Test Equipment Anonymous

Another swapmeet find. It cost £10. It looked clean, it wasn't covered in stickers. It had no missing knobs or cracks in the plastic front and rear panels. It's a 60MHz analogue scope from the mid 80s.

I found this series of three videos on Youtube about them.

https://www.youtube.com/watch?v=pUcE2aKm8Ec

https://www.youtube.com/watch?v=xVrgGKJYSj8

https://www.youtube.com/watch?v=CRN2yGs60d4

Essential viewing for the dedicated HM605 enthusiast. He's a kindred spirit.

As is usual with Hameg, the manuals are easy to find in the WWW, but sometimes the cct diagrams are not the best scans, and Hameg don't seem to have published full adjustment procedures such that you can take a scope with scrambled presets and set it up from scratch.

I powered it up and it produced traces on both channels, always a good sign. The trigger only worked on auto and the curious delay trigger feature didn't work at all. The trigger slope switch was dodgy. Oddly enough all other controls moved easily and the multiway switches worked properly. Usually they are stiff and need a dose of contact cleaner.

I checked the supply voltages on the trigger circuit, 12V and 5V, and they were fine with no ripple. I checked the switch with a meter and it seemed good. I noticed manual trigger did work once or twice when the button was pressed. I looked at the soldered joints of the switch with a jeweller's eye glass and sure enough there was a circular crack in one of them. After the old solder was removed and the joints were resoldered, the manual/auto problem was solved.

The trigger slope switch problem was down to the control shaft being stiff in the hole in a metal panel through which it passed and the button being a bit stiff, solved with a light touch of silicone grease.

The delay trigger feature took some head scratching to sort out, but it was down to a bad, or badly seated, 74AL74. All the ICs on this scope are socketed. With the chip replaced by a 74ALS74, that problem was solved.

Now to set the scope up and make sure all features work properly. The place to start is always with the supply voltages, which I'd done in a lazy way before.

The +12 V, -12V and +5V supplies are supplied by 7800 series regulators and were spot on and without ripple. The 140V supply was at 111V and the 68V supply, from which it's referenced, was at 56V. I had a look at the power supply and there were few oddities which looked like a repair. T2503 is a BUX86, not a BD232. R2510 a 150K resistor, not 182K. R2511 was a 5.4K resistor, not 5.62K. R2512 was a 4.7K with a 10K soldered across it, not a 6.81K. IC2500 was a TL431C, not a TL430; TL431 has a nominal reference voltage of 2.5V and TL430 2.7V. I checked the components in circuit, as best I could and they all seemed right and the connections appeared sound. I thought this power supply circuit was odd, until it dawned that it was designed to maintain the reference voltage on the TL430 and did that by maintaining the output at 68V (or whatever the preset VR2500 required). This particular TL431C had a reference voltage of 2.486. After some work with a calculator to work out which resistors were needed to put the wiper of VR2500 at 2.486V when at the centre of its travel, when the supply was at 68V, the answer was 4.7K of resistance between the preset and ground. I decided to solder a 12K resistor across R2511 making it 3.7K, to avoid removing the board. Removing that board is a fiddly task involving desoldering and removing transistors which use the metalwork of the scope as a heatsink.

The cct diagram of the 68V/140V supply.
Picture of the 68V/140V supply with the codged resistors. I couldn't see the point of the 10K in parallel with the 4.7K and cut the connection at one end. I suspect the original 6.81K resistor was chosen for a reason, although I doubt it's critical. I'm slightly tempted to put a 6.8K in that position.
That done, the 140V supply could be adjusted to 140V as specified, and the 68V supply was at 70V. That's not specified and I guess it's close enough. It could be got exactly right by changing the resistors, R2504 and R2505.

For the rest of the adjustment. The 60 MHz response was a fraction of what it should be and nothing like 3dB down. There are no instructions for adjusting the presets in the Y preamp, intermediate Y amp and final Y amp. There are hints at what they do on a couple of layouts showing their positions. The answer seemed to be to twiddle a preset cap in the final Y amp and another in the preamp to give a 3dB down signal at 60MHz and then adjust the preset resistors in the final Y amp and intermediate Y amp to give the best square wave at 1MHz, then repeat checking the the 60MHz response was as it should be and repeat. Then do the same for the other channel, leaving the final Y amp alone.

An OK result was arrived at in this messy, trial and error way, looking at the procedures for similar scopes.

Then the sequence for adjustments Hameg do give was followed. Things like var balance, trigger adjustment etc. There were other adjustments, as for the calibrator and overshoot indicators which were easy enough to work out.


I could truthfully describe it as a working scope. I'm not totally happy with it. The reservoir capacitors, 1000µF 25V and 220µF 100V, plus smaller ones in the power supplies are notorious for failing and should be replaced. The codged parallel resistor solution should be replaced with a 1% high quality metal film resistor. All that involves the PITA removal of a board for access. The trace starts slightly late on the faster timebase settings, which I find irritating. The component tester horizontal trace is slightly too long, but there appears to be no adjustment on this scope. The focus works, but the control is too close to the end of its travel for comfort. That's probably an HV resistor gone high.

I believe the 140V/68V supply failed and was sort of repaired at one time, but wasn't got right. Also I believe Y amp settings had been twiddled, maybe because it wasn't behaving properly with a low PSU voltage.

I'm fed up with messing with it for now. I might revisit it and deal with the irritating residual faults later on. I suppose for practical purposes, it's a working 60MHz analogue scope, clean and in a tidy condition.

The scope in ebay working pose. I really must get into taking oscilloscope porn. I'm sure it needs the right lighting and the use of a tripod.

Good work Sherlock.
One must always go into old instruments with eyes wide open......

I love Hamegs! I got into them when I realized many had a built in "Huntron" component tester, and you could get that for often under $100 instead of thousands being asked for said Huntron. Easy to work on little scopes.

I've got four 20MHz Hamegs, a 203-5, two 203-6 and a 203-7. One has been earmarked as a gift. I also have this HM605 (60MHz) and an HM1005 (100MHz) which more or less works, but is yet to be addressed properly.

I have mixed feelings about them. When working and sorted out, at least the 20MHz ones, are a pretty good, stable analogue scope which I think would go on for years. The power supply capacitors are notoriously prone to fail and they strike me as under specified. They have to be less than 25mm high. I also don't dig the voltage test socket, which has low voltages and a couple of -1900V points. It would be easy to make an unpleasant mistake with that.

As for being easy to work on, it depends whether you can reach the part from the side, which in the case of the main reservoir capacitors is possible, or whether you have to remove one of the main boards, which involves desoldering things I'd rather not mess with and unbolting things which use the metal work of the scope as a heatsink. A lot of work to replace a failed small electrolytic, which is causing a power supply rail to oscillate. The later ones have silk screened component IDs which saves a lot of hassle.

The component tester is something of a Marmite item. Some think it's fantastic, others can't really see the point. I fall into the second camp, although it's something I feel obliged to get working properly.

My main gripe is that the technical documentation isn't great. There are circuit diagrams, which are pretty good and count for a lot, but no parts list, and in particular Hameg don't seem to have released a factory set up procedure, which would allow you to take a scope with scrambled presets and set it up to its specification. They provide a schedule for usual adjustments and the manuals show the position of various other presets and hint at what they are for.

I don't regard the HM605 as quite finished. I could describe it as a working scope, but it's a bit ragged round the edges in some aspects. I got fed up with messing with it, although I may revisit it later.

As for sub top tier (HP, Tek etc) scopes, I'd take Hitachi over Hameg every time.

It's all a bit of fun.