HAMEG HM605 trace squishing up on the right of the screen - Page 1
Forum section: electronics / repair

author: dentaku
author_rank: Frequent Contributor
timestamp: July 04, 2014, 01:24:23 am
content: I have an old Hameg HM 605 that does strange things when it's moved or the top circuit board is pushed on/flexed only a tiny bit.The X-POS knob can be pulled out to switch to 10X mag and it's connected to a long rod that connects to pot/switch. This puts allot of stress on the solder joints because it pries it back and forth. Originally the plastic thing that joins the rod to the pot shaft was very badly repaired and it was off center so the pot rocked back and forward a tiny bit by simply turning the knob. I've since fixed that.Here's what it does...The trace will move left and right just fine sometimes but eventually it will start compressing along the right side of the screen the more I mess with the X-POS knob or sometimes even when I turn the TIME/DIV. knob all to the way right.Pulling on the knobs associated with the top circuit board, pushing up or down on the circuit board (the cover's off at the moment) or even just propping one corner of the scope on something so it just slightly twists the whole machine will eventually make it go back to normal. Clicking the X-POS/MAG.10X knob a few times sometimes does the trick.Here's a videoof me moving the X-POS knob while it's working then I put a few mm thick SD card case under the right corner so that it' no longer level and the whole thing goes wonky.http://1drv.ms/1kkrKXRIt's built very similarly to the one Dave worked on in this videoimpossible to access the bottom of the boards without allot of fiddly work.ALSO:probably not linked to the other problem, sometimes when it warms up the slightly hummy transformer starts buzzing louder and I get a trace that jitters around a bit or even sometimes gives me a little downward dip. I've only ever heard a sound like that before, but a bit louder, when I connected a bridge rectifier incorrectlyMaybe the power supply caps are going bad?It goes away if I turn it off then back on.Here's a photo of the bottom of the board where the X-POS pot/X10switch is soldered. The one thicker solder fillet looks weird.I SO don't want to remove this board. There's a ridiculous amount of rods and even three wires soldered directly to it that would make it difficult to disassemble. Of course if it gets worse I'll have to take it apart.Could it just be a bad solder joint in the area of the X-POS, the HOLD OFF and the TIME/DIV. pots and switches that does this because pushing in that general area will case or fix that strange behavior?

author: miceuz
author_rank: Frequent Contributor
timestamp: July 04, 2014, 08:47:49 am
content: Quote from: dentaku on July 04, 2014, 01:24:23 amHad similar problem on mine, main DC regulator caps were almost short, I'd suggest fixing this first, just replace the biggest electrolytics.Quote from: dentaku on July 04, 2014, 01:24:23 amYou'll get used to that after several runsIt's not as bad as it looks, just get yourself a small hexagon wrench, all those shafts can be removed pretty easily.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 04, 2014, 02:46:17 pm
content: Quote from: miceuz on July 04, 2014, 08:47:49 amI'm assuming you turn all the knobs fully left or right before disconnecting stuff so you know what position they should go in when you re-assemble it?The rods aren't terrible but it even has wires soldered directly that will need to be unsoldered. Three of them go to the rear BNCs and two go to the read of the CRT.I'm thinking I might be able to leave them alone and hopefully flip the board over while they're still connected.I see there are three 1000uF 25V caps on the bottom board next to the five (five!) little round bridge rectifiers. Of course to access these I need to remove to top board to unsolder themThere's also two big 220uF 100V caps that I'm assuming they have more to do with the CRT than the power supply itself?

author: miceuz
author_rank: Frequent Contributor
timestamp: July 04, 2014, 09:51:50 pm
content: Quote from: dentaku on July 04, 2014, 02:46:17 pmNo, those are for linear regulation too, in mine unit exactly these were bad.Measure voltages on a test header first, maybe you will be lucky to access the offending cap via side, but that will require some solder iron bendingAnd you are better off changing all of them if it's an old unit.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 04, 2014, 10:39:20 pm
content: Well, I'm removing everything. The rods are no big deal but there are screws that are in ridiculously awkward placesThere's one last screw I'm still trying to figure out how to get at.

author: zaoka
author_rank: Frequent Contributor
timestamp: July 05, 2014, 03:26:44 am
content: They have different kind of problems because of bad solder connections on ICs.You have to re-solder all ICs of this kind:This usually fixes the problem.

author: Gertjan
author_rank: Frequent Contributor
timestamp: July 05, 2014, 06:47:40 am
content: QuoteI also had these symptoms with my Hameg 204-2 scope.I found the Power Supply Caps were bad. The worst one even was warm to the touch.They are placed in a very warm environment….Also I found a bad solder joint on one of the (very hot) rectifier bridges.I managed to change the power supply capacitors without disassembling the scope/ removing the boards.It was a bit tricky, just being able to touch the right spot with my Solder Iron between the boards, but certainly better than disassembling the boards & those long rod switches etc,!Good luck, Gertjan.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 06, 2014, 05:09:45 pm
content: Quote from: zaoka on July 05, 2014, 03:26:44 amMine is much older than that. All the ICs are through hole and socketed.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 06, 2014, 05:14:32 pm
content: Quote from: Gertjan on July 05, 2014, 06:47:40 amI just removed the last screw that was nearly impossible to get to. I had to use long pliers to slowly turn it but they're finally all out. Now I need to unsolder the TRIG. INP. connector because there's absolutely no way to turn that nut.I'm still not sure if I can remove it though. It's an incredibly tight fit.The 204 was probably built a bit more intelligently

author: Gertjan
author_rank: Frequent Contributor
timestamp: July 06, 2014, 06:29:43 pm
content: QuoteI am afraid the built of the 204-2 is more or less the sameIn any case. this is the moment to replace ALL power supply caps!You don't want to do this operation again when another cap goes bad....And please check the soldering of the rectifiers that get hot.I did this repair about two years ago, and ever since my scope has been performing flawless again.Regards, Gertjan.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 06, 2014, 07:19:42 pm
content: Quote from: Gertjan on July 06, 2014, 06:29:43 pmDid yoursjusthave to buzzing problem I described or did it also have the weird problem where the trace compresses along the right side of the screen when you move the X-POS knob like in my video?http://1drv.ms/1kkrKXRAnyway.. I finally was able to flip it upside down so I'll post photos of it later. It looks like someone has touched up some of the solder points on that board before because they're much shinier and thicker. I'm not sure though.I'm beginning to think it's a problem with a BF458 and a BF470 transistor that are very badly soldered. The BF458 actually has a leg (the base) that is completely not stuck to the solder at all and since it's right next to one of the back corners of the board I can see how flexing it could make the problem come and go.The datasheet even says that it's intended for "Video Output Stages of TV Sets , for AF Output Stages with a High Operating Voltageand as Driver Transistors in Horizontal Deflection Circuit Aplications." so this could explain why my X axis is messing up.I had no electricity for over a day because of a hurricane and I've been traveling around lately so I haven't had allot of time to mess with this so thank you for help.AS for the power supply, I can easily replace the 1000uF 25V caps but I'll probably have to order the 220uF 100V ones online.

author: Gertjan
author_rank: Frequent Contributor
timestamp: July 06, 2014, 08:23:04 pm
content: Indeed, I just had the buzzing problem, with the downward dips in the trace, so I can't help you with the trace compress problem.Make sure that you replace your caps with ones made for high temperatures, 105 degrees Celsius at least. They are in a hot environment, and you don't want to replace them again....Regards, Gertjan.

author: tautech
author_rank: Super Contributor
timestamp: July 06, 2014, 09:09:51 pm
content: QuoteWith this sort of fault, one can reverse trace the signal path from the appropriate side of the CRT to the faulty output stage or in your case the suspected transistor.These video output transistors do fail and it seems like yours have been replaced...badly.You might be lucky and it is only the solder joint.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 09, 2014, 12:15:29 am
content: I'm waiting to get some capacitors to try and fix the power supply problem but here's what I believe to be the cause of the trace squishing up against the right side of the CRT.The soldering on the two BF470 and two BF458 transistors looks horrible. One of the BF470s has a Base that has almost no solder on it and the other two look like they where touched up at some point. The BF458 in these photos looks like it still has it's original solder but for some reason it's sunken into the holes. There's no actual fillet.I probably made it worse while trying to remove the last stubborn screw that held the bracket being used as a heatsink for these transistors but I was able to pick at the leads with my thumbnail and they moved so that's not good.Once I get the capacitors I'll get back to fixing the soldering on these four transistors too.

author: miceuz
author_rank: Frequent Contributor
timestamp: July 09, 2014, 07:27:58 pm
content: Looks sad. I wander, does this look like a self desoldering because of overheating?

author: dentaku
author_rank: Frequent Contributor
timestamp: July 09, 2014, 09:35:21 pm
content: Quote from: miceuz on July 09, 2014, 07:27:58 pmIt kind of does.The funny thing is that all four are attached to the heatsink (just one of the brackets that holds the circuit board up) with a plastic screw with a white ceramic looking washers between the transistor and the metal.The bottom of the board looks a bit darker in the area of the transistors. There are no sil-pads or heatsink compound use anywhere in this oscilloscope.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 31, 2014, 02:38:21 pm
content: OK, I finally got the caps to replace the old ones.I was able to unsolder the three 1000uF 25V caps and one of the 220uF 100V caps very easily. The other 220uF cap is in a horribly difficult to reach spot under the CRT.I used my little MK-168 tester and the 1000uF 25V caps measure very close to 1000uF and ...vloss 0.9%, ESR 0.36ohmvloss 0.9%, ESR 0.36ohmvloss 1.4%, ESR 0.36ohmThe 220uF 100V measuresVloss 1.3%, ESR 0.70ohmThis doesn't look bad to me but is it acceptable for this size of cap?What exactly does Vloss measure?The 1000uF 35V 105deg Nichicon replacements I got measure Vloss 0.8%, ESR 0.34ohmand the 220uF, 100V, 105deg Nichicons measure Vloss 0.6%, ESR 0.31ohmSo they're definitely better but are the readings I get from the old ones really that bad?

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2014, 08:05:02 am
content: QuoteThey look like they may have been OK, but it is peace of mind that they are replaced.I would interpret Vloss to be leakage.Look up the specs of the new Nichicons and see if you can come to a similar figure for leakage.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 01, 2014, 05:55:39 pm
content: Quote from: tautech on August 01, 2014, 08:05:02 amThe .PDF for these caps is tricky to interpret.I'm guessing my 1000uF, for example are on page 5. The 1000uF 35V 16X20 ones.http://canada.newark.com/nichicon/upm1v102mhd6tn/capacitor-alum-elec-1000uf-35v/dp/65R3677?ost=65R3677The Impedance (ohm) MAX. at 20°C / 100kHz is rated as 0.034. I'm guessing that's the ESR? My little Chinese tester reads ESR=0.04ohm so that seems logical.I'm not sure about the leakage figures.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2014, 04:00:18 am
content: Yes, comparing datasheet to test equipment results can be confusing.I suspect the Vloss % will be related to Coulombshttp://en.wikipedia.org/wiki/CoulombImpedance and ESR can be considered the same, the important spec is the rated frequency for ESR as this could determine the end use. (SMPS or Linear PS)(~40 KHz or ~100 Hz)

author: dentaku
author_rank: Frequent Contributor
timestamp: August 03, 2014, 02:38:29 am
content: I just checked the long documentation for this tester and it sort of explains what Vloss is about.Oh well. ESR and capacitance are what most people care about.5.3.6 Voltage loss after a load pulse, VlossWith the measurement of capacitors with big capacity values the voltage loss after the loading isanalysed. The reached load voltage is lost with electrolytic capacitors after a short periode. Thisvoltage loss can be caused by a parallel connected resistor. But I assume, that this voltage lossof electrolytic capacitors is caused by a internal load dispersion directly after the load pulse. Byloading the capacitors with the 470kresistor, as it is done for little capacity values, this dispersionis already done after switching o the current. No voltage loss is detectable for this case. But if youload the same capacitor with a short current pulse, you can also detect the voltage loss for capacitorswith lower capacity. The same e ect with lower loss can also be noticed for ceramic type capacitors.I have noticed, that capacitors with more than some % voltage loss are suspect. Especially noticablewith respect to the voltage loss are older paper type capacitors, which are for other measurement aproblem too.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 09, 2014, 12:43:11 am
content: Well, it's been a month since I posted the first messageI removed and re-soldered those 4 transistors quite a while ago but waited for an ESR tester and some new capacitors before re-assembling the whole thing and yesterday I finally started it back up and I can move the X-Pos knob now and it seems to work fine.There's still one big capacitor that I didn't replace (it was in an impossible to reach place) but since the others seemed to be OK and an in-circuit test of the impossible to get to capacitor measured similar to it's partner next to it that I managed to remove it's probably fine.I have some doubt about one of the little bridge rectifiers because it seems to be quite warm compared to the others but that's not too hard to replace.I guess now I just need to use it for a while and see how stable it is.

author: dentaku
author_rank: Frequent Contributor
timestamp: September 15, 2014, 07:02:59 pm
content: Quote from: Gertjan on July 05, 2014, 06:47:40 amI know this post is old but since I've re-soldered those 4 transistors and the X-POS now works right I only have the occasional time where that trace goes crazy if the scope has been on for a while but turning the scope off then back on again fixes it. It still hums so I used my multimeter thermocouple to test out the little bridge rectifiers and I don't remember the numbers anymore but two out of the five of them got very hot compared to the others.Would you be able to tell me how hot yours get? I know I have an HM-605 and not a 204 but it would be interesting to know if it's normal that two of them are so much hotter.I should be able to do some measurements on it tonight an post them here.

author: Gertjan
author_rank: Frequent Contributor
timestamp: September 17, 2014, 10:15:32 am
content: I don't know exactly how hot the rectifiers are, but I do remember that one or two were too hot to touch.Problem is that the transformer has to deliver large peak currents into the big reservoir capacitors.(It charges the capacitors only at the peak of the sine-wave)That is also why the transformer is humming.In my case it didn't help that the Mains voltage where I live was raised from 220V to 230V.....A solution is to put low-Ohmic resistors between the transformer and the rectifiers to reduce these currents.Your power supply will also gets cleaner, it is reducing the rectifier hash.(For that reason it is standard practice in medical electronics)I prototyped this solution and the Transformer Hum went away.However in the end I couldn't find room for these extra power resistors (which also get warm), as it is crampy in the scope around the transformer / power supply.The rectifiers should be able to cope with the heat (it is within their specification) Only thing is their solder connections can go bad because of the expansion and shrinking caused by the temperature changes.

author: dentaku
author_rank: Frequent Contributor
timestamp: September 17, 2014, 11:38:57 am
content: Yup, two of hem are way too hot to touch. They're in a much easier spot to reach so I can check if the solder has gone bad like the 4 transistors I needed to re-solder but there's nothing obviously bad about them just looking at them.Oh well, this is a learning experience for me considering I've never done anything like this.