Old Analog HAMEG 20MHz dual channel scope died on me. Where would I start? - Page 1
Forum section: electronics / testgear

author: dumle29
author_rank: Regular Contributor
timestamp: May 13, 2014, 11:28:35 pm
content: So a while back I got an old scope from my school for free, as they had gotten fancy new scopemeters (fluke 123) abd it worked a treat, however when I turned it on today, it made a slightly different clank that it usually does on power on, and now the trace will only briefly scan on the CRT, and then nothing. I've changed every knob, but nothing happens.Since it's such an old analog beastie, I'm thinking it should have a larger chance of me being able to fix it, and seeing as it traces the trace for a split second, the importan bits should work :/

author: Hydrawerk
author_rank: Super Contributor
timestamp: May 14, 2014, 11:29:29 pm
content: Start with checking the power supply. And please write the exact model of your oscilloscope.

author: dumle29
author_rank: Regular Contributor
timestamp: May 18, 2014, 12:14:43 pm
content: So I checked the rails, all seem fine (video attached) The model is a HAMEG 203-7Here's some pictures:http://imgur.com/a/h2epq(Too big for the forum I guess)

author: eurofox
author_rank: Unknown
timestamp: May 18, 2014, 12:23:01 pm
content: Did you check the high voltage?You need a special probe for this and be careful, it is dangerous if you don't no what you are doingIf high voltage is OK for the next step you need a diagram and maybe get a scope for testing.eurofox

author: dumle29
author_rank: Regular Contributor
timestamp: May 18, 2014, 12:43:29 pm
content: No I didn't check the HV. I only have that meter in the video, and it's only rated at 600V input, while the CRT itself has a label saying "danger 2000v" which to me seems a little low, but then again, this isn't the 26" crt I knicked my flybacks from.I suppose I could make a resistive divider to read the voltage, but really that just sounds like trouble. I hope that that isn't the issue, as I were able to get a trace or two over to happen, (tho not anymore) before the screen would blank.

author: eurofox
author_rank: Unknown
timestamp: May 18, 2014, 12:54:59 pm
content: Here you have the service manual in Germanbut I'm sure that you can manage with the diagramhttp://mti.kvk.uni-obuda.hu/adat/tananyag/adatlapok/muszer/HM203_7_scope_deutsch.pdf

author: 128er
author_rank: Regular Contributor
timestamp: May 18, 2014, 02:00:51 pm
content: This one is a little bit better to read, instead of the pixelated one:www.hameg.com/downloads/man/HM203-7_deutsch.pdfYour two wires with "CX" that are flapping in the breeze, seems to be marked as "wired capacitor" in the "TB/Trigger Control Circuit" diagram (Page 40 of my PDF). Yes, of course - two wires hanging around are a capacitor. But thats it? Maybe . . .I have no old Hameg to compare

author: JOERGG
author_rank: Contributor
timestamp: May 18, 2014, 07:31:47 pm
content: If the scope does not trigger then you will not see the beam.Release X-Mag x 10 button, because it dims the beam, then be sure intens pot is turned up right.Release AT/Norm button to set to auto trigger.Input for exampel probe compensation signal square wave on channel one.Do you see the beam? Is Y position knob turned to middle position?If you still do not see the beam, does the trigger led light up?Addition: Be sure that external trigger button is not pressed.

author: tautech
author_rank: Super Contributor
timestamp: May 18, 2014, 08:12:19 pm
content: Quote from: 128er on May 18, 2014, 02:00:51 pmTwisted insulated wire capacitors are not uncommon in old CRT scopes.They sometimes are adjusted as part of an adjustment procedure in calibration

author: 128er
author_rank: Regular Contributor
timestamp: May 18, 2014, 08:41:38 pm
content: Quote from: tautech on May 18, 2014, 08:12:19 pmOk, nice to know. I've experimented with twisted wires as capacitors for myself. But I did not know that something like this is used in reliable test equipment. I guess this two wires are maybe a few hundreds or tens femtofarad. But it makes sense to trim C331 (0.56pF) with it.

author: dumle29
author_rank: Regular Contributor
timestamp: May 20, 2014, 09:10:35 pm
content: These aren't twisted tho, they really are just flapping in the breeze

author: Stonent
author_rank: Super Contributor
timestamp: May 20, 2014, 11:20:53 pm
content: Does the scope have a voltage selection switch and is it set correctly for your unspecified country?Noticed R3115 next to the paired transistors has a dark spot on it.

author: dumle29
author_rank: Regular Contributor
timestamp: May 22, 2014, 09:57:59 pm
content: I live in Denmark, so the voltage is ~230v, and the voltage selection on the back, is one of those square fuse holder/voltage selectors. It was at 220V, which has worked flawlessly for 2 years, but I guess that might have been a bit on the edge, as I believe voltages have been slowly raised to ~230v. I now have it on the 240V option, but that's after the fault started. (measuring our power here, gives me 228V at 50.02 Hz)

author: Wim13
author_rank: Frequent Contributor
timestamp: May 23, 2014, 05:45:41 pm
content: Quote from: dumle29 on May 18, 2014, 12:43:29 pmYou dont need a probe, there is already a probe in the power supply build in, measure point is IC301 pin 3.The High voltage is also regulated, on ic 301 pin 3 in the power supply, has to be 0 Volt.On that point you have a connection to the -1913 V via a resistor of 6M81 and 1.5 M ( 8.31 M ) and aother resistor 47K5 and 4K75 to ( 52.25 K ) to +12 Volt.