Please help me to fix HM605 oscilloscope - no trace

[miceuz]

My scope stopped working after it was on for a relatively long time - about 3 - 4 hours. I was making measurements when it happened - I've heard a silient click inside somewhere around the screen and the trace has disappeared completely, no glow, nothing. Now when I turn on the scope, I don't hear static crackle that was present before.

I'm pretty sure amplifiers are ok - I can see it triggering, lighting up a led when a trace (now invisible) goes off screen, etc.

I've measured voltages according to service manual. All low voltages are within spec, I don't have a very high impedance multimeter to properly measure high voltages, but crappy one I got shows somewhat over 1200V on a high voltage rails (manual specifies 1216V, 1250V).

Does this suggest that probably high voltage circuit is working ok? I've poked around diodes and transistors in HV circuit, all of them seem to be intact.

How can I safely check 13kV voltage output that goes to CRT? What else could I test? Can it be that HV cascade went dud? It's a potted block with markings BG1899-882-201.

Edit: I'm attaching a photo of a schematics for the power section.

[tautech]

If you discharge the PDA HT lead and then power on and discharge again you should be able to eliminate that. Listen carefully for the spark of the discharge.
Or does the Beam finder work?

[uwezi]

I keep my fingers crossed for you, but does the cathode still glow red?

[miceuz]

I keep my fingers crossed for you, but does the cathode still glow red?

i've checked the filament power supply, it's 6.3V as it should be and filament glows inside of the tube.

[miceuz]

If you discharge the PDA HT lead and then power on and discharge again you should be able to eliminate that. Listen carefully for the spark of the discharge.
Or does the Beam finder work?

I'm not sure I'm following you. Do you mean I should disconnect PDA lead from tube, power cycle the scope and try putting PDA connector near to chasis?

There is no beamfinder in this model, but I'm pretty sure the trace is gone.

[tautech]

If you discharge the PDA HT lead and then power on and discharge again you should be able to eliminate that. Listen carefully for the spark of the discharge.
Or does the Beam finder work?

I'm not sure I'm following you. Do you mean I should disconnect PDA lead from tube, power cycle the scope and try putting PDA connector near to chasis?

There is no beamfinder in this model, but I'm pretty sure the trace is gone.

Never touch PDA lead without discharging it first. My PDA grounding procedure is to slip something(screwdriver) grounded under the grommet on the CRT.(scope off)
There is no need to remove PDA lead. Then re-power scope, turn off and ground again listening for a telltale click. All you are trying to prove is the Quintupler is ok and eliminate it from further testing.

Another thought... if trigger is in normal mode a trace will not be displayed without a signal input. Check its nothing to do with trigger setting also.

The manual will have setup procedures and settings for DC readings throughout the scope.
These are useful for find the locality of a fault.

[miceuz]

I've managed to get it to work! One connection on the tube was loose - pin 13 - that's AST control. Grid voltage was too high --  +48V instead of recommended +30V and X plate voltage was too high too - 77.8V and 79.1V instead of 60V. I've tweaked pots to put these voltages to recommended values

Now I have another question. After it started working, it's somwhat strange:






Can anyone explain me what's wrong?
Can this be fixed by tuning internal scope controls? Could my changes to mesh voltage and X plate voltage cause this behaviour?

[grumpydoc]

Looks like the blanking to suppress flyback is off a bit

[nctnico]

The reason I never looked at Hameg again: the one I used regulary at my first job needed a 'tap' (sometimes two) before use.

[oldway]

Hameg HM605 is my favorite bench scope.
I have 2 units of them.
HM1005 is still better but lack component tester. (I have one too)
Ajust of 48V instead of 30V is not important: it's geometry ajust and voltage may be different from one CRT to another.
To ajust voltage of X plates, this is not right as it will let to loose the calibration of the scope.
You seems to have tweaked also a wrong pot: VR604.
This causes  the problem you have.
Try to ajust it again so there is no retrace anymore.

[Retep]

The reason I never looked at Hameg again: the one I used regulary at my first job needed a 'tap' (sometimes two) before use.

Funny, the Hameg scope I have has exactly the same problem. Despite repeated attempts I haven't figured out what is causing this.

[mos6502]

The reason I never looked at Hameg again: the one I used regulary at my first job needed a 'tap' (sometimes two) before use.

Funny, the Hameg scope I have has exactly the same problem. Despite repeated attempts I haven't figured out what is causing this.

Well, that's obviously something mechanical. Probably a broken solder joint on one of the heavier components.

[grumpydoc]

Well, that's obviously something mechanical. Probably a broken solder joint on one of the heavier components.

I bought a 605 last year, supposedly "fully working". There were so many dry joints on the board that when I pulled one of the switch coupling rods out an entire lump of switch fell off the PCB.

[mos6502]

What a coincidence, I sold my HM605 on eBay last year But it really was fully working. Never had a problem with it. Apart from the trigger delay being jittery with large delays. But I think that was normal?

The good thing about the old Hamegs is that they don't use any exotic or custom components. Just a bunch of standard transistors, 74XX, 40XX.

[Retep]

The reason I never looked at Hameg again: the one I used regulary at my first job needed a 'tap' (sometimes two) before use.

Funny, the Hameg scope I have has exactly the same problem. Despite repeated attempts I haven't figured out what is causing this.

Well, that's obviously something mechanical. Probably a broken solder joint on one of the heavier components.

Yes, that was my thought too. I did a visual inspection on the outer circuit boards, did some poking around to see if I could locate the problem, wiggled and reseated connectors and resoldered any joint that looked vaguely suspect. Unfortunately with my HM408 there seems to be no easy way to access the inner circuit boards, other than to desolder the many wires that run between the boards and possibly even drill out some of the rivets that keep the frame together. At that point I figured I would probably do more harm than good by taking it further apart.

[uwezi]

The reason I never looked at Hameg again: the one I used regulary at my first job needed a 'tap' (sometimes two) before use.

Funny, the Hameg scope I have has exactly the same problem. Despite repeated attempts I haven't figured out what is causing this.

I bought my HM605, my private oscilloscope, second hand from its first owner on eBay more than 10 years ago. It works nicely, only needs a recalibration of the timebase, which is off by about 10%. I have the service manual, know what to do, just have been too lazy.

I like Hameg analog oscilloscopes as much as I like Tek - which are the only two brands which I have been and am currently using. Never had any problems with Hameg. As far as I know Hameg-clones are still manufactured today (in India?), but the ones I know are still original ones - could it be an explanation for the bad reputation?

[miceuz]

You seems to have tweaked also a wrong pot: VR604.
This causes  the problem you have.
Try to ajust it again so there is no retrace anymore.

Thanks for a tip!  The tweaking of the VR604 pot did not give any results, so I went looking for something fishy...

My cheap multimeter has a logic level beeper function, so I used that - I've found and listened to ZUP line that is comming from triggering circuit - that was working. I've slowed down the timebase so that I could clearly distinguish when blanking signal is on and off. I've noticed, that blanking is not happening at all - I could clearly see trace returning back and sitting at the left most of the screen until blanking signal is off. This got me thinking - I've poked around transistor T604, blanking signal was going thru it, then I've removed and measured IC600 optocouple - both led and transistor measured ok, then I've measured  T600 - one of the socketed ones; and surely enought it was short all the way! A moment of shuffling thru junk bin and vola - I have a properly blanking trace! Will have to order a proper RF transistor though. And now using VR604 I was able to get rid of blanking artifacts. 

Another thing I've noticed is that X plate voltages are very different if I center the trace. Something like 84V vs 50V. Is this normal because of properties of the tube or something fishy is going on in X amplifier?

[miceuz]

I bought my HM605, my private oscilloscope, second hand from its first owner on eBay more than 10 years ago. It works nicely, only needs a recalibration of the timebase, which is off by about 10%. I have the service manual, know what to do, just have been too lazy.

My timebase was off too before this failure. It's a hell to remove the case cover, so I was lazy too Surely enought, after I've played all the pots around timebase settings today, I'm off even more. Any tips on how to calibrate it? I suppose it's an iterative process adjusting sweep time and trace length? I have the manual, but it does not provide clear instructions I could understand.

[uwezi]

The best way to calibrate the timebase is to have a good and accurate square wave generator. Then there are supposed to be 3 variable resistors VR3001, VR3002, and VR3003 for the 5µs-0.2ms, 0.5ms-20ms and 50ms-1s timebase ranges.

However, since I had the impression (needs to be checked) that I was off by the same amount in all ranges, that actually the x-amplifier needed adjustment, VR3102.

Miceuz, now  that you have opened the scope: there are several unlabeled adjustment points which are accessible from the outside through holes in the chassis. Are the above adjustment points accessible through these holes? I am quites sure that I once had a drawing of the scope with corresponding labels, but I cannot find this anymore...

[oldway]

Another thing I've noticed is that X plate voltages are very different if I center the trace. Something like 84V vs 50V. Is this normal because of properties of the tube or something fishy is going on in X amplifier?

Try to measure the x plate voltages in X-Y with spot in the middle of the screen. Be careful, not too much brightness as this can burn the screen.

I bought my new Hameg HM605 in 1982 and I have used it for 2 decades in tropical climates.
However, I have had many failures, but relatively easy to repair.
NB: this scope has thousands of hours of working !

Here's the list:
- CA3127 replaced twice in the same channel
- A high voltage multiplier defective replaced
- Two electrolytics replaced (they really exploded)
- Two bridge rectifiers with interrupted diode. (very commun failure, better replace them by 4 diodes 1N4004)
- Various cold solders reworked

On my second HM605, I had to replace the TL820CN of the overscan circuit.

NB: for frequency precision measurement, I use the backpanel BNC Y output conected to a 5315A HP universal counter.

[firewalker]

My HM605 had a similar problem. The screen went off some times and I had to nudge it couple of times. It was an easy fix tbh.








Alexander.

[miceuz]

Miceuz, now  that you have opened the scope: there are several unlabeled adjustment points which are accessible from the outside through holes in the chassis. Are the above adjustment points accessible through these holes? I am quites sure that I once had a drawing of the scope with corresponding labels, but I cannot find this anymore...

Nope, one of those pots is different orientation.