FIXED! HM205-2 storage not working - Page 1
Forum section: electronics / repair

author: calli
author_rank: Regular Contributor
timestamp: March 18, 2016, 07:50:48 pm
content: Hi,I had an ebay accidentNot "quite" $19Today it came in, a Hameg 205-2. It is old, it is filthy, its not even grey its brown, like the main color in a retirement home. Bahh.But no pixels! And fast.All but the digital storage seems to work, the switches are better that I feared after some usage. Will open it to reseat the digital chips and to try some contact spray.Will keep this thread updated, I also have some questions on hold.Cheers,Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 19, 2016, 04:52:28 pm
content: So.I opened that thing and bahhhh. Filthty... No idea how to clean all that old smoke and tar off the board under the air vents.Ok, managed to get some contact spray into some switches and it works ok for me now. I was surprised that this model is MUCH different than the one Dave had (the $19 one). Did not expect such differences (205-3 mine is 205-2).And guess what, Storage mode is not working. I removed it (much harder to access than on 205-3) and removed all chips, cleanded the sockets and reseated the chips. No change.It looks to me that Hold I is not working at all, Hold II is working somewhat, I see a curve but it looks as not triggered and when holding it it is not the right horizontal scale. Single Shot is also not triggered. I made two videos in case someone has an idea at which placed I can look for the error. Don't expect Dave grade fun videosCheers,Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 19, 2016, 06:04:34 pm
content: Found the Timebase chip its on the TB-Board (sic!). It generates a clock according to the switch when STOR. is on. The clock also reaches the STOR-BRD. But now its hard, because the Storage board is UNDER the TB-Board... Grrrrrr...Still searching where the trigger comes into play in digital mode.Oh, if someone wants to have a look at the manual/service/schematics:https://dl.dropboxusercontent.com/u/4898693/Hameg_Manual_HM205-2.pdfHope R&S don't sue me.Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 19, 2016, 08:37:16 pm
content: Found the Trigger signal. Reaches the Store Board.So say I can somehow check the Store board while powered, there was a talk that there are many test pinson the boards, and that they are marked in the schematics. Hmmm. I can't find them? Help!1) I would start checking the voltages at the ic chips.2) TBA. ;-)I know maybe it is hard to find help in such a special case but put away theand give me some general advices pleaseCarsten

author: TAMHAN
author_rank: Frequent Contributor
timestamp: March 19, 2016, 09:10:24 pm
content: Oh, a docent at a German university - had nice memories of being a guest speaker at Isarlohn.IMHO, this might be interesting;http://www.mikrocontroller.net/topic/176108My approach for testing combiscopes:x) Does he trigger?x) Is there motion on the RAM pinsDo you have a logic analyzer? if no, Rucka Rucka Ali will gladly ship you one - order one from AliExpress, the 30USd units are more than enough for what we plan here.

author: calli
author_rank: Regular Contributor
timestamp: March 19, 2016, 09:56:30 pm
content: I have a bit scope dso which has 8 Channels. It seems not to Trigger. (In Storage mode!)But right now I have first to solve the Problem that the Logic Board is under the TB Board. Maybe I can use Jumper wires to extend the cables so i have it at the side to do actual measures.May try that tomorrow. It is not that I really need this but I really like to solve that Puzzle and learn a bit.Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 20, 2016, 01:51:07 pm
content: So. I used jumper wires to bring the storage board into a position where I can measure on it.I checked some chips for voltage, 4.8V mostly. But some had also 4.4V... fishy?I traced the trigger pulse from the header to the first flip flop. I could not find any digital pulses there on the flipflop, so I tried the clock lines and found some. After some tinkering I started again to check the voltages on the chips.I found at least one Chip (a 74HC00) which gets 4,8V when STOR is not on, but has 0V when it is on. I need now a break, any hints what next? Just replace such a chip? Or is the problem elsewere?Short: I am lost.Carsten

author: sotos
author_rank: Frequent Contributor
timestamp: March 20, 2016, 02:50:58 pm
content: If you are suspicious about it, change it, it’s cheap, easy to find and on a socket.Why not try changing them with new ones; they are all on sockets and easy to change and see.

author: calli
author_rank: Regular Contributor
timestamp: March 20, 2016, 07:57:53 pm
content: Is it normally safe to remove a Chip and then Power on to check the voltage on the socket alone?Carsten

author: sotos
author_rank: Frequent Contributor
timestamp: March 20, 2016, 08:13:41 pm
content: No problem. Power off, remove the chip and then power on. Measure voltage, which you suspect.Don’t forget to power on /off when removing the Ics.

author: KJDS
author_rank: Super Contributor
timestamp: March 20, 2016, 08:43:44 pm
content: Quote from: sotos on March 20, 2016, 08:13:41 pmDon't forget to put it back in the right way round. I know from experience that getting it the wrong way round isn't good.

author: oldway
author_rank: Super Contributor
timestamp: March 20, 2016, 08:50:38 pm
content: Why worry about the digital function of this oscilloscope ?It is very old school digital technology and almost unusable.Use it only as an analog scope.

author: calli
author_rank: Regular Contributor
timestamp: March 20, 2016, 09:35:19 pm
content: @oldway i am doing this like other people do cross Word Puzzles or such. I want some brain Training and learning something. And also that is a classic scope would you ask a owner of a classic car why he fixes it? 😆😊Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 20, 2016, 10:42:25 pm
content: maybe i should ask a mod to move this into the repair forum?Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 21, 2016, 11:41:04 am
content: Am I right that if I measure 0V or 2.5V VCC on a 74HCxx chip thats a bad sign?I found 4 of those close by another on the board (IC11, IC17, IC15, IC7).I swapped two of the same kind (from elsewere) and the error was the same. No idea if this helps but I have the idea that thas means at least the chip on the 0V socket is not damaged...So now find the cause of the 0V. Blocking Capacitor? Any advises what I can/should do/look for?Censored because of extremely user stupidness.Cheers,Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 21, 2016, 12:16:17 pm
content: I am still thinking about this:I traced the trigger pulse from the header to the first flip flop. I could not find any digital pulses there on the flipflop, so I tried the clock lines and found some. After some tinkering I started again to check the voltages on the chips.The trigger impulse comes as nice square wave, frequency derived from the horizontal setting. Then it goes through a rc filter (?) and after this it is spikes which goes to the flipflop. I rebuild this part in a circuit simulator and this seems right. But the I should (i think) get some logic levels on the flip flop which is next in the signal flow?Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 21, 2016, 03:04:55 pm
content: Someone still watching?This is way top of my head :-) But hey, still somewhat fun. I now really have an idea how that thing works and the purpose of all ICs.As I could not get further from the Triger input side I did start from the other input. I checked both ADCs and they seem to work ok. Also the DAC is doing its job and outputs digital values. I removed the RAM chip and then I can see the first time the ADC->DACed Channel ONE when I press HOLD II. Then there is nothing from the RAM and voila, the CH1 input is there (Still not triggered of course).So still searching where a signal from CLK and Trigger is generated.Carsten

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 22, 2016, 03:20:30 pm
content: Datasheet of IC can help.

author: calli
author_rank: Regular Contributor
timestamp: March 22, 2016, 03:45:47 pm
content: I think I found a faulty 74HC02. Just ordered some chips.It is a bit hit and miss measurement with my USB DSO (mixed mode). But on one 74HC02 there is a gate used to invert a signal, on the inputs it is there on the output not.I think(!) this also causes many quirks such as the mixed up and not triggered digital signals.Lets wait for the chips.Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 22, 2016, 06:42:08 pm
content: Just a quick update: after the last post I thought that it would be interesting to remove the defunc. chip. And yes, then I was able to see both chanels in HOLD mode, depending on pressing HI or HII. So I am confident that I did found at least ONE error ;-)Carsten

author: Anks
author_rank: Frequent Contributor
timestamp: March 22, 2016, 07:54:48 pm
content: Are all the gates on the 74HC02 being used? Sometimes when I've been fixing arcade boards you could use some of the unused gate to make a temporary repair just to check that you had found the problem if you didn't have any spares to hand at the time.

author: calli
author_rank: Regular Contributor
timestamp: March 22, 2016, 08:52:29 pm
content: @Anks: Great idea, but no waste of silicon in there :-)Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 24, 2016, 10:41:35 am
content: WTF!It WORKS! Sorry. I am so happy. Only one f.... Gate in a 74HC02!But I found it and I now know how that thing works, great.Thanks to all who did follow here and had nice words and head up.Carsten

author: calli
author_rank: Regular Contributor
timestamp: March 24, 2016, 10:48:51 am
content: Despite of the age and the quality of the digital part it is still quite usefull! Especially the single shot. At the time I need to start the USB DSO, klick all that pesky little buttons (which don't remember their last settings) and find a trigger in that little window I had made the measurement with the Hameg.Carsten

author: BFX
author_rank: Frequent Contributor
timestamp: March 24, 2016, 08:34:50 pm
content: Good to know that next Hameg scope is workingIt's nice scope's I like it.I have few to repair (only for training) that I order two for 25 EUR. (only one trouble is there back cover is missing)I looking forward for repairWill be soon I hope

author: TAMHAN
author_rank: Frequent Contributor
timestamp: March 26, 2016, 05:37:13 pm
content: Wanted to post congratulations before, but was held up. Congrats on the repair, hope the scope works a-ok.P.S. I will be in a similar hell soon. Iwatsu DS-6121 with ALL KINDS of flaws...

author: calli
author_rank: Regular Contributor
timestamp: March 26, 2016, 06:54:53 pm
content: Thanks! It was a fun experience and teached me a lot about scopes and digital technics. So heads up for your project.Carsten