Hameg 605

[WaveyDipole]

I recently and gratefully received a Hameg 605 from a member. It seems to be in reasonable order I was told that it is not working properly but on power up you get two dots. I had a look at it today and found to start with that some of the switches were stuck while others did not latch between the in and out position. Also the X position control squeaked as it was rotated, had not stop point and pulled out a very long way.

I removed the cover and cleaned out the usual dust, dirt and cobwebs (yes cobwebs) and had a look at the switches. The switches themselves work but the push rods were sticking at the point where they pass through the chasis. This was quite easily solved with a bit of a clean and some lubricant.

I noticed also that shaft that connects the X position pot had a missing coupler which explained why it rotated freely was able to pull out so far. I will need to get a replacement 4mm coupler to reconect it at some point.

I set the controls as per Preliminary Checks in the manual. Next, I fired it up and within a few seconds got a trace. It was bright, sharp and easily focussed but was compressed and confined to about 3cm width. I connected a probe, switched to AC and upon touching the probe got a trace. I then checked some of the controls and established that the timebase is running and that both Y channels work. Dual produced two traces on the screen and I could change between Alt/Chop mode.

I noticed one of the ribbon cables - the one that connects to the Delay controls - appeared to have a loose wire. I powered off and disconnected and upon close inspection discovered that one half of the plug was actually missing and although the cable was still attached to the pins, one wire had come loose. I re-attached it, powered up - no trace - just dots. I disconnected it again and hey presto I how had a trace again.

Since the trace itself is bright and can be focussed sharply this seems to suggest that the HT side of things is OK. So I'm thinking the problem might relate to the X amplifier and possibly there is a fault with the delay circuit.

I am in the process of studying the circuit diagrams to understand better how it all fits together and I will need to measure the various line voltages once I work out where the test points are. The figures on the circuit diagrams I have unfortunately are not very clear.

It looks as if when I need to replace any parts I will have to remove the main board which does not look straightforward. There are lots of switch shafts and push rods to disconnect first and then and the ribbon cables. I'm thinking that I will probably have a good look at it while its out and replace all the caps while I'm at it.

I'm a little bit stuch on where to check the line voltages as the test points are not so clear. I'm also wondering whether to worry about that delay controls for now, or whether I can/should I run with it disconnected for now until I solve the problem with the trace width?

[kernowcam]

I opened mine up recently and was amazed at the number of linkages and switches. I had to take the Y board out and needed to remove the front panel and quite a few screws / connections to do so. I would imagine you need to remove quite a bit to get the main board out.

[WaveyDipole]

Yes, its also pretty cramped around the rectifiers which makes checking the rail voltages a bit of a pain as its awkward to get probes in there. Its quite possible though that one or more caps has gone leaky and is dragging one of the rail voltages down. Anyway, had to call it a day at that point, but will have to get around to it before removing the board.

[WaveyDipole]

Well spent a while checking rail voltages etc tonight and it would appear that all of them (with the exception of HT which I did not check) are correct when measured with respect to the chasis. The calibration voltages appear to be correct as well. Will need to have a closer look at that X amp circuit I think.

[WaveyDipole]

Anyone know where the wiper of the X-Pos pot VR700 connects to? On D7 it shows the breakout board for the controls in the top right corner and shows that the wiper is connected directly to pin 1 or the edge connector, but I can't find the point on the diagram where the ribbon that links to the main boad then connects to?

I'm trying to establish why connecting the wiper kills the trace altogether.

I also noticed that there is a 10uF capacitor underneath the TB board and located directly under the timing rotor and one of the leads was snipped snipped. The capacitor is an electrolytic, but orange in colour rather then the grey/beige for all the others, the same as the one on the other side of the board above it. I'm guessing these are high tollerence electrolytics? But why would one be snipped?

[WaveyDipole]

I'm working on a Hameg 605 which appears to have a fault in the X Amp. I was doing some work on it today when I discovered two resistors (R3105 and R3106) that appear to be open circuit. My DVM reads them as open and I have 138.9v on one side and -1.4v the other side of both of them. The scope is not displaying its trace properly (only a fraction of its proper width) and the gain/trace width/x pos controls have no effect. If these resistors have failed, T3100 and T3101 would not have the correct supply voltages rendering their inputs ineffective. This might explain the conditions being experienced. I have marked the relevant part of the attached circuit in red.

I now have a couple of questions:

1) On the diagram these resistors appear to be marked 68r2. The resistor itself is marked blue, grey, orange, gap, red. According to the service manual these are 0.25W 2% (carbon film) resistors, so does the '2' of 68r2 denote the 2% tollerance or is this actually a 68,200ohm resistor? So will 68k replacements suffice or do I have to add a 200ohm in series as well?

2) There are no outward signs of damage on either component. The transistors also appear fine as does R3107 which has the expected voltages at both ends. Can resistors fail open without any outward sign of damage? What is the chance of this happening on both of them?

I intend to change the 10uF electrolytics in the circuit at the same time just to be sure but as far as I can tell they all appear to be OK.

[David G4EBT]

The three colour bands of the resistor indicate that it is a 68k resistor. The gap between the three bands indicates that the fourth band is the tolerance, for which red is 2%. 2% of 68k is 1,360R, so to be within +/- 2% tolerance a 68k resistor could range from 66,640R to 69,360R.

I wonder if the circuit value is indistinct and actually says 68K2 to indicate (oddly perhaps by present convention) 68K 2%? I'd trust the actual component on the PCB rather than the circuit. I think most modern resistors tend to be 1% tolerance, most commonly rated at 0.5W or 0.6W. (Maplin et al). From the voltages you've measured, it seems not to be subject to high voltages and with no exclamation mark to indicate that it's safety critical.

Hope that helps.

Best of luck with the restoration.

[WaveyDipole]

David, thanks. The image above is exactly how the print appears - its a screen grab of a section of the PDF open on screen which is just slightly bigger than actual size. The manual was downloaded directly from Hameg and the scan is of a poor resolution. The figures on the circuit are actually quite small and unfortunately rather indistinct. Just to add to the confusion, I noticed another unusual value in the same circuit, i.e 51R1. In accordance with the symbol used, this should be a 2% tolerance resistor. However that means it would range from 49.8 to 52.1, so that 51.1 would make no sense. However 1% and 0.5% resistors are supposed to be marked with a slightly different resistor symbol.

I suspect you are correct when you say that the colour bands should be used to determine the value.

[David G4EBT]

Ordinarily, R = Ohms, (and of course K =1,000s Ohms and M = 1,000,000s Ohms), and the modern convention to avoid confusion with decimal places is to express these values as (say) 4R7, 47R, 4K7, 47K, 4M7, 47M. Using that convention, 51R1 would be 51.7 Ohms, which isn't as preferred value, so it seems that the convention applied by Hameg (at least when the 605 scope was produced), was to apply 1, 2 or whatever, to indicate the tolerance. Again, if a resistor has retained its markings, I'd go by what's on the resistor rather than on the circuit.

The colours used for tolerance bands seem to defy logic, in that unless we knew better, I think most of us would assume that gold and silver would be used for the closest tolerances. Not so! Brown = 1%, red = 2% (fits in the the usual code for resistors and caps). However, violet is 0.1%, Gold is 5%, silver is 10% and if no tolerance band, is 20%. Due to improved manufacturing techniques modern resistors are as often as not spot on.

If you google 'electronic colour codes' you'll find a useful Wiki article which covers resistors, caps, transformer leads, and telecoms cables, based on EN600062 2005. It also covers numbering systems as used in SMDs.

As an aside, I'm quite a fan of Hameg scopes, which are nicely designed and built. I use a 203-7 bought as a non-worker which I fettled up. A bit daunting when you take the covers off, with all the switchery and linkages! It's not for nothing that it was the biggest selling scope in Europe in its day. Many scopes are far too complex for the limited use to which most of us will put them. Complexity increases the likelihood of malfunction and reduces the chances of fixing any faults. So complex in fact that some of us won't even know if it's working properly or not. (Me for example!).

[Herald1360]

51R1 is a standard value in the E96 1% series of values but 68k2 is an oddball- 68k1 would be expected.

http://www.logwell.com/tech/componen...or_values.html.

[WaveyDipole]

Thanks for that link Herald1360. I've added it to my bookmarks for future reference.

Well, I replaced the two resistors with 64k 1% 0.6W sourced from Maplins although these have 4 bands - Blu/Gry/Blk/Red/---/Bro but they appear to add up to the same thing. Even so, I was inclined to measure them just to be sure!

I also replaced the three 10uF capacitors in the circuit as planned. Since the cable grips on the plug connected to the ribbon cable linking the delay controls was so temperamental, I removed the plug and soldered the ribbon cable in place onto the pins protruding from the circuit board (will need to find a better and more permanent way of sorting this). Finally, after checking for leakage I reconnected the capacitor on the timebase that had one leg snipped.

I connected the scope to the mains and after a few seconds a trace appeared across the full width of the screen! I left it running for a while to confim all is well and then ran some basic tests.

Although I have not yet tested the delay controls, it seems that the scope otherwise works as expected. Timebase works OK on all time intervals, attenuators on both channels are OK. The component tester also appears to work.

All that is left is to reattach the X-Pos control, but I need to get a coupler for that. I have seen one or two on eBay just a question of making the purchase. As a bit of pro-active maintenance I will also replace the caps on the power lines. Finally I will probably aslo get a couple of scope probes and make up some component tester leads.

I still find it a bit odd for two identical resistors to go open circuit like that but I'm glad I now have a working scope!

I have attached some photos. The trace is sharp and well focussed but appears a little blurred in the photos due to the low lighting level.

[WaveyDipole]

PS, I have noticed an intermediate problem. It oftem seems to appear when I activate the component tester and connect a components, particularly a capacitor. Sometimes it appears when I switch the scope on. Sometimes it will clear when I turn the scope off and then on again, but sometimes won't go away for a little while. I have also noticed that sometimes switching to inverse mode and back again clears it.

What you get a second 'echo' line with shading between the base line and the echo line. This also leads to strange looking traces. A little difficult to explain so hopefully the pictures illustrate clearly what is seen.

Anyone know what ight be causing this intermittent behaviour? Often intermittents are caused by dry joints, but I'm not so sure here. Why would just touching a capacitor to the component tester connector (the [-] is already attached to the earth BTW and I'm just touching [+] to tester connector) trigger this?

[Herald1360]

Originally Posted by WaveyDipole

Well, I replaced the two resistors with 64k 1% 0.6W sourced from Maplins although these have 4 bands - Blu/Gry/Blk/Red/---/Bro but they appear to add up to the same thing. Even so, I was inclined to measure them just to be sure!

Yes that's a standard 4-band (6-8-0-+2zeroes)=68k0 1% resistor (not64k).

[WaveyDipole]

Yes, my mistake, I did mean 68kohm not 64kohm.

[WaveyDipole]

With regards to the intermediate problem in the post above, it looks like the capacitor onb the component tester was a red herring. It turned out tp be the HT cable. I noticed that when you turned the chasis on its side the problem did not occur. Sometimes when the problem occurred and one lifted or moved the chasis it would go away. So I used the end of a pen and tapped various capacitors, trimmers etc on the board but nothing. I then ccidetally disturbed the HT lead and the effect flickered on and off.

It looked like the cable had been snagged at some point because it looked as if it had been pulled a little out of the rubber end cap that connects to the CRT tube. I pushed the cable a little way back into the cap and powered up again. It now seems a bit more stable but still occasionally prone to the problem. I'm thinking about removing the cap from the CRT and checking the connection inside. How long does it take for the charge in the CRT to dissipate once the scope has been turned off?

[David G4EBT]

Originally Posted by WaveyDipole

How long does it take for the charge in the CRT to dissipate once the scope has been turned off?

I dare say that TV enthusiasts on the forum would say that rather than speculate on how long it takes for the charge to dissipate, a safer and more certain approach would be to discharge it. I think the common time honoured way of doing that was to use a well insulated lead with an alligator clip at each end, with one end clipped to the blade of a well insulated screwdriver. Firstly, clip the other end of the lead to chassis, then poke the screwdriver blade beneath the rubber cap to make contact with the EHT lead so that you discharge the EHT to ground. There'll be a spark and maybe a crack, which can be disconcerting, (high Volts, but low current of course). However, I'd have thought that on a 'scope tube, as opposed to a large TV CRT, it wouldn't be too dramatic, especially if you've left it switched of off overnight.

This might help, albeit it's about TV CRTs, though the technique is the same:

http://humphreykimathi.blogspot.co.u...sion-tube.html

Hope that helps a bit.

Usual safety precautions of course - ride at your own risk!

Well done on the headway that you've made with the scope.

[WaveyDipole]

Thanks for the link David. I discharged it as you (and Humphrey) suggest and then cleaned the contacts. I also lifted the Z-board off the back of the scope to gain access via the rear of the chasis and replace the two 220uF/100v capacitors on the Y-Board for the 68v and 140v power line. I had to disconnect the linkages to the Focus and Intensity pots and making a note of all the CRT connections, but this seemed easier than trying to remove the Y-board. While I had the Z-board lifted, I also replaced a handfull of small capacitors on that board.

Having re-assembled everything, reconnected the CRT and the HT line connector I powered up and I'm glad to say it works a treat.

[David G4EBT]

Congrats on a successful outcome!

The forum member from whence the scope came will doubtless be pleased that you didn't throw in the towel and put it on the 'to do pile' and now have a fully functioning 'scope - a great way to start the new year!

Your efforts bring to mind this quote:

"Never let defeat have the last word."
(Tibetan Proverb).

I'll try to adopt that as my New Year Resolution. (Yet again!).

[WaveyDipole]

Thanks David. As it turned out, there was still a little further to go....

I have now completed re-capping the power board and I fitted a new coupler to the X-Pos shaft to link it to the pot. I found one more slight fault that needed rectifying - the X10 multiplier did not work. The switch itself was fine but I could not get continuity between one leg of VR3100 and R3104 when it was operated. Well it turned out that there must have been a break in the short length of track (which could not have been more than about 7-8mm long) between VR3100 and the switch (couldn't actually see it properly). I was able to solder a bypass lead between the two points though, which solved the problem.

Anyway, that should be all for now and I guess it probably could probably do with calibrating!

Anyway, thanks for you suport.