Hameg HM1005 100 Mhz oscilloscope

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Zenith
Posts: 811
Joined: Sat Oct 22, 2022 9:06 pm

Hameg HM1005 100 Mhz oscilloscope

Post by Zenith »

This came from a swapmeet for a very few £s. I was told it didn't produce a display and it had to go. It was clean and with all knobs and buttons. There was slight damage to the feet/cable store on the rear panel.

When I got it home I tried it, and sure enough there was no display. Nothing could get it to produce a hint of a display, X-Y mode, full brilliance, auto trigger, nothing.

I took off the cover and checked for anything burnt, damaged or otherwise obviously wrong. I checked the voltages, apart from the EHT. The levels were right with no excessive ripple. The room was getting dark and I thought I saw a glow from the bottom of the screen. I fiddled with the controls and there was a line on the screen. I connected a probe to the calibration point and there was a square wave.

The problem with it displaying nothing was definitely there. Since then it's been in and out of the case four or five times, and been powered on dozens of times. and there hasn't been a hint of it not producing a display. It may have been a dry joint or bad connection.

I put it away to take its turn on the queue. A couple of weeks back I took it up again.

It had a lot of the usual problems, stiff controls, contacts that needed cleaning, and generally what's to be expected with an old scope, such as the timebase not being quite right and similar adjustments. The two faults that were really obvious were that the response dropped off by much more than 3dB at 100 MHz and it couldn't produce a 100 Hz square wave with a flat top. I downloaded the manual from several sources and found they were all the same and for the HM1005/2-3. It doesn't have a proper set up procedure. It talks about usual adjustments and there are diagrams giving the positions of the presets, with some hints as to what they are for. The problem was they didn't correspond to this scope. The 100 Hz square wave adjustment is usually in the Y preamps. The Y preamps on this have covers with no access holes and they are soldered in place.

I contacted Sky Messtechnik. Their business is mainly modern TE, Rigol etc, but they sell some Hameg spares and offer support for Hameg (pre R&S takeover) products. Michael Leichum is very knowledgeable and pleasant to deal with. It turns out this is an early model HM1005 and there were several board revisions before things stablised with the HM1005-3. He sent me circuit diagrams and some layout information for the earliest Y amplifier versions. He also said that Hameg had not produced a full factory set up procedure. You just had to make the best of it, using such information as was in the manual.

I desoldered the preamp covers. I found the best thing for that was a soldering gun with a plastic cutting bit. It was then easy to adjust for a flat topped square wave at 100 Hz. Oddly, none of the balance presets needed adjusting. Then came the adjustment of the Y amps to give a square wave with no overshoot at 1 MHz and meet the 3dB requirement at 100 MHz. I set up an AWG to sweep from 1 MHz to 100 MHz. I did check the sweep against a fairly new 150 MHz scope and it's pretty much flat up to 120 MHz. There's no point chasing a Will o' the Wisp. I then adjusted the presets in the intermediate Y amps and final Y amp to determine how they affected the response. Then I altered them for the best flat response tapering off to 3dB down at 100 MHz and then changed to 1 MHz and checked the square wave for overshoot and undershoot. If the high performance was right, the square wave showed overshoot. Some of the preset caps are very twitchy. In the end I settled for minimal overshoot at 1 MHz while meeting the 100 MHz requirement.

I had to work out what the timebase presets did, by looking at the layout I had and experimenting to find out which ones were which. There were various things to adjust such as the A and B timebases, the X response and the calibrator. I then noticed that the trigger input switch was very stiff and took so much force to move there was a danger of breaking it. I could see no option but to remove the front panel to gain access. The trigger selection control is a plastic lever arrangement, which connects to a slide switch via a piece of stiff wire. I couldn't understand why it was so stiff. In the end, on a hunch, I lubricated the plastic lever with PTFE dry lubricant. After it had been moved back and forth a few times, it worked as expected, with very light pressure. Removing the panel made access to other switches and controls far easier. Refitting the panel is a miserable job and took a couple of hours. I could probably have lubricated those plastic lever mechanisms by gaining access behind the panel. One to remember for next time. I put the preamp covers back as a push fit. I suppose I really ought to solder them or find some more definite way to retain them.

All features work properly and I've had enough of messing with it for now. It has three Y channels, two normal Y channels and a third with fixed attenuation at 50 mV/V. I don't find the third channel phenomenally useful. The delay sweep puts up a second display and you can pick which parts of the main waveform you want to expand. Some things I haven't done, such as replacing the notorious reservoir capacitors and the scale illumination bulbs. There wasn't anything wrong with it that needed replacing. Much of the effort was taken up by the fact there wasn't a proper set up procedure or documentation for this early version. I feel uneasy about the original problem of it not producing a display.

It's a nice scope to use. It produces a sharper display than this but I turned up the brilliance for the photos.

Front panel removed. The problem grey plastic levers are about half way up and in the right half.
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Displaying three channels.
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Trace and expanded trace on two channels.
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