Please help me to fix HM605 oscilloscope - no trace - Page 1
Forum section: electronics / testgear

author: miceuz
author_rank: Frequent Contributor
timestamp: February 21, 2014, 09:41:45 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: February 21, 2014, 09:52:09 pm
content: 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?

author: uwezi
author_rank: Unknown
timestamp: February 21, 2014, 10:01:00 pm
content: I keep my fingers crossed for you, but does the cathode still glow red?

author: miceuz
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:31:15 am
content: Quote from: uwezi on February 21, 2014, 10:01:00 pmi've checked the filament power supply, it's 6.3V as it should be and filament glows inside of the tube.

author: miceuz
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:34:11 am
content: Quote from: tautech on February 21, 2014, 09:52:09 pmI'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.

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2014, 01:50:37 am
content: Quote from: miceuz on February 22, 2014, 12:34:11 amNever 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.

author: miceuz
author_rank: Frequent Contributor
timestamp: February 22, 2014, 02:28:05 pm
content: 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 valuesNow 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?

author: grumpydoc
author_rank: Super Contributor
timestamp: February 22, 2014, 02:38:36 pm
content: Looks like the blanking to suppress flyback is off a bit

author: nctnico
author_rank: Super Contributor
timestamp: February 22, 2014, 02:59:30 pm
content: The reason I never looked at Hameg again: the one I used regulary at my first job needed a 'tap' (sometimes two) before use.

author: oldway
author_rank: Super Contributor
timestamp: February 22, 2014, 03:55:32 pm
content: 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.

author: Retep
author_rank: Regular Contributor
timestamp: February 22, 2014, 04:07:40 pm
content: Quote from: nctnico on February 22, 2014, 02:59:30 pmFunny, the Hameg scope I have has exactly the same problem. Despite repeated attempts I haven't figured out what is causing this.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 22, 2014, 04:09:09 pm
content: Quote from: Retep on February 22, 2014, 04:07:40 pmWell, that's obviously something mechanical. Probably a broken solder joint on one of the heavier components.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 22, 2014, 04:45:12 pm
content: QuoteI 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.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 22, 2014, 04:56:45 pm
content: What a coincidence, I sold my HM605 on eBay last yearBut 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.

author: Retep
author_rank: Regular Contributor
timestamp: February 22, 2014, 05:14:21 pm
content: Quote from: mos6502 on February 22, 2014, 04:09:09 pmYes, 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.

author: uwezi
author_rank: Unknown
timestamp: February 22, 2014, 05:24:04 pm
content: Quote from: Retep on February 22, 2014, 04:07:40 pmI 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?

author: miceuz
author_rank: Frequent Contributor
timestamp: February 22, 2014, 05:35:59 pm
content: Quote from: oldway on February 22, 2014, 03:55:32 pmThanks 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?

author: miceuz
author_rank: Frequent Contributor
timestamp: February 22, 2014, 05:51:52 pm
content: Quote from: uwezi on February 22, 2014, 05:24:04 pmMy timebase was off too before this failure. It's a hell to remove the case cover, so I was lazy tooSurely 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.

author: uwezi
author_rank: Unknown
timestamp: February 22, 2014, 06:48:03 pm
content: 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...

author: oldway
author_rank: Super Contributor
timestamp: February 22, 2014, 07:06:32 pm
content: QuoteTry 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 reworkedOn 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.

author: firewalker
author_rank: Super Contributor
timestamp: February 22, 2014, 07:17:59 pm
content: 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.

author: miceuz
author_rank: Frequent Contributor
timestamp: February 23, 2014, 09:58:24 pm
content: Quote from: uwezi on February 22, 2014, 06:48:03 pmNope, one of those pots is different orientation.