Test Equipment Anonymous

These came from a swapmeet for the price of a packet of 20 not the cheapest cigarettes for the pair. One I was told worked a bit, the other was for spares. Both were covered in stickers and grime.

HM203-7, works a bit.
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It was missing a knob from the Channel II Volts/Div fine control and the shaft was bent. I thought that may have been deliberate. Both attenuator controls and the timebase control were very stiff. It produced an OK trace from channel I but channel II was about a third the amplitude expected. It didn't trigger properly.

The bent shaft was straightened with blunt nose pliers. A knob was borrowed from another piece of Hameg gear. The attenuator and timebase controls were stiff because the original grease had hardened. A couple of spots of lighter fuel (purified petrol) were put on the end of the shafts with the scope tilted so it ran into the bearing. After couple of minutes to soak in, the controls were turned back and forth a few times and loosened up. A blob of silicone grease about 1.5mm diameter was put where the shaft goes into the bearing and the controls turned back and forth. The silicone grease is semi liquid in the warm weather we've been having. The controls were now free moving with a definite snap as they went into each position.

All switches were cleaned. Most of them are Schadow slide switches, which have plastic housings and look as if they snap together. I didn't try to take the covers off. I cleaned off the surface with a cloth and put about five drops of pure IPA on each with a small paint brush, enough so it could be seen in the gaps at the sides of the switches. The switches were worked back and forth about 20 times. The rotary switches were cleaned by squirting contact cleaner through small gaps.

The proper way to do this is probably to disassemble everything, clean off the hardened grease, relubricate and reassemble. That's a lot more work and might involve desoldering. It occurred that there was a good chance of damaging something in the course of doing it.

The scope was powered up. Supply voltages were set properly and checked for ripple. All features now worked. The calibration was done. Hameg didn't publish a factory calibration procedure, so it was based on the information they did publish and experience with similar scopes.

It all seems to work as it should. A bit boring really. It's a nice basic analogue scope.

HM203-5 Spares or repairs.
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It was particularly grimy. It produced a shortened sweep at low timebase speeds. It produced two full traces in chop mode. The component tester didn't work and X-Y mode was strange. Triggering didn't work in normal mode.

The grime and stickers are fairly easy to deal with with a good dose of kitchen surface cleaner. A plastic dish scourer and a nail brush work wonders. Some persistent stickers need the topical application of boiling water. IPA can remove some of the gum left from stickers.

Switches and stiff shafts were sorted out as with the first scope, voltages were set and ripple checked. The attenuator wafer switch is behind a metal shield but it was possible to give it a squirt of Deoxit from the side. The problem with the component tester not working and the short trace remained. Another problem appeared. After the scope had been on for about ten minutes the traces became fuzzy and in XY mode instead of seeing a spot, it produced a small circle. Another scope showed a 7.4MHz 1V ptp ripple on the -12V, 24V and 12V supply lines. The electrolytic caps in the PSU that were accessible were changed. The problem remained. I decided it had to be a 10µF capacitor on the output of the 24V regulator. Unfortunately, this is impossible to access properly without removing a board, and it's surrounded by wires from the mains transformer. I did a very nasty bodge. I cut through the old cap with a pair of side cutters and removed the remains, leaving its wires. I soldered the new cap onto the existing wires. The problem with the 7.4MHz ripple disappeared.

The problem with the short trace remained. I checked the outputs of the X and Y amplifiers to the CRT. Everything was as expected with no real difference in chop mode. Eventually it dawned that it might be to do with flyback suppression. The unblanking pulse looked as it should. I didn't try examining the -1900V with 22V blanking level imposed. Hameg recommend a makeshift HV probe using a 10nF 2kV capacitor. I did sort out a 4kV ceramic capacitor but didn't get round to it. The blanking signal at TTL levels is superimposed on the -1900V using an optoisolator. There's a bias current adjustment for the optoisolator. The penny dropped. I turned the adjustment and proper traces appeared. I suppose I should set it up properly as suggested in the manual. The component tester now worked and XY mode was as expected.

I went through the calibration procedure and the scope seemed to be behaving as expected. Then I noticed another problem. The scope could trigger on a slow square wave, but not a triangular wave using internal triggering, but it worked with the external trigger. It turned out to be the int/ext trigger switch. It must have been not making contact, but there was capacitative coupling, explaining why a fast edge worked. I may have missed that switch the first time as it is tucked away, but a dose of the IPA treatment sorted it out.


Another basic analogue scope. I'd say it works as well as it ever did. I'd be happy to sell it on with a clear conscience, as a properly working scope.

Comparing the two, the 203-5 was probably from 86/87 and the 203-7 from after 1990. Of course they are similar. The 203-7 has a few extra features and is an updated design with different ICs and transistors. They also seem to have fixed a few irritating things about the 203-5. The component IDs are silk screened on the circuit boards, which makes repair work easier. Some of the 1000µF reservoir capacitors are increased to 2200µF. I always thought they were too small in the 203-5 and 203-6. High wattage resistors are more sensibly placed. The 203-7 circuit diagrams are better and have the voltages and waveforms expected.

I really don't like the EHT arrangement on the 203-5. It's Cockroft-Walton multiplier run from a 480V winding on the mains transformer, not the more usual high frequency transformer. That has advantages. The problem is it's all on one of the two main boards and the EHT circuitry is not covered. The EHT is taken to the rear of the CRT with a wide spaced ribbon cable along with low voltages a It all seems a bit doubtful to me. Worse is the check point header. The first 7 positions are -12V, +5V etc, position 8 is not connected, positions 9 and 10 carry -1900V. It would be so easy to make an unpleasant mistake. A warning printed or a plastic plug to be removed so access was a deliberate act, would have been considerate.

Dismantling is a problem on most scopes. Gaining access to the solder side of the main boards on these scopes is something of a mission. You can access some of the components from the side.

It's amazing how many faults on old scopes are caused by dirty contacts or bad caps, or maybe some preset having been twiddled.

I have more challenging and interesting projects in line.

Altogether, the scopes were more fun than 20 mid-priced cigarettes, not to mention healthier.

Your cap bodge is no worse than many random factory bodges I've seen... good work.

Of course, the 203-7 would probably go on the back bench for the curve tracer, just as I kept that old V-212 for most of my life as a repair tech. I suspect you prolly have a 577 or its ilk tho, so don't likely actually need it.

mnem
tzzzzzzt.

My go to scope is a 100MHz Siglent SDS 1102 CML+ - with the enhancement to make it 150MHz. It's the size of a shoe box and very convenient. Siglent and Rigol take software updates seriously and generally have a good reputation. The next go to scope is a Tek 475. There are several others.

I was interested in the Hameg 203 scopes because 30 odd years back the basic introductory scope was dual trace 20MHz. There were a load of them on the market starting from about £250 and going up. IIRC the Hamegs were £350. I paid £225 for a Topward on special offer. I paid an extra £10 for the service manual - just as well as it turned out. I used it when I wanted a scope for 15 years. It eventually played up because of an EHT problem, which was easy enough to fix with the service manual - the manual was not on the WWW. The Topward still works but could do with some TLC.

I always wondered whether the Hamegs were worth the extra, so when I saw them for next to nothing I bought them. They are better made with better quality switches and have stood the test of time better. However, if I could go back and advise my younger self, I wouldn't be insisting on blowing the extra on a Hameg. Inflation has been several times since then.

I'd say for a lot of people, especially those on a tight budget, they are an adequate scope, providing there's an assurance they work. I'd say they are a better bet than the el cheapo DSOs and easier to use.

Around 1993 the company I worked for, part of which made oscilloscopes, needed some basic analogue scopes for a production line. They bought Hitachi, which were substantially more expensive than Hameg. From what I've seen Hitachi are definitely a cut above Hameg in terms of build quality and standing the test of time. I assume they only wanted a 20MHz scope, but they absolutely couldn't afford to mess about with something that might act up.

I've always thought there's a lot of specsmanship in scopes. The 250MHz scope is so much better than the 100MHz scope it's untrue, and anyone can see it. However, if you are working with audio or checking a waveform in a burglar alarm factory, a 250MHz scope is a serious waste of money.