[solved] HM204 auto trigger does not work - Page 1
Forum section: electronics / repair

author: ChrisG
author_rank: Unknown
timestamp: February 22, 2018, 01:28:56 pm
content: Hi all, since many years I have an old 3rd hand freebie HM204. All seems to be fine and okay-ish. However the Auto-Trigger function does not work at all and never had for me. Only when I pull out the trigger level knob and play with the level I can get a signal triggered. Any immediate thoughts on what to do. I'm a bit put-off by the CRT and the high-voltages (caps) when dismanteling the whole thing. Re-soldering the joints in the trigger section? Any measurements I can do? ( need to find schematics still too)

author: oldway
author_rank: Super Contributor
timestamp: February 23, 2018, 06:55:53 am
content: You can find the technical documentation with the diagrams for free here.http://www.sm5cbw.se/hameg/hmosc/hm204-2-man.pdfHameg oscilloscopes are known for their excellent triggering, so first you need to know if your problem is caused by a fault or an incorrect adjustment.Readjust VR301 and VR302 as described in the sequence of important adjustments and report here.EDIT: Currently, I am repairing vintage audio devices and have already repaired many analog oscilloscopes in the past, many of them hameg.My workhorse is a HM605 hameg that I bought in 1982 and it has thousands of hours of use.It is an excellent oscilloscope for repair job, the component tester is very useful.I also have 2 other spare HM605's, an HM1005 and an HM1505.The main failures of these oscilloscopes are cold solders and open rectifier bridges

author: ChrisG
author_rank: Unknown
timestamp: February 23, 2018, 08:40:22 am
content: Quote from: oldway on February 23, 2018, 06:55:53 amI have a HM204 with the dash. I tried to attach the German manual with circuit diagram just now but the file is too big. So a screenshot for the relevant diagrams here below. Hopefull the resolution is sufficient. There is no VR 301 and VR302 on this board. But there is a VR320 and VR321 which seem to perform similar function.The board underneath look immaculate and all shiney solder joints. I can take another look but around the trigger area it's for me really perfect. The scope does trigger in Manual/Normal mode but not in AT mode. The scope does also trigger on external trigger, again in Normal mode (pulled knob out) and not in AT mode. What I also notice that when in AT mode, no trigger, and then pushing the +/- slope button the trigger light does blink briefly with also an image on screen but it does not sustain. Perhaps this is a side effect and would be there also when AT mode would work I do not know.I should focus first on VR320 and VR321 right? Powered on and with an square or sine-wave input signal of 50mVpp @ 1Khz?Link to HM204 (non dash-2):http://www.sky-messtechnik.de/images/PDFs/HM204-1.pdfHi, some time further in the analysis. Not related I assume but IC SN74ALS74N has it's legs completely covered in soot. None of the other IC's or components seem to have this. Very odd indeed and perhaps has todo with static and switching frequency? Anyways: I've measured the voltage from the level pin during normal trigger: it triggers then between -1.12V and +0.98V whilst turning the level knob and the graph then slides. When in AT mode the Max Voltage I get when turning the knob is: -2.02V and +8.26V!!! It does not come close to the other range of voltages and stays also only negative. Could the CA082E (RCA component) be the culprit?

author: oldway
author_rank: Super Contributor
timestamp: February 23, 2018, 08:16:26 pm
content: Do you still want to repair this oscilloscope ? Why did you not what I asked you to do ?

author: Armadillo
author_rank: Super Contributor
timestamp: February 23, 2018, 11:23:45 pm
content: Firstly let's eliminate bad potentiometer to complex the troubleshooting. Use proper contact cleaner to blow the Level potentiometer VR340, VR320 and VR321 then; [some people here may scream you not to do that, in that case you can ignore what I say]You need to confirm below can be done before we suspect faulty components;Auto Adjustment :Set CH 1 attenuator to 10mV/div,input 50mV @50 kHz sine (5 div display height).thenSet attenuator to 0.1 V/div (5 mm display height).Adj. VR320 (LEVEL button depressed) for just triggering. Then set attenuator to 0.2 V/div, confirm that no triggering is possible.Depress SLOPE± button, set attenuator to 0.1 V/div.Adj. VR321 for same trigger threshold. Repeat triggering adjustment as above.Can you achieve that?

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 09:09:33 am
content: @OldwayYes I still want to repair it for a couple of reasons: to learn, practice and have some relaxing past time as this is just a hobby for me. The VR's you were indicating I could not find in HM204-2 manual as you indicated too. That's when it hit me that I needed to find the correct manual with schematics which I did and attached part of the document and the url to the full pdf. Appreciate the support and guidance.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 09:30:49 am
content: Quote from: Armadillo on February 23, 2018, 11:23:45 pm1. I just bought WD-40 specialist contact spray. So yes will try. It reads that it is alcohol based and leaves no residu .... (hmm)2. Set ARB to the right settings.3. Blasted the three VR's: VR320, VR321 and VR340.4. In AT mode (knob depressed in). Adj. VR320 does not do any AT trigger. Tried fast and slow to have it checked.Nothing, just a scrolling screen of the sine-wave. When in Normal mode I can adjust the level and presto: it triggers and the screen is stable. Switched to 0.2 mV attenuation and also no trigger in AT mode possible. Triggering in normal mode is a bit finicky and requires more precision on the level, in the end I got it working too, though the trigger flickers then a bit.5. Repeated the above with Slope +/- depressed and on VR321, same result as with 4. Though here the 0.2mV during normal triggering is more easy to adjust.So I can get it to trigger @ 0.2mV attenuation in Normal mode. No trigger in AT mode.Hope this helps you guys guide me further now. Thank you.

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 09:53:54 am
content: Then let's measure [the voltage] and see what's wrong with it;See attached and measure with respect to Ground,. [Switch CH1 to Ground].Edit: Attachment corrected.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 10:40:33 am
content: Hi Armadillo, herewith the measurements.

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 10:46:34 am
content: IC320 appears to be defective.I would change it before proceed any further.

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 10:58:54 am
content: And also recheck the inconsistent reading shown in blue.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 10:59:54 am
content: VR320 (and VR321 for) behave funny when I measure, powered-off, the normal overall resistance, not the 100K Ohm and fluctuates. R321 measures 1.4MOhm iso of the 2.7MOhm. Did not take the components out of course and not sure if I'm measuring now through the CA082E as well. R320 measures 100Ohm dead-on precise! Other VR's on this board also measure and lock-in immediately.Corrected the picture after re-measuring. Please see attached. -1.107V from R329 to the zeners

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 11:05:18 am
content: Could be remnant capacitor charge or take out components to R321 to measure since you are going to change the faulty IC.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 11:06:27 am
content: So you think the IC is faulty?

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 11:08:05 am
content: Quote from: ChrisG on February 24, 2018, 11:06:27 amI am sure to the best of my knowledge.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 11:28:20 am
content: Quote from: Armadillo on February 24, 2018, 11:08:05 amHi Armadillo, I've taken out the CA082E. Most of the resistors measure dead on. The R321 of 2M7 is just 1.4M Ohm though. VR320 is all over the place to be honest. So I'm wondering if just popping in a new CA082E will be sufficient now. BTW the TL082CP is more readily available and seems to have the better spec's. Is this the way to go then?

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 11:35:37 am
content: You should change R321, VR320 and the IC since those are not expensive components.If you can, change to CA082E if can be obtained together with the other components otherwise get the TL082CP.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 11:43:59 am
content: That'll be quite the task in unmounting the PCB :-) . Btw with some tricks and looking around the corner the marking on the VR320 says 600K iso the 100K in the circuit diagram. I understand a 600KOhm pot would be able to do the trick but I assume better to put in a 100KOhm right? And then change the VR321 as well.

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 11:51:48 am
content: Maybe someone attempted a repair before, yeah!.Follow the circuit so that you can have finer adjustment using 100K than 600K, even though the circuit is just a comparator.As for VR321, might as well change it to 100K same. For me, I would change it, since ordering the cheap components, the delivery charge more expensive.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 12:07:52 pm
content: I've ordered 3 CA082E via eBay (Germany) which will arrive end of next week. Will order the other components too. Strangely enough the R331 of 2.7M also measures 1.41MOhm so assume either a similar fault or I'm still having things in parrallel here. Just one question: popping in a new (2nd hand/NoS) CA082E without replacing the VR's and so on is wise to do? I've ordered three so what the heck

author: Armadillo
author_rank: Super Contributor
timestamp: February 24, 2018, 12:15:44 pm
content: Then you better put a IC socket to avoid soldering too many times.Yeah! stable the VR320 first, clean the contact with a piece of newspaper slight it through, then pop it in. No harm. Enjoy.Remember to calibrate it as previous message.Edit: The resistor, then read the color bands.

author: ChrisG
author_rank: Unknown
timestamp: February 24, 2018, 12:43:44 pm
content: The CA082E is (was) already socketed so that helps. The color bands read correctly 2.7M Ohm both of them. The VR's are the enclosed type from Piher, all over the board and also on the other boards.

author: sarahMCML
author_rank: Regular Contributor
timestamp: February 25, 2018, 03:29:57 am
content: Hi,If I'm reading the schematic correctly, please check that pin 4 of the CA082E is at -12 volts. If not, check R332 is not open or high in value, or that the electrolytic capacitor attached to pin 4 isn't shorted.Regards, Sarah.

author: Armadillo
author_rank: Super Contributor
timestamp: February 25, 2018, 06:20:27 am
content: Valid point.

author: ChrisG
author_rank: Unknown
timestamp: February 25, 2018, 09:13:55 am
content: Quote from: sarahMCML on February 25, 2018, 03:29:57 amHi Sarah,Not near the equipment now but what I measured yesterday and as depicted in the diagram pin 4 is at 0.00V. So indeed not -12V. I'm now wondering if it should be at -12V though when reading the schematics.

author: Armadillo
author_rank: Super Contributor
timestamp: February 25, 2018, 10:12:02 am
content: Sarah is right. It should be -12V. I figured it should be typo or another of the inconsistent measurement since you can get minus output out of the opamp.Worth to check it nevertheless.

author: ChrisG
author_rank: Unknown
timestamp: February 26, 2018, 05:07:42 pm
content: Hi Sarah, Armadillo,Without the CA082E in the socket pin 8 measures 11.8V and pin 4 measure -11.8V. Socketed Pin 8 and 4 drop to 11.31V and -11.34V respectively. Indeed a measurement mistake from me last Saturday. Awaiting new CA082E's to be delivered this week. Let's see what this brings. Appreciate your help

author: ChrisG
author_rank: Unknown
timestamp: March 01, 2018, 12:59:42 pm
content: Hi all, I've received the 3 CA08Es at home. Shall I just put a new one in and check what happens? Or better to change the VR's and measure the surrounding components before doing so?

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 02, 2018, 04:01:34 pm
content: Hi again,Unless you've checked the values of the various resistors around IC320 with at least one end disconnected from the pub, I'd not worry too much about the lower readings. As suggested, clean the pot wipers gently if you can, then insert a new CA082E into IC320 and see what happens!Regards, Sarah

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 08, 2018, 03:33:54 am
content: Hello ChrisG,Just out of interest, did you get any further with the repair of your oscilloscope's trigger circuits?Regards, Sarah.

author: ChrisG
author_rank: Unknown
timestamp: March 08, 2018, 06:36:05 pm
content: Hi Sarah,I've not been able to due this yet due to other priorities. I hope to spend time again next week on this. Actually quite keen on getting is sorted out and will order some cheap components around the CA082E just in case they're faulty. Once you get into it, it can be quite annoying to not have such components readily available.

author: ChrisG
author_rank: Unknown
timestamp: March 12, 2018, 06:59:17 pm
content: Hi Sarah, Armadillo,I've just slapped in the new CA082E... and... it works! Now remeasuring the voltages and need to put the knobs on too. I did not (yet) change the VR's as it seems to work still. :-|

author: Armadillo
author_rank: Super Contributor
timestamp: March 12, 2018, 07:22:38 pm
content: Great! Enjoy....

author: ChrisG
author_rank: Unknown
timestamp: March 12, 2018, 08:01:44 pm
content: Well... I've also ordered the wrong VR's (too large @ 250mW). So I'm pleased it works now. Thank you for all the help and I have now some spares too :-)

author: sarahMCML
author_rank: Regular Contributor
timestamp: March 13, 2018, 02:07:41 am
content: Hi Chris,Well done, and I hope that it'll give you good service into the future.Regards, Sarah.

author: ChrisG
author_rank: Unknown
timestamp: March 14, 2018, 09:16:54 am
content: Thank you Sarah and Armadillo. It feels great to have achieved this small accomplishment. Mechanically and functionally it seems to function well. I do believe some sort of calibration would be needed still though and that would be quite the difficult task and there is not so much material on that.