[solved] HM204 auto trigger does not work

[ChrisG]

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)

[oldway]

You can find the technical documentation with the diagrams for free here.
http://www.sm5cbw.se/hameg/hmosc/hm204-2-man.pdf

Hameg 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

[ChrisG]

You can find the technical documentation with the diagrams for free here.
http://www.sm5cbw.se/hameg/hmosc/hm204-2-man.pdf

Hameg 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

I 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.pdf


Hi, 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?

[oldway]

Do you still want to repair this oscilloscope ? Why did you not what I asked you to do ?

[Armadillo]

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 [Peak]:

Set CH 1 attenuator to 10mV/div,
input 50mV @50 kHz sine (5 div display height).
then
Set 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?

[ChrisG]

@Oldway
Yes 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.

[ChrisG]

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 [Peak]:

Set CH 1 attenuator to 10mV/div,
input 50mV @50 kHz sine (5 div display height).
then
Set 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?

1. 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.

[Armadillo]

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.

[ChrisG]

Hi Armadillo, herewith the measurements.

[Armadillo]

IC320 appears to be defective.
I would change it before proceed any further.

[Armadillo]

And also recheck the inconsistent reading shown in blue.

[ChrisG]

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

[Armadillo]

Could be remnant capacitor charge or take out components to R321 to measure since you are going to change the faulty IC.

[ChrisG]

So you think the IC is faulty?

[Armadillo]

So you think the IC is faulty?

I am sure to the best of my knowledge.   

[ChrisG]

So you think the IC is faulty?

I am sure to the best of my knowledge.   

Hi 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?

[Armadillo]

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.

[ChrisG]

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.

[Armadillo]

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.

[ChrisG]

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 

[Armadillo]

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.

[ChrisG]

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.

[sarahMCML]

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.

[Armadillo]

Valid point. 

[ChrisG]

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.

Hi 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.