am	#7
LancsRick Retired Dormant Member Join Date: May 2015 Location: Preston, Lancashire, UK. Posts: 10	Re: HAMEG 203-7 Calibration issues I thought it would be rude not to post back here as I've now for the scope up and running. Long story short, the calibration signal generator on the scope was the issue, not the scope itself. Having made a trip to my dad's place and borrowed some of his kit, I managed to do a 3-way verification of both the frequency and voltage gain accuracy of the scope, so I'm really happy now. Wavey's absolutely right in that I might be compensating for a fault elsewhere, but given my usage is typically very crude (e.g. tracking down the 100hz output buzz I have on my first valve amp restoration), this isn't being used as a precision instrument by any means. I know, I'm a heathen! Thanks for all the advice, and if anyone sees this and wants the manuals for a 203-7 then feel free to drop me a message and I'll email them over.

20th Nov 2016, 12:07 pm	#8
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: HAMEG 203-7 Calibration issues If you have questions about internal calibrator accuracy, but don't have proper calibration gear, there is a simple check you can do:- Assume calibrator is correct. Set Y-gain to suit. Set to DC coupled if not already set, and connect a 1.5v cell across scope input, with your best AVO/DVM also in parallel across scope input. Avo/DVM should read same as scope, if not, it is fair to assume calibration waveform is incorrect. If internal calibration needs a different attenuator setting for the battery test (maybe 10mV/div), then a simple divider resistor across the battery will get the input level suitable, and any decent DVM will still give good check. Les.